CHAPTER XIII
EARTHQUAKES IN GEOLOGY
It is more than likely that earthquakes in the geological past were very much more violent, widespread, and frequent than they are now; and they may have had a more potent effect in overturning the rocks of the earth. Even now their effects and the circumstances which accompany them are tremendous and terrifying. When the great earthquake comes, says Major Edward Dutton in his book on Earthquakes, it comes quickly and is quickly gone. Its duration is generally a matter of seconds rather than of minutes, though instances have been known in which it lasted from three to four minutes. Perhaps forty-five seconds would be a fair average. The first sensation is a confused murmuring sound of a strange and even weird character. Almost simultaneously loose objects begin to tremble and clatter. Sometimes almost in an instant, sometimes more gradually, but always quickly, the sound becomes a roar, the clattering becomes a crashing. The rapid quiver grows into a rude violent shaking of increasing amplitude. Everything beneath seems beaten with rapid blows of measureless power; loose objects begin to fly about; those that are lightly hung break from their fastenings. The shaking increases in violence. The floor begins to heave and rock like a boat on the waves. Plaster ceilings fall, the walls crack, the chimneys go crashing down, everything moves, heaves, tosses. Huge waves seem to rush under the foundations as if driven by a gale. The swing now becomes longer and still more powerful. The walls crack open. A sudden lurch throws out the front wall into the street, or tears off or shakes down in rubble the whole corner of the building. Then comes a longer swaying motion, not unlike a ship at sea, but more rapid; not alone from side to side, but forward and backward as well, and both motions combined with a wriggle which it seems impossible for anything to withstand. It is this compound, figure-8 motion which is so destructive, rending asunder the strongest structures as if they were built of clay. It is the culmination of the quake. It settles into a more regular and less violent swing; then suddenly abates and ceases.
Out in the open country the signs and portents are of a different character. The first intimation is a strange sound. Some have likened it to the sighing of pine trees in the wind, or to falling rain; others to the distant roar of the surf; others to the far-off rumble of the railway train. It grows louder. The earth begins to quiver, then to shake rudely. Soon the ground begins to heave. Then it is actually seen to be traversed by visible waves—something like waves at sea, but of less height and moving much more swiftly. The sound becomes a roar. It is difficult to stand, and at length becomes impossible to do so. People fling themselves to the ground to avoid being dashed against it. The trees are seen to sway violently, sometimes so much that they touch the ground with their branches.... As the waves rush past the ground opens in cracks and closes again. As the cracks close the squeezed-out air blows out sand and gravel, and sometimes sand and water are spurted high in air. The roar becomes appalling. Through its din are heard loud, deep, solemn booms that seem like the voice of some higher Power speaking out of the depths of the universe. Suddenly the storm subsides, the earth comes speedily to rest, and all is over.
And yet, says Major Dutton, this description suggests but a single instance, or a few instances, of what earthquakes are like. In some the full vigour of the shock comes with explosive suddenness. People find themselves suddenly thrown to the earth, the ground swept from under their feet. Sometimes the rolling waves of earth are absent, and the movement is a rude quiver, rapidly vibrating in every direction—twisting, contorting, wrenching the ground as if in a determined effort to shake it into dust. Sometimes the most pronounced motion is up and down, as if the earth beneath were being hammered with giant strokes. Sometimes the growth, the climax, the dying out of the earthquake movement are repeated before the first shocks have ceased, or a few minutes afterwards, or even with an interval of several hours. The last-named case is, however, uncommon, though after the first shocks of a great earthquake there are minor shocks and tremblings for days, weeks, or months afterwards. Some of these are of considerable force, though they do not inflict the devastation of the principal earthquake. The greatest earthquakes are not always those which wreak the largest amount of destruction. Evidently an earthquake, the centre of which is situated near a great city, is more appalling in its effects than one which takes place in some desert place like the steppes of Siberia. Recent earthquakes in Italy and near San Francisco were regarded as great earthquakes because they took place in thickly populated neighbourhoods. In cities, writes Professor W. H. Hobbs, to the rumbling of the earthquake is quickly added the crash of falling masonry, and to this succeeds an uncanny grey darkness as the air becomes filled with the dust from broken bricks, mortar, and plaster.... In places the ground opens and swallows the objects which lie upon it. Ponds are sucked down and disappear, and great fountains of water gush out and flood the country. During the New Madrid earthquakes of 1811 and 1812 water was shot upwards in vertical sheets and carried to the tops of the highest trees. Near Lake Baikal, during the earthquake on January 26th, 1862, the surface of the steppe, over two hundred square miles, was suddenly dropped; fountains opened at many parts within the sunken area, and water shot up to heights of twenty feet. The water gushed also in great volume from the open wells, and where these were tightly covered by wooden caps, their lids were shot up into the air like corks from champagne bottles. On the night of September 5th, 1896, during heavy earthquake shocks in Iceland, a new warm spring suddenly opened to the accompaniment of loud roaring and whistling, and threw water, steam, and fragments of rock to a height estimated at six hundred feet. The force of the new geyser was, however, soon spent, and ten days later it ceased to flow. Nearly all the Icelandic geysers suffered changes during this earthquake, and the famous Strokkur, which had been born during the earthquake of 1789, suddenly ceased its eruption and came to an end.
