Examples of Calculations on Sea Waves.

1. The wave of 1854.—This wave originated near Japan, and it was recorded on tide gauges at San Francisco, San Diego, and Astoria.

On December 23, at 9.15. a.m., a strong shock was felt at Simoda in Japan, which, at 10 o’clock, was followed by a large wave thirty feet in height. The rising and falling of the water continued until noon. Half an hour after, the movement became more violent than before. At 2.15 p.m. this agitation decreased, and at 3 p.m. it was comparatively slow. Altogether there were five large waves.

On December 23 and 25, unusual waves were recorded upon the self-registering tide gauges at San Francisco, San Diego, and Astoria.

At San Francisco three sets of waves were observed. The average time of oscillation of one of the first set was thirty-five minutes, whilst one of the second and third sets was almost thirty-one minutes.

At San Diego three series of waves were also shown, but with average times of oscillation of from four to two minutes shorter than the waves at San Francisco.

The San Francisco waves appear to indicate a recurrence of the same phenomena.

The record at San Diego shows what was probably the effect of a series of impulses, the heights increasing to the third wave, then diminishing, then once more renewed, after which it died away.

The result of calculations based on these data were:—

	Distance geographical miles	Time of transmission		Velocity in feet per sec.	Depth of ocean in fathoms
		h.	m.
Simoda to San Diego	4917	12	13	545	2100
Simoda to San Francisco	4527	12	39	528	2500 or 2230

The difference for the depths in the San Francisco path depends whether the length of the waves is reckoned at 210 or 217 miles. The length of the waves on the San Diego path were 186 or 192 miles.[82]

The wave of 1868.—On August 11, 1868, a sea wave ruined many cities on the South American coast, and 25,000 lives were lost. This wave, like all the others, travelled the length and breadth of the Pacific.

In Japan, at Hakodate, it was observed by Captain T. Blakiston, R.A., who very kindly gave me the following account:

On August 15, at 10.30 a.m., a series of bores or tidal waves commenced, and lasted until 3 p.m. In ten minutes there was a difference in the sea level of ten feet, the water rising above high water and falling below low water mark with great rapidity. The ordinary tide is only two and a half to three feet. The disturbance producing these waves originated between Iquique and Arica, in about lat. 18.28 S. at about 5 p.m. on August 13. In Greenwich time this would be about 13h. 9m. 40s. August 13. The arrival of the wave at Hakodate in Greenwich time would be about 14h. 7m. 6s. August 14: that is to say, the wave took about 24h. 57m. to travel about 8,700 miles, which gives us an average rate of about 511 feet per second. These waves were felt all over the Pacific. At the Chatham Islands they rushed in with such violence that whole settlements were destroyed. At the Sandwich Islands the sea oscillated at intervals of ten minutes for three days.

Comparing this wave with the one of 1877 we see that one reached Hakodate with a velocity of 511 feet per second, whilst the other travelled the same distance at 512 feet per second.

An account of this earthquake wave has been given by F. von Hochstetter (‘Über das Erdbeben in Peru am 13. August 1868 und die dadurch veranlassten Fluthwellen im Pacifischen Ocean,’ Sitzungsberichte der Kaiserl. Akademie der Wissenschaften, Wien 58. Bd., 2. Abth. 1868). From an epitome of this paper given in ‘Petermann’s Geograph. Mittheil.’ 1869, p. 222, I have drawn up the following table of the more important results obtained by F. von Hochstetter.

The wave is assumed to have originated near Arica.

	Distance sea miles from Arica	Time taken by wave		Velocity in feet per second	Depth of ocean in feet
		h.	m.
Valdivia	1,420	5	0	479	7,140
Chatham Islands	5,520	15	19	608	11,472
Lyttleton	6,120	19	18	533	8,838
Newcastle	7,380	22	28	538	9,006
Apia (Samoa)	5,760	16	2	604	11,346
Rapa	4,057	11	11	611	11,598
Hilo	5,400	14	25	555	9,568
Honolulu	5,580	12	37	746	17,292

Calculations on the same disturbance are also given by J. E. Hilgard.[83]

Assuming the origin of the wave to have been at Arica, his results are as follows:

	Distance from Africa	Time of transmission		Nautical miles per hour	Mean depth of ocean
	miles	h.	m.		feet
San Diego	4,030	10	55	369	12,100
Fort Point	4,480	12	56	348	10,800
Astoria	5,000	18	51	265	6,200
Kodiak	6,200	22	00	282	7,000
Rapa	4,057	10	54	372	12,200
Chatham Islands	5,520	15	01	368	12,100
Hawaii	5,460	14	10	385	13,200
Honolulu	5,580	12	18	454	18,500
Samoa	5,760	15	38	368	12,100
Lyttelton	6,120	19	01	322	9,200
Newcastle	9,380	22	10	332	9,800
Sydney	7,440	23	41	314	8,800

The wave of 1877.—Two sets of calculations have been made upon the wave of 1877 by Dr. E. Geinitz of Rostock.[84]

The following table is taken from Dr. Geinitz’s second paper, in which there are several modifications of his first results. The origin of the disturbance is assumed to have been near Iquique.

Observation stations	Distance from Iquique geol. miles	Arrival of wave			Time taken by wave		Velocity in feet per second	Mean depth of ocean in fathoms
		h.	m.		h.	m.
Taiohāc (Marquesa Islands)	4,086	8	40	a.m.	12	15	563·8	1,647
Apia (Samoa)	5,740	12	0	„	15	30	610·4	1,930
Hilo (Sandwich Islands)	5,526	10	24	„	14	0	667·9	2,310
Kahuliu „	5,628	10	30	„	14	5	675·2	2,361
Honolulu „	5,712	10	50	„	14	25	669·7	2,319
Wellington (New Zealand)	5,657	2	40	p.m.	18	15	524·2	1,430
Lyttelton „	5,641	2	48	„	18	23	519·8	1,400
Newcastle (Australia)	6,800	2	32	„	18	7	633·0	2,075
Sydney „	6,782	2	35	„	18	10	631·4	2,065
Kamieshi (Japan)	8,790	7	20	„	22	55	649·0	2,182
Hakodate „	8,760	9	25	„	25	0	592·5	1,818
Kadsusa „	8,939	9	50	„	25	15	604·9	1,895

The mean depths represent a mean of two sets of calculations, one made with the aid of Airy’s formula, and the other by Scott-Russell’s formula. The result of my own investigation about this disturbance, the origin of which, by several methods of calculation, is shown to have been beneath the ocean, near 71° 5′ west long., and 21° 22′ south lat., are given on next page.

Dr. Geinitz considers that his calculated depths of the ocean and those obtained by actual soundings are in accordance, a result which is diametrically opposed to that which I have obtained.

This difference between my calculations and those of Dr. Geinitz, Hochstetter, and others, chiefly rests on the origin we have assigned for the sea waves. Dr. Geinitz, for instance, although he says that the origin of the 1877 earthquake was not on the continent but to the west in the ocean, bases all his calculations on the assumption that the centrum was at or near to Iquique, and the time at which that city was disturbed was the time at which the waves commenced to spread across the ocean. This time is 8.25 p.m. At this time, however, it appears that the waves must have been more than double the distance between the true origin and Iquique, from Iquique on their way towards the opposite side of the Pacific. Introducing this element into the various calculations which have been made respecting the depth of the Pacific Ocean as derived from observations on earthquake waves—which element, insomuch as the waves appear to have come in to inundate the land some time after the shock, needs to be introduced—we reduce the velocity of transit of the earthquake wave and, consequently, the resultant depths of the ocean.

Longitude

Arrival of
wave in
Greenwich
mean time

Time taken
by wave

Distance
from the
origin
in miles
(calculated
in great
circles)

Velocity
in feet
per second

Depth of
the ocean
in feet

Height
of waves

Interval
between waves
in minutes

′

day

Origin of wave

San Francisco

122

4,578

498

7,721

9 in.

Callao

658

231

1,657

Iquique

14½

348

3,770

20 ft.

Cobija

352

3,857

30 „

Mejillones

108

339

3,587

35 „

15 or 45

Chanaral

455

272

2,309

Coquimbo

508

328

3,363

Valparaiso

695

310

3,000

Concepcion

928

350

3,824

12 to 15

Honolulu

157

5,694

561

9,807

34 to 54 ft.

Hilo

155

5,506

563

10,217

30 or 8 „

{	3 or 15 18 or 27

Kahuliu

156

5,611

579

10,437

Samoa

171

5,773

566

9,972

12 ft.

Taurauga

176

5,615

427

5,697

Wellington

174

5,574

445

6,168

11 „

Akaroa

172

5,542

440

6,031

Lyttelton

172

5,558

441

6,055

Kameishi

140

8,844

549

9,378

6 „

Hakodate

140

8,778

512

8,169

7 „

In Dr. Geinitz’s paper there are also some slight differences in the times at which the earthquake phenomena were observed at various localities. These, however, are but of minor importance. At the end of the paper by Dr. Geinitz two interesting tide gauge records are introduced, one from Sydney and the other from Newcastle. These appear to show a marked difference in the periods of the sea waves at these two places.[85]

Comparison of velocities of wave-transit which have been actually observed, with velocities which ought to exist from what we know of the depth of the Pacific by actual soundings.—From a chart given in ‘Petermann’s Geograph. Mittheilungen,’ Band xxiii. p. 164, 1877, it is possible to draw approximate sections on lines in various directions across the bed of the Pacific.

From the origin of the shock to Japan (Kameishi) the line would be as follows:—

about 7,441	miles	15,000	feet deep
1,100	„	18,000	„
160	„	27,000	„
80	„	12,000	„
60	„	6,000	„

On account of the Tuscarora and Belkap Deeps this would be the most irregular line over which the wave had to travel.

From the origin to New Zealand (Wellington) the line would be

about 5,274	miles	15,000	feet deep
„ 300	„	12,000	„

From the origin to Samoa the line would be

about 5,773

miles

15,000

feet deep

From the origin to the Sandwich Islands (Honolulu) the line would be

almost 5,634	miles	15,000	feet deep
and 60	„	12,000	„

By Scott-Russell’s rule, or, what is almost identically the same, by Airy’s general formula, we can calculate how long it would take such waves as we have been speaking about to travel over the different portions of each of these lines, and by adding these times together we obtain the time taken to travel across any one line. I have made these calculations, but as I get in every case answers which are too small, I think it unnecessary to give them.

The actual times taken to travel the distances just referred to were,

To Japan (Kameishi)	23 hr. 38 min.
„ New Zealand (Wellington)	18 „ 23 „
„ Samoa	14 „ 58 „
„ Sandwich Islands (Honolulu)	14 „ 53 „

From San Francisco to Simoda the line is almost 3,567 miles, 3,000 fathoms deep, 840 miles, 2,500 fathoms deep, and 120 miles 1,000 fathoms deep. This gives an average depth of about 2,854 fathoms. Bache calculated the depth at 2,500 fathoms.

If we are to consider that, because the sea wave at Simoda came in some time after the land shock had been felt, the origin of this earthquake, instead of being at Simoda, was some distance out at sea, this calculated depth would be reduced.

CHAPTER X.
DETERMINATION OF EARTHQUAKE ORIGINS.

Approximate determination of an Origin—Earthquake-hunting in Japan—Determinations by direction of motion—Direction indicated by destruction of buildings—Direction determined by rotation—Cause of rotation—The use of time observations—Errors in such observations—Origin determined by the method of straight lines—The method of circles, the method of hyperbolas, the method of co-ordinates—Haughton’s method—Difference in time between sound, earth, and water waves—Method of Seebach.

One of the most practical problems which can be suggested to the seismologist is the determination of the district or districts in any given country from which earthquake disturbances originate. With a map of a country before us, shaded with tints of different intensity to indicate the relative frequency of seismic disturbance in various districts, we at once see the localities where we might dwell with the least disturbance, and those we should seek if we wish to make observational seismology a study. Before erecting observatories for the systematic investigation of earthquakes in a country, it would be necessary for us, in some way or other, to examine the proposed country to find out the most suitable district. The special problem of determining approximately the origin or origins of a set of earthquakes would be given to us. Having made this preliminary investigation, the next point is, by means of observatories so arranged that they could always work in conjunction with each other, to determine the origins more accurately. By knowing the origin from which a set of shocks spring we know the general direction in which we may expect the most violent disturbances, and we can arrange our seismometers accordingly.

Approximate determination of origins.—In 1880 I obtained a tolerably fair idea of the distribution of seismic energy throughout Japan, by compiling the facts obtained from some hundreds of communications received from various parts of the country respecting the number of earthquakes that had been felt.

The communications were replies to letters sent to various residents in the country and to a large number of public officers. By taking these records, in conjunction with the records made by instruments, it was ascertained that in Japan alone there were certainly 1,200 shocks felt during the year, that is to say, three or four shocks per day. The greater number of these shocks were felt along the eastern coast, commencing at Tokio, in the south, and going northwards to the end of the main island. These shocks were seldom felt on the west coast. It appeared as if the central range of mountains formed a barrier to their progress. Similarly, ranges of mountains to the south-west of Tokio prevented the shocks from travelling southwards. Proceeding in this way the conclusion was arrived at that the west coast, the southern part of Japan, and the islands of Shikoku and Kiushiu, had their own local earthquakes.

Earthquake-hunting.—These preliminary enquiries having shown that the northern part of Japan was a better district for seismological observations than the southern half, the next step was to subject the northern half to a closer analysis. This analysis was commenced by sending to all the important towns, from thirty to one hundred miles distant from Tokio, bundles of postcards. These were entrusted to the local government offices with a request that each week one of these cards would be returned to Tokio stating the number of shocks felt. In this way it was quickly discovered that the majority of shakings emanated from the north and east, and seldom, if ever, passed a heavy range of mountains to the south. The barricade of postcards was then extended farther northwards, with the result of surrounding the origin of certain shocks amongst the mountains, whilst others were traced to the sea shore. By systematically pursuing earthquakes it was seen that many shocks had their origin beneath the sea—they shook all the places on the north-east coast, but it was seldom that they crossed through the mountains, forming the backbone of the island, to disturb the places on the west coast.

The actual results obtained in three months by this method of working are shown in the accompanying map, which embraces the northern half of the main island of Nipon and part of Yezo. The shaded portion of the map indicates the mountainous districts, which are traversed by ranges varying in height from about 2,000 and 7,000 feet. The dotted lines show the boundaries of the more important groups of earthquakes which were recorded.

I. is the western boundary of earthquakes, which at places to the eastward are usually felt somewhat severely. Some of these have been felt the most severely at or near Hakodadi, whilst farther south their effects have been weak. Occasionally the greatest effect has been near to Kameishi. Sometimes these earthquakes terminate along the western boundaries of III. or IV., not being able to pass the high range of mountains which separate the plain of Musashi from Kofu.

Fig. 29.—Northern Japan. Mountainous districts shaded with oblique lines.

II. is the boundary of a shock confined to the plain which surrounds Kofu. These earthquakes are evidently quite local. Many of the disturbances have evidently originated beneath the ocean, having come in upon the land in the direction of the arrows a or b.

III. This line indicates the boundary of a group of shocks which are often experienced in Tokio. These may come in the directions d, e, or f. It is probable that some of them originate to the eastward of Yokohama, on or near to the opposite peninsula.

IV. V. and VI. The earthquakes bounded by these lines probably originate in the directions c or d.

VII. The earthquakes bounded by this line probably come from the direction e.

VIII. This line gives us the boundary of earthquakes which may come from the direction b.

The above boundaries sometimes do not extend so far to the westward as they are shown. At other times, groups like V. and VI. extend farther to the south-west. These earthquake boundaries, which so clearly show the effects of high mountains in preventing the extension of motion, have been drawn up, not from single earthquakes, but from a large series of earthquakes which have been plotted upon blank maps, and are now bound together to form an atlas. To give an idea of the material upon which I have been working, I may state that between March 1 and March 10, 1882, I received records of no less than thirty-four distinct shocks felt in districts between Hakodate and Tokio, and for each of these it is quite possible to draw a map. In addition to the boundaries of disturbances given in the accompanying map, other boundaries might be drawn for shocks which were more local in their character. The groups which contained the greatest number of shocks are III., IV., V., VI., and VII. By work of this description it was found that a very important group of earthquakes might be studied by a line of stations commencing at Saporo in the north, passing through Hakodate, down the east coast of the main island, to Tokio or Yokohama in the south. A further aid to the study of this group, together with the study of an important local group, might be effected with the help of a few additional stations properly distributed on the plain of Musashi, which surrounds Tokio. With this example before us it will be recognised that the choice of sites for a connected set of seismological observatories will often be more or less a special problem. If earthquake stations were to be placed in different directions around Tokio without preliminary investigation, it is quite possible that some of them might be so situated that they would seldom if ever work in conjunction with the remaining observatories, and therefore be of but little value. And this remark must equally apply to districts in other portions of the globe. The method is crude, and, so far as actual earthquake origins are concerned, it only yields results which are approximate. The crudeness and the want of absoluteness in the results is, however, more than counterbalanced by the certainty with which we are enabled to express ourselves with regard to such results as are obtained. Even when working with the best instruments we have at our command, unless we are employing some elaborate system, this method of working gives a most valuable check upon our instrumental records, and enables us to interpret them with greater confidence.

Determination of earthquake origins from the direction of motion.—If we assume that an earthquake is propagated from a centre as a series of waves, in which normal vibrations are conspicuous, and obtain at two localities, not in the same straight line with the origin, and sufficiently far separated from each other, the direction of movement of these normal motions, by drawing lines parallel to these directions through our two stations, the lines would intersect at a point above the required origin. If instead of two points we had three, or, better still, a large number, the results we should obtain ought to be still more certain. Unfortunately, it seems that earthquakes seldom originate from a given point, and, further, normal motions are not always (sufficiently) prominent. Sometimes, as has already been shown in the chapters on earthquake motion, they may be non-existent. It is probable, however, that difficulties of this sort are more usually associated with non-destructive earthquakes. Mallet regards the destructive effects of an earthquake as almost solely due to normal motions. If this be true, for destructive earthquakes, the problem is shorn of many of its difficulties. In cases where normal vibrations are not prominent, where we have only transverse vibrations, motions due to the interference of normal or transverse motions, or directions of motions due to the topographical or geological nature through which the disturbance has passed, the determination of the origin of an earthquake by observations on the direction in which the ground has been moving appears to be a problem which is practically without a solution. We will, therefore, only consider the determination of the origin of those earthquakes which have predominating directions in their movements, which directions we will consider as normal ones. The question which is, then, before us, is the determination of the direction of these normal movements. First of all we may take the evidence of our senses. In exceptional cases these have given results which closely approximate to the truth, but in the majority of cases such results are not to be relied upon, as the inhabitants of a town will, for the same shock, give directions corresponding to all points of the compass. Much, no doubt, depends upon the situation of the observer, and much, perhaps, upon his temperament. If he is sitting in a room alone, and is accustomed to making observations on an earthquake, on feeling the earthquake, if he concentrates his attention on the direction in which he is being moved, his observations may be of value. If, however, he is not so situated, and his attention is not thus concentrated, his opinions, unless the motion has been very decided in its character, are usually of but little worth.

Direction determined from destruction of buildings.—When an observer first sees a town that has been partially shattered by an earthquake, all appears to be confusion, and it is difficult to imagine that in such apparent chaos we are able to discover laws. If, however, we take a general view of this destruction and compare together similarly built buildings, it is possible to discover that similar and similarly situated structures have suffered in a similar manner. By carefully analysing the destruction we are enabled to infer the direction in which the destroying forces have acted. It was chiefly by observing the cracks in buildings, and the direction in which bodies were overthrown or projected, that Mallet determined the origin of the Neapolitan earthquake. From the observations given in Chapter VII. it would appear that, with destructive earthquakes, walls which are transverse to the direction of motion are most likely to be overturned, whilst, with small earthquakes, these walls are the least liable to be fractured.

From a critical examination of the general nature of the damage done on the buildings of a town, earthquake observers have shown that the direction of a shock may often be approximately determined. The direction in which a body having a regular form like a prismatic gravestone or a cylindrical column is overturned sometimes gives the means by which we can determine the direction from which a movement came.

The rotation of bodies.—It has often been observed that almost all large earthquakes have caused objects like tombstones, obelisks, chimneys, &c., to rotate.

One of the most natural and at the same time most simple explanations is to suppose that during the shock there had been a twisting, or backward and forward screw-like motion in the ground. Amongst the Italians and the Mexicans earthquakes producing an effect like this are spoken of as ‘vorticosi.’ In the Calabrian earthquake, not only were bodies like obelisks twisted on their bases, but straight rows of trees seem to have been left in interrupted zigzags. These latter phenomena have been explained upon the assumption of the interference of direct waves and reflected waves, the consequence of which being that points in close proximity might be caused to move in opposite directions. Reflections such as these would be most likely to occur near to the junction of strata of different elasticity, and it may be remarked that it is often near such places that much twisting has been observed.

Another way in which it is possible for twisting to have taken place would be by the interference of the normal and transverse waves which probably always exist in an earthquake shock, or by the meeting of the parts of the normal wave itself, one having travelled in a direct line from the origin, whilst the other, travelling through variable material, has had its direction changed.

Mallet, however, has shown that the rotation may have been in many cases brought about without the supposition of any actual twisting motion of the earth—a simple backward and forward motion being quite sufficient. If one block of stone rests upon another, and the two are shaken backwards and forwards in a straight line, and if the vertical through the centre of gravity of the upper block does not coincide with the point where there is the greatest friction between the blocks, rotation must take place. If the vertical through the centre of gravity falls on one side of the centre of friction, the rotation would be in one direction, whilst, if on the other side, the rotation would be in the opposite direction.

Although the above explanation is simple, and also in many cases probably true, it hardly appears sufficient to account for all the phenomena which have been observed.

Thus, for instance, if the stones in the Yokohama cemetery, at the time of the earthquake of 1880, had been twisted in consequence of the cause suggested by Mallet, we should most certainly have found that some stones had turned in one direction whilst others had been twisted in another. By a careful examination of the rotated stones, I found that every stone—the stones being in parallel lines—had revolved in the same direction, namely in a direction opposite to that of the hands of a watch.

As it would seem highly improbable that the centre of greatest friction in all these stones of different sizes and shapes should have been at the same side of their centres of gravity, an effect like this could only be explained by the conjoint action of two successive shocks, the direction of one being transverse to the other.

Although fully recognising the sufficiency of two transverse shocks to produce the effects which have been observed in Yokohama, I will offer what appears to me to be the true explanation of this phenomenon: it was first suggested by my colleague, Mr. Gray, and appears to be simpler than any with which I am acquainted.

Fig. 30.

If any columnar-like object, for example a prism which the basal section is represented by a b c d (see fig. 30), receives a shock at right angles to b c, there will be a tendency for the inertia of the body to cause it to overturn on the edge b c. If the shock were at right angles to d c, the tendency would be to overturn on the edge d c. If the shock were in the direction of the diagonal c a, the tendency would be to overturn on the point c. Let us, however, now suppose the impulse to be in some direction like e g, where g is the centre of gravity of the body. For simplicity we may imagine the overturning effect to be an impulse given through g in an opposite direction—that is, in the direction g e. This force will tend to tip or make the body bear heavily on c, and at the same time to whirl round c as an axis, the direction of turn being in the direction of the hands of a watch. If, however, the direction of impulse had been e′ g, then, although the turning would still have been round c, the direction would have been opposite to that of the hands of a watch.

To put these statements in another form, imagine g e′ to be resolved into two components, one of them along g c and the other at right angles, g f. Here the component of the direction g c tends to make the body tip on c, whilst the other component along g f causes revolution. Similarly g e may be resolved into its two components g c and g f′, the latter being the one tending to cause revolution.

From this we see that if a body has a rectangular section, so long as it is acted upon by a shock which is parallel to its sides or to its diagonals, there ought not to be any revolution. If we divide our section a b c d up into eight divisions by lines through these directions, we shall see that any shock the direction of which passes through any of the octants which are shaded will cause a positive revolution in the body—that is to say, a revolution corresponding in its direction to that of the movements of the hands of a watch; whilst if its direction passes through any of the remaining octants the revolution will be negative, or opposite to that of the hands of a watch. From the direction in which any given stone has turned, we can therefore give two sets of limits between one of which the shock must have come.

Further, it will be observed that the tendency of the turning is to bring a stone, like the one we are discussing, broadside on to the shock; therefore, if a stone with a rectangular cross section has turned sufficiently, the direction of a shock will be parallel to one of its faces, but if it has not turned sufficiently it will be more nearly parallel to its faces in their new position than it was to its faces when in their original position.

If a stone receives a shock nearly parallel with its diagonal, on account of its instability it may turn either positively or negatively according as the friction on its base or some irregularity of surface bearing most influence. Similarly, if a stone receives a shock parallel to one of its faces, the twisting may be either positive or negative, but the probability is that it would only turn slightly; whereas in the former case, where the shock was nearly parallel to a diagonal, the turning would probably be great.

Determination of direction from instruments.—When speaking about earthquakes it was shown, as the result of many observations, that the same earthquake in the space of a few seconds, although it may sometimes have only one direction of motion, very often has many directions of motion. In certain cases, therefore, our records, if we assume the most permanent motions to be normal ones, give definite and valuable results. In other cases it is necessary to carefully analyse the records, comparing those taken at one station with those taken at another.

One remarkable fact which has been pointed out in reference to artificial earthquakes produced by exploding charges of gunpowder or dynamite, and also with regard to certain earthquakes, is that the greatest motion of the ground is inwards, towards the point from which the disturbance originated. Should this prove the rule, it gives a means of determining, not only the direction of earthquake, but the side from which it came.

Determination of earthquake origins by time observations.—The times at which an earthquake was felt at a number of stations are among the most important observations which can be made for the determination of an earthquake origin. The methods of making time observations, and the difficulties which have to be overcome, have already been described. When determining the direction from which a shock has originated, or determining the origin of the shock by means of time observations, it has been usual to assume that the velocity of propagation of the shock has been uniform from the origin. The errors involved in this assumption appear to as follows:—

1. We know from observations on artificial earthquakes that the velocity of propagation is greater between stations near to the origin of the shock than it is between more remote stations; and also the velocity of propagation varies with the initial force which produced the disturbance. If our points of observation are sufficiently close together as compared with their distance from the origin of the disturbance, it is probable that errors of this description are small and will not make material differences in the general results.

