CHAPTER XXIV

SOME OF THE PHENOMENA OF EARTHQUAKES

The nature of an earthquake and the movements of its waves from their starting place having now been briefly described, it remains to explain some of the strange phenomena that precede, accompany, or follow one.

Next to the violent shaking of the earth's crust, perhaps the most wonderful and impressive thing is the great variety of sounds and noises. These occur not only while the earth-waves are passing through the crust at any place, but also long before the principal shocks reach the place, as well as long after they have passed.

Earthquake sounds vary almost infinitely, both in intensity and character. Some are like the gentle sighings of the wind, or resemble faint mysterious whisperings; some are not unlike the confused murmurings of a crowded room; some resemble the sounds of a busy street. Some sounds are full and strong, like the deep bass notes of a large organ. Others resemble the din of a great battle with the reports of the large guns. Still others reach the intensity of continuous peals of thunder. But we can better understand the nature of earthquake sounds from an actual description of them in a number of great earthquakes, and by inquiring at the same time into any of the peculiar facts connected.

Humboldt in his great work, "Cosmos," thus describes the varied voice of the earthquake:

"It is either rolling or rustling, or clanking, like chains being moved, or like near thunder, or clear and ringing, as if obsidian or some other vitrified masses were struck in subterranean cavities."

That the sounds produced during earthquakes are carried through the ground faster than through the air appears clear from the fact that such sounds are sometimes heard in deep mines when they are not at all heard on the earth's surface.

In describing the earthquake that occurred in Kamtschatka, in 1759, Krashenikoff of St. Petersburg states that noises were heard like the rushing of a strong underground wind, accompanied by a hissing sound, which resembled the sizzlings heard when red hot coals are thrown in water.

In an earthquake that occurred in Lincolnshire, England, February 6th, 1817, a noise was heard closely resembling the sounds of wagons running away on a road. So complete and convincing was the resemblance that several wagoners on one of the roads drew their teams to one side so as to permit the runaway to pass safely.

Another kind of noise heard during earthquakes is a loud hollow bellowing. Sometimes, however, the sounds are more musical in their nature, being not unlike those produced by a very large organ pipe. At other times they resemble the noises produced when steam is blown into cold water.

The following account of earthquake sounds is given by Daubeny, in his book on volcanoes. It appears that during March, 1822, the people living on the island of Melida, opposite Ragusa, in Dalmatia, were greatly alarmed by sounds that at first they believed due to cannonading either at sea or on the neighboring coast. They afterwards found that these sounds were due to something that was taking place under the ground. The noises continued at intervals until August 23d, 1823, when a great earthquake occurred, during which one of the highest mountains on the island was cleft or split in one place. The underground noises continued from time to time and so frightened the people that they were about to leave the island permanently and emigrate to the mainland of Dalmatia. They were dissuaded from doing so by the government, and while the noises continued at intervals it so happened that no damage came to them. It is said, however, that twenty years after an active volcano broke out on the island.

There are various causes that produce earthquake sounds. A very slight rubbing or grinding together of rock surfaces may produce fairly loud noises, the volume of the sound being increased by transmission through the rock masses that lie in the path of the waves. An example of such an increase in the loudness of sounds is seen in the case of several of the large blocks of stone used for some of the piers of Kingston Harbor, in Ireland. When these rocks are moved together by blows of the waves they produce loud and appalling sounds, as if the whole island were being washed away. The same rocks, however, when left high and dry on the falling of the tide, can be caused to rub together, when moved by the hand. Under these circumstances, they produce but feeble sounds that can only be heard in their immediate neighborhood.

No doubt, some find it difficult to understand how it is possible for comparatively feeble sound-waves to be strengthened by their passage through large masses of solids. This is important and should be made clear. As everyone well knows, the ticking of a watch can only be heard at a short distance when the watch is held in the hand, because the sound-waves cannot readily pass through the body of the person holding the watch to the earth, the materials of the body not being sufficiently elastic. If, however, the watch be placed on the bare surface of a large wooden table from which the tablecloth has been removed, so that the watch can come directly in contact with the wood, and nothing else is placed on the table but the watch, the sound-waves are transmitted to the mass of the table and its entire surface sends them out into the air. The ticking of the watch can then be heard distinctly in almost any part of a large room.