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A Geyser in Action
In geysers the suggestion that the fountains of steam and hot water originate by the contact of water with hot rock is irresistible.
In steep-walled mountain valleys earthquakes nearly always cause landslips, and these may completely block the course of a river. The lake formed in this manner during the great earthquake of January 25th, 1348, in Carinthia destroyed no fewer than seventeen villages, and to-day, nearly six centuries afterwards, the area is a great marsh. After the earthquake near Lake Baikal in Siberia, of which we have spoken and in which the ground sank, the sunken area was soon after invaded by the waters of the lake. Sometimes when the earthquake takes place near the mouth of a great river, the channels of the streams are changed. After the Californian earthquake of 1857 the current of the River Kern was turned upstream; and the San Gabriel River left its bed to follow a new course offered to it by an earthquake fissure. After the Japanese earthquake of 1891 a former lake was cut in half by one of the earthquake displacements, and one half of the lake was left high and dry. Near Flagstaff, Arizona, there is an old earthquake crack along which the waters of several rivers which intersect it all disappear down the crevice. The most remarkable revelation of the process of lake draining during earthquake shocks was furnished, however, according to Professor Hobbs, by the former Lake Eulalie near New Madrid. After the shocks of 1812 the lake completely disappeared. On the lake bottom thus exposed there was revealed a series of fissures down the funnel-shaped openings which the waters had disappeared.
It will be seen from many of the foregoing instances that whatever are the principal causes of earthquakes, they must have played a great part in the shaping of events in the geological past; and the only limitation which we can place in the importance of the part they played will depend on whether we regard the earthquake as having been caused by a movement of the underlying strata, or whether we believe that the same cause which produces earthquakes may produce alterations in the lie of the strata themselves. In the next chapter we shall describe some of the effects produced on land by earthquakes. But impressive as some of these effects are, it is by no means certain that the greatest earthquakes take place on land at all. They may take place at sea, deep underneath the ocean. Our opportunities for observing such quakes, however, are much smaller than are afforded by land earthquakes. The instruments which have been devised for observing earth tremors will measure the smallest of such disturbances, and will record earthquakes the centres of which are thousands of miles away. These delicate instruments often record distant earthquakes, the exact locality of which is never determined. No doubt, some of these distant tremors originate in the ocean bed; but the seaquake can only be localised when the water is put into a state of vibration sufficiently energetic to rock the ship and its loose objects and thus affect the senses. Vast waves are sometimes rolled in on the shores of continents, and are undoubtedly caused by some great disturbance beneath the ocean.
Such waves have been known through a long period of history in the Eastern Mediterranean, where they have ravaged the shores of Syria and Asia Minor; and it is sometimes supposed that the great deluge on which the Ark of Noah floated was accompanied by a mighty sea-wave, rolled in upon the lands of Chaldea from the Persian Gulf. Off the Pacific coast of South America these waves arise most often and most mightily. They have been especially formidable in the angle where the coast of Peru meets that of Chili, and the harbours of Pisco, Arica, Tacna, Iquique, and Pisagua have been repeatedly subjected to these destructive invasions. Usually they are foreshadowed by a violent earthquake, and the inhabitants, taking warning, fly to the hills. The sea-wave does not, however, always follow the earthquake, but it appears often enough to arouse serious fear that it may come whenever the ground is strongly shaken. The first sign of the coming disaster is the withdrawal of the sea from the shore, leaving bare the bed of the harbour. A few minutes later the sea returns in a high, irresistible wave, which overflows the adjoining lands. Again it withdraws and again returns, and these oscillations may last for many hours, slowly diminishing in the amount of rise and fall till they die out.
