CHAPTER XV
THE CAUSES OF EARTHQUAKES
Now that we have before us some of the examples of the changes which historic earthquakes have brought about in the face of the country, it is easy to see what an important effect they must have exerted in geological history. But there are still weighty questions to be answered about earthquakes. We have seen that an earthquake can contort, upset, and twist the surface strata of the earth as easily as we can crumple a sheet of cardboard. We have yet to find whether the crumpling of the strata is always produced by earthquakes, or whether an earthquake is the culminating symptom that some agency is at work crumpling the strata. Let us try to imagine an example on a small scale. Suppose we take the top of a pill-box, and, holding it in the crook between our thumb and forefinger, compress it very tightly on all sides. What will happen? The lid of the pill-box, being subjected to stress or strain on all sides, will presently buckle and crack. We shall have produced an earthquake on a small scale, and there will be an earthquake fracture—perhaps an earthquake fissure. If the whole pill-box had been used for the purpose of our experiment, and had been packed to the brim with ointment or thick liquid, and if it had been squeezed in a vice instead of in our hands, then perhaps we should have provoked still more striking symptoms of an earthquake. The ointment might have broken out through the lid. Perhaps even tiny jagged holes or craters would have been formed in the lid. Thus we see how strain may produce earthquakes. Take some more examples. Suppose a cork is very tightly fixed in a wine bottle, and in order to get it out we employ a very powerful lever corkscrew. The neck of the bottle, under the effect of the too powerful pressure put on the inside surface of the glass, will crack or break. Similarly if we screw down a microscope too hard on a slip of glass the glass will often crack suddenly. Both these instances recently occurred within the writer's experience, and few readers can have escaped noticing one or other of them. The breakage in these instances is always caused because a strain is set up somewhere in the glass—there is more pressure at one point than another, and the glass, unable to resist this unequal pressure, gives way.
Stereo Copyright, Underwood & U.
London and New York
The Track of an Earth Wave
Showing portion of a street in San Francisco after the terrible earthquake of 1906. The resemblance of the break in the ground to the appearance of a stationary wave should be noted.
What happens when it gives way? To answer this question we had better carry our minds to the example of the pill-box lid. If the top of the box were of very brittle material, like pottery or glass, then after the breakage we know that most likely one of the broken pieces would be a little higher than the other—would perhaps overlap it. That is what we often see when examining the geological earth strata. One stratum, instead of lying evenly with another where a crack has occurred, rests a little above it or below it. This inequality or unevenness geologists call a fault. Now we can easily see that whenever, and by whatever causes, a fault is produced, there is probably at the same time an earthquake. The fault cannot be produced without a great and shaking disturbance. Mr. John Milne, the most distinguished of British authorities on earthquakes, says that all large earthquakes originate from the formation or extension of these "faults" or great cracks in the strata.
The occurrences of these "faults" are most frequent when the process of mountain building has been going on over the earth. In other words, if we imagine a period in which some great continental area of the world's surface was slowly rising above the oceans, then at that time we should expect that there would have been great "faults" occurring in the strata, and great earthquakes following them. To quote his words: "If therefore we wish to know when earthquakes were greatest and most frequent, we must turn to those periods in geological history when mountain ranges were built, when volcanic activity was pronounced, and when great 'faults' were made. The first of these periods would be coincident with the creation of the Ural Mountains, the Grampians, and other mountain ranges. This took place in the earliest geologic times. Another period of mountain formation was when the Himalayas and the Alps were slowly but intermittently brought into existence. In both these periods volcanic activity was pronounced, and beds of coal were formed. When the crust of the earth was crumbling, mountains grew spasmodically—(they sprang up, as it were, from out of the giant forces which we have described earlier in this book)—'faults' gave rise to earthquakes, volcanic forces found their vents, and conditions existed which gave rise to the accumulation of materials to form coal."