Mallet states that in nearly all great earthquakes sounds are heard before the principal shock, and in his description of the Calabrian earthquake Hamilton says:

"All agreed that every shock seemed to come with a rumbling noise from the westward, beginning with the horizontal and ending with the vorticose (rotary) motion."

According to Dolomieu, during the Lisbon earthquake, the shocks were preceded "by a loud subterranean noise like thunder, which was renewed for every shock.... This great shock," he says, referring to one of the great upward shocks, "occurred without the prelude of any slight shocks, without any notice whatever as suddenly as the blowing up of a mine.... Some, however, pretend that a muffled interior noise was heard almost at the same moment."

The noises do not generally continue long after the earthquake shocks. In some cases, however, a very loud noise is heard at intervals for a considerable length of time after the principal shock. This was the case at Quito and Ibarra, in which a great noise was heard for from eighteen to twenty minutes after the principal shock. In a similar manner during the earthquake of October, 1746, at Lima, and Callao, South America, peals of underground thunder were heard at Truxillo for fifteen minutes after the principal shock. In such cases it seems probable that the noises were not caused by the same impulses that caused the original shock, but by the forces that caused the subsequent shock.

Humboldt relates that in 1784 there were noises heard at Guanajuato, from the 9th to the 12th of February. They were not, however, followed by an earthquake.

Humboldt also states that in an earthquake which occurred on the 30th of April, 1812, on the banks of the Orinoco River, in South America, a loud thundering noise was heard, without, however, any shock, but at this time a volcano on the island of St. Vincent, in the Lesser Antilles, although some 632 miles to the northeast, was pouring out streams of lava. Again in the great eruption of Cotopaxi, in 1734, underground noises were heard as if cannon were being fired. These sounds were distinctly heard at as great a distance as Honda on the banks of the Magdalena River. Now, bearing in mind that the crater of Cotopaxi is situated on the high plateau of Quito, in a region full of valleys and fissures, it would seem that for the sounds to have been sent through the 436 miles between the mountains and the valley of the Magdalena River, the waves must, for the greater part, have been transmitted through the solid earth at some considerable distance below the surface.

Mallet states that the underground noises which continued for more than a month from the midnight of January 9th, 1784, at Guanajuato, were not followed by any earthquake shocks, that it was if as thunder clouds occupied the space below the surface at that part of the earth and from these clouds there came the slow rolling sounds like short, quick, snaps of thunder.

Major Dutton in his book entitled "Earthquakes in the Light of the New Seismology" gives the following as the principal signs that herald the coming earthquake in the open country.

"The first sensation is the sound. It is wholly unlike anything we have ever heard before, unless we have already had a similar experience. It is a strange murmur. Some liken 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; others to distant thunder. 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 somewhat likes 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 it becomes impossible to do so. The victim flings himself to the ground to avoid being dashed to it, or he clings to a convenient sapling, or fence-post, to avoid being overthrown. The trees are seen to sway sometimes through large arcs, and are said, doubtless with exaggeration, to touch the ground with their branches, first on one side, then on the other. As the waves rush past, the ground on the crests opens in cracks which close again in the troughs. As they close, the squeezed-out air blows forth 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 the Eternal One, speaking out of the depths of the universe. Suddenly this storm subsides, the earth comes speedily to rest and all is over."

There are many other curious phenomena besides earthquake sounds or noises. Among some of the more interesting are the fire and smoke that are seen to come out of fissures that have been rent in the ground.

It is possible that in many cases these flashes of fire are in reality produced by electric discharges that momentarily light the clouds of dust thrown up out of the fissure. But sometimes true flames are seen escaping from the fissures. This was the case during the earthquake of Lisbon, in 1755, when fire burst through fissures at several places, burning with a lambent flame for some hours.

The clouds of dust that follow the rending of mountain masses by earthquakes are probably to be traced to the fracture of the rock masses, the dust so formed being violently thrown forth by the air squeezed out of the fissures, when they are suddenly closed. The violent compression of this air may raise this dust to incandescence.

