Earthquakes and other earth movements

by John Milne

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.