But, the reader will naturally inquire, if "faults" gave rise to earthquakes, and faults are the result of pressure, what produces the pressure? And what produces the mountains? Before we answer that we must again have recourse to examples taken from common experience. A sheet of glass or of marble we usually regard as a thing that may break but does not bend. But all of us have seen glass strips, if they are long enough, bend under their own weight; and there are even marble mantelpieces which, if examined, show that the slab of marble has bent. Thus we can readily imagine that sometimes the strata of the earth will bend by reason of the weight put upon them. If that weight is not put on them quite evenly the strata will be still more likely to bend; it will go from bending to buckling, and from buckling to breaking. As soon as it breaks there will be a "fault" formed. Those who recollect what we have said about the enormous weight of the rocks one above the other will not have to search far for the cause of weight sufficient to bend or buckle the rock strata of the earth. And those who have followed carefully all that has been written about the shifting of materials from the land to the sea by the processes of denudation and erosion set up by rain and wind and carried out by streams and rivers, will easily discover where and how the shifting of great weights of the earth's surface goes on. Little by little great weights are taken from one place on the earth's surface to another, as we might shift the weights in balanced scale-pans grain by grain, till at last the heavier scale-pan goes down quickly, or it may be with a crash, and the "fault" occurs, while the earthquake follows.
There remains another question, however, to be answered, and it is, How were the mountains formed? Mountains are very closely connected with earthquakes, for nearly all the regions of the earth where the great disturbances take place are in the neighbourhood of great mountain ranges; and many, indeed most, of the students of earthquakes believe this to be the case, because the great weight of the mountains, especially when near the deep sea, induces pressure on the rocks, and consequently slipping and "faults." But the causes of mountain formation are very difficult to show with certainty. One such explanation, that of the continual shifting of portions by weight of the earth, we have already given. There is another one which may perhaps supply an additional cause. It is that just as "faults" produce earthquakes, perhaps in some cases earthquakes produce "faults." In the illustration we gave of the bottle-neck being broken by continual pressure, or the slide of a microscope suddenly breaking with a crash, because the increasing pressure of a screw was greater than it could bear, we have considered cases where the pressure was slowly applied. But a tap with a hammer or any sudden shock would also produce a breakage—and "faults," and an earthquake on a small scale—and it is possible that some of the convulsions of nature and some of their permanent effects are caused by sudden and violent causes.
One such cause might be the violent and sudden formation of steam by the contact of water with rock at a very high temperature. Everybody knows what happens to the kitchen boiler after a severe frost. The frost clogs the pipes with ice, so that the kitchen boiler becomes dry because no water is reaching it; but it continues to grow hotter and hotter till the iron plates or iron lining become red-hot. Then the frost perhaps gives way and a small amount of water finds its way into the red-hot boiler. The water is converted instantly into steam; and, as a result, the boiler, if it is a weak one, is blown out into the kitchen, causing grave personal inconvenience to the cook. If the boiler is a strong one, it may merely crack. Now, apply these considerations to the instance of the earth, its oceans, its thin crust, and its hot rocks situated at a depth of not more than thirty miles, and perhaps at a good deal less depth than that. What would happen if the ocean leaked through into the strata of red-hot or molten rocks? There would be enormous quantities of steam formed; and if, owing to the vast pressure of strata and water above, these quantities of steam did not instantly produce violent explosions, yet underneath there would be imprisoned, peak, forces of tremendous power which would only await a favourable opportunity in order to manifest themselves. The hot steam until this favourable opportunity arose would be absorbed by the rocks, just as hot steel can be shown to absorb gases.
Stereo Copyright, Underwood & U.
London and New York
A Geyser at Rest in Yellowstone Park, U.S.A.
The encrusted deposits of mineral salts should be observed.