The most memorable seaquake of the Chilian coast was that of August 13th, 1868, when the coast of South America was shaken from Ecuador to Valdivia. In the town of Arica most of the buildings were thrown down. A few minutes later the sea began to retire slowly from the shore, so that ships anchored in seven fathoms of water were left high and dry. Then the sea returned like a great wall of water, which caught up the ships in the roadstead and swept them inland like chips of wood. Among them was the United States war vessel Wateree, which was carried inland nearly half a mile and was left, little injured, on dry land when again the wave receded. The wave of this catastrophe was felt far away from Chili. It was perceived on the coasts of Australasia, Japan, Kamchatka, Alaska, and California. In the harbour of Hakodate, in Japan, a series of waves was registered on the tide-gauge. The ordinary tide in that port is only about two and a half to three feet. On this occasion the water rose and fell a height of ten feet in twenty minutes. It had taken the first wave twenty-five hours to travel the distance of 7600 miles from South America. On May 9th, 1877, another seaquake almost as great as this was felt in many of the same places. This was on the occasion of the Iquique earthquake. At Arica the stranded hulk of the Wateree, which had remained high and dry for nine years, was picked up and swept farther inland. Like its predecessor, the wave was felt all over the Pacific. At Samoa the height of the waves varied from six to twelve feet; in New Zealand from three to twenty feet; in Japan from five to ten feet.
When a wave reaches shallow water it piles itself up to a height, as any one knows who has watched the waves coming in on the sea-shore, so that the height of a wave measured on the tide-gauge of a seaport is a good deal greater than that of the height of the wave when it is far out on the ocean. In fact, the mid-ocean height of the wave is likely to be inches while the in-shore wave is measured in feet. An illustration of this can be seen on the coast of Cornwall, where sometimes, on quite a calm day the sea that looks so still breaks on the shore in big rollers. We cannot tell exactly how high an earthquake wave may be in mid-ocean, but we know it cannot usually be very great, though it travels at great speeds—sometimes as much as five miles a minute, or three hundred miles an hour.
Thus we should not expect that ships far out at sea would often notice seaquakes unless the quake took place very near them. There are, however, some instances. Captain Gales, of the ship Florence Nightingale, reported that on January 25th, 1859, while near St. Paul's Rocks, not far from the Equator, "we felt a strong shock of an earthquake. It began with a rumbling sound like distant thunder and lasted about forty seconds. I was quite well acquainted with earthquakes, as I had experienced a good many on the west coast of America, but never had I felt so severe a one. Glass and dishes rattled so vigorously that I was surprised to find them uninjured. A good many objects fell down, and it was as if the ship were grounding on a reef." Another report from a locality not far from this speaks of a strange submarine noise not unlike distant thunder, or still more like the distant firing of heavy guns. At the same time there was a vibration of the ship as though the anchor had been let go.
The foregoing are representative of the large majority of the reports of seaquakes. The ship quivers, vibrates; loose objects clatter and tumble. There is a strange thunderous noise in the sea. The first impression is as if the ship were grinding upon the bottom, and there is an instinctive rush of the crew to the deck to see if the ship is not on a reef. In some instances there are some forcible disturbances. M. Vulet d'Aourst speaks of a seaquake so severe that "the Admiral feared the complete destruction of the corvette." Heavy objects, including cannon and their carriages, were thrown upon the deck. The ship itself seemed to be hurled upwards.
One of the explanations offered of a phenomenon such as the last described is that the vessel has been near a submarine volcanic eruption of great power. The places where some or most of the seaquakes have been observed have been charted, and certain districts of the ocean have been found to produce more of these disturbances than others. Among the first to be thus determined were two, located in the Atlantic Ocean, very near the Equator and nearly midway between Cape Palmas on the southeastern coast of Liberia and Cape St. Roque, Brazil. One of them is the St. Paul's Rocks district, of which mention has already been made. Another district from which seaquakes have been reported with exceptional frequency is the North Atlantic in the neighbourhood of the Azores. Between these islands and the coast of Portugal it may be remembered that the great quake originated which, on November 1st, 1775, destroyed Lisbon. The West Indian Deep, that profound basin of the Atlantic lying north of the Lesser Antilles and east of the Bahamas, where the Atlantic has its greatest depths and where its bottom has its greatest inequalities, is another district from which an unusual number of seaquakes have been reported. The usual explanation of their origin is that in these neighbourhoods, owing to the great pressure of water above them, there are continual slips and fractures of the sea bottom, like landslips on land, and that into the great cavities thus produced the water rushes, and thus sets up disturbances which show themselves on the surface like waves, very much in the same way that the water rushing through the escape of a bath produces small disturbances on the surface of the water in the bath. To satisfy the requirements of such a wave as rolled in upon the South American coast at Arica in 1868 would require the sudden drop of many hundred square miles of sea bottom—perhaps of several thousand square miles.