2. We have reasons for believing that the transit velocity of an earthquake is dependent on the nature of the rocks through which it is propagated. Errors which arise from causes of this description will, however, be practically eliminated if our observation points are situated on an area sufficiently large, so that the distribution of the causes tending to alter the velocity of a shock balance each other. It must be remarked, that causes of this description may also produce an alteration in the direction of our shock.

Other errors which may sometimes enter into our results, when determining the origin of shocks by means of observations on velocities, are the assumptions that the disturbance has travelled along the surface from the epicentrum and not in a direct line from the centrum. Again, it is assumed that the origin is a point, whereas it may possibly be a cavity or a fissure. Lastly, if we desire extreme accuracy, we must make due allowance for the sphericity of the earth and the differences of elevation of the observing stations.

I. The method of straight lines.—Given a number of pairs of points a₀, a₁, b₀, b₁, c₀, c₁, &c., at each of which the shock was felt simultaneously, to determine the origin.

Theoretically if we bisect the line which joins a₀ and a₁ by a line at right angles to a₀, a₁, and similarly bisect the lines b₀, b₁, c₀, c₁, all these bisecting lines a₀, a₁, b₀, b₁, c₀, c₁, &c., ought to intersect in a point, which point will be the epicentrum or the point above the origin.

This method will fail, first, if a₀, a₁, b₀, b₀, c₀, c₁ form a continuous straight line, or if they form a series of parallel lines.

Hopkins gives a method based on a principle similar to the one which is here employed—namely, given that a shock arrives simultaneously at three points to determine, the centre. In this case, the relative positions of the three points, where the time of arrival was simultaneous, must be accurately known, and these three points must not lie in a straight line, or the method will fail. For practical application the problem must be restricted to the case of three points which do not lie nearly in the same straight line.

II. The method of circles.—Given the times t₀, t₁, t₂, &c., at which a shock arrived at a number of places a₀, a₁, a₂, &c., to determine the position from which the shock originated.

Suppose a₀ to be the place which the shock reached first, and that it reached a₁, a₂, a₃, &c., successively afterwards.

Let t₁ - t₀ = a
t₂ - t₀ = b
t₃ - t₀ = c, &c.

With a₁, a₂, a₃, &c. as centres, describe circles with radii proportional to the known qualities a, b, c, &c., and also a circle which passes through a₀ and touches these circles. The centre of the last circle will be the epicentrum. The radii proportional to a, b, c, &c. may be represented by the quantities ax, bx, cx, &c., where x is the velocity of propagation of the shock.

It will be observed that the times at which the shock arrived at three places might alone be sufficient. If, instead of taking the times of arrival of a shock, the arrival of a sea wave be taken, the result would be a closer approximate to the absolute truth.

It will be observed that this method is not a direct one, but is one of trial. If, however, an imaginary case be taken, and three given points of observation, a₀, a₁, a₂, be plotted on a piece of paper, it will be found that it is not a difficult matter to determine two numbers proportional to a and b which will allow you to draw two circles so that they may be touched by a third circle drawn through a₀. This problem has practically been applied in the case of the arrival of a sea wave at a number of places on the South American coast, at the time of the earthquake of May 9, 1877. This is illustrated as follows. The places which were chosen were Huanillos, Tocopilla, Cobija, Iquique, Mejillones.

In the following table the first column gives the times at which the sea wave arrived at each of these places in Iquique time; in the second column the difference between these times and the time at which it reached Huanillos is given; in the third column the distances through which a sea wave, propagated at the rate of 350 feet per second, could travel during the intervals noted in the second column is given.

	Arrival of sea wave		Time after arrival at Huanillos	Distance at 350 feet per second
	h.	m.	minutes	miles
Huanillos	8	30	0	0
Tocopilla	8	32	2	8
Cobija	8	38	8	32
Iquique	8	40	10	40
Mejillones	8	46	16	64

The distances marked in the third column are used as radii of the circles drawn round the places to which they respectively refer.

The centre of the circle drawn to touch the circles of the first column, and at the same time to pass through Huanillos, is marked c.

The position from which the shock originated appears therefore to have occurred very near to a place lying in Long. 7° 15′ W. and Lat. 21° 22′ S.

Fig. 31.

The actual operations which were gone through in making the accompanying map were as follows. First, the places with which we had to deal were represented on a map in orthographical projection, the centre of projection being near to the centre of the map. This was done so that the measurements which were made upon the map might be more correct than those we should obtain from an ordinary chart where this portion of the world was not the centre of projection. Next, a number was taken as equal to the velocity with which the sea wave had travelled. The first velocity taken was about 400 feet per second—this being about the velocity with which, theoretically, it must have travelled in an ocean having a depth equal to that indicated upon the charts—also it seemed to have travelled at this rate from the various times of arrival as recorded at places along the coast. Circles were then drawn round Tocopilla, Cobija, Iquique, and Mejillones with radii equal to 2, 8, 10, and 15, each multiplied by (60 × 400). It was then seen by trial that it was impossible to draw a single circle which should touch four circles and also pass through Huanillos. These four circles were, in fact, too large. Four new but smaller circles, which are shown in the map, were next drawn, their radii being respectively equal to the numbers 2, 8, 10, and 16, each multiplied by (60 × 350), and it was found that a circle, with a centre c, could be drawn which would practically touch the four circles, and at the same time would pass through Huanillos.

III. The method of hyperbolas.—The method which I call that of hyperbolas is only another form of the method of circles. It is, however, useful in special cases, as, for instance, where we have the times of arrival of earthquakes at only two stations. Between Tokio and Yokohama, at which places I frequently obtain tolerably accurate time records, the method has been applied on several occasions with advantage. In the preceding example let us suppose that the only time records which we had were for Huanillos and Mejillones, and that the wave was felt at the latter place sixteen minutes or 960 seconds after it was experienced at the former. Calling these places h and m respectively, round m draw a circle equal to the 960 multiplied by the velocity with which the wave was propagated. It is then evident that the origin of this disturbance must be the centre of a circle which passes through h and touches the circle drawn round m. Join h m, cutting the circle round m in y. Bisect y h in v. It is evident that v is one possible origin for the disturbance. Next, from m, in the direction of h, draw any line m z p; join z h; bisect z h at right angles by the line o p n. Because ph = pz, it is evident that p is a second possible origin. Proceeding in this way a series of points lying to the right and left of v on the curve r v t may be found, and we may therefore say that the origin lies somewhere in the curve r v t. By increasing or decreasing our velocity we vary the position of the curve r v t, and, instead of a line on which our origin may be, we obtain a band. As the assumed velocity increases, the circle round m becomes larger, and has its limit when it passes through h, where the two arms of the curve r v t will close together and form a prolongation of the line m y h as the assumed velocity diminishes. The circle round m becomes smaller until it coincides with the point m. At such a moment the curve r v t opens out to form a straight line bisecting m h at right angles. The curve r v t is a hyperbola with a vertex v and foci h and m. Inasmuch as pm - ph = a constant quantity. If we have the time given at which the shock or wave arrived at a third station as at Iquique, it is evident that a second hyperbola r′ v′ t′ might be drawn with Iquique and Huanillos as foci, and that the mutual intersection of these two hyperbolas with a third hyperbola, having for its foci Iquique and Mejillones, would give the origin of the wave. The obtaining of a mutual intersection would depend on the assumed velocity, and the accuracy of the result, like that of the method of circles, would depend upon the trials we made. The method here enunciated may be carried farther by describing hyperboloids instead of hyperbolas, the mutual intersection of which surfaces would, in the case of an earth wave, give the actual origin or centrum rather than the point above the origin or epicentrum.

IV. The method of co-ordinates.—Given the times at which a shock arrived at five or more places, the position of which we have marked upon a map, or chart, to determine the position on the map of the centre of the shock, its depth, and the velocity of propagation.

Commencing with the place which was last reached by the shock, call these places p, p₁, p₂, p₃, and p₄, and let the times taken to reach these places from the origin be respectively t, t₁, t₂, t₃, and t₄.

Through p draw rectangular co-ordinates, and with a scale measure the co-ordinates of p₁, p₂, p₃, and p₄, and let these respectively be a₁, b₁; a₂, b₂; a₃, b₃; a₄, b₄. Then if x, y, and z be the co-ordinates of the origin of the shock, d, d₁, d₂, d₃, and d₄, the respective distances of p, p₁, p₂, p₃, and p₄ from this origin, and v the velocity of the shock, we have

x² + y² + z² = d² = v² t²
(a₁ - x)² + (b₁ - y)² + z² = v² t₁²
(a₂ - x)² + (b₂ - y)² + z² = v² t₂²
(a₃ - x)² + (b₃ - y)² + z² = v² t₃²
(a₄ - x)² + (b₄ - y)² + z² = v² t₄²

Because we know the actual times at which the waves arrived at the places p, p₁, p₂, p₃, p₄, we know the values t—t₁, t—t₂, t—t₃, t—t₄. Call these respectively m, p, q, and r. Suppose t known, then

t₁ = t - m
t₂ = t - p
t₃ = t - q
t₄ = t - r.

Subtracting equation No. 1 from each of the equations 2, 3, 4, and 5, we obtain,

a₁² + b₁² - 2a₁ x - 2b₁ y = v² (t₁² - t²) = v² (m² - 2t m)
a₂² + b₂² - 2a₂ x - 2b₂ y = v² (t₂² - t²) = v² (p² - 2t p)
a₃² + b₃² - 2a₃ x - 2b₃ y = v² (t₃² - t²) = v² (q² - 2t q)
a₄² + b₄² - 2a₄ x - 2b₄ y = v² (t₄² - t²) = v² (r² - 2t r)

Now let v² = u, and 2v² t = w.

Then

2a₁ x + 2b₁ y + u m² - n m = a₁² + b₁²
2a₂ x + 2b₂ y + u p² - n p = a₂² + b₂²
2a₃ x + 2b₃ y + u q² - n q = a₃² + b₃²
2a₄ x + 2b₄ y + u r² - n r = a₄² + b₄²

We have here four simple equations containing the four unknown quantities x, y, u, and w.

x and y determine the origin of the shock. The absolute velocity v equals √ u. From v and w we obtain t. Substituting x, y, v, and t in the first equation we obtain z.

We have here assumed that the points of observation have all about the same elevation above sea level.

If it is thought necessary to take these elevations into account, a sixth equation may be introduced.

If the propagation of the wave is considered as a horizontal one, as would be done when calculating the position of the epicentrum or point above the origin, by means of the times of arrival of a sea wave, the ordinate z of the first five equations would be omitted. Working in this way the resulting four equations, viz.

2a₁ x + 2b₁ y + um² - wm² = a₁² + b₁²
&c. &c. &c.

remained unchanged.

Applying this method to the same example as that used as illustration for the two previous methods, we obtain for the co-ordinates of Mejillones, Iquique, Cobija, Tocopilla, and Huanillos, measured in geographical miles, and the times in Iquique time at which the wave reached each, as given in the following table; ox and oy being, drawn through Mejillones.

	Co-ordinates		Time of arrival
	OX	OY	h.	m.
Mejillones	a or 0	b or 0	8	46	p. m.
Iquique	a₁ or 150	b₁ or 96	8	40	„
Cobija	a₂ or 36	b₂ or 14	8	38	„
Tocopilla	a₃ or 66	b₃ or 31	8	32	„
Huanillos	a₄ or 102	b₄ or 58	8	30	„

From this data we find the co-ordinates x and y of this origin to be 85·8 and 56·7; whilst the velocity of propagation = 45 feet per second.

Measuring these ordinates upon the map, we obtain a centre lying very near Long. 71° 5′ W. and Lat. 21° 22′ S., a position which is very near to that which has already been obtained by other methods.

If instead of Huanillos we substitute the ordinates and time of arrival of the sea wave for Pabalon de Pica, another point for the origin will be obtained lying farther out at sea. To obtain the best result, the method to be taken will evidently be, first to reject those places at which it seems likely that some mistake has been made with the time observations, and then with the remaining places to form as many equations as possible, and from these to obtain a mean value. This is a long and tedious process, and as the time observations of this particular earthquake are probably one and all more or less inaccurate, it is hardly worth while to follow the investigation farther.

In this example, as in the preceding ones, it will be observed that it has been sea waves that have been dealt with, rather than earth vibrations. It is evident, however, that these latter vibrations may be dealt with in a similar manner.

In these determinations it will also have been observed that it is assumed that the disturbance has radiated from a centre, and, therefore, approached the various stations in different directions. If we assume that we have three stations very near to each other as compared with their distances from the origin, so that we can assume that the wave fronts at the various stations were parallel, the determination of the direction in which the wave advanced appears to be much simplified. The determination of the direction in which a wave has passed across three stations was first given by Professor Haughton.

Haughton’s method.—Given, the time of an earthquake shock at three places, to determine its horizontal velocity and coseismal line.

The solution of this is contained in the formula

tan ϕ = ^{a (t₂ - t₁) sin β}/_{c (t₃ - t₂) + a (t₂ - t₁) cos β}.

When a, b, and c are three stations at which a shock is observed at the times t₁, t₂, and t₃; a, b, and c are the distances between a, b, and c, and ϕ is the angle made by the coseismal lines x a x, y b y, and the line a b, which are assumed to be parallel.

This I applied in the case of the Iquique earthquake, but owing to the smallness of the angles between the three stations a, b, and c, the result was unsatisfactory. The problem ought to be restricted, first, to places which are a long distance away from a centre, and, secondly, to places which are not nearly in a straight line. This problem may be solved more readily by geometrical methods. Plot the three stations a, b, and c on a map, join the two stations between which there was the greatest difference in the time observation. Let these, for example, be a and c. Divide the line a c at point d, so that a d : d c as the interval between the shock felt at a and b is to the interval between the shock felt at b and c. The line b d will be parallel to the direction in which the wave advanced.

The difference in time of the arrival of two disturbances.—In the various calculations which have been made to determine an origin based on the assumption of a known or of a constant velocity, we have only dealt with a single wave, which may have been a disturbance in the earth or in the water. A factor which has not yet been employed in this investigation is the difference in time between the arrival of two disturbances; one propagated, for instance, through the earth, and the other, for example, through the ocean. The difference in the times of the arrival of two waves of this description is a quantity which is so often recorded that it is well not to pass it by unnoticed. To the waves mentioned we might also add sound waves, which so frequently accompany destructive earthquakes, and, in some localities, as, for instance, in Kameishi, in North Japan, are also commonly associated with earthquakes of but small intensity. It was by observing the difference in time between the shaking and the sound in different localities that Signor Abella was enabled to come to definite conclusions regarding the origin of the disturbances which affected the province of Neuva Viscoya in the Philippines, in 1881; the places where the interval of time was short, or the places where the two phenomena were almost simultaneous, being, in all probability, nearer to the origin than when the intervals were comparatively large. I myself applied the method with considerable success when seeking for the origin of the Iquique earthquake of 1877. The assumptions made in that particular instance were, first, that the velocity of the disturbance through the earth was known, and, secondly, that the velocity with which a sea wave was propagated was also known.

A method similar to the above was first suggested by Hopkins. It depended on the differences of velocity with which normal and transversal waves are propagated.[86]

Seebach’s method.—To determine the true velocity of an earthquake, the time of the first shock, and the depth of the centre.

Fig. 32.

Let the straight line m, m₁, m₂, m₃ represent the surface of the earth shaken by an earthquake. For small earthquakes, to consider the surface of the earth as a plane will not lead to serious errors.

If an earthquake originates at c, then to reach the surface at m it traverses a distance h in the time t. To reach the surface at m₁ it traverses a distance h + x₁ in a time t₂. If v equals the velocity of propagation,

then t = ^h/_v, t₁ = ^{h + x₁}/_v,

t₂ = ^{h + x₂}/_v, &c.

Seebach now says that if we have given the position of m or epicentrum of the shock, and draw through it rectangular axes like m m₃ and m t₃, and lay down on m m₃ in miles the distances from M of the various stations which have been shaken, and in equal divisions for minutes lay down on m t₃ the differences of time at which m, m₁, m₂, &c. were shaken, then m₁ t₁, m₂ t₂, &c. are the co-ordinates of points on an hyperbola. The degree of exactness with which this hyperbola is in any given case constructed is a check upon the accuracy of the time observations and the position of the epicentrum. The apex of the hyperbola is the epicentrum.

The intersection of the asymptote with the ordinate axis is the time point of the first shock, which, because the scale for time and for space were taken as equal, gives the absolute position of the centrum. This intersection is shown by dotted lines. Knowing the position of the centrum, we can directly read from our diagram how far the disturbance has been propagated in a given time.

CHAPTER XI.
THE DEPTH OF AN EARTHQUAKE CENTRUM.

The depth of an earthquake centrum—Greatest possible depth of an earthquake—Form of the focal cavity.

Depth of centrum.—The first calculations of the depth at which an earthquake originated were those made by Mallet for the Neapolitan earthquake of 1857. These were made on the assumption that the earth wave radiated in straight lines from the origin, and, therefore, at points at different distances from the epicentrum it had different angles of emergence. These angles of emergence were chiefly calculated from the inclination of fissures produced in certain buildings, which were assumed to be at right angles to the direction of the normal motion. If we have determined the epicentrum of an earthquake and the muzoseismal circle, and make either the assumption that the angle of emergence in this circle has been 45° or 54° 44′ 9″ (see page 54), it is evidently an easy matter by geometrical construction to determine the depth of the centrum. Höfer followed this method when investigating the earthquake of Belluno.

Other methods of calculation which have been employed are based on time observations, as, for instance, the method of Seebach, the method of co-ordinates, the method of hyperboloids or spheres (see pages 200–212).

By means of a number of lines parallel to twenty-six angles of emergence, drawn in towards the seismic vertical, Mallet found that twenty-three of these intersected at a depth of 7⅛ geographical miles. The maximum depth was 8⅛ geographical miles, and the minimum depth 2¾ geographical miles.

The mean depth was taken at a depth of 5¾ geographical miles where, within a range of 12,000 feet, eighteen of the wave paths intersected the seismic vertical.

The point where these wave paths start thickest is at a depth not greater than three geographical miles, and this is considered to be the vertical depth of the focal cavity itself.

For the Yokohama earthquake of 1880, from the indications of seismometers, and by other means, certain angles of emergence were obtained, leading to the conclusion that the depth of origin of that earthquake might be between 1½ and 5 miles.

Possibly, perhaps, the earthquake may have originated from a fissure the vertical dimensions of which was comprised between these depths.

A source of error in a calculation of this description is that the vertical motions may have been a component of transverse motions or perhaps due to the slope of surface waves.

The following table of the depths at which certain earthquakes have originated has been compiled from the writings of several observers.

		In feet
		Minimum	Mean	Maximum
Rhineland	1846 (Schmidt)		127,309
Sillien	1858 (Schmidt)		86,173
Middle Germany	1872 (Seebach)	47,225	58,912	70,841
Herzogenrath	1873 (Lasaulx)	16,553	36,516	56,477
Neapolitan	1857 (Mallet)	16,705	34,930	49,359
Yokohama	1880 (Milne)	7,920	17,260	26,400

A table similar to this has been compiled by Lasaulx.[87]

With the exception of the determination for the two last disturbances these calculations have been made with the assistance of the method of Seebach, which depends, amongst other things, on the assumptions of exact time determinations, direct transmission of waves from the centrum, and a constant velocity of propagation.

Admitting that our observations of time are practically accurate, it appears that the other assumptions may often lead to errors of such magnitude that our results may be of but little value.

From what has been said respecting the velocity with which earth disturbances are propagated, it seems that these velocities may vary between large limits, being greatest nearest to the origin.

If we refer to Seebach’s method, we shall see that a condition of this kind would tend to make the differences in time between various places, as we recede from the epicentrum, greater than that required for the construction of the hyperbola. The curve which is obtained would, in consequence, have branches steeper than that of the hyperbola, and the resultant depth, obtained by the intersection of the asymptotes of this curve with the seismic vertical, indicates an origin which may be much too great.

Another point worthy of attention, which is common to the method of Mallet as well as to that of Seebach, is the question whether the shock radiates directly from the origin, or is propagated from the origin more or less vertically to the surface, and then spreads horizontally. We know that earthquakes, both natural and artificial, may be propagated as undulations on the surface of the ground, and that the vertical motion of the latter, as testified by the records of well-constructed instruments, has no practical connection with the depth from which the disturbance originated.

In cases like these, the direction of cracks in buildings, and other phenomena usually accredited to a normal radiation, may in reality be due to changes in inclination of the surface on which the disturbed objects rested. When our points of observation are at a distance from the epicentrum of the disturbance which, as compared with the depth of the same, is not great, calculations or observations based on the assumption of a direct radiation of the disturbance may possibly lead to results which are tolerably correct. The calculations of Mallet for the Neapolitan earthquake appear to have been made under such conditions.

For smaller earthquakes, and for places at a distance from the seismic vertical of a destructive earthquake, the results which are deduced from the observations on shattered buildings, and all observations based upon the assumption of direct radiation, we must accept with caution.

Another error which may enter into calculations of this description is one which has been discussed by Mallet at some length. This is the effect which the form and the position of the focal cavity may have upon the transmission of waves.

Should the impulse originate from a point or spherical cavity, then we might, in a homogeneous medium perhaps, regard the isoseismals as concentric circles, and expect to find that equal effects had been produced at equal distances from the epicentrum. Should, however, this cavity be a fissure, it is evident that even in a homogeneous medium the inclination of the plane of such a cavity will have considerable effect upon the form of the waves which would radiate from its two walls.

For example, let it be assumed that the first impulse of an earthquake is due to the sudden formation of a fissure, rent open from its centre, and that the waves leave the walls at all points normal to its surface. Then, as Mallet points out, it is evident that the disturbance will spread out in ellipsoidal waves, the greatest axis of which will be perpendicular to the plane of the fissure.

By taking a number of cases of fissures lying in various directions and drawing the ellipsoidal waves which would result from an elastic pressure, like that of steam suddenly admitted into such cavities, the differences in effect which would be simultaneously produced by these waves on reaching the surface can be readily understood. The following example of an investigation on this subject will serve as an example to illustrate the general nature of the many other cases which might be taken.

Fig. 33.

Let a disturbance simultaneously originate from all points of the fissure f f. This will spread outwards in ellipsoidal shells to the surface of the earth e e. The major axis of these ellipsoidal shells will be the direction of greatest effect. In the direction c d the waves will plunge into the earth, and places to the right side of the fissure will, to use an expression due to Stokes, when speaking of analogous phenomena connected with sound, be in earthquake shadow. The same expression has been employed, somewhat differently, when speaking of the effects produced on buildings.

For places, like s and p, situated at equal distances from the seismic vertical, it is evident that the intensity of the shock will be different, and also its time of arrival. It will also be observed that the isoseismals will take the form of ovals or distorted ellipses, the larger or fuller end of which being to the left of the fissure.

Other cases, like those just given, which are discussed by Mallet in his account of the Neapolitan earthquake, are where the fissure forms the division between materials of different elasticities. In the hard and more elastic material the waves will be more crowded, the velocity of a wave particle will be greater, and the transit will be quicker than in the less elastic medium.

The result is that the distance of equal effect from the seismic vertical will be greatest in the direction of the more compressible material.

Unless these considerations are kept carefully before the mind when investigating the depth and, we may add, the position and form of the centrum of an earthquake, serious errors may arise.

Greatest depth of an earthquake origin.—A curious but instructive calculation which Mallet made was a determination of the greatest possible depth at which an earthquake may occur. This calculation is based upon the idea that the impulsive effect of an earthquake has an intimate relationship with the height of neighbouring volcanoes, the column of lava supported on a volcanic cone being a measure of the internal pressure tending to rupture the adjacent crust of the earth.

Mitchell, in 1700, virtually propounded this idea, when he suggested that the velocity of propagation of an earthquake was related to the height of such a column.[88]

Mallet showed that there was probably considerable truth in such a supposition by appealing to the results of actual observation. The pressure gauge of the Neapolitan district would be Vesuvius, the height of which has in round numbers varied between 3,500 to 4,000 feet. One of the most destructive earthquakes in this district—namely, the one of 1857—projected bodies with an initial velocity of about fifteen feet per second. The Riobamba earthquake, which projected bodies with an initial velocity of eighty feet per second, appears to have been the most violent earthquake, so far as its impulsive effort is concerned, of which we have any record. It occurred amongst the Andes, where there are volcanoes from 16,000 to 21,000 feet in height.

Comparing these two earthquakes together, we see that the Riobamba shock had a destructive power 5·33 times that of the Neapolitan shock, and we also see that the Riobamba volcanoes were about 5·33 times higher than Vesuvius. The accordance in these quantities is certainly interesting, and tends to substantiate the idea that volcanoes are barometrical-like pressure gauges of a district.

Carrying the argument still further. Mallet says that if the depth of origin of earthquakes were the same, then the area of disturbance would, for like formations and configuration of surface, be a measure of the earthquake effort, and also some function of the velocity of the wave. From this we may generally infer ‘that earthquakes, like that of Lisbon, which have a very great area of sensible disturbance, have also a very deep seismal focus, and also the greatest depth of seismal focus within our planet is probably not greater than that ascertained for this Neapolitan earthquake, multiplied by the ratio that the velocity of the Riobamba wave bears to that of its wave, or, what is the same thing, by the ratio of the altitudes of the volcanoes of the Andes to that of Vesuvius.’

Now, as the depth of the Neapolitan shock may be taken at 34,930 feet, the greatest probable depth of origin of any earthquake impulse occurring in our planet is limited to 5·333 × 4,930 feet, or 30·64 geographical miles.

Ingenious as this argument is, we can hardly admit it without certain qualifications.

First, we are called upon to admit the identity of the originating cause of the volcano and the earthquake—as to what may be the originating cause of earthquakes we have yet to refer, but certainly in the case of particular earthquakes, as, for instance, those which occur in countries like Scotland, Scandinavia, and portions of Siberia, the direct connection between these phenomena are not at first sight very apparent.

Secondly, even if we admit the identity of the origin of these phenomena, it is not difficult to imagine that the fluid pressure brought to bear upon certain portions of the crust of the earth may possibly in many instances be infinitely greater than that indicated by the height of the column of liquid lava in the throat of a volcano, the true height of which we are unable to obtain. Further, in certain instances such a column only appears to be a measure of the pressure upon the crust of the earth in the immediate vicinity of the cone.

Thus, in the Sandwich Islands, we have lava standing in the throat of the volcano of Mauna Loa 10,000 feet higher than it stands in the crater Kilauea, only twenty miles distant. That these columns should be measures of the same pressure, originating in a general subterranean liquid layer with which they are connected, is a supposition difficult to satisfactorily substantiate.