We are thus face to face with the following situation, as it is expressed by Dr. T. J. J. See, the American physicist:—The internal temperature of the earth is extremely high with heated rocks quite near the surface, while the crust is fractured and leaky everywhere, and especially where the depth of the sea is greatest. The sea covers three-fourths of the earth's surface, and earthquakes are found to be most violent where the sea is deepest, and volcanoes most numerous on the adjacent shores. Can we then suppose that both earthquakes and volcanoes depend on the explosive power of steam which has developed in the heated rocks of the earth's crust? We have said that earthquakes and volcanoes are most common in regions where high mountains are near deep oceans—as on the westward of South and Central America, the Aleutian Islands, the Kurile Islands, Japan, Sumatra, Java, and other islands of the East Indies bordering on the deep waters of the Indian Ocean, New Zealand, and the Lesser Antilles in the West Indies, Iceland, Italy, and Greece. These are also the regions where, owing to the existence of high mountains, the weight, the pressure, the tear and stress on the underlying strata are greatest, and where consequently there is the greater chance of strata slipping or bending or giving way. Mr. John Milne divides the world into eleven such great "world-earthquake" districts; and he has endeavoured to show that all the great convulsions of the earth have their origin in one or other of these areas—where usually a great mountain range slopes steeply down to the sea. There are eleven such world-earthquake districts. There is the Alaskan region, where on the shore Mount Elias rises to a height of 18,000 feet and where the water is 7000 feet deep sixty miles from the shore—altogether a drop of 25,000 feet from the top of the mountain to the bed of the ocean in 200 miles. This drop, without going into measurements, may be taken as typical of the rest, which are classified as the Cordillerean region, the Antillean region (in the earthquake district of which Mount Pelée at Martinique and the Soufrière in St. Vincent are situated); the Andean district; the Japan district; the Javan district; the Mauritius district; the Antarctic district; three submarine districts of sunken ridges in the North-Eastern Atlantic, the North-Western Atlantic, and the Northern Atlantic; and one great land district, distant from the sea, which lumps together all the mountains of the Alps, the Balkans, the Caucasus, and the Himalayas. In thirteen years, from the time in which earthquake investigation has become a science, 750 great earthquakes originated in these districts. On the average, about sixty great earthquakes occur every year, or a little more than one a week. In addition to these world-shaking effects there are about 30,000 small earthquakes every year, England's annual contribution to this number being about half a dozen.
What is the meaning of a "great earthquake," and how do we define "small" earthquakes? A small earthquake is one such as we have described as taking place in the valley of the Mississippi, and which, even though it may produce considerable disturbance in its neighbourhood, is not perceptible at any great distance away. A great earthquake is one which sends its vibrations thousands of miles. A very large earthquake, originating in any part of the world, may be recorded in any other part of the globe. Although only a few people in Great Britain have been privileged to feel a home-made earth tremor, every one of us is very many times a year moved by earthquakes. We do not perceive them because the back-and-forth motion of the ground is performed too slowly, while if there is a movement of the ground the undulations are so very flat that they cannot be perceived. But at several places in England and at earthquake observatories (seismological stations) all over the world, from Japan to Australia and from South Africa to Greenland, instruments are set up which are sensitive enough to record these tremors, though not always to locate them. Sometimes when Professor Behar in Germany, or Mr. Milne from his observatory in the Isle of Wight, telegraphs to the newspapers that signs of a great earthquake have appeared on their instruments, the world hears no more of these disturbances. They have occurred we are certain, but the place where the great cataclysm which has thus shaken the whole round world took place has been fortunately remote from inhabited portions of the earth, and has very likely been beneath the waters of some ocean.
Earthquake waves start out from the great area where the cataclysm took place, and begin to disturb the earth in all directions, just as if we were to put a row of marbles on a table and were to strike the end marble of the row. The marble farthest from it would presently receive the shock as it travelled along the row of marbles. Any one of our readers who has ever seen a train of luggage wagons being shunted is familiar with the way in which the shock of a sudden pull or push on the part of the engine travels all down the line of wagons, and we may think of the shock of an earthquake as travelling along and through the earth in the same way. Observation, however, shows that these waves are propagated farthest in one particular direction. For example, the chief movement following the San Francisco earthquake, which originated from fault lines running parallel to the coast of California, was much more marked in countries lying to the east or west of California than in countries lying towards the south. England and Japan obtained large records of the disturbance, while in Argentina the records were extremely small. In the case of the Jamaica earthquake, where the lines of origin ran east and west, the phenomenon was reversed. Toronto received a large quantity of motion, and England a very little. Another peculiarity of this phase of earthquake motion is that it may be propagated in one direction round the world to a greater distance than in an opposite direction. The suggestion is that the initial impulse was delivered in the direction towards which motion was propagated farthest. That which happens corresponds to what we see if we dip the blade of a spade in water and suddenly push the blade in some particular direction. The water waves thus created travel farthest in the direction of the impulse.