Mallet asserts that in many cases the clouds of smoke observed do not consist of true smoke like that produced when wood or vegetable matters are incompletely burned, but is only ordinary air mixed with sulphurous acid gas, and various other gases.

But not only fire and smoke are seen at times coming out of fissures in the earth. A thing still more frequently thrown out is water, which often spouts forth along with great quantities of mud, sand, and the finely ground fragments of earthy materials generally. Among many other instances where the emission of water from the crevices was particularly noticeable, may be mentioned the earthquakes at Jamaica in 1687 and 1692. Here the water, in some places, was thrown out of the ground to considerable heights in the air.

Mallet calls attention to the fact that the waters of springs collect in reservoirs consisting either of fissures or crevices of the rocks, of small width but great depth, which are vertical or inclined to the horizon, or in reservoirs that are formed of extended beds of sand or gravel.

Now, when the earthquake waves moving horizontally over the surface produce movements that squeeze these fissures together, the water in the fissures is spurted out in high jets, and carries with it the finely divided rock or sand formed by the rubbing together of the rock surfaces. In the case of the reservoirs consisting of beds of sand or gravel, lying between impervious layers, if, during an earthquake motion, the land areas are suddenly lowered, the water rushing into the cavity thus left will afterwards be shot out with considerable force, when the land is suddenly raised again.

Where there are no direct openings in the ground the water will burst through the crust in the shape of great vertical jets, thus forming a circular hole, broken or fractured at its edges. Water jets of this character were especially numerous during the earthquake of Calabria in 1783. In a swampy plain, known as Rosarno, many of these circular wells or openings about the size of an ordinary carriage wheel, though in some cases much larger, were to be seen crowded together. The appearance of the openings are represented in Fig. 40.

Some of these were filled with water, but the greater number were dry and filled with loose sand. These latter, when examined by digging, were shown to be funnel-shaped, as seen in Fig. 41. As seen, the margins of the wells exhibit a series of cracks or crevices extending radially outward from the centre. Their origin is evident. As the water was violently expelled by the squeezing motion of the upper and lower impervious strata, it shot upwards, thus producing the funnel-shaped tube. At the same time the force of the eruption was sufficiently great to produce the radial fissures or fractures at the sides.

But greater fissures than these have been formed by earthquakes, especially those of the class created by a slipping of the earth's strata. In the case of an earthquake on the South Island of New Zealand, in 1848, a fissure having an average width of eighteen inches could be clearly seen extending in a direction parallel to the mountain chain for a distance of sixty miles, and during a later earthquake in the same region, in 1855, a fracture was formed that could be clearly traced for a distance of nearly ninety miles.

In some cases these fissures or fractured parts of the crust are left with one of their sides at a higher level than the opposite side. This was the case of the great Japanese earthquake of October 28th, 1891.

There are three kinds of waves produced by earthquakes; namely, the earthquake waves proper through the earth; the sound waves in the air, and great forced waves in the sea.

The sound waves of course reach the air from the point of origin below the earth's surface through the solid materials of the crust, and take on the curious varieties already described in connection with the sounds accompanying earthquakes.

We have already briefly described the manner in which the earthquake waves travel through the materials of the earth's crust. There remain to be discussed the great waves that are rolled up in the ocean during an earthquake shock. These waves are, perhaps, among the most destructive phenomena of great earthquakes. The following are only some of the more remarkable of such waves, and have been taken from Mallet's collection of earthquake data.

During some of the great earthquakes on the coasts of Chile and Peru, huge waves from the ocean did great damage when they reached the land. In the earthquake of 1590, ocean waves rushed for several leagues inland over the coast of Chile, carrying with them ships that were left high and dry as the wave receded. In the earthquake of 1687, Callao was inundated by a great wave from the Pacific Ocean, and ships were carried a full league into the country. During the earthquake of 1746, Callao was again swept away by a huge ocean wave. At later times earthquake waves have caused great damage to several other parts of the coast of South America.

Ocean waves of this character are formed by successive upward and downward movements at the bottom of the ocean, following each other at very brief intervals. Le Conte points out that the sudden upheaval of the bed of the ocean forms a huge mound in the surface of the water which results in a large wave that spreads rapidly in all directions. Waves produced in this manner sometimes reach a height of fifty to sixty feet. They are not readily observed in the deep ocean, but as soon as they reach the shallow waters near the shore they rush forward, forming waves from fifty to sixty feet in height and, rushing over the land, sweep everything before them.