Another measure of the impulsive efforts which subterranean terranean forces may exert upon the crust above them is evidently the height to which volcanoes eject materials. Cotopaxi is said to have hurled a 200-ton block of stone nine miles. Sir W. Hamilton tells us that in 1779 Vesuvius shot up a column of ashes 10,000 feet in height; and Judd tells us that this same mountain in 1872 threw up vapours and rock fragments to the enormous height of 20,000 feet. This would indicate an initial velocity of 1,131 feet per second.

Notwithstanding Mallet’s calculation that thirty miles is the limiting depth for the origin of an earthquake, the origin of the Owen’s Valley earthquake of March 1872 was estimated as being at least fifty miles.[89]

Form of the focal cavity.—Among the various problems which are put before those who study the physics of the interior of our earth it would at first sight appear that there was none more difficult than the attempt to determine the form of the cavity, if it be a cavity, from which an earthquake originates. Almost all investigators of seismology have recognised that the birthplace of an earthquake is not a point, and have made suggestions about its general nature. The ordinary supposition is that the earthquake originates from a fissure, and if the focus of a disturbance could be laid bare to us it would have the appearance of a fault such as we so often see exposed on the faces of cliffs.

A strong argument, tending to demonstrate that some of the shakings which are felt in Japan are due to the production of such fissures, is the fact that the vibrations which are recorded are transverse to a line joining the point of observation and the district from which, by time observations, we know the shock to have originated. The most probable explanation of this phenomena appears to be that one mass of rock has been sliding across another mass, giving rise to shearing strains, and producing waves of distortion.

The first seismologist who attacked the problem of finding out the dimensions and position of such a fissure was Mallet, when working on the Neapolitan earthquake of 1857. The reasons that the origin should, in the first place, have been a fissure, rather than any other form of cavity, was that such a supposition seemed to be a priori the most probable, and, further, that it afforded a better explanation of the various phenomena which were observed, than that obtained from any other assumption.

The method on which Mallet worked to determine the form and position of the assumed fissure, which method was subsequently more or less closely followed by other investigators, was as follows:—

From an observation of the various phenomena produced upon the surface of the disturbed area, a map of isoseismals was constructed. These were seen, as has been the case with many earthquakes, not to distribute themselves in circles round the epicentrum, but as distorted oval or elliptical figures, the major axes of which roughly coincided with each other. Further, the epicentrum, did not lie in the centre of these ovals, but was near to the narrow end where they converged.

This at once showed, if the reasoning respecting the manner in which waves are propagated from an inclined fissure be correct, that the fissure was at right angles to the major axis of the curves, dipping from their narrow end downwards, in the direction of their larger widespread ends.

The next weapon which Mallet employed to attack this problem was the sound which was heard at different points round about the focus. These sounds appear to have been of the nature of sudden explosive reports accompanied by rushing, rolling sounds. The form of the area in which these sounds were heard was closely similar to that of the first two isoseismals. Except in the central area of great disturbance, no sound was heard to accompany the shock.

Those at the northern and southern extremity of the sound area all described what they heard as a ‘low, grating, heavy, sighing rush, of twenty to sixty seconds’ duration.’ Those in the middle and towards the east and west boundaries of this area described a sound of the same tone, but shorter and more abrupt, and accompanied with more rumbling.

The nature of the arguments which were followed in discussing the sound observations will be found in the chapter relating to these phenomena.

A portion of the argument which it is difficult to follow relates to the maximum rate at which it can be supposed possible for a fissure to be rent in rocks, which rate depends on the density and elasticity of these rocks and other constant factors.

Next it was observed that the paths of the waves drawn on the surface, although generally intersecting in a point, did not do so absolutely, but along a line passing through the main focus some 7½ miles in length. This, coupled with the observations of sounds, led to the supposition that the centre of disturbance, considered horizontally, originated along a curved line passing through the chief focus and the various intersections of the wave paths.

The last phenomena brought forward to assist in the solution of this interesting problem were a study of the tremulous movements that preceded and followed the shock, and their relation to the sound phenomena.

If the earthquake originated by the formation of a fissure, after the rending has gone on for a certain time the focal cavity is enlarged to a certain extent, and the great shock takes place. This would be followed by concluding tremulous waves. A succession of phenomena like those accompanied the shock about which Mallet writes.

By observations such as these, coupled with what has been said about the maximum and mean depths of the focal cavity, Mallet came to the conclusion that the focal cavity was a fissure, the rending open of which produced the earthquake. The vertical dimensions of this cavity were not more than 5·3 miles, but were probably limited to three miles.

From the intersection of the wave paths upon the surface and the observed emergences, this fissure followed horizontally a curve of double flexure, about nine geographical miles in length. The area of this fissure was twenty-seven geographical miles. The time of rending it open in Apennine limestone would be about 7½ seconds, which should be the same as the period during which tremors were felt. The time actually recorded was six or eight seconds.

Briefly, this is, then, the line of reasoning which was followed by Mallet in an investigation the results of which are as interesting as they are startling. Since the line of investigation has been opened, and the existence of new problems has been indicated, other investigators, although not exactly following Mallet’s method in all their details, have, when endeavouring to attain the same ends, employed similar weapons.

Thus, for example, Seebach, when determining the depth and nature of the origin of the earthquake of Middle Germany, reasoned somewhat as follows:—

Had the origin been more or less of a spherical cavity, then the region of most violent disturbance upon the surface would, according to a theorem we have already mentioned, have been upon or near a circle of about 8·8 miles in radius round the epicentrum. This region, however, was found by observation to lie along a curved band about forty miles in length, altogether on one side of the epicentrum.

To explain this anomaly Seebach followed Mallet, and assumed that the origin was not a spherical cavity, but a fissure.

The depth and strike of this fissure was determined by the observation that the area of greatest disturbance was along a curved line lying radial to the epicentrum. Such a condition it was assumed indicated that the fissure of origin must be inclined towards this area of greatest disturbance. A line was then drawn from this area to the centrum. A second line at right angles to this one gave the dip of the fissure.

Höfer, when working on the earthquake of Belluno, came to the conclusion that the disturbance originated from two faults meeting each other at an angle of 60°. In this determination he was chiefly influenced by the form of a certain homoseist which was of the form of an elongated ellipse met on one side by a second ellipse, the principal axes of the two ellipses giving the strike of the two faults.

CHAPTER XII.
DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.

General distribution of earthquakes—Occurrence along lines—Examples of distribution—Italian earthquake of 1873—In Tokio—Extension of earthquake boundaries—Seismic energy in relation to geological time; to historical time—Relative frequency of earthquakes—Synchronism of earthquakes—Secondary earthquakes.

General distribution of earthquakes.—The records of earthquakes collected by various seismologists lead us to the conclusion that at some time or other every country and every ocean in the world has experienced seismic disturbances. In some countries earthquakes are felt daily, and from what will be said in the chapter on earth pulsations it is not unlikely that every large earthquake might with proper instrumental appliances be recorded at any point on the land surfaces of our globe. The area over which any given earthquake extends is indeterminate. The area over which an earthquake is sensible is sometimes very great. The Lisbon shock of 1755 is estimated as having been sensible over an area of 3,300 miles long and 2,700 miles wide, but in the form of tremors and pulsations it may have shaken the whole globe.

The regions in which earthquakes are frequent are indicated in the accompanying map, which, to a great extent, is a reproduction of a map drawn by Mallet. The regions coloured with the darkest tint are those where great earthquakes are the most frequent. The actual number of earthquakes which have been felt in the differently coloured areas are given, when speaking of the relation of seismic energy to season.

When looking at this chart it must be remembered that if we were to make a detailed map of any one of the different countries where earthquakes are frequent, we should find in it all the differences that we observe in the general chart. For instance, one portion of Japan, where perhaps sixty shocks are felt per year, would be coloured with a dark tint, whilst other portions of the same country, where there is only one slight shaking felt every few years, would be left almost uncoloured. The black dots indicating the position of volcanic vents are even more general in their signification than the tinted areas. Professor Haughton gives for the world a list of 407 volcanoes, 225 of which are active. These numbers are the same as those given by A. von Humboldt. Of the active volcanoes 172 are on the margin of the Pacific, and of the total number eight are in Japan. From my own observations in Japan independently of the Kurile Islands, I have enumerated fifty-three volcanoes which are either active or have been active within a recent period. In a few years’ time this list will probably be increased. I mention this fact to show how very imperfect our knowledge is respecting the number of volcanic vents existing on our globe. If we were in a position to indicate the volcanoes which had been in eruption during the last 4,000 years, the probability is that they would number several thousands rather than four or five hundred.

An inspection of the map shows that earthquakes chiefly occur in volcanic and mountainous regions. The most earthquake-shaken regions of the world form the boundaries of the Pacific ocean. It may be remarked that these boundaries slope beneath the neighbouring ocean at a much steeper angle than the boundaries of countries where earthquakes occur but seldom. The coasts of South America, Kamschatka, the Kuriles, Japan, and the Sandwich Islands, for example, have slopes beneath the Pacific from one in twenty to one in thirty. The coasts of Australia, Scandinavia, and the eastern parts of South America, where earthquakes are practically unknown, have slopes from one in fifty to one in two hundred and fifty. Many earthquakes have taken place in mid-ocean. In the Atlantic Ocean M. Perrey has given about 140 instances of such occurrences.

The majority of the earthquakes which shake Japan appear to have their origin in the neighbouring ocean. If we could draw a map of earthquake origins, it is probable that the greater number of the marks indicating these origins would be found to be suboceanic and along lines parallel to the shores of continents and islands which rise steeply from the bed of deep oceans. In countries like Switzerland and India, our marks would hold a relationship to the mountains of these countries.[90] Looking at the broad features of the globe, we see on its surface many vast depressions. Some of these saucer-like hollows form land surfaces, as in central Asia. The majority of these, however, are occupied by the oceans. Active volcanoes chiefly occur near the rim of the hollows which have the steepest slopes. The majority of earthquakes probably have their origin on or near the bottom of these slopes. To these, however, there are exceptions, as for instance the earthquakes in the Alps, in the hills of Scotland, and the shakings which are occasionally felt in countries like Egypt. The earthquakes which shake the borders of the Pacific have their origins in, and their effects are almost exclusively felt on, the sides of the bounding ridge facing this ocean. In Japan it is the eastern sides of the islands which suffer, the western side being almost as free from these convulsions as England.

Similar remarks may be made about the eastern side of South America, especially the southern portion of the continent. At Buenos Ayres, for example, there has been no disturbance since Mendoza was destroyed, some twenty years ago. In British Guiana slight shocks are occasionally felt in the low delta which forms the settled portion of the colony, but they are extremely rare.

Disturbances in lines or zones.—It has often been observed that disturbances are propagated along the length of mountains or valleys, and it is but seldom that earthquakes cross them transversely. Thus the valleys of the Rhone, the Rhine, and the Danube are lines along which disturbances travel.

The major axes of the elliptical areas of disturbances which have shaken India have a general direction parallel to the valley of the Ganges along the flanks of the Himalayas.

The disturbances which have shaken London appear to have been chiefly east and west, or along the valley of the Thames. In South America the line of disturbance is along the western sides of the Andes. Another line is along the northern coast of the continent through Andalusia and Caraccas towards the Antilles and Trinidad. The shocks of the Pyrenees are chiefly felt along the southern side of these mountains. In the middle and on the northern side they are but seldom felt. This propagation in lines or zones may in certain cases be apparent rather than real. Thus the north and south ranges of mountains in Japan are mountains almost simultaneously shaken along their eastern flanks, giving the impression that an earthquake had originated simultaneously from a fissure parallel to this line, or else, starting at one end, had run down their lengths. Time observations have, however, shown that such disturbances had their origin at some distance in the ocean, and, travelling inwards, had reached all points on the flanks of these mountains almost simultaneously. The same explanation will probably hold for the so-called linear disturbances of western South America.

All earthquake disturbances have probably a tendency to radiate from their source, and are only prevented from doing so by meeting with heavy mountainous districts, which by their mass and structure absorb the energy communicated to them. Much energy is also lost by emergence on the open flanks of a range of mountains. Rather than say that high mountains often bound the extension of an earthquake, or that earthquakes appear to run along the flanks of such mountains, we might say that earthquakes have boundaries parallel to the strike of the rocks in a given district, that such a direction is the one in which the propagation is the easier.

Rossi is of opinion that volcanic fractures play an important part in governing the distribution of seismic disturbances. When a volcano is formed, a series of starlike fractures are formed round the central crater. Secondary craters may indicate the line of these fissures. The mountains about Rome are regarded as typical of this radial structure. The more distant the secondary craters are from the centre of the system, the smaller will they be, and also the younger. If two fissures intersect we get a larger crater at the junction. Earthquakes are propagated along the direction of these fissures, whilst the rising and falling of these lips throw off transverse waves. Rossi adduces observations which appear to meet with explanation on such suppositions.

Suess, who has written upon the earthquakes of lower Austria, shows how the majority of the disturbances have had their origin along certain lines which form a break in the continuity of the Alps. One line runs north-east from Bruck towards Vienna. Near Wiener Neustadt, where the greatest number and heaviest shocks have occurred, this line is met by a north-north-west line crossing the Danube and following the valley of the river Kamp.[91] Hoeffer has drawn similar lines from the head of the Adriatic, one set running north-north-east to intersect near Litschau, and the other north-north-west to intersect near Frankfort in the valley of the Rhine.[92]

Examples of distribution.—A curious example of the distribution of seismic movement is that of the earthquake of March 12, 1873, worked out by Professor P. A. Serpieri. This earthquake appears to have been simultaneously felt on the Dalmatian coast and in central Italy, in a region lying north-east from Rome and south-east from Florence. In both of these areas the motion was from south-east to north-west. The shock then radiated from the central Italian regions, so that at places on the western shore of the Adriatic it was felt after it had been felt on the Dalmatian coast.

Many explanations might be offered for this peculiar distribution of seismic activity. Possibly the shock originated at a great depth beneath the bed of the southern part of the Adriatic, and by following existing lines of weakness simultaneously reached the surface of the earth in central Italy and Dalmatia.

In Tokio, which is built partly on a flat plain, partly in valleys denuded from a low tableland, and partly on the spurs of the tableland itself, the distribution of earthquakes is a subject yet requiring attention. Sometimes it has happened that persons in one house have been sufficiently alarmed to escape into the open air, whilst others, not more than a mile distant, have not been aware that the city had been shaken.

Fig. 34.

Areas almost simultaneously struck from S.E. to N.W.

Subsequent radial disturbance

Extension of earthquake boundaries.—Natural obstructions which may be sufficient to retard small earthquakes may in certain instances not be found sufficient to retard the larger disturbances. Thus the shocks of Calabria are usually only felt on the western side of the Apennines, but instances have occurred when they have crossed this barrier. In 1801 the earthquake of Cumana crossed a branch of the coast range.

Sometimes earthquake boundaries give way, and countries which they sheltered subsequently become exposed to all disturbances. The true explanation of this is probably in a shifting of the centre of seismic activity. Thus up to December 14, 1797, although Cumana was often devastated, the peninsula of Araya was not hurt. On this date Araya commenced to suffer, and has continued to suffer ever since.

Fuchs gives an example of the movement of a seismic centre in the case of the Calabrian earthquake. The first shock commenced near Oppiedo, the second shock commenced four or five miles farther to the north, and the third shock had its origin five or six miles still farther, near to Girifalco.

CHAPTER XIII.
DISTRIBUTION OF EARTHQUAKES IN TIME (continued).

Seismic energy in relation to geological time.—If we admit that seismic energy is only a form of volcanic energy, it must also be admitted that any cause tending to produce a general decrease in the amount of the latter will also produce an alteration in the amount of the former.

The nebular hypothesis of Laplace tells us that the solar system is the result of the whirling of a heated gaseous mass, which as it cooled continually contracted and consequently whirls the faster. With this hypothesis before us, we understand why all the planets and their satellites have a similarity in the directions of their movements, why they revolve nearly in the same plane, in orbits nearly circular, why some have a flattened figure and are surrounded by rings or belts, why the exterior planets should have a greater velocity of rotation, a greater number of satellites, and a less density as compared with the interior planets, the similarity of the elements in meteoric stones, the sun, the stars, and those found upon our earth, and lastly why there should be an increase in temperature as we descend into our earth.[93] This increase in temperature as we descend into the earth as deduced from many observations appears to be about 1° F. for every fifty or sixty feet of descent.

To explain this and other kindred phenomena it is assumed that the earth was once very much hotter than it is at present, and to reach its present stage it has been gradually cooling. As the laws of cooling are perfectly known, to calculate how many years it must have taken a body like our earth to cool down to its present temperature is a definite problem. Sir William Thomson, starting with the temperature of 7,000° F., when all the rocks of the earth must have been molten and a skin or crust upon the surface, such as is so quickly produced upon the surface of molten lava, finds by calculation that the time taken to reach the present temperature must have been about one hundred million years. Into this period he and other physicists desire to compress the history of all the stratified deposits. Geologists find this period too short. Others seeking to reconcile the views of physicists and geologists endeavour to show that the various agencies engaged in degrading rocks and accumulating sediments in former ages are not to be judged of by the agencies we now see around us; in former times they were more active. At one period the elastic tides in the earth may have been so great that they resulted in the fracturing off from our planet its satellite the moon, and subsequently the moon, acting on the waters of the earth, may, even as late as 150,000 years ago, have produced every three hours tides 150 feet in height.

Whatever may be the value of the figures here quoted, reasonings like these bring us to the conclusions that the various agencies which we now know to be acting upon our earth were once far more potent than they are at present, and if the moon, as a producer of elastic tides, has any influence upon the occurrence of earthquakes, it must have had a much greater influence in bygone times.

We might speak similarly with regard to the internal heat of the earth.

From the present heat gradient of our globe it is possible to calculate how much heat flows from the earth every year.

This is equivalent to a quantity which would raise a layer of water ·67 centimetres thick, covering the whole of our globe, from a temperature of 0° to 100° C.

Similarly, we might calculate the quantity of heat which would be lost when the average heat gradient, instead of being 1° F. for fifty feet of descent, was 1° F. for twenty-five feet of descent.

We might also calculate how many years ago it was since such a gradient existed.

The general result which we should arrive at would be that in past ages the loss of heat was more rapid than it is at present. Now the contraction of a body as it cools is for low temperatures proportional to its loss of heat, and this law is also probably true for contraction as it takes place from high temperatures.

Contraction being more rapid, it is probable that phenomena like elevations and depressions would be more rapid than they are at present, and generally all changes due to plutonic action, as has already been pointed out by Sir William Thomson, must have been more active.

We have, therefore, every reason to imagine that earthquakes which belong to the category of phenomena here referred to were also numerous and occurred on a grander scale during the earlier stages of the world’s history than they do at present, and seismic and volcanic energy, when considered in reference to long periods of time, is probably a decreasing energy.

In making this statement we must not overlook the fact that in geological time, as testified by the records of our rocks, volcanic action, and with it probably seismic action, has been continually shifting, first appearing in one area and then in another, and even in the same area we have evidence to show that these have periods of activity and repose successively succeeding each other. Thus in Britain, during the Palæozoic times, we have many evidences of an intense volcanic activity. During the Mesozoic or Secondary period volcanic energy appears to have subsided, to wake up with renewed vigour in the Cainozoic or Tertiary period.

During this latter period it is not at all improbable that Scotland was in past times as remarkable for its earthquakes as Japan is at the present day.

Later on it will also be shown that earthquakes are concomitant phenomena, with those elevatory processes which we have reason to believe are slowly going on in certain portions of the earth’s crust. If, therefore, we are able by the examination of the rocks which constitute the accessible portions of our globe to determine which periods were characterised by elevation, we may assume that such periods were also periods of seismic activity.

Amongst these periods we have those in which various mountain ranges appeared. Thus the Grampians, and the mountains of Scandinavia, were probably produced before the deposition of the Old Red sandstone. The Urals were upheaved prior to Permian times. The chief upheaval in the Alps took place after Eocene times. The Rigi and other sub-Alpine mountains were formed after the deposition of the Miocene beds. About this same time the Himalayas were upheaved.[94]

The earthquakes which from time to time shake those newer mountains apparently indicate that the process of mountain-making is hardly ended.

Seismic energy in relation to historical time.—The distribution of seismic energy with regard to historical time is a subject which has been very carefully examined by Mallet, who collected together a catalogue of between six and seven thousand earthquakes, embraced between the periods b.c. 1606 and a.d. 1850. The earthquake of b.c. 1606 was on the occasion of the delivery of the law at Mount Sinai. Between b.c. 1604 and b.c. 1586 an earthquake probably occurred in Arabia, when Korah, Dathan, and Abiram were swallowed up. Another biblical record is that of b.c. 1566, when the walls of Jericho were overthrown.

The earliest records from China is in b.c. 595; in Japan b.c. 285; in India a.d. 894.

By using the number of earthquakes which have been recorded in each century as ordinates, Mallet constructed a curve, which apparently shows a continual increase in seismic energy, especially during recent times. This, Mallet remarks, contradicts all the analogies of the physics of the globe, and points out that the rapid increase in the number of earthquakes in latter years is chiefly due to the greater number of records which have been made, and the increase of the area of observation. No doubt many of the records made by the ancients have been lost.

If we compare Mallet’s records, as he invites us to do, with the great outlines of human progress, we see that the two increase simultaneously, and we come to the conclusion that, taken as a whole, during the historical period the seismic activity of the world has been tolerably constant.

These conclusions, based on the evidence at our command, are not to be confuted. If, however, instead of considering the seismic energy of the whole world, we consider the seismic energy of particular areas, it seems reasonable to expect that in certain instances sometimes a decrease and sometimes an increase in this energy might be discovered, especially, perhaps, in areas which are highly volcanic.

In France we know that volcanic activity ceased at a period closely bordering on historical times, and it is not unlikely that seismic activity may have ceased at a corresponding time.

In a country like Japan, it is possible that in one district seismic energy may be on the increase, whilst in another upon the decrease.

In a country like England, it is probable that the seismic state is constant, and, whatever changes may be now occurring, they are taking place at so slow a rate that, even if our records of the historical period were complete, we could hardly be expected to find these changes sufficiently marked to be observable.

For purposes of reference, and also for examining the present question, the table, page 240, has been compiled. The earthquakes given are chiefly those which have been recorded in histories as being more or less destructive.

In the second column of this table will be seen the number of earthquakes which have occurred in Japan during each century, the centuries being marked in the first column. In columns 3 to 18 inclusive are given the number of earthquakes which have occurred during different centuries in the various countries and districts mentioned at the head of each column. These latter, which are taken from the writings of Mallet, are given for the sake of comparison with the Japanese earthquakes. If we commence with the seventh century in the column for Japan, we see that a great increase in the number of earthquakes, as we come towards the present time, is not so observable as it is in the other columns.

Key:

1	Centuries
2	Japan
3	Scandinavia and Iceland
4	British Isles and Northern Isles
5	Spanish Peninsula
6	France, Belgium, Holland
7	Rhine Basin
8	Switzerland and Rhine Basin
9	Danube Basin
10	Italy, Sicily, Sardinia, and Malta
11	Supplemental table for Italy, Sardinia, and Malta
12	Turco-Hellenic Territory, Syria, Ægean Isles, and Levant
13	United States and Canada
14	Mexico and Central America
15	Antilles
16	Cuba
17	Chili and La Plata Basin
18	Northern Zone of Asia
19	Approximate Intensity in the Kioto District of Japan

1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19
I.	1	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
II.	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
III.	1	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
IV.	—	—	—	—	—	—	—	—	6	—	23	—	—	—	—	—	—	—
V.	1	—	—	—	1	—	—	—	5	—	19	—	—	—	—	—	—	—
VI.	1	—	—	—	6	—	—	19	3	—	27	—	—	—	—	—	—	—
VII.	12	—	—	—	—	—	—		1	—	8	—	—	—	—	—	—	15
VIII.	11	—	—	—	—	—	—		2	1	12	—	—	—	—	—	—	17
IX.	40	—	—	—	21	—	19		6	—	7	—	—	—	—	—	—	60
X.	17	—	—	—	2	—	2		3	3	5	—	—	—	—	—	—	24
XI.	20	—	8	3	16	—	9		7	5	18	—	—	—	—	—	—	28
XII.	18	—	11	4	12	—	8		18	22	23	—	—	—	—	—	—	20
XIII.	16	28	15	3	9	—	3		15	26	13	—	—	—	—	—	—	16
XIV.	19		4	8	21	—	18		20	51	8	—	—	—	—	—	—	25
XV.	36		1	4	14	—	12		18	47	11	—	—	—	—	—	—	29
XVI.	17		8	10	61	10	52	35	32	5	22	—	6	1	4	5	—	17
XVII.	26		14	10	91	29	120	31	121	9	53	10	7	16	4	9	—	11
XVIII.	31	111	63	93	237	71	141	88	438	20	124	88	24	85	2	10	32	8
XIX.	27	113	110	85	211	81	173	145	390	88	194	51	30	145	50	170	57	8

The explanation for this probably lies in the fact that Japan has practised civilised arts for a longer period than many of the European and other countries mentioned in the table.

In Japan, no doubt, the records of later years have been more perfect than they were in early times, but this, although so potent an effacer of what was probably the true state of natural phenomena in the case of Europe, has not quite obliterated the truth in Japan; for instead of an apparent increase of seismic energy since early times it shows a slight decrease.

To draw up a table of earthquakes such as the one which has just been given, and then, after the inspection of it, draw conclusions as to whether there has been an increase or decrease in seismic energy, is, however, hardly a just method of reasoning. The earthquakes, taken as they are, for the whole of Japan, represent a collection of places some of which are 1,000 miles apart. When we consider that many earthquakes which occurred at one end of this line were never felt at the other end, in order to justly estimate the periodicity of seismic phenomena it would seem that we ought either to take some particular seismic area or else the whole world.

The particular area which has been taken is that of Kioto in Central Japan, and the earthquakes which have been felt there are enumerated in the table.

In order to show the variation in seismic activity of this district a curve has been plotted, fig. 35, with ordinates equal to the values given for the Kioto earthquakes during succeeding centuries. The upper points of these ascending and descending lines are joined by a free curve. The lower points are similarly joined. The points of bisection of ordinates drawn between these two curves are taken as points in a curve to show the true secular change in seismic energy.

Fig. 35.—Curve of Seismic Intensity for Kioto.

By looking at this wavy line it will be seen that the intervals between maxima and minima are closer together in early times than they are later on.

Thus, between the eighth century and the ninth century, points of maximum and minimum seismic efforts occurred at times a century apart, whilst later on, from the eleventh to the fifteenth century, they were at intervals of 300 years apart.