Another curious phenomenon connected with the large waves of certain earthquakes is that they may be very marked for one thousand miles round their origin, and may be perceived on the exactly opposite side of the earth (though, of course, much reduced in size), but cannot be recorded on the earthquake instruments of the regions in between. For example, an earthquake originating near New Zealand may be recorded in that country, but not in India, Egypt, West Asia, or east of Europe, though in Britain it may make itself evident on the seismometer's record. The phenomenon may be compared to a water wave running down an expanding estuary. At the mouth of such an estuary it may have become so flat that it is no longer recognisable. Should it, however, run up a second estuary, we can imagine concentration taking place, so that near the top of the second estuary it would eventually become recordable on instruments. In these antipodean survivors we see the final efforts of a dying earthquake. It is only occasionally that the precursors and the followers of these large waves have sufficient energy to reach their antipodes. They die en route.
From the earliest times philosophers have held that the causes of earthquakes were associated with the contact between fire and water. Plato, Aristotle, Strabo, and Pliny all held that water and air penetrate into the earth through hollows, fissures, and crevices, thus developing in the heated interior great vapour, a part of which is expelled from volcanoes. Aristotle correctly associated seismic sea waves with earthquakes, and even Homer assigned these great disturbances of the sea to Poseidon's trident, which was also the means employed for raising up islands from the sea bottom. The withdrawal of the water from the shore after an earthquake and its return as a great wave were familiar to Aristotle, and are implied in his description of the sinking of Helike in 373 B.C.
Before leaving the subject of earthquakes we may quote some passages from Mr. John Milne on the influence which these great disasters have exercised on the emotions. Immediately after the Kingston earthquake we read of the dazed and almost insane condition of the people. Many were affected with an outburst of religious ecstasy, thinking the last day had come. The negro population camped on the racecourse and spent their time in singing hymns. Somewhat similar scenes took place in Chili; men and women ran hither and thither, mad with terror and devoid of reason. Amid shrieks and sobs and the wailing of a multitude an "Ora pro nobis" or a "Pater noster" might now and then be heard. In early civilisations underground thunderings have so far excited the imagination that subterranean monsters or personages have been conjured into existence, and these in many instances have played a part in primitive religions. At the time of an earthquake in Japan the children are told that the shaking is due to the movement of a fish which is buried beneath their country, and in Japan we find references to this fish in the pictorial art, pottery and carving, literature, and everyday conversation, all of which would be unintelligible if we did not know the story of the earthquake fish. In other countries the subterranean creature will be a pig, a tortoise, an elephant, or some other animal.
The most interesting myths, however, relate to underground personages. The forty-five Grecian Titans, who were of gigantic stature and of proportionate strength, were confined in the bowels of the earth. According to the poets, the flames of Etna proceeded from the breath of Enceladus, and when he turned his weary body the whole island of Sicily was shaken to its foundations. Neptune was not only a god of the oceans, rivers, and fountains, but with a blow of his trident he could create earthquakes at pleasure. The worship of Neptune was established in almost every part of the Grecian world. The Livians, in particular, venerated him, and looked upon him as the first and greatest of the gods. The Palici were born in the bowels of the earth, and were worshipped with great ceremonies by the Sicilians. In a superstitious age the altars of the Palici were stained with the blood of human sacrifices. In Roman mythology two very familiar deities are Pluto and Vulcan. These and a host of other deities, the outcome of imagination, excited by displays of seismic and volcanic activity, we meet with every day in picture galleries, in museums, in literature, and in our daily papers. Earthquakes have led to the abolition of oppressive taxation, the abolition of masquerades, the closing of theatres, and even to the alteration in fashions. A New England paper, of 1727, tells us that "a considerable town in this province has been so far awakened by the awful providence in the earthquake that the women have generally laid aside their hooped petticoats."
In the next chapter we shall consider more particularly the terrible effects of earthquakes on geological history.