During the great Lisbon earthquake of 1755 a huge wave started at a point fifty miles off the coast of Portugal. Half an hour after the earthquake was over several waves, the largest of which was sixty feet in height, rushed over a part of the city and greatly increased the ruin already wrought by the earthquake. According to Le Conte the great waves so formed moved in all directions across the Atlantic Ocean. They were thirty feet high when they reached Cadiz, eighteen feet in height at Madeira, and five feet on the coast of Ireland. They even crossed the Atlantic, being observed on the coasts of the West Indies.

A great ocean wave accompanied the Japanese earthquake in 1854. As in the case of the Lisbon earthquake this wave started in the bed of the ocean off the coast of Japan and only reached the island half an hour afterwards. It was thirty feet in height, and completely swept away the town of Simoda.

Owing to water's greater freedom of motion earthquake waves travel greater distances through the water than they do on land.

Of course, great earthquake shocks as a rule cause a very large loss of life. The following figures from Mallet give some idea of the extent of this loss, which is generally a matter of a few moments.

In the Lisbon earthquake, where the worst shock lasted a few seconds, 60,000 people were killed. During other earthquakes the losses have been as follows: 10,000 at Morocco; 40,000 in Calabria; 50,000 in Syria, and probably 120,000 in earthquakes that occurred in Syria in A. D. 19 and in A. D. 526.

But even these figures give only a meagre idea of the vast loss of life that has occurred during the past. It is said that during the reign of Justinian, earthquakes repeatedly shook the whole Roman world. The city of Constantinople was visited by earthquake shocks that continued at intervals for forty days. Deep chasms were opened in the earth and huge masses were thrown into the air. Enormous sea-waves were formed. At Antioch, during the earthquake of May 20th, A. D. 526, 250,000 people are believed to have been killed.

On the 31st of July, A. D. 365, in the second year of Valentinian, a dreadful earthquake shook the Roman world, and a great wave rolled in from the Mediterranean and swept two miles inland, carrying ships over the tops of houses. During this earthquake 50,000 people lost their lives at Alexandria.

In the earthquake of Messina in 1692, 74,000 people are said to have been killed; and, according to other accounts, 100,000. In the year A. D. 602, another earthquake at Antioch killed 60,000 people.

During the earthquake of Quito, in 1797, Humboldt estimates that 40,000 natives were either buried in crevices in the earth, under the ruins of buildings, or were drowned in lakes and ponds that were temporarily formed.

In this connection Mallet writes as follows:

"Such are the numbers to be met with in narratives, and if we suppose that there occurs one great earthquake in three years over the whole earth and that this involves the entombment of only 10,000 human beings, and that such has been the economy of our system for the last 4,000 years, we shall have a number representing above 13,000,000 men thus suddenly swallowed up, with countless bodies of animals of every lower class. Sir Charles Lyell then with good reason suggests that even in our own time we may yet find the remains of men and of their habitations and implements thus buried deep and embalmed, as it were, by earthquakes that occurred in the days of Moses and the Ptolemies."

Necessarily the progress of a great earthquake wave will produce great changes in the earth's surface features; for example, landslides, where immense layers of clay or other material slip or slide to a lower level and are thrown across the course of a river, causing its waters to be dammed up and then by spreading to form great lakes.

Sometimes, after vast bodies of water have been collected in this manner, disastrous floods result later from a sudden giving way of the barrier, and the loss thus caused is occasionally far greater than that directly due to the earthquake.

Permanent changes of level are frequently caused by earthquakes, as, for example, the coast of Chile during the earthquake of November 19th, 1822, where the coast for many miles was raised from three to four feet above its former plane.

In other cases the level of the ground is permanently lowered. This occurred in the Bengal earthquake in 1762, when an area of some sixty square miles suddenly sank, leaving only the tops of the higher points above water.

In some cases of changes in the level of the ground, large areas being raised in one place and lowered in another, rivers take new courses, and their old courses are completely obliterated.