By inspecting either the wavy line or the resultant curve, it will be seen that since the ninth century down to the present time there has been a decided decrease in seismic energy. From the ninth century down to the fifteenth century this decrease is represented by a regular curve. At this point, however, the decrease becomes slightly more rapid, and is represented by a second curve. If, instead of calculating ordinates for my curve, in which intensity has been considered, simply the number of earthquakes are counted, a similar result is obtained. From this it appears that the rate at which seismic energy decreased during the last 500 years was about the same as that at which it decreased during the 500 years previous to this period.

If the lists for the Italian and Turco-Hellenic districts could be similarly analysed, and the earthquakes of any particular district picked out from the others, it is very probable that a similar decrease or alteration in seismic energy might be observed.

Provided that we have at our disposal records of the various earthquakes which have occurred in any given district during a sufficiently long period of time, one conclusion that we may expect to arrive at is that we shall be able to trace some variation in the seismic activity of that district. For the Kioto area, it has been shown that there is a diminution in seismic activity, In other districts, however, there may possibly be an increase.[95]

Relative frequency of earthquakes.—A question which is of great interest to those who dwell in shaken districts is as to how often disturbances may be expected to occur.

From a general examination of this question, considering the earthquakes of the whole world. Mallet arrived at the following conclusions:—

1. While the smallest or minimum paroxysmal intervals may be a year or two, the average interval is from five to ten years of comparative repose.

2. The shorter intervals are in connection with periods of fewer earthquakes—not always with those of least intensity, but usually so.

3. The alternations of paroxysm and of repose appear to follow no absolute law deducible from these curves.

4. Two marked periods of extreme paroxysm are observable in each century, one greater than the other—that of greatest number and intensity occurring about the middle of each century, the other towards the end of each.

The form of the curves which Mallet has drawn seem to indicate that seismic energy came in sudden bursts, and then subsided, gradually gathering strength for another exhibition. This is continually seen in the shocks experienced in various seismic areas—a large shock, or the maximum of the activity dying out by repeated small shocks on succeeding days.

Mr. I. Hattori, writing on the large earthquakes of Japan, remarks that on the average there has been one large earthquake every ten years. They, however, occur in groups, as shown in the following table.

No. of shocks	Period		Interval
6	A.D.	827–836	10	years
6	„	880–890	10 10	„
4	„	1040–1043	4	„
5	„	1493–1507	5	„
4	„	1510–1513	4	„
5	„	1645–1650	6	„
5	„	1662–1664	3	„
4	„	1853–1856	4	„

Dr. E. Naumann, who has also written on the earthquakes quakes in Japan, remarks that if periods of seismic activity do not occur every 490 years, there is a repetition of the cycle after 980 years, but there is much variability. A period of 68 years is very marked. On the average, large earthquakes have occurred every 5·9 years. Fuchs gives some interesting examples of the repetition of earthquakes at definite intervals, of which the following are examples. Sometimes earthquakes appear to have repeated themselves after 100 years. One remarkable example of this is that of Lima, on June 17, 1578, which was repeated on the same day in the year 1678. In Copiapo it is believed that earthquakes occur every twenty-three years, and examples of such repetitions are found in the years 1773, 1796, and 1819. In Canada, near to Quebec, earthquakes lasting forty days are said to occur every twenty-five years. The plateau of Ardebil is said to be regularly shaken by earthquakes every two years.

A. Caldcleugh, writing on the earthquake of Chili, in 1835,[96] remarks that the Spaniards first had the idea that a great earthquake occurs every century. Afterwards they thought the period was every fifty years. As a matter of fact, however, there were large earthquakes in 1812, at Caracas; in 1818, at Copiapo; in 1822, at Santiago; in 1827, at Bogota; in 1828, at Lima; in 1829, at Santiago; and in 1832, at Huasco.

The average period of seismic disturbances in any country probably depends upon the subterranean volcanic activity of that country. When the activity is great the large earthquakes may occur at short intervals; but when the activity is small, as in England, shocks of moderate intensity may not be felt more than once or twice per century. A general idea of the relative frequency of the large earthquakes in various parts of the world may be easily obtained by an inspection of the table on page 240.

Between the years 1850 and 1857 Kluge found that in the world there had been 4,620 earthquakes, which is, upon the average, nearly two per day. This estimate of the frequency of earthquakes of sufficient intensity to be recorded without the aid of instruments is, however, much below the truth. In Japan alone there probably occurs, as a daily average, a number at least equal to that which has been just given for the whole world. Boussingault considered that, in the Andes, earthquakes were occurring every instant of time.[97]

To state definitely how many earthquakes are felt in the world on the average every day is, from the data which we have at our command, an impossibility. Perhaps there may be ten, perhaps there may be 100. The question is one which remains to be decided by statistics which have yet to be compiled.

After a large earthquake, smaller shocks usually occur at short intervals. At first the succession of disturbances are separated from each other by perhaps only a few minutes or hours. Later on, the intensity of these shocks usually decreases, and the intervals between them become greater and greater, until, finally, after perhaps a few months, the seismic activity of the area assumes a quiescent state.

The great earthquake which overtook Concepcion on February 20, 1835, was followed by a succession of shocks like those just referred to, there being registered, between the date of the destructive shock and March 4, 300 smaller disturbances.

During the twenty-four hours succeeding the destruction of Lima (October 28, 1746), 200 shocks were counted, and up to the 24th of February in the following year 451 shocks were felt.

At St. Thomas, in 1868, 283 shocks were counted in nine and a quarter hours.

Similar examples might be taken from the description of almost all destructive earthquakes of which we have records. For a large earthquake to occur, and not to be accompanied by a train of succeeding earthquakes, is exceptional. Sometimes we find that a large number of small earthquakes have occurred without a large one being felt. Seismic storms of this description have happened, even in England—for instance, in the year 1750, which appears to have been a year of earthquakes for many portions of the globe.

In this year, which is known as the ‘earthquake year,’ shocks were felt in England as follows: On March 14, in Surrey; March 18, in south-west of England; April 2, at Chester; June 7, at Norwich; August 23, in Lincolnshire; September 30, Northamptonshire.

Synchronism of earthquakes.—One of the first writers who drew attention to the fact that two shocks of earthquakes have been felt simultaneously at distant places was David Milne, who published a list of these occurrences.[98]

In two instances, February and March 1750, shocks were simultaneously felt in England and Italy. In September 1833 shocks appear to have been simultaneously felt in England and Peru. These and many other similar examples are discussed by Mallet, who thinks with Milne that these coincidences are in every probability matters of accident. According to Fuchs, Calabria and Sicily appear often to have had earthquakes at the same time, as for instance in 1169, 1535, 1638, when the town Euphemia sank, and in the years 1770, 1776, 1780, and 1783.

A remarkable example of coincidence occurred on November 16, 1827, when a terrible earthquake was felt in Columbia, and at the same time a shock occurred on the Ochotsk plains, nearly antipodal to each other.

Kluge also gives a large number of instances of simultaneous earthquakes; thus, on January 23, 1855, on the same day that Wellington, New Zealand, so severely suffered, there was a heavy earthquake in the Siebengeberge, and also in North America. To this might be added the fact that the last destructive earthquake in Japan occurred within a few days of this time.

Sometimes neighbouring countries where earthquakes are common are equally remarkable by their utter want of synchronism. For example, Southern Italy and Syria are said never to be shaken simultaneously.

Secondary earthquakes.—Although it is possible that the simultaneous occurrence of earthquakes in distant regions may sometimes be a matter of chance, it must also be remarked that the shaking produced by one earthquake may be sufficient to cause ground which is in a critical state to give way, and thus the first disturbance becomes the originator of a second earthquake. Admitting that an earthquake, as it radiates from its centre, may act in such a manner, we see that a feeble disturbance might be the ultimate cause in the production of a destructive earthquake, just as the disturbance of a stone upon the face of a scarp might, by its impact upon other stones, cause many tons of material to be dislodged.

It is also easy to conceive how the seismic activity of two districts may be dependent upon each other. Inasmuch as these secondary shocks are direct effects of primary disturbances, they might have been treated in a previous chapter.

As examples of consequent or secondary earthquakes Fuchs tells us that when small earthquakes take place in Constantinople and Asia Minor, earthquakes are felt in Bukharest, Galazy, and Kronstadt.

The great Lisbon earthquake also appears to have given rise to several consequent disturbances. One was in Derbyshire, occurring at 11 a.m. It was sufficiently violent to cause plaster to fall from the sides of a room and a chasm to open on the surface of the ground. Some miners working underground were so alarmed that they endeavoured to escape to the surface. During twenty minutes there were three distinct disturbances.

Another shock was felt at Cork.[99]

Although these disturbances own a consequence of the Lisbon earthquake they might properly perhaps be attributed to the pulsations produced by the shock at Lisbon, which spread through England and other countries without being felt.

The shocks which men felt in New Zealand and New South Wales in 1868 were probably secondary shocks, due to the disturbance at Arequipa and other places on the South American coast.

These so-called secondary earthquakes, although in many instances they may be due to earth pulsations produced by earthquake, or to the immediate sensible shaking of a large earthquake, may perhaps, in certain instances, be attributed to some widespread disturbance beneath the crust of the earth. The occurrence of periods where all earthquake countries suffer, unusual disturbances indicate the probability of such underground phenomena.

CHAPTER XIV.
DISTRIBUTION OF EARTHQUAKES IN TIME (continued).

The occurrence of earthquakes in relation to the position of the heavenly bodies—Earthquakes and the moon—Earthquakes and the sun; and the seasons; the months—Planets and meteors—Hours at which earthquakes are frequent—Earthquakes and sun spots—Earthquakes and the aurora.

The position of the heavenly bodies and the occurrence of earthquakes.—Since the earliest times, in searching for the cause of various natural phenomena, man has turned his energies towards the heavens. One of the earliest observations was the connection that exists between the season of the year and the motions of the heavenly bodies. Tides were seen to be influenced by the moon. In later times it has been discovered that periods of maximum magnetic disturbances occur every ten or eleven years with the sun spots, and Herr Kreil, of Vienna, tells us our satellite, the moon, has also an influence upon the magnet.

From day to day we see the bond connecting our planet with the sun, the moon, and other heavenly bodies which are outside us gradually becoming closer.

Inasmuch as many phenomena, like the motion of the tides, the rise and fall of the barometer, fluctuations in temperature, are all more or less directly connected with the relative position of our planet with regard to the sun and moon, any coincidence between the phases of these bodies and the occurrence of earthquakes more or less involves a time relationship with the other phenomena resultant on lunar and solar influences.

Earthquakes and the position of the moon.—Many earthquake investigators have attempted to show the connection between earthquakes and the phases of the moon.

The first and most successful worker in this branch of seismology was Professor Alexis Perrey, of Dijon, who, after many years of arduous labour in tabulating and examining catalogues of earthquakes, showed that earthquakes were more likely to occur at the following periods than at others.

1. They are more frequent at new or full moon (syzygies) than at half moon (quadratures).

2. They are more frequent when the moon is nearest the earth (perigee) than when she is farthest off (apogee).

3. They are more frequent when the moon is on the meridian than when she is on the horizon.

These results were obtained by Perrey after analysing his catalogues by three different and independent methods, and they were confirmed by the report of a committee appointed by the Academy of Sciences. It must, however, be remarked that in several instances anomalies occur, and also that the difference between the number of earthquakes at any two periods is not a very large one. Thus, for instance, the annual catalogues compiled by Perrey from 1844 to 1847, the earthquakes in perigee are to those in apogee as 47 : 39. Between the years 1843 and 1872 Perrey finds that 3,290 shocks occurred at the moon’s perigee, and 3,015 at the apogee.[100]

Between 1761 and 1800 earthquakes occurred as follows:—

In Perigee	526
Apogee	465

The following table shows the results which enabled Perrey to deduce his first law.

Dividing the period of lunation into quarters, with the time of syzygies and quadratures as the centres of these quarters, he found that the earthquakes were distributed as follows.

	Totals	Syzygies	Quadratures	Difference in favour of the Syzygies
1843–1847	1,604	850·48	753·52	69·96
1848–1852	2,049	1,053·53	995·47	58·06
1853–1857	3,018	1,534·13	1,483·87	50·26
1858–1862	3,140	1,602·99	1,537·41	65·98
1863–1867	2,845	1,463·42	1,381·58	81·84
1868–1872	4,593	2,333·48	2,259·52	73·96
1843–1872	17,249	8,838·03	8,410·97	427·06

The reported earthquakes between 1751 and 1843 are shown to conform with the same rule.[101] Julius Schmidt, astronomer at Athens, found for the earthquakes of Eastern Europe and adjacent countries for the years 1776 to 1873 that there were more earthquakes when the moon was in perigee. Other maxima were at new moon, and two days after the first quarter. There was a diminution at full moon, and a minimum on the day of the last quarter. As one example of results which are antagonistic to the general results obtained by Perrey may be quoted the results of an examination by Professor W. S. Chaplin of the earthquake recorded at the meteorological observatory in Tokio. The list of earthquakes, 143 in number, extending over a period of three years, was recorded by one of Palmieri’s instruments. The results were as follows:—

1. There have been maxima of earthquakes when the moon was two and nine hours east and seven hours west. At the upper transit there is a minimum.

2. Considering the moon’s position with regard to the sun, at conjunction there were 32, at opposition 37, and at quadrature 74. East of the meridian the maximum was at least four hours.

3. When the moon was north of the equator these were 68, when south 82.

4. A maximum of earthquakes seven and eleven days after the moon’s perigee. The fact that these results were obtained for the earthquakes of a special small seismic area renders them more interesting.[102]

Frequency of earthquakes in relation to the position of the sun.—The question as to whether there is a connection between the frequency of earthquakes and the relative position of the sun is to a great extent identical with the question as to the relative frequency of earthquakes in the various seasons. It is a subject which we find referred to by writers in the earliest ages. Pliny and Aristotle thought that earthquakes occurred chiefly in spring and autumn. In later times it has been a subject which has been most carefully considered by Merian, von Hoff, Perrey, Mallet, Volger, Kluge, and others who have devoted attention to seismology. In a résumé of the earthquakes of Europe, and of the adjacent parts of Asia and Africa, from a.d. 306–1843, Mallet gives the following results:—

	For Nineteenth Century				For the whole period
Winter Solstice	177	}	Solstices		253	}	Solstices
Spring Equinox	151		}	306	170		}	403
Summer Solstice	129			Equinoxes	150			Equinoxes
Autumnal Equinox	164			315	159			329

The above periods were called by Perrey critical epochs, because as a general result of his researches he found that at such periods there was a greater frequency of earthquakes. Fuchs, quoting from Kluge’s tables, extending from 1850–1857, tells us that the recorded earthquakes occurred as follows:—

In the Northern Hemisphere—

Equinoxes	1324
Solstices	1202

In the Southern Hemisphere—

Equinoxes	301
Solstices	261

Earthquakes are, therefore, more frequent at the equinoxes, and this especially at the autumnal equinox. In the northern hemisphere, at the solstices, the greater number of shocks occur about the winter solstices, whilst in the southern hemisphere, about the summer solstices.

Exceptions, however, are found in Central America and the West Indies, in the Caucasus, and the Ægean Sea.

The idea that earthquakes had a periodicity dependent upon the position of the heavenly bodies is by no means confined to Europe. In a Japanese work called ‘Jishin Setsu’ (an opinion about earthquakes) by a priest called Tensho, it is stated that the relative positions and movements of the twenty-eight constellations with respect to the moon cause earthquakes. This Tensho asserts after careful calculation, and Falb tells us that all future earthquakes can be predicted.

In the Kuriles and Kamschatka, Sicily, and in parts of South America, it is said that the equinoxes are regarded as dangerous seasons.

Frequency of earthquakes in relation to the seasons and months.—What is here said respecting the relative frequency of earthquakes at the different seasons and months is little more than an extension and critical examination of the results which have been given respecting the frequency of earthquakes in regard to the position of the sun.

That there is a difference between the number of earthquakes which are felt at one season of the year as compared with those felt at another is a fact which, as seismoscopic observations are extended, is becoming more and more recognised.

Some of the more important results which were arrived at by Mallet from 5,879 observations made in the northern hemisphere, and 223 in the southern hemisphere, may be expressed as follows:—

	Maxima	Minima
Northern Hemisphere	January, also a slight rise in August and October	May, June, and July
Southern Hemisphere	November, also May and June	March, extending over one month, also August

Julius Schmidt, of Athens, who so carefully examined the earthquakes of eastern Europe, came to the following conclusions:—

For the earthquakes between 1200 and 1873, a maximum on September 26 and January 17; a minimum on December 3 and June 13.

For the earthquakes between 1873 and 1874, a maximum on March 1 and October 1; a minimum on July 7 and December 15.

For all the earthquakes of eastern Europe, a maximum on January 3; a minimum on July 8, or there was a maximum at perihelion and aphelion.

When the months are grouped together according to the seasons, spring, summer, autumn, and winter, we find that in the northern hemisphere the minimum is in summer and the maximum in winter, whilst in the southern hemisphere (giving the proper months corresponding to its seasons) we find two maxima, one at the commencement of winter, and the other at midsummer, whilst the minima are in spring and autumn.

Fig. 36.—Curves of Monthly Seismic Intensity (Mallet).

In the following table the difference in the number of earthquakes felt at different seasons is given more in detail.

In examining this table, we must remember that for countries like Peru, Chili, and New Zealand, lying in the southern hemisphere, the records given for the months April to September correspond to the winter months of those countries. The Roman numerals indicate the centuries between which the records date.

			October to March	April to September
Northern Regions	1.	Scandinavia and Iceland, xii–xix	129	91
	2.	British and Northern Isles, xi–xix	123	94
	3.	Belgium, France, and Holland, iv–xix	395	272
	4.	Rhone Basin, xvi–xix	115	69
	5.	Switzerland and Rhine Basin, ix–xix	327	205
	6.	Danube Basin, v–xix	147	128
	7.	Spanish Peninsula, xi–xiv	114	87
	8.	Italy, Sicily, Sardinia, and Malta, iv–xix	650	581
	9.	Turco-Hellenic Territory, Syria, Ægean Isles, and Levant, iv–xix	214	222
	10.	Northern Zone of Asia, xviii–xix	46	36
	11.	Japan (Tokio area), 1872–1880 (small earthquakes)	213	157
	12.	Japan b.c. 295-a.d. 1872 (large earthquakes)	165	188
	13.	Algeria and Northern Africa	26	20
	14.	United States and Canada, xvii–xix	86	48
Central Regions	15.	Java, Sumatra, and neighbouring Islands, 1873–4–7–8	194	182
	16.	Mexico and Central America, xvi–xix	26	26
	17.	West Indies (Mallet), xvi–xix	108	114
	18.	West Indies, xvi–xix	296	343
	19.	Cuba, xvi–xix	28	23
Southern Regions	20.	Chili, and La Plata Basin, xvi–xix	89	89
	21.	Peru, Columbia, Basin of Amazons, xvi–xix	506	541
	22.	New Zealand, 1869–1879	166	176

Neglecting those records which show as many earthquakes for the winter months as for the summer months, we see at a glance that generally the greater number of shocks have happened during the colder seasons. In the southern hemisphere, so far as the records go, this is not true. In the northern regions, out of fourteen examples there are two exceptions. In the central regions there are two cases where the greatest number of earthquakes have been recorded in the winter months, and two cases where the greatest number have been recorded for the summer.

Altogether, out of twenty-two examples, there are only six exceptions to the rule. These exceptions altogether occur among records many of which are ancient, and are, therefore, more open to error than lists which have been compiled in modern times.

Because small earthquakes are seldom noticed by persons out in the open air, it might be expected that the number of earthquakes observed in warm countries at one portion of the year would be equal to those observed in any other season. Such an argument, however, would hardly apply to most of the records which are quoted, as they refer to destructive disturbances.

If, however, we take the records made in tropical countries from the table just given, we see that in such countries there have been almost as many observations of earthquakes at one season as at any other.

Another fact which might be adduced against the rule that the greater number of earthquakes occur during the winter months would be the comparison of a table of earthquakes recorded previous to the nineteenth century. By doing this we see that for certain countries the winter rule is inverted, and that the greater number of shocks are felt during the summer.

Notwithstanding these objections to Perrey’s conclusions, the balance of evidence is in favour of his general result, and we may conclude that during the colder portions of the year we may expect more shakings than during the warmer portions. Comparing the number of earthquakes of winter and autumn to those of summer and spring, they are to each other in the proportion of 4 : 3.

A fairer way to examine this question, and to determine what is probably the present state of seismic activity in our globe, would be only to consider the earthquakes which have taken place in comparatively recent times, laying especial stress upon those observations which have been made with the assistance of automatic instruments, or those which have been collected by persons interested in these investigations.

For this purpose the following table, showing the distribution of earthquakes in different countries during the nineteenth century, has been compiled.

The arrangement is mensual. Where the number of earthquakes in any month is above the average, the number is printed in large type; where below the average, in small type.

Earthquakes of the Nineteenth Century, chiefly from Perrey.

Key:

Jan	January
Feb	February
Mar	March
Apr	April
May	May
Jun	June
Jul	July
Aug	August
Sep	September
Oct	October
Nov	November
Dec	December
Ave	Average per month

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Ave
Scandinavia and Iceland	17	11	11	7	7	6	8	8	10	10	11	6	9·3
British Isles and Northern Isles	9	9	10	7	8	6	5	11	12	8	11	12	9·0
France, Belgium, Holland	27	17	21	13	13	8	15	17	15	17	21	25	17·0
Basin of the Rhone	12	12	8	3	3	2	2	4	6	6	8	14	6·6
Basin of the Rhine and Switzerland	15	17	13	12	11	6	12	11	10	17	24	25	14·0
Basin of the Danube	14	15	9	8	12	8	16	11	11	16	10	12	11·8
Spanish Peninsula	10	5	6	7	4	6	10	5	9	11	7	5	7·0
Italian Peninsula, Sicily, Sardinia, and Malta	44	44	48	43	40	34	41	46	27	45	26	39	39·0
Turco-Hellenic Territory, Syria, Ægean Islands, and Levant	22	20	10	10	16	15	14	22	14	17	12	14	16·0
Northern Zone of Asia	4	6	6	4	4	3	5	7	6	3	4	5	4·7
1876–1881, Japan (Tokio area)	39	41	41	30	33	30	27	21	10	28	34	43	31·4
Japan (large earthquakes)	—	5	3	3	1	—	5	4	—	—	1	5	2·0
Algeria and North’rn Africa	5	2	6	7	3	2	2	5	1	4	8	1	3·8
United States and Canada	4	4	3	3	3	—	4	6	3	2	7	5	3·8
Java, Sumatra, &c., 1873–4–5–7 and 9	35	30	38	33	22	36	27	40	24	35	30	26	31·0
Mexico and Central America	3	2	2	2	6	2	2	1	1	3	2	3	2·5
Antilles	9	8	19	12	12	10	9	16	12	10	13	12	11·8
Cuba	4	3	2	3	3	4	5	2	6	5	6	4	4·0
Chili and La Plata	14	10	14	8	19	11	16	15	16	9	27	8	13·9
Peru, Columbia, Basins of Amazons, xvi–xix	92	83	92	27	106	79	94	93	97	77	72	90	87·0
New Zealand, 1869–79	31	27	37	23	22	31	27	36	37	21	27	23	28·5
Jan. 1850, Dec. 1857
Northern Hemisphere	153	162	143	161	126	124	141	156	154	171	151	168	150·0
Southern Hemisphere	72	43	61	66	46	42	53	39	54	55	57	46	53·0
1821–1830
Northern Hemisphere	31	36	31	29	33	33	20	31	24	41	26	34	30·0
Southern Hemisphere	2	—	1	1	3	1	3	2	3	2	1	1	1·6

A glance at this table shows that for most countries in the northern hemisphere the rule that there are generally more earthquakes during the winter months—that is, from October to March—holds good. For countries which lie comparatively near to the Equator, and also for those countries in the southern hemisphere, the rule is not so clear. When examining this table it must be remembered that it does not enable us to judge of the relative frequency of earthquakes in different countries, inasmuch as the periods over which the records were taken are different in different cases.

To the above table might be added the records of P. Merian, who examined the earthquakes felt in Basle up to 1831. As a result he found that during the winter months eighty shocks had been felt, whilst during the summer only forty. Taking the records for the two hemispheres from 1850–1857, compiled by Kluge,[103] in the northern hemisphere we have in the months between October and March 948 shocks against 862 in the remainder of the year. In the same months in the southern hemisphere we have for the corresponding periods the numbers 337 and 300, and thus both hemispheres would appear to follow the same rule. If, however, we examine the table we see that the two seasons are not so pronounced for the southern hemisphere as they are for the northern, and that there may be two or three periods of maximum disturbance as has been previously indicated.

Earthquakes and the planets and meteors.—Just as the moon and the sun may exert an attractive influence upon the earth and cause earthquakes to predominate at certain seasons rather than at others, several investigators of seismic phenomena have thought that the planets might act in a similar manner.

M. J. Delauney, from a study of Perrey’s tables of earthquakes from 1750–1842, found two groups of maxima each with a period of about twelve years, one commencing in 1759 and the other in 1756. Two other groups with twenty-eight year periods respectively commence in 1756 and 1773. These groups coincide with the times when Jupiter and Saturn reach the mean longitudes of 265° and 135°. From this Delauney concludes that earthquakes have a maximum when the planets are in the mean longitudes just mentioned.

The increased number of earthquakes, especially in November, are attributed to the passage of the earth through swarms of meteors, and in like manner supposes the influence of Jupiter and Saturn to be due to their passing through meteor streams situated in mean longitudes 135° and 265°.

As a consequence of this he predicts an increase of earthquakes in the years 1886, 1891, 1898, 1900, &c.[104]

Dr. E. Naumann, who critically examined the large earthquakes of Japan, showed that there was an approximate coincidence between many of the disturbances and the thirty-three year period of meteoric showers.[105]

Humboldt states that a great shower of meteors was seen at Quito before the great earthquake of Riobamba (Feb. 4, 1797). The earthquakes of 1766 and 1799 at Cumana are also said to have been accompanied with meteoric showers. Mallet gives a list of large earthquakes which occurred at the times when meteors were observed.[106]

The hours at which earthquakes are most frequent.—From the examination of a catalogue of over 2,000 earthquakes which occurred in various parts of the world between the years 1850 and 1857, made by Kluge, it is found that both for the northern and southern hemispheres the observations which were made during the night generally exceed those which were made during the day.

	Number of Earthquakes
	Day	Night
In the Northern Hemisphere	938	1592
In the Southern Hemisphere	292	357

In the northern hemisphere the greatest number were observed between 10 p.m. and 12 p.m. (360 shocks), and the fewest between 12 and 2 p.m. (139 shocks). In the southern hemisphere, the greatest number were observed at night between 12 and 1, and the smallest number between 1 and 2 and 4 and 5 in the afternoon.[107] These distinctions, however, are less distinctly marked as we approach the Equator. Schmidt found for the earthquakes of the Orient between 1774 and 1873, that shocks had been most frequent about half-past two a.m., and less frequent about 1 p.m. With regard to these conclusions, which have been reached with much labour, we might be inclined to think that they are partially to be explained on the supposition that more observations are made during the night than during the day—the personal experience of residents in an earthquake country being, that many earthquakes which occur during the day are passed by unnoticed, whilst those which occur during the night are recorded by thousands of observers. Such a view is certainly confirmed by the instrumental records obtained in Japan. From 1872 to 1880 inclusive there were 261 shocks recorded, 132 of which occurred between the hours of 6 p.m. and 6 a.m.

Earthquakes and sun spots.—Of late years considerable attention has been drawn to a coincidence between the occurrence of sun spots, magnetic disturbances, rainfall, and other natural phenomena.

These periods of sun spots occur about every eleven years, and appear to be coincident with the periodical return of the planet Jupiter. In Japan, Dr. E. Naumann sought for a coincidence between these periods of sun spots and earthquakes, but without any marked results.

Schmidt, who carefully compared his lists of earthquakes with the appearance of sun spots, came to the conclusion that there was no marked coincidence. The occurrence of earthquakes had sometimes synchronised with sun spots, whilst at other times there had been a maximum of sun spots and no earthquakes.

M. R. Wolf[108] apparently considers that earthquakes, like volcanic eruptions and the appearance of the aurora, are coincident with sun spots.

Kluge, however, came to the conclusion that when there are few sun spots, earthquakes, like volcanic eruptions and magnetic disturbances, have been at a maximum.

M. A. Poey, who examined a catalogue of the earthquakes of Mexico and the Antilles, extending from 1634 to 1870, shows by a table that earthquakes have come in groups, first at the maxima and then at the minima period of sun spots. Out of thirty-eight groups, seventeen being at the maximum and seventeen at the minimum, the remaining four are exceptions to the rule, being between the maximum and minimum. Phenomena which are dependent upon heat occur with the minima of sun spots, and those dependent upon cold with the maxima.[109]

Earthquakes and the aurora.—The possible connection between earthquakes and the aurora is a subject which has attracted some attention. Boué has especially made a careful examination of this subject.[110]

He comes to the conclusion that if we compare the monthly periods of earthquake frequency and the aurora there is an agreement between the two. Comparing Perrey’s tables of earthquakes from the fourth to the nineteenth century, with tables of the aurora, one-third of both phenomena have occurred, not only in the same day, but often at the same hour. Between 1834 and 1847, 457 earthquakes are given and 351 notices of the aurora.

Out of these:—

48 occur on the same day,
5 occur in the same hour,
30 approximate to the same time.

The nearer together that these phenomena have occurred the stronger have they been.

Professor M. S. di Rossi brings forward many examples where there has been a coincidence between the appearance of the aurora and earthquakes. On 139 nights out of 211 days the aurora was seen in some parts of Italy, and ninety-three times earthquakes were felt. On forty-six occasions earthquakes and aurora took place together.[111] In considering the probability of a connection existing between these two phenomena, we must bear in mind that the aurora is at no great height above the surface of our earth, and, further, that it can be partially imitated. The fact that in earthquake countries, like Japan, the aurora is practically never seen, would indicate that we can neither regard this imperfectly understood phenomenon either as an effect or cause of earthquakes. That earthquakes and the appearance of the aurora in certain countries should not sometimes coincide is an impossibility.

Dr. Stukeley, who, it must be remembered, attempted to correlate the phenomena of earthquakes and electricity, when writing of the disturbances which shook England in 1849 and 1850, says that the weather had been unusually warm, the aurora borealis frequent and of unusually bright colours, whilst the whole year was remarkable for its fire-balls, lightnings, and corruscations.[112]

The aurora was observed before the commencement of the Maestricht earthquakes in 1751[113] whilst at the time of the shock flashes of light like lightning were observed in the sky.

Glimmering lights were seen in the sky before the New England earthquakes (Nov. 18, 1755), and again, before the disturbances which occurred in the same region in 1727, peculiar flashes of light were seen.

Preceding the Sicilian earthquake of 1692 strange lights were seen in the sky. Ignis fatui have also been observed with earthquakes. At the time of auroral displays Bertelli has observed microseismical disturbances, and M. S. di Rossi, who has made similar observations, thinks that there is an intimate connection between the aurora and earthquakes; the aurora either occurring in a period of earthquakes, or else taking the place of earthquakes.

CHAPTER XV.
BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE AND EARTHQUAKES.

Changes in the barometer and earthquakes.—Mallet, who collected together a number of examples of earthquakes which have occurred with a fall of the barometer, and a number which have happened with a rise, concludes that there are as many instances of the one as of the other. At the great earthquake of Calabria, in 1783, the barometer was very low. The earthquake of the Rhine (February 23, 1828) was preceded by a gradual fall of the barometer, which reached its lowest point upon that day. After the earthquake the barometer again rose. The earthquake of February 22, 1880, in Japan, was accompanied by exactly similar phenomena. Caldcleugh, who observed the heavy shocks in Chili (February 20, 1835), noticed that on February 17 and 18 the barometer fell 5/10 inches. Similar phenomena were observed before the succeeding smaller shocks. After the shocks the barometer again rose. Principal Dawson, speaking of the earthquakes of Canada, observes that some of the shocks have been accompanied with a low barometer.

P. Merian, who examined the connection between the Swiss earthquakes and atmospheric pressure, found that out of twenty-two earthquakes observed in Basle between 1755 and 1836, thirteen of these were local shocks, of which eight were accompanied with sudden changes of pressure. Of the remaining nine, which were only felt slightly in Basle, no change in atmospheric pressure was observed. Of thirty-six earthquakes which, between 1826 and 1836, were felt in Switzerland, thirty were chiefly confined to Switzerland, and ten of these occurred with a low or falling barometer.

Humboldt is of opinion that earthquakes only occur with changes in barometric pressure in those countries where earthquakes are few; and he gives examples where the regular variations of the barometer have gone on without interruption at the time of earthquakes.

Frederick Hoffmann, who examined fifty-seven earthquakes which occurred at Palermo between 1788 and 1838, came to the following result:—

The barometer was sinking	in 20 cases
„ „ rising	in 16 „
„ „ at a minimum	in 7 „
„ „ maximum	in 3 „
„ „ undetermined	in 11 „[114]

The observations of M. S. di Rossi apparently show that the earthquakes in Italy chiefly occur with a barometrical depression and with sudden jumps in atmospheric pressure.

Schmidt, who examined the earthquakes of the Orient, which occurred between 1858 and 1873, says that they were rare with a high barometer, but numerous when the barometer was low.

From an examination of a table of 396 earthquakes (May 8, 1875-Dec. 1881) felt in Tokio, furnished to me by Mr. Arai Ikunosuke, the director of the meteorological department, I obtained the following results:—

The barometer was rising	in 169 cases
„ „ falling	in 154 „
„ „ steady	in 73 „
„ „ below the monthly mean	in 189 „
„ „ above	in 192 „

From this it would appear that in Japan at least the movements of the barometer do not show any marked connection with the occurrence of earthquakes.

When considering this question we must remember the marked effects which a lowering of the barometer produces upon certain volcanoes and solfataras. The volumes of steam emitted from Stromboli and from some of the solfataras in Tuscany hold a marked connection with atmospheric pressure as the quantity of fire damp given off from coal seams—these being greatest when the barometer is low. At certain changes of the weather it is said that the volcano of Vulture, near Melfi, emits noises. These phenomena at once place volcanic phenomena and barometrical pressure in direct relationship.

Changes in temperature.—If, with an earthquake, it should happen that there is a change in the height of the barometer, we should naturally expect that this might be accompanied with the changes in the temperature, in the wind, and in other atmospheric phenomena which are more or less connected with the height of the barometer.

Many times it has been observed that after an earthquake there has been a sudden fall in the temperature. Such was the case with the Yokohama earthquakes of 1880.

Cotte endeavours to show that the earthquakes of Lisbon produced a change upon the temperature of all Europe. In the year which followed this earthquake storms were more common than usual.

Kluge has collected together a large number of examples when there has been a fall of temperature at the time of an earthquake.[115]

At Kiachta, in Siberia, at the time of the earthquake of December 27, 1856, the thermometer fell from 12° to 25° R. We must, however, remember that there are many cases known where the thermometer rose.

M. S. di Rossi remarks that we have the highest records of temperature in the years richest in earthquakes. Thus, in 1873, at the time of the earthquakes in Central and Northern Italy, an abnormal high temperature was remarked. Japanese writers have remarked upon the unusual heat which has shaken their countries. The temperature of subterranean waters have been known to increase before earthquakes.

CHAPTER XVI.
RELATION OF SEISMIC TO VOLCANIC PHENOMENA.

Want of synchronism between earthquakes and volcanic eruptions—Synchronism between earthquakes and volcanic eruptions—Conclusion.

Connection between earthquakes and volcanic eruptions.—Insomuch as it is a recognised fact that regions which are characterised by their seismic activity are chiefly those which are also characterised by the number of their volcanoes, it is generally assumed that these two phenomena have an intimate relation. The residents in a volcanic country, when seeking for the origin of an earthquake, invariably turn towards the volcanoes which surround them. If a neighbouring volcano is in a state of activity, it is often regarded as a safeguard against seismic convulsions, in other cases it is looked upon as being the cause of such disturbances. In certain instances both of these views have apparently been corroborated. When we consider that an earthquake and a volcanic eruption may both be the result of some great internal convulsion, and that first one and then the other may take place in the same neighbourhood, it is natural to expect that when these internal forces have expended themselves in the production of one of these phenomena, it is not so likely that they should exhibit themselves in the other. The inhabitants of Sicily and Naples, we are told, regard eruptions of Etna and Vesuvius as safeguards against earthquakes. A similar belief is to be found in portions of South America with regard to the volcanoes for which that country is so celebrated.

From an examination of the records of the large earthquakes and the volcanic eruptions which have taken place in Japan during the last 2,000 years, Dr. Naumann found that there was often an approximate coincidence between the times of the occurrence of these phenomena, suggesting the idea that the efforts which had been sufficient to establish the volcano had at the same time been sufficient to shake the ground.

Of destructive earthquakes which have occurred at the time of volcanic eruptions, and of examples when these phenomena have occurred at widely separated intervals, the records are extremely numerous.

Want of synchronism between earthquakes and volcanic eruptions.—Many of the great earthquakes of South America do not appear to have been connected with volcanic eruptions.

The great earthquakes of the world, like those of Calabria and Lisbon, which took place in regions which are not volcanic, have not, Fuchs tells us, taken place in conjunction with volcanic outbursts.

In Japan, as in the Sandwich Islands and in many other parts of the globe, the small earthquakes which occur almost daily do not appear to show any marked connection with volcanic disturbances.

In 1881, during the eruption of Natustake, a volcano lying about a hundred miles north of Tokio, there was neither an increase nor a decrease in the earthquakes which were felt in Tokio. Similar remarks apply to the state of seismic activity of 1876–77, when Oshima, a volcanic island about seventy miles to the south of Tokio, was in eruption. In the Sandwich Islands Mauna Loa seems to have its eruptions independently of the disturbances which shake these islands.[116]

Synchronism of earthquakes and volcanic eruptions.—Although many examples like the above may be quoted, which apparently show an utter want of connection between earthquakes and volcanoes, we must not overlook that class of earthquakes which almost invariably accompany all great volcanic disturbances. In fact the sudden explosions which take place at volcanic foci, as, for instance, at the commencement of an eruption, are enumerated as one of the causes which produce earthquakes. Earthquakes like these usually continue until the pressure of the steam and lava have found for themselves an opening. As compared with the total number of earthquakes which are recorded, they form but an insignificant portion.

The direct connection which exists between these phenomena has, no doubt, done very much to spread the popular belief that all earthquakes may be connected with volcanic eruptions. As examples where this connection has existed we might quote from almost all the volcanic countries in the world.

Thus, Fuchs tells us that on October 6, 1737, almost the whole of Kamschatka and the Kurile Islands were disturbed by movements which were simultaneous with the outbreak of the great volcano Klutschenskja of North Kamschatka.

One of the earliest records of a severe earthquake and a volcanic eruption occurring simultaneously is found in the accounts of the destruction of Herculaneum and Pompeii. The throwing up of Monte Nuovo in the neighbourhood of Pozzuoli was accompanied with a dreadful earthquake.[117]

In 1868 the earthquake of Arequipa was accompanied by the opening of the volcano Misti, on its north side. The distance to the volcano is about fourteen miles.

At the time of the eruptions of Kilauea in 1789 the ground shook and rocked so that persons could not stand.

The first eruption of the volcano Irasu, in Costa Rica (1783), was accompanied by violent earthquakes.[118] The smoke and flames which are said to have issued from the side of Mount Fojo at the time of the Lisbon earthquake are regarded by some as having been volcanic. Others thought that the phenomena, rather than being on the side of Fojo, which showed no traces of volcanic action, had taken place in the ocean.

At the time of the great earthquake at Concepcion (1835), whilst the waves were coming in, two great submarine eruptions were observed. One, behind the Isle of Quiriquina, appeared like a column of smoke. The other, in the bay of San Vicente, appeared to form a whirlpool. The sea-water became black, and had a sulphurous smell, there being a vast eruption of gas in bubbles. Many fish were killed.[119]

With this same earthquake, near to Juan Fernandez, about one mile from the shore, the sea appeared to boil, and a high column of smoke was thrown into the air. At night flames were seen.

In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a volcano at the foot of which Mendoza is situated burst into eruption.

The earthquake of 1822 at Valdivia was accompanied by eruptions of the neighbouring mountains, which only lasted a few minutes.

At the time of the Leghorn shocks (January 16–27, 1742) some fishermen observed a part of the sea to rage violently, to raise itself to a great height, and then rush landwards.[120]

In 1797, when Riobamba was destroyed, the neighbouring volcanoes were not affected, but Mount Pasto, 120 miles distant, suddenly ceased to throw out its usual column of water.

On the night of December 10, 1874, a strong shock was felt in New England, whilst at 4.45 a.m. on December 11 a shock was felt in the Pic du Midi, in the Pyrenees. In the middle of December there were volcanic outbursts in Iceland.[121]

It is possible that these occurrences might be the results of some widespread disturbance beneath the crust of the earth, or perhaps even of widely extended earth pulsations. The probability, however, is that these coincidences are accidental. When we remember that in a small area like the northern half of Japan alone there are periods when there are at least two shocks per day on the average, it is impossible for these coincidences not to exist. Less frequently coincidences between the larger disturbances must occur. Over and above these accidental coincidences, it would appear that in the world’s history periods have occurred when earthquakes were unusually frequent, and at such times distant countries have suffered simultaneously. This approximate coincidence in period, which has been referred to when speaking of the distribution of destructive earthquakes in historical time, does not imply an exact synchronism in the single shocks.

Small earthquakes, or, more properly speaking, local tremblings, are a necessary accompaniment of almost all volcanic eruptions. Tremors of this description are seldom, however, felt beyond the crater, or at the most upon the flanks of the mountain where the eruption is going on.

They are due to the explosive action of steam bursting through the molten lava.

Volcanic eruption succeeding earthquakes.—Sometimes it has happened that an earthquake, or a series of earthquakes, have terminated with the formation of volcanic vents.

As an example of a volcanic outburst terminating a seismic disturbance, may be mentioned the appearance of a new volcano in the centre of Lake Ilopango, as a sequel to the shocks which had disturbed that neighbourhood in 1879.[122]

In 1750 there were continuous shakings lasting over three months at Manilla. These terminated with an eruption of a small island in the middle of a neighbouring lake. Three days after the commencement of this eruption, four other small islands rose in the same lake.[123]

Antonio d’Ulloa, when speaking of the Andes, remarks that after a volcanic eruption the shocks cease.[124]

Conclusion.—Looking at this question generally, insomuch as the greatest number of volcanic eruptions appear, according to Fuchs, to have taken place in summer, whilst the greatest number of its earthquakes have apparently taken place in winter, it would seem that the two phenomena are without any direct connection, unless it be that both are different effects of a common cause.

Regarded in this manner, an earthquake may be looked upon as an uncompleted effort to establish a volcano. To use the words of Mallet, ‘The forces of explosion and impulse are the same in both; they differ only in degree of energy, or on the varying sorts and degrees of resistance opposed to them.’[125]

Although we have many examples of earthquakes having occurred without volcanic eruptions, and, on the other hand, of volcanic eruptions without earthquakes, volcanoes may still be regarded as ‘safety-valves of the earth’s crust,’ which, by giving relief to internal stresses, guard us against the effects of earthquakes.

That many earthquakes are felt at Copiapo is attributed to the fact that in the neighbouring mountains there are no volcanic vents.

We must not, however, overrate the protective influence of volcanoes. In the Sandwich Islands we see the columns of liquid lava in neighbouring mountains standing at different heights, indicating a want of subterranean connection between these vents. In consequence of this it would seem that enormous pressures might be generated in the neighbourhood of one of these mountains without finding relief at the other. When we have conditions like these, it would seem that the eruption of a volcano may have little or no influence in protecting neighbouring districts.

This may possibly be the explanation of the fact that in 1835 Concepcion was destroyed, notwithstanding there being an unusual activity in the volcanic vents of the neighbouring mountains.

CHAPTER XVII.
THE CAUSE OF EARTHQUAKES.

Modern views respecting the cause of earthquakes—Earthquakes due to faulting—To explosions of steam—To volcanic evisceration—To chemical degradation—Attractive influence of the heavenly bodies—The effect of oceanic tides—Variation in atmospheric pressure—Fluctuation in temperature—Winds and earthquakes—Rain and earthquakes—Conclusion.

As the results of modern inquiries respecting the cause of earthquakes, we see many investigators chiefly attributing these phenomena to special causes. A few attribute them to several causes. It seems to us that they might be attributed to very many causes which often act in a complex manner. The primary causes are telluric heats, solar heat, and variations in gravitating influences. These may be the principal, and sometimes the immediate, cause of an earthquake. The secondary causes are those dependent upon the primary causes, such as expansions and contractions of the earth’s crust, variations in temperature, barometrical pressure, rain, wind, the attractive influences of the sun and moon in producing tides in the ocean or the earth’s crust, variations in the distribution of stress upon the earth’s surface caused by processes of degradation, the alterations in the position of isogeothermal surfaces, &c.

The part which may be played by these various causes in the production of oscillations, pulsations, and tremors will be referred to.

Earthquakes consequent on faulting.—In the chapter on Earth Oscillations, the causes producing the phenomena of elevation and depression are briefly indicated.

By the variations in stress accompanying elevations and depressions, cracks are produced. Inasmuch as compression would crush the rocks constituting the earth’s crust, we must conclude with Captain Dutton that these cracks are formed by tension. By elevation, the upper rigid crust of the earth is stretched, and fissures are produced. The sudden formation of these fissures or faults gives rise to earthquakes, and perhaps also to volcanic vents. That earthquake and volcanic regions are situated on areas where there is evidence of rapid elevation is strikingly illustrated round the shores of the Pacific.

Lasaulx considered that the earthquake of Herzogenrath was more or less intimately connected with the great mountain fissure—the Feldbiss—which crosses the coal region of the Wurm.[126] The sudden elevation or sinking of large areas at the time of an earthquake may be a consequence of these dislocations.

It has already been pointed out that the earthquake region of Japan is the one where we have evidence of recent and rapid elevation. That certain earthquakes of this region may possibly be the result of faulting we have the evidence of our senses and of our instruments. The sudden blows and jolts which are sometimes felt are indicative of the sliding of one mass of rock across another.

Should the ground be simply torn asunder, this tearing would give rise to a series of waves of distortion, vibrating in directions parallel to the plane of the fissure. Supposing this motion to be propagated to a number of surrounding stations, it would be recorded at each of these as having the same direction. To those situated on a line forming a continuation of the strike of the fissure, the vibrations would advance so to speak end on, whilst to those stations lying in a line perpendicular to the strike of the fissure, the motion would advance broadside on.

Motions like these latter have been recorded in Tokio, where earthquakes which from time observations were known to have come from the faulted and rising region to the south have been registered as a series of east and west motions, or vibrations transverse to this line of propagation.

It must, however, be here mentioned that the registration of only transverse motion may possibly be due to the extinction of normal motion, although this is not generally regarded as probable.

It would therefore appear that certain earthquakes and faults are closely related phenomena, the former being an immediate effect of the latter. Faults are due to earth oscillations, and to a variety of causes producing disturbances in the equilibrium of the earth’s crust; the principal cause of all these phenomena being alterations in the distribution of heat, and the attractive force of gravity.

Earthquakes consequent on the explosion of steam.—Humboldt regarded volcanoes and earthquakes as the results of a common cause, which he formulated as ‘the reaction of the fiery interior of the earth upon its rigid crust.’ Certain investigators, who have endeavoured to reduce Humboldt’s explanation to definite limits, have suggested that earthquakes may be due to sudden outbursts of steam beneath the crust of the earth, and its final escape through cracks and fissures.

Admitting that steam may accumulate by separating out from the cooling interior of our globe, its sudden explosion might be brought about by its own expansive force, or by the movements in the bubbling mass from which it originated.

Others, however, rather than regard the steam as being a primeval constituent of the earth’s interior, imagine it arises from the gradual percolation of water from the surface of the earth down to volcanic foci, into which it is admitted against opposing pressures, by virtue of capillary action.

Mallet, in his account of the Neapolitan earthquake, shows that the whole of the observed phenomena can be accounted for by the admission of steam into a fissure, which by the expansive force exerted on its walls was rent open. Just as at the Geysers we hear the thud and feel the trembling produced by the sudden evolution and condensation of steam, so may steam by its sudden evolution and condensation in the ground beneath us give rise to a series of shocks of varying intensity, accompanied by intermediate vibratory motions—that is to say, a motion which, as judged of by our feelings, is not unlike many earthquakes. Often it may happen that the result of the explosion may be the production of a fault, or at least a fissure; and thus in the resulting movements we may have a variety of vibrations, some being those of compression and distortion, produced by the blow of the explosion, and others being those of distortion alone, produced by the shearing action which may have taken place by the opening of the fault. Sometimes one set of these vibrations may be prominent, and sometimes the other. Thus, when we say that an earthquake has shown evidence by the nature of its vibrations that it was produced by a fault, this by no means precludes the possibility that an explosion of steam may also have been connected with the production of the disturbance. Mallet threw out the suggestion that the opening of fissures beneath the ocean might admit water to volcanic foci. During the time that the water was in the spheroidal state, the preliminary tremors, so common to many earthquakes, would be produced. These would be followed by the explosion, or series of explosions, constituting the shock or shocks of the earthquakes.

The chief reasons for believing that the earthquakes of North-Eastern Japan are partly due to explosive efforts are:—

1. That the greater number of disturbances, perhaps ninety per cent., originate beneath the sea, where we may imagine that the ground, under the superincumbent hydrostatic pressure, is continuously being saturated with moisture.

2. Many of the diagrams show that the prominent vibrations, of which there are usually from one to three, in a given disturbance have the same character as those produced by an explosive like dynamite, the greatest and probably the most rapid motions being inwards towards the origin.

It may here be remarked that a very large proportion of the destructive earthquakes of the world have originated beneath the sea, as has often been testified by the succeeding sea waves. Also, it must be observed, that earthquake countries, like volcanic countries, are chiefly those which have a coast line sloping at a steep angle beneath the sea—that is to say, earthquakes are frequent along coasts bordered by deep water.

The earthquakes which occur at volcanic foci constitute another class of disturbances which may be accredited to the explosive efforts of steam.

Earthquakes due to volcanic evisceration.—By the ejection of ashes and lava from volcanic vents, there is an extensive evisceration of the neighbouring ground. When we look at a volcano like Fujiyama, 13,000 feet in height, and at least fifty miles in circumference, and remember that the mass of cinders and slag of which it is composed came from beneath the area on which it rests, the point to be wondered at is, that earthquakes, consequent on the collapse of subterranean hollows, are not more frequent than they are. At the time of a single eruption of a volcano, the quantity of lava ejected amounts to many thousand millions of cubic feet. In 1783 the quantity of lava ejected from Skaptas Joknee, in Iceland, was estimated as surpassing ‘in magnitude the bulk of Mont Blanc.’[127] Admitting that hollow spaces are the results of these eruptions, and that in consequence of this evisceration the ground is rendered unstable, the instability being increased by the additional load placed above the eviscerated area, it would seem that from time to time earthquakes are inevitable.

Facts, however, teach us that volcanoes act as safety valves, and that, as a rule, at or shortly after an eruption, earthquakes cease. The relationship of earthquakes to volcanic eruptions would therefore indicate, notwithstanding the arguments put forward to show that an area loaded by a volcano has in consequence of the evisceration and the load a quaquaversal dip, that evisceration does not take place beneath volcanoes as is usually supposed, and we may conclude that it is but few earthquakes which have an origin due to these causes.

Earthquakes and evisceration by chemical degradation.—A powerful agent, which tends to the formation of subterranean hollows, is chemical degradation. The effects of this have been often measured by quantitative analysis of the solid materials which are daily carried away by many of our springs. In limestone districts this is very great. Prof. Ramsay estimates that the mineral matter discharged annually by the hot springs of Bath is equivalent in bulk to a column 140 feet in height and 9 feet in diameter. At San Filippo, in Tuscany, the solid matter discharged from the springs has formed a hill a mile and a quarter long, a third of a mile broad, and 250 feet in thickness.[128] Many other examples of subterranean chemical degradation will be found in text-books of geology.

By this chemical action large cavernous hollows are produced. Beneath a volcano it is probable that liquid material immediately takes the place of that which is ejected, and that hollows are not formed as in the case of chemical degradation. If a cavern becomes too large, it eventually collapses.

Of the falling in of large excavations we have examples in large mines. As a consequence, not only is a trembling produced, but also a noise, which is so like that produced by certain earthquakes that the South American miners have but one word, ‘bramido,’ to express both.[129]

Boussingault, who was an advocate for the theory that many earthquakes are produced by the sinking of the ground, calls attention to the fact that we have evidences of the subsidence of great mountains, like the Andes, the districts around which are so well known for their earthquakes. Capac Urcu is one of these mountains which legends tell us has decreased in height.

The variation in the height of mountains is a subject which deserves attention. That mountains may possibly be hollow, we have the remarkable results attained by Captain Herschel, who found that the attractive force of gravity in the neighbourhood of the Himalayas was not so great as it ought to have been had these mountains been solid. The Rev. O. Fisher gives another explanation of this phenomenon. Palmieri considers that the terrible earthquake which devastated Casamicciola (1881) was due to the hot springs having gradually eaten out cavernous spaces beneath the town. The extremely local character of this shock was certainly favourable to such a view.

The earthquake which, in 1840, caused Mount Cernans, in the Jura, to fall, is also attributed to the solvent action of waters in undermining its foundations. This undermining action was in great measure probably due to a large spring, which, twenty-five years previously, had disappeared, and which subsequently may possibly have been slowly disintegrating the foundations of the mountain. Earthquakes of this order would be principally confined to districts where there are rocks which are more or less soluble, as, for instance, rock salt, gypsum, and limestone.

Earthquakes and the attractive influences of the heavenly bodies.—The most important attractions exercised upon our planet are those due to the sun and moon. To these influences we owe the tides in our ocean, and possibly elastic tides in the earth’s crust. Some theorists would also insist upon liquid tides in the fluid interior of our earth. The nature of the earth’s interior is, however, a question on which there is a diversity of opinion.

One doctrine, which, until recent years, received much support, was that the interior of the earth was a reservoir of molten matter contained within a thin crust. Hopkins showed that the least possible thickness of such a crust must be from 800 or 1,000 miles, otherwise the motions of precession and nutation would be subject to interference.

M. Delauney objected to the views of Hopkins, on the supposition that the fluid interior of the earth had a certain viscosity.

Sir William Thomson arrives at the conclusion that the earth on the whole must be more rigid than a continuous solid globe of glass. Mr. George H. Darwin’s investigations on the bodily tides of viscous or semi-elastic spheroids tend to strengthen the arguments of Sir William Thomson.

Some philosophers hold the view that the central portion of the earth, although intensely hot, is solid by pressure, whilst the outer crust is solid by cooling. Between the two there is a shell of liquid or viscous molten matter.

Another argument is, that although the interior of the globe may be solid, it is only retained in that condition by an immense pressure, on the relief of which it is liquefied—it is potentially liquid.

As these views, and the arguments for and against them, are to be found in all modern text-books of geology, we will at once proceed to consider the effect of solar and lunar attractive influences in producing earthquakes upon a globe which is either solid, partially solid, or which has an interior wholly liquid.

Effect of the attractive influences of the sun and moon.—In 1854 M. F. Zantedeschi put forward the view—that it is probable there is a continual tendency of the earth to protuberance in the direction of the radii vectores of the two luminaries which attract it. In consequence of these protuberances, pendulums ought at one time to swing more slowly than at others. Zantedeschi remarks that the periods of earthquakes appear to confirm such a view, insomuch as they occur more often at the syzygies, or epoch of the spring tides, than at neap tides—an observation found in the works of Georges Baglivi (1703) and Joseph Toaldo (1770).[130]

Prof. Perrey, of Dijon, who did so much for seismology, held the view that the preponderance in the number of earthquakes felt at particular seasons was possibly due to the attractive influence of the sun and moon producing a tide in the fluid interior of the earth, which, acting on the solid crust, produced fractures.

Rudolf Falb, whose writings have of late years attracted considerable attention, brings forward views which may be regarded as amplifications of those suggested by Perrey.

According to Falb, the inner portion of the earth must be regarded as fluid. In the crust above this fluid reservoir are cracks and channels, into which, by the attraction of the moon and sun, the fluid is drawn. On entering these cracks cooling takes place, together with explosions of gas and subterranean volcanic disturbances. The attractions producing the internal tides required by Falb are chiefly dependent upon the following factors:—

1. The nearness and distance of the sun from the earth (January 1 and July 1).

2. The position of the moon with regard to the earth, which in every twenty-seven days is once near and once distant.

3. The phases of the moon—whether full or new moon (syzygies), or whether first or last quarter (quadratures).

4. The equinoxes, the position of the sun in the equator, and the relative position of the earth.

5. The position of the moon relative to the equator.

6. The concurrence of the ‘centrifugal force’ of the earth with the last quarter of the moon.

7. The entrance of the moon on the ecliptic—the so-called nodes.

Assuming that earthquakes are wholly consequent on these attractions, it at once becomes possible to predict their occurrence. This Falb does, and when his predictions have been fulfilled he has certainly gained notoriety.

He commenced by the predictions of great storms. In 1873 he predicted the destructive earthquake of Belluno, which earned for himself a eulogistic poem, which he has republished in his ‘Gedanken und Studien über Vulkanismus.’ After this, in 1874, he predicted the eruption of Etna. He also explained why, in b.c. 4000, there should have been a great flood, and for a.d. 6400 he predicts a repetition of such an occurrence.

When we approach the question of the extent to which the attraction of the sun and moon may influence the production of earthquakes, a question which we have to answer is, whether it is likely that the attractive power of the moon is so great that it could draw up the crust the earth beyond its elastic limits. We know what it can do with water. It can lift up a hemispherical shell 8,000 miles in diameter about two or three feet higher at its crown than it lifts the earth. Even supposing the solid crust to be lifted 100 times the apparent rise of the tide, is it likely that a hemispherical arch 8,000 miles in diameter when it is raised 200 feet at its crown could by any possibility suffer fracture? If an arch is 12,000 miles in length, all that we here ask is, whether the materials which compose the arch are sufficiently elastic to allow themselves to be so far stretched that the crown may be raised 200 feet. The result which we should arrive at is apparently so obvious that actual calculation seems hardly necessary. If we regard the earth as being solid, the question resolves itself into the inquiry as to whether a column of rock, which is equal in length to the diameter of the earth, or about 8,000 miles, can be elongated 200 feet without a fracture. This is equivalent to asking whether a piece of rock one yard in length can be stretched one seventy thousandth of a foot. Considering that this is a quantity which is scarcely appreciable under the most powerful of our microscopes, we must also regard this as a question which it is hardly necessary to enter into calculations about before giving it an answer. To vary the method of treating such a question, may we not ask what is the utmost limit to which it would be possible to raise up or stretch the crust of the earth without danger of a fracture? Thus, for instance, to what extent might a column of rock be elongated without danger of its being broken? From what we know of the tenacity of materials like brick and their moduli of elasticity, it would seem possible to stretch a bar of rock 8,000 miles in length for approximately half a mile before expecting it to break. As to whether there is a wave, the height of which is equal to half this quantity, running round our earth as successive portions of its surface pass beneath the attracting influences of the sun and moon, is a phenomenon which, if it exists, would probably long ago have met with a practical demonstration.

The deformation which a solid globe or spherical shell would experience under the attractive influences of the sun and moon has been investigated by Lamé, Thomson, Darwin, and other physicists and mathematicians.

A conclusion that we are led to as one result of these valuable investigations is, that if the interior of the earth be fluid, and covered with a thin shell, then enormous elastic tides must be produced. A consequent phenomenon, dependent on the existence of these tides, would be a marked regularity in the occurrence of earthquakes. As this marked regularity does not exist, we must conclude that earthquakes are not due to the attractive influences of the sun and moon acting upon the thin crust of the earth covering a fluid interior. The periodicity of earthquakes corroborates the conclusions of Sir William Thomson, who remarks that if the earth were not extremely rigid the enormous elastic tides which must result would be sufficient to lift the waters of the ocean up and down so that the oceanic tide would be obliterated.

Assuming that the earth has the rigidity assigned to it by mathematical and physical investigators, we nevertheless have travelling round our earth, following the attractions of the moon and sun, a tidal stress. This stress, imposed upon an area in a critical state, may cause it to give way, and thus be the origin of an earthquake. Earthquakes ought therefore to be more numerous when these stresses are the greatest.

The periods of maximum stress or greatest pull exerted by the moon and sun will occur when these bodies are nearest to our planet—that is, in perigee and perihelion, and again when they are acting in conjunction or at the syzygies. That earthquakes are slightly more numerous at these particular periods than at others is a strong reason for believing that the attractions of the moon and sun enter into the list of causes producing these phenomena.

Had there been a strongly marked distinction in the number of earthquakes occurring at these particular seasons as compared with others, we might have attributed earthquakes to the existence of elastic tides of a sensible magnitude. As the facts stand, it appears that the maximum pulls exerted by the moon and sun are only sufficient to cause a slight preponderance in the number of earthquakes felt at particular seasons, and therefore that these pulls only result in earthquakes when the distorting effort has been exerted on an area which, by volcanic evisceration, the pressure of included gases, and other causes, is on the verge of yielding.

Earthquakes and the tides.—If we assume that earthquakes are in many cases due to the overloading of an area and its consequent fracture, such loading may occur by the rising of the tide. A belief that the earthquakes of Japan were attributable to the tides may be found in the diary of Richard Cocks under the date November 7, 1618, who remarks:—

‘And, as we retorned, about ten aclock, hapned a greate earthquake, which caused many people to run out of their howses. And about the lyke hower the night following hapned an other, this countrey being much subject to them. And that which is comunely markd, they allwais hapen at a hie water (or full sea); so it is thought it chauseth per reason is much wind blowen into hollow caves under ground at a loe water, and the sea flowing in after, and stoping the passage out, causeth these earthquakes, to fynd passage or vent for the wind shut up.’[131]

Although we may not acquiesce in Cock’s views respecting the imprisoned wind, it would seem that a comparison of the occurrence of earthquakes and the state of the tide would be a legitimate research. Inasmuch as the stresses which are brought to bear upon an area by the rising of the tide are so very much greater than those due to barometrical changes, it is not unlikely that a marked connection would be found. But it must be remembered that because researches, so far as they have gone, tend to show that earth movements are more frequent when an area is relieved of a load, it is not unlikely that the greatest number of earthquakes may be found to occur at low water. Prof. W. S. Chaplin attempted to make this investigation in Japan, but not being able to obtain the necessary information respecting the tides, was compelled to relinquish this interesting work.

Every foot of rise in a tide is equivalent to a load being placed on the area over which the tide takes place of sixty-two pounds to the square foot. This load is not evenly distributed, but stops abruptly at a coast line. Lastly, it may be observed that many coast lines are not simultaneously subjected to stresses consequent upon this load. Japan, for instance, may be regarded as an arch placed horizontally. The area near the crown of this arch is loaded by the tidal wave crossing the Pacific before the areas near the abutment, and farther there is a horizontal pressure at the crown which, if Japan were like a raft, would tend, as the tide advanced, to straighten its bow-like form, but as the wave passed its abutments to increase its curvature.

Prof. G. Darwin has calculated the amount of rise and fall of a shore line due to tidal loads (see [p. 336], ‘Earth Pulsations’). The result of these calculations apparently indicates that these loads may have a considerable influence upon the stability of an area in a more or less critical condition.

Mr. J. Carruthers suggests that tidal action may hold a general but indirect relationship to volcanic and seismic action by the retardation it causes on the earth’s rotation. By this retardation the polar axis tends to lengthen, and tensile stresses are induced, resulting in fracture. The fluid interior of the earth, being no longer restrained, would move polewards, and, leaving equatorial portions unsupported, this would gradually collapse. The primary fractures would be north and south, while the secondary fractures would be east and west.[132]

That the rise of the tide is accompanied by a greater percolation of water to volcanic foci, which, in consequence, assume a greater state of activity, is a theory which was advanced many years ago. To determine how far tides may directly be connected with earthquakes, the necessary records have yet to be examined.

Variations in atmospheric pressure.—When we consider the immense load which, by a sudden rise of the barometer, is placed upon the area over which this rise takes place, it is not difficult to imagine that this rise may occasionally be the final cause which makes the crust of the earth to give way. A barometric rise of an inch is equivalent to a load of about seventy-two pounds being put upon every square foot of area over which this rise takes place. On the other hand, a fall in the barometric column indicates that a load has been removed, and whatever elastic effort may be exerted by subterranean forces in endeavouring to escape, being met by less resistance, they may burst these bonds, and an earthquake will result. For reasons such as these the final cause of earthquakes has often been attributed to variations in atmospheric pressure. In Japan there are practically as many earthquakes with a high barometer as with a low one.

The extent to which barometric fluctuations have acted as final causes in the production of earthquakes may be judged of by a comparison of the times of barometric variation and the times at which earthquakes have occurred.

Three important laws of barometric variation are the following:—

1. In the world generally the average barometric pressure is highest in winter. (Exceptions occur near Iceland and in the North Pacific.)

2. The summer and winter monthly mean barometer differs least near the equator and over the great oceans. They differ most over the great continents and generally with increasing latitude.

3. The greatest number of barometrical fluctuations usually take place in winter.

Inasmuch as there are generally more earthquakes in winter than in summer, the first of these laws would indicate that this might be due to the greater load which acts upon the crust of the earth at that season. The second law would indicate that the distinction between the winter and summer earthquakes ought to be most marked in high latitudes, which, if we refer to the table on p. 257, we observe to be borne out by the results of observation. The countries where there are as many earthquakes in winter as in summer are chiefly those in low latitudes. The number of these countries from which we have records are, however, few.

Facts opposed to the idea that earthquakes may be caused by an increase of barometric pressure are the results of observations like those of Schmidt and Rossi, which show that earthquakes chiefly occur with a low barometer.

Assuming that these latter observations will be found by future investigators to be generally true, we must conclude that the relief of atmospheric pressure has an influence upon the occurrence of earthquakes. Such a conclusion would partially accord with the third barometrical law, or the fact that there are more occasions on which we get a low barometer during the winter months.

Other writers who have examined this question are Volger, Kluge, Andrès, and Poly. The latter investigator sought a connection between earthquakes and revolving storms, in the centres of which there is usually an abnormal decrease of atmospheric pressure. If an area over which such a sudden change in pressure took place was in a critical state, it is not difficult to see that storms such as Poly refers to might sometimes be accompanied by earthquakes.

Fluctuations in temperature.—Inasmuch as fluctuations in temperature are governed by the sun, it may at once be said that there is a connection between earthquakes and readings of the thermometer. Certainly earthquakes occur mostly during the cold months or in winter. Similarly, as changes in temperature are so closely connected with barometric fluctuations, and these are said to have a direct influence upon the yielding of the earth’s crust, seismic phenomena are indirectly linked to fluctuations in temperature. A rise in temperature is usually accompanied by a fall in the barometer, and this in turn may be a condition favourable for the occurrence of an earthquake.

If we regard solar heat as an agent causing expansions or contractions in the earth’s crust, then fluctuations in temperature become an immediate cause of earthquakes. The probability, however, is that solar heat has little or no connection with the final cause producing earthquakes, although at the same time coincidences between the occurrence of earthquakes and unusual fluctuations in temperature may from time to time be observed.

Winds and earthquakes.—Although it may be admitted that high winds exert enormous pressures upon mountain ranges, and might occasionally give rise to stresses causing rocky masses in unstable equilibrium to give way, the coincidences which have been established between the occurrence of storms and earthquakes can usually only be regarded as occurrences which have synchronised by chance.

Storms are usually accompanied with a barometric depression, and the relation of diminutions in atmospheric pressure to earthquakes has been discussed.

Rain and earthquakes.—It has already been shown that earthquakes have occasionally been found to coincide with rain and rainy seasons. Whether the saturation of the ground with moisture or the percolation of the same to volcanic foci may be a direct effect producing earthquakes it is difficult to say. The probability, however, is that, rain being dependent on phenomena like changes in temperature, barometric fluctuations, and winds, we must regard it and the earthquakes which happen to coincide with these precipitations of moisture as congruent effects of more general causes.

Conclusion.—Although it would be an easy matter to discuss the relationship of earthquakes and other phenomena, we must conclude that the primary cause of earthquakes is endogenous to our earth, and that exogenous phenomena, like the attraction of the sun and moon and barometric fluctuations, play but a small part in the actual production of these phenomena, their greatest effect being to cause a slight preponderance in the number of earthquakes at particular seasons. They may, therefore, sometimes be regarded as final causes. The majority of earthquakes are due to explosive efforts at volcanic foci. The greater number of these explosions take place beneath the sea, and are probably due to the admission of water through fissures to the heated rocks beneath. A smaller number of earthquakes originate at actual volcanoes. Some earthquakes are produced by the sudden fracture of rocky strata or the production of faults. This may be attributable to stresses brought about by elevatory pressure. Lastly, we have earthquakes due to the collapse of underground excavations.

CHAPTER XVIII.
PREDICTION OF EARTHQUAKES.

General nature of predictions—Prediction by the observation of unusual phenomena (alteration in the appearance and taste of springs; underground noises; preliminary tremors; earthquake prophets—warnings furnished by animals, &c.)—Earthquake warning.

General nature of predictions.—Ever since seismology has been studied, one of the chief aims of its students has been to discover some means which would enable them to foretell the coming of an earthquake, and the attempts which have been made by workers in various countries to correlate these occurrences with other well-marked phenomena may be regarded as attempts in this direction.

Ability to herald the approach of these calamities would unquestionably be an inestimable boon to all who dwell in earthquake-shaken countries, and the attempts which have been made both here and in other places are extremely praiseworthy. In almost all countries where earthquakes are of common occurrence these movements of the earth have been more or less connected with certain phenomena which, in the popular mind, are supposed to be associated with an approach of an earthquake.

If predictions were given in general terms, and they only referred to time, inasmuch as on the average there are in the world several shakings per day, we should always find that predictions were verified. We might even go further and predict that on certain days earthquakes would occur in certain countries, and still find that in many instances our supposed power of foresight had not deceived us. Thus, for instance, in Japan, where on the average there are probably one or two shakings every day, if prognostications were never correct there would be a violation of the laws of chance.

What is required from those who undertake to forewarn us of an earthquake is an indication not only of the time at which the disturbance will happen, but also an indication of the area in which it is to occur. Those who dwell in an area where there are certain well-defined periods during which seismic activity is at a maximum—if ten or fourteen days should have passed without a shock—might, in many instances, find that a prophecy that there would be an earthquake within the next few days would prove itself correct. Also, if a severe shock had taken place, a prophecy that there would be a second or third smaller disturbance within a short period would also meet with verification.

Certain persons with whom I am intimate appear to have persuaded themselves that they can foretell the coming of an earthquake by the sultry state of the atmosphere or a certain oppressiveness they feel, and an instinctive feeling arises that an earthquake is at hand.

It is said that a few minutes before many of the shocks which shook New England between 1827 and 1847 people could foretell the coming disturbance by an alteration in their stomach.[133] No doubt many who dwell in earthquake countries, and have been alarmed by earthquakes, are at times subject to nervous expectancy.

The author has had such sensations himself, due, perhaps, to a knowledge that it was the earthquake season, that there had been no disturbance for some weeks, and a consequent increasing state of nervous presentments. In consequence of this, not only has he carefully prepared his instruments for the coming shock, but he has written and telegraphed to friends to do the same.

Sometimes these guesses have proved correct. One remarkable instance was a few hours prior to the severe shock of February 22, 1880, when he communicated with his friends in Yokohama and asked them to see that their instruments were in good order. Oftener, however, his prognostications have been incorrect. The point in connection with this subject which he wishes to be remarked is, that the instances where earthquakes occurred shortly after the receipt of his letters are carefully remembered, and often mentioned, but the instances in which earthquakes did not occur appear to be entirely forgotten. He is led to mention these facts because they appear to be an experimental proof of what has taken place in bygone times, and what still takes place, especially amongst savages—namely, that the record of that which is remarkable survives, whilst that which is of every-day occurrence quickly dies. Had the records of all prognostications been preserved, the probability is that we should find that they had, in the majority of cases, been incorrect, whilst it would have been but in very few instances they had been fulfilled.

Prediction by the observation of natural phenomena.—The above remarks may perhaps help us to understand the prognostications of the ancient philosophers about which Professor Antonio Favaro, of Padua, has written.[134] Cicero in the ‘De Divinatione,’ speaking on this subject, says that ‘God has not predicted so much as the divine intelligence of man.’—‘Non Deus prævidet tantum, sed et divini in genii viri.’ Favaro regards these predictions, however, as the result of observations of nature which show it is possible that indications of coming earthquakes had been announced by variations in the gas given out from subterranean sources, the change in colour, taste, level, temperature of the water in springs, &c.

In 1843 a bishop of Ischia forewarned his people of a conning earthquake, and thus was instrumental in the saving of many lives. Naturally, in an age of superstition, the bishop would be regarded as a prophet, but Favaro considers that the prognostication was probably due to a knowledge of premonitory signs as exhibited in changes in the characters of mineral waters.

The shock of 1851, at Melfi, was in this way predicted by the Capuchin fathers, who observed that a lake near their door became frothy and turbulent.

Underground noises have led persons to the belief that an earthquake was at hand. It was in this way that Viduari, a prisoner at Lima, predicted the destruction of that city.

Before the earthquake of 1868, so severely felt at Iquique, the inhabitants were terrified by loud subterranean noises.

That underground noises have preceded earthquakes by considerable intervals appears to be a fact, but, at the same time, it must be remembered that similar noises have often occurred without an earthquake having taken place.

Farmers predicted the earthquake of St. Remo, in 1831, by underground noises.

On the day before the earthquake which, in 1873, shook Mount Baldo, the inhabitants of Puos, a village north of Lake Santa Croce, heard underground noises.

Before the earthquakes which, in 1783, shook Calabria and Sicily, fish are said to have appeared in great numbers on the coast of Sicily, and the whirlpool of Charybdis assumed an unusual excited state.

It is said that Pherecydes predicted the earthquakes of Lacedemon and Helm out, by the taste of the water in the very deep well at the castle of Lovain.[135]

The writer of an article on the Lisbon earthquake says that ‘after the 24th I felt apprehensive, as I observed the same prognostics as on the afternoon of October 31, that is, the weather was severe, the wind northerly, a fog came from the sea, the water in a fountain ran of a yellow clay colour, and’ he adds, ‘from midnight to the morning of the 25th I felt five shocks.’[136]

At the present time Rudolf Falb, following a theory based upon the attractive influences of the sun and moon, tells us the time at which we are to expect earthquakes.

That occasionally there are signs attendant on earthquakes, although we cannot give them a physical explanation, we cannot doubt. Also we know that in certain areas earthquakes are more likely to occur at one season than at another. Should earthquakes be foretold with the assistance of knowledge of this description, the predictions at once become the result of the application of certain natural laws, and are not to be regarded as predictions in the popularly accepted sense of that term, any more than the arrival of a friend is predicted by the previous receipt of a telegram announcing his coming.

Rather than accredit the ancients and those of more modern times who, in consequence of their feelings, have recorded the coming of an earthquake, with a knowledge of premonitory signs, we might in many instances regard the records of those prognostications as the survival of accidental guesses, and, as such, examples of the survival of the useless.

The effect of accidental occurrences of this description upon an uneducated mind, in engendering superstition, is a subject which has often been dwelt upon, and the difficulty of eradicating the same—as may be judged of by the following accident which came under the observation of Mr. T. B. Lloyd and the author, in 1873, when travelling in Newfoundland—will be easily appreciated.

At the time to which I refer, my companion was bringing a canoe down the rapids below the Grand Pond in a country which is practically uninhabited, and where an Indian trapper would perhaps be the only person met with, and this not more than once a year. Whilst shooting the rapids one of the Indians, Reuben Soulian, shot at a deer passing up one bank of the river. That the deer had been hit was testified by a trail of blood which bespattered the rocks. Subsequently several more shots were fired, and it was believed by all that the deer was killed. Soulian quickly followed the animal to the spot where it was supposed to have fallen. Some time after he returned, having failed to find any trace of the animal. He was greatly agitated, but eventually became melancholy, saying that the sudden disappearance of the animal was a sure sign that some of his relations had suddenly died. About two hours afterwards Mr. Lloyd’s party met with a party of Indians coming up the river, the first they had seen for four weeks, who told them that Soulian’s sister had just died on the coast.

In the northern part of South America certain shocks are anticipated by preliminary vibrations which cause a little bell attached to a T-shaped frame (cruz sonante) to ring. There are, however, persons (trembloron) who are supposed to be endowed with seismic foresight, whose verdicts are much relied upon.

In Caraccas it is said that nearly every street in the river suburb has an earthquake Cassandra or two. Some of these go so far not only as to predict the coming seisms, but also the vicissitudes of particular streets.[137] Earthquake prophets are, however, by no means confined to the new world, and many examples of them may be found in the histories of countries where earthquakes have been felt.

The story of the crazy lifeguardsman who prophesied an earthquake to take place in London on April 4, 1691, is an example. The Rev. Sig. Pasquel E. Perdini, writing on the earthquakes at Leghorn in 1742, says that ‘a Milanese astrologer predicted this earthquake for January 27, by which “misfortune” the faith and credit given to the astrologer gained him more reverence and honour than the prophets and holy gospel.’ Before the time at which he predicted a second shock, people removed away from Leghorn.

Warnings furnished by animals.—A study of the warnings furnished by animals is also interesting. Several of the natives in Caraccas possess oracular quadrupeds, such as dogs, cats, and jerboas, which anticipate coming dangers by their restlessness.

Before the catastrophe of 1812, at Caraccas, a Spanish stallion broke out from its stable and escaped to the highlands, which was regarded as the result of the prescience of a coming calamity. Before the disturbances of 1822 and 1835, which shook Chili, immense flocks of sea birds flew inland, as if they had been alarmed by the commencement of some suboceanic disturbance. Before this last shock it is also related that all the dogs escaped from the city of Talcahuano.

Earthquake warning.—What has here been said respecting the prediction of earthquakes is necessarily imperfect—many of the signs which are popularly supposed to enable persons to foretell the coming of an earthquake having already been mentioned in previous chapters. That we shall yet be able to prepare ourselves against the coming of earthquakes, by the applications of laws governing these disturbances, is not an unreasonable hope.

With an electric circuit which is closed by a movement of the ground, we are already in a position to warn the dwellers in surrounding districts that a movement is approaching.

An earthquake which travelled at the rate of four seconds to the mile might, if it were allowed to close a circuit which fired a gun at a station fifteen miles distant, give the inhabitants at that place a minute’s warning to leave their houses. The inhabitants of Australia and the western shores of the Pacific might, by telegraphic communication, receive eighteen to twenty-five hours’ warning of the coming of destructive sea waves resulting from earthquakes in South America.

Although warnings like these might have their value, that which is chiefly required is to warn the dwellers at and near an earthquake centre of coming disturbances.

What the results of the observations on earth tremors will lead to is problematical.

Should microseismic observation enable us to say when and where the minute movements of the soil will reach a head, a valuable contribution to the insurance of human safety in earthquake regions will have been attained.

As to whether the movements of tromometers are destined to become barometric-like warnings of increased activity beneath the earth crust, or whether they are only due to vibrations of the earth crust produced by variations in atmospheric pressure, has yet to be investigated.

Other phenomena which may probably forewarn us of the coming of an earthquake are phenomena resultant on the stresses brought to bear upon the rocky crust previous to its fracture, or phenomena due to changes in the position and condition of heated materials beneath the earth’s surface. Amongst these may be mentioned electrical disturbances, which appear to be so closely related to seismic phenomena.

At the time of earthquakes telegraph lines have been disturbed, but as to what may happen before an earthquake we have as yet but little information. The subject of earthquake warning is of importance to many countries, and is deserving of attention.

As our knowledge of earth movements, and their attendant phenomena, increases, there is but little doubt that laws will gradually be formulated, and in the future, as telluric disturbances increase, a large black ball gradually ascending a staff may warn the inhabitants on the land of a coming earthquake, with as much certainty as the ball upon a pole at many seaports warns the mariner of coming storms.

CHAPTER XIX.
EARTH TREMORS.

Artificially produced tremors—Observations of Kater, Denman, Airy, Palmer, Paul—Natural tremors—Observations of Zöllner, M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With seismoscopes, microphones, pendulums—Work in Italy—Bertelli, Count Malvasia, M. S. di Rossi—Instruments employed in Italy—Tromometers, microseismographs, microphones—Results obtained in Italy—in Japan—Cause of microseismic motion.

During the past few years considerable attention has been drawn towards the study of small vibratory motions of the ground, which to the unaided senses are usually passed by without recognition. These motions are called earth tremors. Their discovery appears to have been due to accident, and not to the results of inductive reasoning. No sooner had philosophers contrived astronomical and other instruments for the purpose of making refined measurements and observations than they at once discovered that they had an enemy to contend against in the form of microscopic earthquakes.

Artificially produced tremors.—Artificial disturbances of this description exist in all our towns, and near a railway line they are perceptible with every passing train. Those who have used microscopes of high power will readily appreciate how small a disturbance of the ground is visible in the apparent movement of the object under examination.

Captain Kater found that he could not perform his pendulum experiments in London on account of the vibrations produced by the rolling of carriages. Captain Denman, who made some observations on artificially produced tremors, found that a goods train produced an effect 1,100 feet distant in marshy ground over sandstone. Vertically, however, above a tunnel through the sandstone, the effects only extended 100 feet.

A remarkable example of the trouble which artificially produced earth vibrations have occasioned those who make astronomical observations occurred some twenty years ago at the Greenwich Observatory. When determining the collimation error of the transit circle by means of the reflexion of a star in a tray of mercury, it was found that on certain nights the surface of the mercury was in such a state of trembling that the observers were unable to complete their observations until long after midnight. After obtaining a series of dates on which these disturbances occurred, it was found that they coincided with public and bank holidays, on which days crowds of the poorer classes of London flocked to Greenwich Park, and there amused themselves with running and rolling down the hill on which the observatory is situated. On these occasions it was found that the disturbances in the mercury were such that observations could not be made until two or three hours after the crowds had been turned out of the neighbouring park.[138]

To obviate this difficulty Sir George Airy suspended his dish of mercury in a system of india-rubber bands, and in this way succeeded in eating the intruders up.

Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit of Venus expedition in New Zealand, in 1874, was troubled with vibrations produced from a neighbouring railway. To escape the enemy he intrenched his instruments by placing them in pits. With pits 3½ feet deep he found himself sufficiently protected. The distance from the line was about 400 yards, and the soil through which the disturbances were propagated was a coarse pebbly gravel.[139]

Before the United States Naval Observatory was established at Washington, Professor H. M. Paul was deputed to make a tremor survey to discover stable ground. The results of these experiments were exceedingly interesting. By watching the reflected image of a star in a dish of mercury a passing train would be noticed at the distance of a mile. Its approach could be detected by the trembling of the image before its coming could be heard. At one point of observation the disturbance appeared to be cut off by a ravine. The strata was gravel and clay.[140]

These few examples of artificially produced tremors, to which many more might be added, have been given because they teach us something respecting their nature. Hitherto earth tremors have only been regarded as intruders, which it was necessary to escape from or destroy. From what has been said they appear to be a superficial disturbance which is propagated to an enormous distance. This distance appears to depend upon the propagating medium, upon the intensity of the initial disturbance, and upon its duration. In the observation of these artificial disturbances, which are accessible to every one, and which hitherto have been so neglected, we have undoubtedly a fruitful source of study.

Natural tremors.—Next let us turn to those microscopical disturbances of our soil which are due to natural causes. Thus far they seem to have been recorded wherever instruments suitable for their detection have been erected, and it is not improbable that they are common to the surface of the whole globe.

Some of the more definite observations which have been made upon earth tremors were those made in connection with experiments on the deviation of the vertical due to the attractive influence of the moon and sun.

Professor Zöllner, who invented the horizontal pendulum which he used in the attempt to measure the change in level due to lunar and solar attraction, found his instruments so sensitive that the readings were always changing.

The most interesting observations which were made upon small disturbances of the soil were those of M. d’Abbadie, who carried on his experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant from the Atlantic, and 62 mètres above sea level. The soil was a loamy rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height, which was pierced down the centre by a vertical hole or well, which was continued two mètres below the cone into the solid rock. At the bottom of this hole or well a pool of mercury was formed which reflected the image of cross wires placed at the top of the hole. These cross wires and their reflection were observed by means of a microscope. The observations consisted in noting the displacement and azimuth of the reflected image relatively to the real image of the wires. After allowing this structure five years to settle, M. d’Abbadie commenced his observations. To find the surface of the mercury tranquil was a rare occurrence. Sometimes the mercury appeared to be in violent motion, although both the air and neighbouring sea were perfectly calm. At times the reflected image would disappear as if the mercury had been disturbed by a microscopic earthquake.

The relative positions of the images were in part governed by the state of the tide. Altogether the movements were so strange that M. d’Abbadie did not venture any speculations as to their cause, but he remarks that the cause of the changes he observed were sometimes neither astronomical nor thermometrical. These observations, the principal object of which was to determine changes in level rather than earth vibrations, were carried on between the years 1868 and 1872.[141]

Observations at Cambridge.—Another instructive set of observations were those which were made in the years 1880–1882, by George and Horace Darwin, in the Cavendish Laboratory, at Cambridge. The main object in these experiments was to determine the disturbing influence of gravity produced by lunar attraction. The result which was obtained, however, showed that the soil at Cambridge was in such an incessant state of vibration that whatever pull the moon may have exerted upon the instrument which was employed was masked by the magnitude of the effects produced by the earth tremors, and the experiments had, in consequence, to be abandoned.

The principle of this instrument was similar to one devised by Sir William Thomson, and put up by him in his laboratory at Glasgow. As erected by the brothers Darwin, at Cambridge, it was briefly as follows: A pendulum, which was a massive cylinder of pure copper, was hung by a copper wire, about four feet long, inside a hollow cylindrical tube rising from a stone support. A small mirror was then hung by two silk fibres, one of which was fastened to the bob and the other to the stone basement. A ray of light sent from a lamp on to the mirror was reflected to a scale seven feet distant, and by this magnification any motion of the bob relatively to the stone support was magnified 50,000 times. In several ways the apparatus was insulated from all accidental disturbances. The spot of light was observed from another room by means of a telescope. This instrument was so delicate that even at the distance of sixteen feet the shifting of your weight from one foot to the other caused the spot of light to run along the scale. So sensitive was the instrument that, notwithstanding its being cut off from the surrounding soil by a trench filled with water and the whole instrument being immersed in water to damp out the small vibrations, it would seem that the ground was in a constant state of tremor; in fact, so persistent and irregular were these movements that it seemed impossible to separate them from the perturbations due to the attraction of the moon.[142]

As a result of observations like these, the world had gradually forced upon it the fact that the ground on which we live is probably everywhere in what is practically an incessant state of vibration.

This led those who were interested in the study of earth movements to establish special apparatus for the purpose of recording these motions with the hope of eventually discovering the laws by which they were governed.

Experiments in Japan.—The simpler forms of apparatus which have been used in Japan may be described as delicate forms of seismoscopes, which, in addition to recording earth tremors, also record the occurrence of small earthquakes.

A simple contrivance which may be used for the purpose of recording small earthquakes can be made with a small compass needle.

If a light, small sensitive compass needle be placed on a table, it will be found that a small piece of iron like a nail may be pushed so near to it that the needle assumes a position of extremely unstable equilibrium. If the table now receives the slightest tap or shake this condition is overcome, and the needle flies to the iron and there remains. By making the support of the needle and the iron the poles of an electric circuit it is possible to register the time at which motion took place with considerable accuracy.

With crude apparatus like this a large number of small earth disturbances have been recorded.

Another form of apparatus, employed in Japan, has been a delicately constructed circuit closer. The motions of this instrument were recorded by causing an electro-magnet to deflect a pencil which was tracing a circle on a revolving dial. The revolving dial was a disc of wood covered with paper fixed to the hour-hand axle of a common clock.

A third form of apparatus used in Japan consisted of a small piece of sheet lead about the size of a threepenny piece suspended by a short loop from a rigid support. Projecting from the lead a fine wire, about two inches in length, passed freely through a hole in a metallic plate. By the slightest motion of the support the small pendulum of lead was set into a state of tremor and caused its pointer to come in contact with one or other side of the hole in the metal plate and thus to close an electric circuit.

A more refined kind of apparatus which has been employed in Japan was similar to that used by the Darwins at Cambridge. This was so arranged that any deflection of the mirror was permanent until the instrument was reset, and in this way the maximum disturbance which had taken place between each observation was recorded.

In addition to these and other contrivances, experiments were made with microphones.

The microphones used were small doubly pointed pencils of carbon about three centimetres long, saturated with mercury, and supported vertically in pivot holes bored in other pieces of carbon, which were the terminals of an electric circuit. These microphones were screwed down on the top of stakes driven deeply into the ground. They were covered with a glass shade thickly greased at its base. The stakes were in the ground at the bottom of a small pit—about two feet square and two feet deep—which was lined with a box. The box was covered with a lid, and earth to the depth of nine inches or one foot. One of these pits was in the middle of a lawn in the front of my house, and the other was at the foot of a hill at the back of the house. The wires from the microphone passed through the side of the box into a bamboo tube and thence up to my dining-room and bed-room. In one of the circuits there were three Daniell’s cells, a telephone, and a small galvanometer. I used the galvanometer because I found that when there was sufficient motion in the microphone to produce a sound in the telephone a motion in the needle of a galvanometer was produced. If in any case motion took place in the magnetic needle during my absence, it was held deflected by a small piece of iron with which it was brought into contact by the movement.

The sensitiveness of the arrangement may be judged of from the fact that if a person walked on the grass within six feet of the microphone, each step caused a creak in the telephone, and the needle of the galvanometer was caused to swing and come in contact with the iron. Dogs running on the grass had no effect. A small stone one or two inches in diameter thrown from the house so that it fell near to the microphone pit caused a sharp creak in the telephone and a movement in the needle.

The nature of the records I received from this contrivance may be judged of from the following extract from my papers.

	h. m.
1879. Nov. 12th	7 0 p.m. contact of needle
	7 2 „ difficult to set the needle
	7 3 „ needle swings and telephone creaks
	7 4 „ „ „ „
	7 5 „ „ „ „
	7 6 „ „ „ „
	7 10 „ 3 more swings
	7 11 „ again „

Here I went out, took away the covering, and examined the microphone. Nothing wrong was to be observed. All that I saw was one small ant. I do not think that this could have caused the disturbance, because it could not get near the instrument.

On the succeeding nights I experienced similar disturbances, and it seemed as if they might possibly have been the prelude to several small shocks which occurred about this time (November 15, 16, and 17). On November 17, at 8 a.m., the needle was found in contact, and again at 5 p.m., and at 6 p.m. the shock of a small earthquake was felt which caused a rattling sound in the telephone for about one minute after the motion had appeared to cease. The needle swung considerably, but did not come in contact.

The great objection to these observations is that it is possible that the movements and sounds which I have recorded might, with the exception of one case when the shaking was actually felt, possibly have been produced by causes other than that of the movement of the ground. To determine this I subsequently put up two distinct sets of apparatus to determine whether the motions of each were synchronous. So far as I went this appeared only to be sometimes the case:—but this is a question difficult to determine, unless a recorder of time be added to the apparatus.

The greatest objection to observations of this sort is that the sensibility of the instrument is not constant. After a current has been running for several days it is no longer sensible to slight shocks, it appears as if its resistance had been increased. To overcome this it is necessary to resharpen the carbon points and bore out the pivot holes every three or four days. Farther, the battery varies. This might to some extent be overcome by using a battery with large plates. These two causes tend to reduce the sensitiveness of the galvanometer-like recorder—the deflection of the needle gradually becoming less and less, and therefore day by day needing a greater swing to bring it into contact with the iron. For reasons such as these this instrument, to be used successfully, appears to require considerable attention.

Another form of microphone employed by the author consisted of an aluminium wire standing vertically on a metallic plate, its upper end passing loosely through a hole in an aluminium wire standard.

The upper end of the vertical wire was loaded with lead. This contrivance possesses all the sensitiveness of an ordinary microphone, whilst, if it receives a sudden impulse, there is a sudden break in the current, and the vertical wire is thrown from one side to the other of the hole in the standard.

After many months of tiresome observation with instruments of this description, and after eliminating all motions which might have been produced by accidental causes, the general result obtained showed that in Tokio there were movements of the soil to be detected every day, and sometimes many times per day, which to ordinary persons were passed by unnoticed.

Work in Italy.—The most satisfactory observations which have been made upon microseismic disturbances are those which have been made during the last ten years in Italy. The father of systematical microseismical research appears to have been Father Timoteo Bertelli, of Florence.

In 1870 Father Bertelli suspended a pendulum in a cellar, and observed it with a microscope. As the result of his observations it was announced that he had perceived the earthquakes which shook Romagna, although to the ordinary observer in Florence these shakings had not been perceptible.

In 1873 Bertelli, by means of microscopes fixed in several azimuths, made 5,500 observations on free pendulums. He also made observations on reflections from the surface of mercury.[143]

One result of these observations was to show that microseismic motions increased with a fall of the barometer. Similar observations were made at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near Rome. On January 14, 15, and February 25 these three observers at their respective stations simultaneously observed great disturbances.

Similar investigations were made at Nice by M. le Baron Prost.

Although doubt was cast upon Bertelli’s observations they appear to have been the origin of a series of microseismical observations, a distinguished leader in which is Professor Rossi, who, in 1874, found that large earthquakes were almost always preceded or accompanied with microseismical storms. In 1878 Professor Rossi worked upon these small disturbances with the assistance of the microphone and telephone, and his first results were published by Professor Palmieri.

Many of Professor Rossi’s observations were made in the grotto of Roca de Papa, 700 mètres high and eighteen mètres under the soil. Here over 6,000 observations were made by means of microscopes, on pendulums of different lengths, suspended in tubes cut in the solid rock.

Instruments employed in Italy.—It is impossible to describe in detail the various forms of apparatus which have been used by the Italian investigators. A description of one or two of the more important instruments may not, however, be out of place, inasmuch as they will assist the reader to understand the manner in which the various results respecting the laws governing microseismic movements have been arrived at.

The most important of these instruments is the Normal Tromometer of Bertelli and Rossi.

This consists of a pendulum 1½ metres long, carrying, by means of a very fine wire, a weight of 100 grammes. To the base of the bob a vertical stile is attached, and the whole is enclosed in a tube terminated, at its base, by a glass prism of such a form that when looked through horizontally the motion of the stile can be seen in all azimuths. In front of this prism a microscope is placed. Inside the microscope there is a micromatic scale, so arranged that it can be turned to coincide with the apparent direction of oscillation of the point of the stile. In this way not only can the amplitude of the motion of the stile be measured, but also its azimuth. The extent of vertical motion is measured by the up and down motion of the stile due to the elasticity of the supporting wire. This instrument is shown in the accompanying drawing.

Fig. 37.—Normal Tromometer.
b, bob of pendulum; p, prism; m, microscope; s, rim of scale.

Another apparatus is the Microseismograph of Professor Rossi. Here we have an arrangement which gives automatic records of slight motions. It consists of four pendulums, each about three feet long, suspended so that they form the corners of a square platform. In the centre of this platform a fifth, but rather longer, pendulum is suspended. The four pendulums are each connected just above their bobs to the central pendulum with loose silk threads. Fixed to the centre of each of these threads, and held vertically by a light spring, is a needle, so adjusted that each thread is depressed to form an obtuse angle of about 155°. These needles form the terminals of an electric circuit, the other termination of which is a small cup of mercury placed just below the lower end of the needle. By a horizontal swing of one of the pendulums this arrangement causes the needle to move vertically, but with a slightly multiplied amplitude. By this motion the needle comes in contact with the mercury, and an electro-magnet with a lever and pencil is caused to make a mark on a band of paper moved by clockwork. The five pendulums being of different lengths allows the apparatus to respond ‘to seismic waves of different velocities.’[144]

Lastly, we have Professor Rossi’s Microphone. This consists of a metallic swing arranged like the beam of a balance. By means of a movable weight at one end of the beam this is so adjusted that it falls down until it comes in contact with a metallic stop. This can be so adjusted that a slight tap will cause the beam to slightly jump from the stop. The beam and the stop form two poles of an electric circuit, in which there is a telephone. The slightest motion in a vertical direction causes a fluctuation in the current passing between the stop and the beam, and a consequent noise is heard in the telephone.

With instruments analogous to these, observations have been made by various observers in all portions of Italy, extending over a period of ten years. Every precaution appears to have been taken to avoid accidental disturbances, and the experiments have been repeated in a variety of forms.

Results obtained in Italy.—The results which from time to time have been announced are of the greatest interest to those who study the physics of the earth’s crust, and they appear to be leading to the establishment of laws of scientific value.

It would seem that the soil of Italy is in incessant movement, there being periods of excessive activity usually lasting about ten days. Such periods are called seismic storms. These storms are separated by periods of relative calm. These storms have their greater regularity in winter, and sharp maximums are to be observed in spring and autumn. In the midst of such a period or at its end there is usually an earthquake. Usually these storms are closely related to barometric depressions. To distinguish these movements from those which occur under high pressure, the latter are called baro seismic movements, and the former vulcano seismic movements. The relation of these storms to barometric fluctuation has been observed to have been very marked during the time of a volcanic eruption.

At the commencement of a storm the motions are usually small, and one storm, lasting two or three days, may be joined to another storm. In such a case the action may be a local one. It has been observed that a barometrical depression tended to bring a storm to a maximum, whilst an increase of pressure would cause it to disappear. Sometimes these actions are purely local, but at other times they may affect a considerable tract of land.

If a number of pendulums of different length are observed at the same place, there is a general similarity in their movements, but it is also evident that the free period of the pendulum more or less disturbs the character of the record. The greatest amplitude of motion in a set of pendulums is not reached simultaneously by all the pendulums, and at every disturbance the movement of one will predominate. From this Rossi argues that the character of the microseismical motions is not constant. Bertelli observed that the direction of oscillation of the pendulums is different at different places, but each place will have its particular direction dependent upon the direction of valleys and chains of mountains in the neighbourhood. Rossi shows that the directions of movement are perpendicular to the direction of lines of faults, the lips of these fractures rising and falling, and producing two sets of waves, one set parallel to the line of fracture, and the other perpendicular to such a direction. These movements, according to Bertelli, have no connection with the wind, rain, change of temperature, and atmospheric electricity.

Fig. 38.

The disturbances, as recorded at different towns, are not always strictly synchronous, but succeed each other at short intervals. If, however, we take monthly curves of the disturbances as recorded at different towns in Italy, we see that these are similar in character. The maximum of disturbance occurs about the winter solstice, and the minimum about the summer solstice, and in this respect they exhibit a perfect accordance with the curves drawn by Mallet to show the periodicity of earthquakes. The accompanying curves taken from one of Bertelli’s original memoirs not only show this general result but also show the close accord there is between the results obtained at different towns during successive months.

At Florence, before a period of earthquakes there is an increase in the amplitude and frequency of vertical movements. These vertical movements do not appear to coincide with the barometrical disturbances, but they appear to be connected with the seismic disturbances.

They are usually accompanied with noises in the telephone, but as the microphone is so constructed as to be more sensitive to vertical motion than to horizontal motion, this is to be expected. This vertical motion would appear to be a local action, inasmuch as the accompanying motions of an earthquake which originates at a distance are horizontal.

Storms of microseismical motions appear to travel from point to point.

Sometimes a local earthquake is not noticed in the tromometer, whilst one which occurred at a distance, although it may be small, is distinctly observed. To explain this, Bertelli suggests the existence of nodes. Similar conclusions were arrived at by Rossi when experimenting on different portions of the sides of Vesuvius. Galli noticed an augmentation in microseismic activity when the sun and moon are near the meridian. Grablovitz found from Bertelli’s observations a maximum two or three days before the syzygies, and a minimum three days after these periods. He also found that the principal large disturbances occurred in the middle of periods separating the quadrature from the syzygies, the apogee from perigee, and the lunistigi period from the nodes, whilst the smallest disturbances happened in the middle of periods opposed to these.

P. C. Melzi says that the curves of microseismical motions, earthquakes, lunar and solar motions, show a concordance with each other.

With the microphone Rossi hears sounds which he describes as roarings, explosions, occurring isolated or in volleys, metallic and bell-like sounds, ticking, &c., which, he says, revealed natural telluric phenomena. Sometimes these have been intolerably loud. At Vesuvius the vertical shocks corresponded with a sound like volleys of musketry, whilst the undulatory shocks gave the roaring. Some of these sounds could be imitated artificially by rubbing together the conducting wires in the same manner in which the rocks must rub against each other in an earthquake. Other sounds were imitated by placing the microphone on a vessel of boiling water, or by putting it on a marble slab and scratching and tapping the under side of it.

These, then, are some of the more important results which have been arrived at by the study of microseismic motions. One point which seems worthy of attention is that they appear to be more law-abiding than their more violent relations, the earthquakes, and as phenomena in which natural laws are to be traced they are certainly deserving our attention. As to whether they will ever become the means of forewarning ourselves against earthquakes is yet problematical. Their systematic study, however, will enable us to trace the progress of a microseismic storm from point to point, and it is not impossible that we may yet be enabled to foretell where the storm may reach its climax as an earthquake. These, I believe, are the views of Professor di Rossi, who is at the present time engaged in the establishment of a system of microseismic observations throughout Italy.

Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer was in a state of agitation, and similar disturbances were observed at Livorno, Florence, and Bologna.

Since February 1883 I have observed a tromometer in Japan, and such results as have been obtained accord with results obtained in Italy. The increase in microseismical activity with a fall of the barometer is very marked. Other peculiarities in the behaviour of the instrument will be referred to under ‘Earth Pulsations.’

Cause of microseismic movements.—As to the cause of tromometric movements, we have a field for speculation. Possibly they may be due to slight vibratory motions produced in the soil by the bending and crackling of rocks produced by their rise upon the relief of atmospheric pressure. If this were so we should expect similar movements to be produced at the time of an increase of pressure. Rossi suggests that they may be the result of an increased escape of vapour from the molten materials beneath the crust of the earth, consequent upon a relief of pressure. The similarity of some of the sounds which are heard with the microphone to those produced by boiling water are suggestive of this, and Rossi quotes instances when underground noises like those which we should expect to hear from a boiling fluid have been heard before earthquakes without the aid of microphones. One instance was that of Viduari, a prisoner in Lima, who, two days before the shock of 1824, repeatedly predicted the same in consequence of the noises he heard.

A possible cause of disturbances of this order may be small but sudden fluctuations in barometric pressure, which are visible during a storm. During a small typhoon on September 15, 1881, when in the Kurile Islands, I observed that the needle of an aneroid worked back and forth with a period of from one to three seconds. This continued for several hours. With every gust of wind the needle suddenly rose and then immediately fell. At times it trembled. These movements were observed in the open air. The extent of these sudden variations was approximately from ·03 to ·05 inches. Beckoning an increase of barometrical pressure of one hundredth of an inch as equivalent to a load of twenty million pounds on the square mile, during this storm there must have been the equivalent of loads of from 60 to 100 million pounds to the square mile continually placed on and removed from a considerable tract of the earth’s surface. If the period of application of these stresses approximately coincide with the natural vibrational period of the area affected, it would surely seem, especially when we reflect upon the effect of an ordinary carriage, that tremors of considerable magnitude ought to be produced.

An inspection of the following few observations taken from my note-book for the same typhoon will suggest that even the large and slower variations are capable of producing tremulous motions.

Time h. m.	Barometer reading
12 5 p.m.	29·02
12 10 „	29·05
12 12 „	29·07
12 13 „	29·05
12 25 „	29·10
12 50 „	29·00
1 10 „	29·00
1 20 „	29·07

CHAPTER XX.
EARTH PULSATIONS.

Definition of an earth pulsation—Indications of pendulums—Indications of levels—Other phenomena indicating the existence of earth pulsations—Disturbances in lakes and oceans—Phenomena resultant on earth pulsations—Cause of earth pulsations.

The object of the present chapter is to show that from time to time it is very probable that slow but large wave-like undulations travel over or disturb the surface of the globe.

These movements, which have escaped our attention on account of their slowness in period, for want of another term I call earth pulsations.

The existence of movements such as these may be indicated to us by changes in the level of bodies of water like seas and lakes, by the movements of delicate levels, by the displacement of the bob of a pendulum relatively to some point on the earth above which it hangs, and by other phenomena which will be enumerated.

Indication of pendulums.—Pendulums which have been suspended for the purposes of seismometrical observations have, both by observers in Italy and Japan, been seen to have moved a short distance out from, and then back to, their normal position.

This motion has simply taken place on one side of their central position, and is not due to a swing. The character of these records is such that we might imagine the soil on which the support of the pendulum had rested to have been slowly tilted, and slowly lowered. They are the most marked on those pendulums provided with an index writing a record of its motions on a smoked glass plate, which index is so arranged that it gives a multiplied representation of the relative motion between it and the earth. As motions of this sort might be possibly due to the action of moisture in the soil tilting the support of the pendulum, and to a variety of other accidental causes, we cannot insist on them as being certain indications that there are slow tips in the soil, but for the present allow them to remain as possible proof of such phenomena.

Evidence of displacement of the vertical, which are more definite than the above, are those made by Bertelli, Rossi, Count Malvasia, and other Italian observers, who, whilst recording earth tremors, have spent so much time in watching the vibrations of stiles of delicate pendulums by means of microscopes. As a result of these observations we are told that the point about which the stile of a pendulum oscillates is variable. These displacements take place in various azimuths, and they appear to be connected with changes of the barometer. I have made similar observations in Japan.

From this, and from the fact that it is found that a number of different pendulums differently situated on the same area give similar evidence of these movements, it would hardly seem that these phenomena could be attributed to causes like changes in temperature and moisture. M. S. di Rossi lays stress on this point, especially in connection with his microseismograph, where there are a number of pendulums of unequal length which give indications of a like character. The direction in which these tips of the soil take place—which phenomena are noticeable in seismic as well as microseismic motions—Rossi states are related to the direction of certain lines of faulting.

Indications of levels.—Bubbles of delicate levels can be easily seen to change their position with meteorological variations; but Rossi also tells us that they change their position, sometimes not to return for a long time, during a microseismic storm. Here again we have another phenomenon pointing to the fact that microseismic disturbances are the companions of slow alterations in level.

One of the most patient observers of levels has been M. Plantamour, who commenced his observations in 1878, at Sécheron, on the Lake of Geneva. He used two levels, one placed north and south, and the other east and west. During the summer of 1878 the east end rose, but at the end of September a depression set in. The diurnal movements had their maximum and minimum at 6 and 7.45 a.m. and p.m. The total amplitude was 4·89″. The variations of the east and west level appeared to be due to the temperature, but the movements of the north and south level were dependent upon an unknown cause.

Between October 1, 1879, and September 30, 1880, the east end fell rapidly, from the middle of November up to December 26, amounting to 88·71″. It then rose 6·55″ to January 5, and then fell again. On January 28 it reached 89·95″, after which it rose.

Between October 4, 1879, and January 28, 1880, the movement was 95·8″, against 28·08″ of the previous year.

These movements were not due alone to temperature. The north and south level, which was not influenced by the cold of the winter, moved 4·56″. In the previous year 4·89″.[145]

From February 17 to June 5, 1883, the author observed in Tokio the bubbles of two delicate levels, one placed north and south, and the other east and west. They were placed under glass cases on the head of a stone column. The column, which is inside a brick building, rests on a concrete foundation, and is about ten years old. It is in no way connected with the building. The temperature of the room has a daily variation of about 1° Fahr.

In both these levels diurnal changes are very marked. Occasionally they are enormously great. Thus, on March 25, the readings of the south end of the north south level were as follows:—

	Time h. m.	Readings.
25th.	4 00 p.m.	104·5
	4 5 „	103
	4 10 „	102
	4 25 „	101
	4 30 „	100
	4 40 „	98
	4 42 „	99·5
	4 45 „	100
	4 50 „	101
	4 55 „	101
	5 00 „	100
26th.	7 00 a.m.	105

Usually this level moves through about three divisions per day.

From March 25 to May 4 it travelled from 98 to 127. Since then, to June 5, it has descended to 116. During this period the east west level has been comparatively quiet. One division of the north south level equals about 2″ of arc.

Many of these changes may be due to changes in temperature, variations in moisture, and other local actions. Some of them, however, are hardly explicable on such assumptions. The fact that the general direction in change of the vertical, as indicated by a tromometer standing on the same column with the levels, showed that the change which was taking place was rather in the column than in the instruments.

The fact also that at the time of a barometrical depression a pulse-like surge can be seen in the levels, having a period averaging about three seconds and sometimes amounting to about one second of arc, is a phenomenon hardly to be attributed to sudden fluctuations in moisture or temperature, but indicates real changes in level.[146]

In addition to variation in the bubbles of levels which come on more or less gradually, we have many recorded instances of sudden alterations taking place in these instruments.

Examples of what may have been a slow oscillating motion of the earth’s crust are referred to by Mr. George Darwin in a Report to the British Association in 1882.

One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged in levelling the axis of a telescope, observed spontaneous oscillation in the bulb of the level.

This was on May 10 (April 28), 1877. The complete period was about 20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen minutes before this he observes that there had been a severe earthquake at Iquique, the distance to which in a straight line was 10,600 kilomètres, and on an arc of a great circle, 12,500 kilomètres. On September 20 (8), in 1867, Mr. Wagner had observed at Pulkova oscillations of 3″, seven minutes before which there had been an earthquake at Malta. On April 4 (March 23), 1868, an agitation of the level had been observed by Mr. Gromadzki, five minutes before which there had been an earthquake in Turkestan. Similar observations had been made twice before. These, however, had not been connected with any earthquakes—at least, Mr. Darwin remarks—with certainty.

Phenomena analogous to the pendulum and level observations.—As examples of phenomena which are analogous to those made on pendulums and levels, the following may be noticed. On March 20, 1881, at 9 p.m. a watchmaker in Buenos Ayres observed that all his clocks oscillating north and south suddenly began to increase their amplitude, until some of them became twice as great as before. Similar observations were made in all the other shops. No motion of the earth was detected. Subsequently it was learnt that this corresponded with an earthquake in Santiago and Mendoza.[147]

Another remarkable example illustrating the like phenomena is furnished by the observations which were made on December 21, 1860, by means of a barometer in San Francisco, which oscillated, with periods of rest, for half an hour. No shock was felt, nor is it likely that it was a local accident, as it could not be produced artificially. On the following day, however, a violent earthquake was experienced at Santiago.[148]

At the time or shortly after the great Lisbon earthquake, curious phenomena were observed in distant countries, which only appear to be explicable on the assumption of the existence of earth pulsations.

Thus at Amsterdam and other towns, chandeliers in churches were observed to swing. At Haarlem water was thrown over the sides of tubs, and it is expressly mentioned that no motion was perceived in the ground.

At the Hague a tallow chandler was surprised at the clashing noise made by his candles, and this the more so because no motion was felt underfoot.

Unusual disturbances in bodies of water.—At the time of large earthquakes it would appear that earth pulsations are produced, which exhibit themselves in countries where the actual shaking of the earthquake is not felt, by disturbances in bodies of waters like lakes and seas.

Some remarkable examples of these disturbances are to be found in the records of the great Lisbon earthquake. This earthquake, as a violent movement of the ground, was chiefly felt in Spain, Portugal, northern Italy, the south of France and Germany, northern Africa, Madeira, and other Atlantic islands. In other countries further distant, as, for instance, Great Britain, Holland, Scandinavia, and North America, although the records are numerous, the only phenomena which were particularly observed was the slow oscillations of the waters in lakes, ponds, canals, &c. In some instances the observers especially remark that there was no motion in the soil.

Pebley Dam, in Derbyshire, which is a large body of water covering some thirty acres, commenced to oscillate from the south. A canal near Godalming flowed eight feet over the walk on the north side.

Coniston Water, in Cumberland, which is about five miles long, oscillated for about five minutes, rising a yard up its shores. Near Durham a pond, forty yards long and ten broad, rose and fell about one foot for six or seven minutes. There were four or five ebbs and flows per minute.

Loch Lomond rose and fell through about two and a half feet every five minutes, and all the other lochs in Scotland seem to have been similarly agitated.

At Shirbrun Castle, in Oxfordshire, where the water in some moats and ponds was very carefully observed, it was noticed that the floods began gently, the velocity then increased, till at last with great impetuosity they reached their full height. Here the water remained for a little while, until the ebb commenced, at first gently, but finally with great rapidity. At two extremities of a moat about 100 yards long, it was found that the sinkings and risings were almost simultaneous. The motions in a pond a short distance from the moat were also observed, and it was found that the risings and sinkings of the two did not agree.

During these motions there were several maxima.

These few examples of the motions of waters, without any record of the motions of the ground, at the time of the Lisbon earthquake, must be taken as examples of a very large number of similar observations of which we have detailed accounts.

Like agitations, it must also be remembered, were perceived in North America and in Scandinavia, and if the lakes of other distant countries had been provided with sufficiently delicate apparatus, it is not unlikely that similar disturbances would have been recorded.

Besides these movements in the waters of seas and lakes, at or about the time of great earthquakes, we have records of like movements, which take place as independent phenomena.

Thus we read that on October 22, 1755, the waters of Lake Ontario rose and fell five and a half feet several times in the course of half an hour.[149] On March 31, 1761, Loch Ness rose suddenly for the period of three-quarters of an hour.[150]

As another example of the disturbance of water at the time of a great earthquake in districts where the earthquake was not felt, may be mentioned the swelling of the waters of the Marañon, in 1746, on the night when Callao was overwhelmed.

Sudden variations in the level of the water have been many times observed in the North American lakes. The changes in level which sometimes take place in the Genfer and Boden lakes are supposed to have some relation to the condition of the atmosphere. A rising and falling of especial note took place on April 18, 1855.

In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’

From the observations and calculations of Prof. Forel it would seem that the period of the ‘seiches’ depends upon the dimensions of the lakes; the calculated periods dependent on the depths of the lakes being approximately equal to the observed periods.[151]

W. T. Bingham, writing on the volcanoes of the Hawaiian Islands, remarks that it is not unusual for the sea to be agitated by great and unusual tides, and that such sea waves have not been attended with volcanic eruptions or seismic disturbances. Thus in May 1819 the tide rose and fell thirteen times. On November 7, 1837, there was an ebb and flow of eight feet every twenty-eight minutes. Again, on May 17, 1841, like phenomena, unaccompanied by any other unusual occurrences, were recorded.[152]

Phenomena which may possibly hold a relationship to earth pulsations are the periodical swellings of the ocean on the coast of Peru. Dr. C. F. Winslow, who made a long period observation upon the coast of Peru, found ‘the highest tides to prevail at Callao and Paita in December and January,’ and ‘also a series of enormous waves or sea-swells to be thrown from time to time upon the coast, varying from twenty-four to twenty-seven hours in continuance, accompanied by unusual height of the tide during the same period.’ During June and July the ocean was unusually tranquil. These phenomena do not appear to be connected with great atmospheric storms, nor do they hold any relation to the prevailing wind. They increase with and accompany the swelling of the tides, and occur generally, but not always, about full moon.

Sometimes they break suddenly upon the coast. ‘They are annual and constant in their periodicity.’

The periodical swellings are most noticeable between Tumbez 3° S.L. and the Chincha Islands 14° S.L.

These oceanic phenomena synchronise with the periodic intensity of earthquake phenomena in that part of the globe, and these with tidal movements.[153]

Other phenomena possibly attributable to earth pulsations.—If we assume that earth pulsations have an existence, these many phenomena which are otherwise difficult to understand meet with an explanation. The curious effects which were produced in the springs at Toplitz at the time of the Lisbon earthquake may have been due to a pulse-like wave. The flow of the principal spring was greatly increased. Before the increase it became turbid and at one time stopped. Subsequently it became clear and flowed as usual, but the water was hotter and more strongly mineralised. Sudden changes in the flow of underground waters which from time to time are observed may be attributed to like causes. Secondary earthquakes such as occurred after the Lisbon earthquake, as for instance in Derbyshire, may have been produced by pulsations disturbing the equilibrium of ground in a critical state.

The falling in of subterranean excavations is also possibly connected with these phenomena.

Possible causes of earth pulsations.—Mr. George Darwin, in a report to the British Association (1882), has shown that movements of considerable magnitude may occur in the earth’s crust in consequence of fluctuations in barometrical pressure. (A rise of the barometer over an area is equivalent to loading that area with a weight, in consequence of which it is depressed. When the barometer falls, the load is removed from the area, which, in virtue of its elasticity, rises to its original position. This fall and rise of the ground completes a single pulsation.)

On the assumption that the earth has a rigidity like steel, Mr. Darwin calculates that if the barometer rises an inch over an area like Australia, the load is sufficient to sink that continent two or three inches.

The tides which twice a day load our shores cause the land to rise and fall in a similar manner. On the shores of the Atlantic, Mr. Darwin has calculated that this rise and fall of the land may be as much as 5 inches. By these risings and fallings of the land the inclination of the surface is so altered that the stile of a plummet suspended from a rigid support ought not always to hang over the same spot. There would be a deflection of the vertical.

In short, calculations respecting the effects of loads of various descriptions, which we know are by natural operations continually being placed upon and removed from the surface of various areas of the earth’s surface, indicate that slow pulsatory movements of the earth’s surface must be taking place, causing variations in inclination of one portion of the earth’s crust relatively to another.

Although it is possible that phenomena like the surging of levels may be attributable to causes like these, we can hardly attribute the other phenomena to such agencies.

Rather than seek an explanation from agencies exogenous to our earth, we might perhaps with advantage appeal to the endogenous phenomena of our planet. When the barometer falls, which we have shown corresponds to an upward motion of the earth’s crust, we know, from the results of experiments, that microseismic motions are particularly noticeable.

As a pictorial illustration of what this really means, we may imagine ourselves to be residing on the loosely fitting lid of a large cauldron, the relief of the external pressure over which increases the activity of its internal ebullition—the jars attendant on which are gradually propagated from their endogenous source to the exterior of our planet. This travelling outwards would take place much in the same way that the vibrations consequent to the rattle and jar of a large factory slowly spread themselves farther and farther from the point where they were produced.

Admitting an action of this description to take place, it would then follow that this extra liberation of gaseous material beneath the earth’s crust would result in an increased upward pressure from within, and a tendency on the part of the earth’s crust to elevation. If we accept this as an explanation of the increased activity of a tremor indicator, then such an instrument may be regarded as a barometer, measuring by its motions the variations in the internal pressure of our planet.

The relief of external pressure and the increase of internal pressure, it will be observed, both tend in the same direction—namely, to an elevation of the earth’s crust.

This explanation of the increased activity of earth tremors, which has also been suggested by M. S. di Rossi, is here only advanced as a speculation, more probable perhaps than many others.

We know how a mass of sulphur which has been fused in the presence of water in a closed boiler gives up in the form of steam the occluded moisture upon the relief of pressure. In a similar manner we see steam escaping from volcanic vents and cooling streams of lava. We also know how gas escapes from the pores and cavities in a seam of coal on the fall of the barometrical column. We also know that certain wells increase the height of their column under like conditions. The latter of these phenomena, resulting in an increase in the rate of drainage of an area by its tendency to render such an area of less weight, facilitates its rise. If we follow the views of Mr. Mallet in considering that the pressures exerted on the crust of our earth may in volcanic regions be roughly estimated by the height of a column of lava in the volcanoes of such districts, we see that in the neighbourhood of a volcano like Cotopaxi the upward pressures must be enormously great. Further, the phenomena of earthquakes and volcanoes indicate that these pressures are variable. Before a volcano bursts forth we should expect that there would be in its vicinity an upward bulging of the crust, and after its formation a fall. Further, it is not difficult to conjecture other possible means by which such pressures may obtain relief.

Should these pressures then find relief without rupturing the surface, it is not difficult to imagine them as the originators of vast pulsations which may be recorded on the surface of the earth as wave-like motions of slow period.

As an explanation of the strange movements observed on seas and lakes, Kluge brings forward the following strange and remarkable theory. The oxygen of the air is magnetic, whilst water is diamagnetic and the earth magnetic: we have, therefore, in our seas and lakes a diamagnetic body lying between and being, consequently, repelled from two magnetic bodies. By variation in temperature, the balance of repulsions exerted by the air and the earth is destroyed. Thus, by an elevation of temperature the air expands and flows away from the heated area, where, in consequence, there is less oxygen. The result of this is, that the repulsion of the air upon the waters is less than that of the earth upon the waters, and the waters are in consequence raised up. By a falling of temperature the waters may be depressed, and by either of these actions waves may be produced without the intervention of earthquakes or earth pulsations.

The more definite kinds of information which we have to bring forward, tending to prove the existence of earth pulsations, too slow in period to be experienced by ordinary observers, are those which appear to be resultant phenomena of great earthquakes.

The phenomena that we are certain of in connection with earth vibrations, whether these vibrations are produced artificially by explosions of dynamite in bore-holes, or whether they are produced naturally by earthquakes, are, first, that a disturbance as it dies out at a given point often shows in the diagrams obtained by seismographs a decrease in period; and, secondly, a similar decrease in the period of the disturbance takes place as the disturbance spreads.

As examples of these actions I will quote the following.

The diagram of the disturbance of March 1, 1882, taken at Yokohama, shows that the vibrations at the commencement of the disturbance had a period of about three per second, near the middle of the disturbance the period is about 1·1, whilst near the end the period has decreased to ·46. That is to say, the backward and forward motion of the ground at the commencement of the earthquake was six times as great as it was near the end, when to make one complete oscillation it took between two and three seconds. Probably the period became still less, but was not recorded owing to the insensibility of the instruments to such slow motions.[154]

We have not yet the means of comparing together diagrams of two or more earthquakes, one having been taken near to the origin, and the other at a distance. The only comparisons which I have been enabled to make have been those of diagrams taken of the same earthquake, one in Tokio and the other in Yokohama. As this base is only sixteen miles, and the earthquake may have originated at a distance of several hundreds of miles, comparisons like these can be of but little value.

Other diagrams illustrating the same point are those obtained at three stations in a straight line, but at different distances from the origin of a disturbance produced by exploding a charge of dynamite in a bore-hole. A simple inspection of these diagrams shows that at the near station the disturbance consisted of backward and forward motions, which, as compared with the same disturbance as recorded at a more distant station, were very rapid. Further, by examining the diagram of the motions, say, at the near station, it is clearly evident that the period of the backward and forward motion rapidly decreased as the motion died out.

These illustrations are given as examples out of a large series of other records, all showing like results.

An observation which confirms the records obtained from seismographs respecting the increase in period of an earthquake as it dies out I have had opportunities of twice making with my levels. After all perceptible motion of the ground subsequent upon a moderately severe shock had died away, I have distinctly seen the bubble in one of these levels slowly pulsating with an irregular period of from one to five seconds.

Although we must draw a distinction between earth waves and water waves, we yet see that in these points they present a striking likeness. Let us take, for example, any of the large earthquake waves which have originated off the coast of South America, and then radiated outwards, until they spread across the Pacific, to be recorded in Japan and other countries perhaps twenty-five hours afterwards, at a distance of nearly 9,000 miles from their origin. Near this origin they appeared as walls of water which were seen rapidly advancing towards the coast. These have been from twenty to two hundred feet in height, and they succeeded each other at rapid intervals, until finally they died out as a series of gentle waves. By the time these walls of water traversed the Pacific, to, let us say, Japan, they broadened out to a swell so flat that it could not be detected on the smoothest water excepting along shore lines where the water rose and fell like the tide. Instead of a wall of water sixty feet in height, we had long flat undulations perhaps eight feet in height, but with a distance from crest to crest of from one to two hundred miles.

If we turn to the effects of large earthquakes as exhibited on the land, I think that we shall find records of phenomena which are only to be explained on the assumption of an action having taken place analogous to that which takes place so often in the ocean, or an action similar to that exhibited by small earthquakes, and artificially produced disturbances, if greatly exaggerated.

The only explanation for the phenomena accompanying the Lisbon earthquake appears to be that the short quick vibrations which had ruined so many cities in Portugal had, by the time that they had radiated to distant countries, gradually become changed into long flat waves having a period of perhaps several minutes. In countries like England these pulse-like movements were too gentle to be perceived, except in the effects produced by tipping up the beds of lakes and ponds.

The phenomenon was not unlike that of a swell produced by a distant storm. It would seem possible that in some cases pulsations producing phenomena like the ‘seiches’ of Switzerland might have their origin beneath the ocean, or deep down beneath the earth’s crust. Perhaps, instead of commencing with the ‘snap and jar’ of an earthquake, they may commence as a heaving or sinking of a considerable area, which may be regarded as an uncompleted effort in the establishment of an earthquake or a volcano.

From what has now been said it would seem that earth pulsations are phenomena with a real existence, and that some of these are attributable to earthquakes. On the other hand, certain earthquakes are attributable to earth pulsations. Some of the phenomena which have been brought forward have only a possible connection with these movements, and they yet require investigation. Elastic tides in the earth’s crust have for long been realities in the minds of physicists. These, however, are due to lunar and solar influences, and are regular in their action. The tidal-like movements called pulsations are of greater magnitude, and their goings and comings are irregular.

CHAPTER XXI.
EARTH OSCILLATIONS.

Evidences of oscillation—Examples of oscillation—Temple of Jupiter Serapis—Observations of Darwin—Causes of oscillation.

Evidences of oscillation.—By earth oscillations are meant those slow and quiet changes in the relative level of the sea and land which geologists speak of as elevations or subsidences. These movements are especially characteristic of volcanic and earthquake-shaken countries.

As evidences of elevations we appeal to phenomena like raised beaches, sea-worn caves, raised coral reefs, and the remains of other dead organisms like barnacles, and the borings of lithodomous shells in and on the rocks of many coasts high above the level of the highest tides. As a proof that subsidence has taken place, there is the evidence afforded by submerged forests, the prolongation of certain valleys beneath the bed of the ocean, the formation of coral islands, the peculiar distribution of the plants and animals which we find in many countries, and the submergence of works of human construction. Inasmuch as these phenomena are discussed so fully in many treatises on physical geology, the references to them here will be made as brief as possible. Elevations and depressions which have taken place at the time of large earthquakes in a paroxysmal manner have already been mentioned. The movements referred to in this chapter, although generally taking place with extreme slowness, in certain instances, by an increase in their rapidity, have approached in character to earth pulsations. In most instances it would appear that the upward movement of the ground, which may be likened to a process of tumefaction, goes on so gently that it only becomes appreciable after the lapse of many generations.

Examples of movements.—Lyell estimated that the average rate of rise in Scandinavia has been about two and a half feet per century. At the North Cape the rise may have been as much as five or six feet per century. Observations made at the temple of Jupiter Serapis, between October 1822 and July 1838, showed that the ground was sinking at the rate of about one inch in four years. Since the Roman period, when this temple was built, the ground has sunk twenty feet below the waves. Now the floor of the temple is on the level of the sea. Lyell remarks that if we reflect on the dates of the principal oscillations at this place there appears to be connection between the movements of upheaval and a local development of volcanic heat, whilst periods of depression are concurrent with periods of volcanic quiescence.[155]

As examples of movements even more rapid than those at the Temple of Jupiter Serapis we refer to an account of the earthquakes in Vallais (November 1755), when the ground about a mountain at a small distance from Brigue sank about a thumb’s-breadth every twenty-four hours. This took place between December 9 and February 26.[156]

Another remarkable example of earth movement is given in the account of the earthquake at Scarborough, on December 29, 1737, when the head of the spa water well was forced up in the air about ten yards high. At this time the sands on the shore are said to have risen so slowly that people came out to watch them.[157]

Two other examples of rapid earth movement are taken from Professor Rossi’s ‘Meteorologia Endogena.’ Professor D. Seghetti, writing to Professor Rossi, says that a few lustres ago (one lustre = twenty years) Mount S. Giovanni hid the towns Jenne and Subiaco from each other. From Subiaco the church at Jenne is now visible, which a few years ago was invisible. The people at Jenne also can see more than formerly. The supposition is that the side of Mount S. Giovanni is lowered. This fact corresponds to a fact stated by Professor Carina, who says that forty or fifty years ago from Granaiola you could not see either the church of S. Maria Assunta di Citrone or the church of S. Pietro di Corsena. Now you can see both.[158]

For a remarkable example illustrating the connection between seismic activity and elevation we are indebted to the patient labours of Darwin, who carefully investigated the evidences of elevation which are visible upon the western coasts of South America. These evidences, consisting of marks of erosion, caves, ancient beaches, sand dunes, terraces of gravel, &c., were traced between latitudes 45° 35′ to 12° 5′, a distance north and south of 2,075 geographical miles, and there is but little doubt that they extend much farther. As deduced from observations upon upraised shells alone, a summary of Mr. Darwin’s observations are contained in the following table:—

	Feet
At Chiloe the recent elevation has been	350
„ Concepcion „ „	625 to 1,000
„ Valparaiso „ „	1,300
„ Coquimbo „ „	252
„ Lima „ „	85

Shells, similar to those clinging to uplifted rocks, which are evidences of these elevations, still exist in the neighbouring seas, and in the same proportionate numbers as they are found in the upraised beds. In addition to this, Mr. Darwin shows us that at Lima, during the Indo-human period, the elevation has been at least eighty-five feet. At Valparaiso, during the last 220 years, the rise was about nineteen feet, and in the seventeen years subsequent to 1817 the rise has been ten or eleven feet, a portion only of which can be attributed to earthquakes. In 1834 the rise there was apparently still in progress.

At Chiloe there has been a gradual elevation of about four feet in four years. These, together with numerous other examples, testify to the gradual but, as compared with other parts of the globe, exceedingly rapid rise of the ground upon the western shores of South America.[159] The most important point to be noticed is that this district of rapid elevation is one of the most earthquake-shaken regions of the world. And further, judging from Darwin’s remarks, in those portions of it where the movements have been the most extensive, and at the same time probably the most rapid, the seismic disturbances appear to have been the most noticeable.

Similar remarks may be applied to Japan, it being in those districts where evidences of recent elevation are abundant that earthquakes are numerous. Thus, in the bay of Yedo, where we have borings of lithodomi in the tufaceous cliffs ten feet above high-water mark, which, inasmuch as the rock in which they are found is soft and easily weathered, indicate an exceedingly rapid elevation, earthquakes are of common occurrence.

From the evidences of elevation which we have upon the South American coast, Japan, and in other countries, it appears that these movements are intermittent, there being periods of rest, when sea cliffs are denuded, and perhaps even periods of subsidence. There is also evidence to show that, although these movements have been gradual from time to time, they have been aided by starts occasioned by earthquakes.

As to whether earthquakes are more numerous during periods of elevation, or of subsidence, or during the intermediate periods of rest, we have no evidence.

Sudden displacements which occasionally accompany earthquakes might, it was said, sometimes be regarded as the cause of an earthquake, and sometimes as the effect.

The slow elevations here referred to may be looked upon as being one of the more important factors in the production of earthquakes. By various causes the rocky coast is bent until, having reached the limit of its elasticity, it snaps, and, in flying back like a broken spring, causes the jars and tremors of an earthquake.

If this is the case, then the number of earthquakes felt in a district which is being elevated may possibly be a function of the rate of elevation.