The Wonder Book of Volcanoes and Earthquakes

CHAPTER XXIII EARTHQUAKES

An earthquake is a shaking of the earth. It may vary in intensity from a shaking so feeble that it requires the use of a delicate instrument to detect it, to a shaking violent enough to overthrow heavy buildings, and even to make great rents or fissures in the crust.

An earthquake then is an earth-shake. It may be caused by anything capable of shaking the earth; for example, as the falling of a heavy weight on its surface. Now, a shaking so caused is only felt in the immediate neighborhood of the place the weight strikes the earth. On the contrary, in an earthquake, the shaking spreads in all directions through the earth's crust, until, in the case of very violent earthquakes, it reaches portions that may be situated many thousands of miles away from where the shock started. This spreading of the earthquake waves through the solid earth is not unlike the spreading of the circular waves that are set up in a still water surface when a stone is tossed in.

Any shaking of the earth's crust produces what may be called an earth-shake or earthquake. The mere falling of a raindrop on the earth produces a slight shaking. The falling of a heavy stone produces a stronger shaking, and sets up a series of minute waves, generally called vibrations, that spread around the place in all directions from where the stone struck. These movements, however, while they spread in all directions, just as they do in a surface of a lake, when a stone is thrown into it, are of course much more quickly stopped by the solid earth than similar movements are by the more readily movable water.

But, while any shaking of the earth's crust constitutes an earthquake, yet, strictly speaking, an earthquake is produced only by some force that acts suddenly on the earth, at a point below its surface, and, therefore, out of sight. This, of course, would rule out all such shakings as are caused by bodies striking the outer surface of the earth.

Earthquakes may occur in any part of the world, and at any time of the day or year. They do occur, however, most frequently in certain parts of the world, at certain seasons of the year and at certain hours of the day.

Earthquakes are far from being unusual occurrences. In some parts of the world, such as the island of Java, they are very common, and in Japan, under certain circumstances, scarcely a day passes without one or more shocks in some part of that little empire.

Professor Mallet, who has made a very extensive study of earthquakes, published in 1850 to 1858, in the Philosophical Transactions, brief abstracts or descriptions of all the more important earthquakes he could find records of during the past 3,456 years. The number of earthquakes thus recorded during this period reached 6,830. Of this great number nearly one-half occurred during the last fifty years.

It should not be inferred from the above figures that the number of earthquakes has really increased so greatly in the past half-century. The explanation of the apparent increase is that greater care has been taken recently in recording earthquakes, and that an apparatus called a seismometer, or earthquake-recorder, has been invented which automatically produces a record of the smallest shocks; so that a great many have been recorded that would otherwise have passed undetected.

It is the opinion of Le Conte that if the records of all the earthquakes of 3,456 years had been thus made there would have been found during the entire time of Mallet's researches to have occurred no less than 200,000, while during the last four years of Mallet's records, the number would have probably reached two earthquakes per week.

Since Mallet's time, Prof. Alexis Perry published (1876) a much larger list of earthquakes. Perry finds that from 1843 to 1872 there have been 17,249 earthquakes, or 575 every year. Perry's list, however, is incomplete, since it fails to record earthquakes that occurred in mid-ocean, and in the unexplored and uncivilized parts of the world. So it seems likely that earthquakes are so common that our earth, at some part or other of its surface, is continually shaking or quaking.

Earthquakes are such tremendous phenomena that they were necessarily observed by the ancients. We find more or less complete accounts of them in various writings. Lucretius (Titus Carus Lucretius, a great Roman poet) speaks as follows, in his De Rerum Natura (On the Nature of Things). We use Munro's translation here:

"Now mark and learn what the law of earthquakes is. And first of all take for granted that the earth below us as well as above is filled in all parts with windy caverns, and bears within its bosom many lakes and many chasms, cliffs and craggy rocks; and you must suppose that many rivers hidden beneath the crust of the earth roll on with violent waves and submerged stones; for the very nature of the case requires it to be throughout like to itself. With such things then attached and placed below, the earth quakes above from the shock of great falling masses, when underneath, time has undermined vast caverns. Whole mountains, indeed, fall in, and in an instant from the mighty shock tremblings spread themselves far and wide from that centre. And with good cause, since buildings beside a road tremble throughout, when shaken by a wagon of not such very great weight; and they rock no less, where any sharp pebble on the road jolts up the iron tires of the wheels on both sides. Sometimes, too, when an enormous mass of soil through age rolls down from the land into great and extensive pools of water, the earth rocks and sways with the undulation of the water just as a vessel at times cannot rest, until the liquid within has ceased to sway about in unsteady undulations....

"The same great quaking likewise arises from this cause, when on a sudden the wind and some enormous force of air gathering either from without or within the earth have flung themselves into the hollow of the earth and there chafe at first with much uproar among the great caverns and are carried on with a whirling motion, and when their force, afterwards stirred and lashed into fury, bursts abroad and at the same moment cleaves the deep earth and opens up a great yawning chasm. This fell out in Syrian Sidon and took place at Ægium in the Peloponnese, two towns which an outbreak of wind of this sort and the ensuing earthquake threw down. And many walled places besides fell down by great commotions on land and many towns sank down engulfed in the sea together with their burghers. And if they do not break out, still the impetuous fury of the air and the fierce violence of the wind spread over the numerous passages of the earth like a shivering-fit and thereby cause a trembling."

Of course, no one at the present time believes this ridiculous explanation as to the cause of earthquakes.

Aristotle, a Greek philosopher, speaks thus concerning earthquakes. We quote the translation employed by Mallet:

"Three theories on the subject have been handed down to us by three different persons; namely, Anaxagoras of Klazomene, before him Anaximenes the Milesian, and later than these Democritus of Abdera.

"Anaxagoras says that the ether of nature rises upward, but that when it falls into hollow places in the lower parts of the earth it moves it (the earth); because the parts above are cemented or closed up by rain, all parts being by nature equally spongy or full of cavities, both those which are above (where we live) and those which are below. Of this opinion it may perhaps be unnecessary to say anything, as being foolish, for it is absurd to suppose that things would thus exist above and beneath, and that the parts of bodies which have weight would not on every side be borne to the earth, and those which are light, and fiery, rise; especially since we see the surface of the earth to be convex and spherical, the horizon constantly changing as we change our place, at least as far as we know. And it is also foolish to assert on the one hand that it remains in the air on account of its great size, and on the other to say that it is shaken, when struck from beneath upwards. And besides these objections, it is to be remarked that he has not treated of the attendant circumstances of earthquakes, for neither every time nor place is subject to these convulsions.

"But Democritus says, that the earth being full of water, and receiving much also by means of rain, is moved by this. For when the water increases in bulk, because the cavities cannot contain it, in its struggles it causes an earthquake. And when the earth becomes partially dried up, the water being drawn from the full reservoirs into those which are empty, in passing from one to the other, by its movements it causes an earthquake also.

"Anaximenes, however, says that the earth, when parched up and again moistened, cracks, and by the masses thus broken off falling on it, is shaken; wherefore earthquakes occur in drouths and again in times of rain; in drouths, because, as we have said, it cracks, when highly dried, and then, when moistened over again, it cracks and falls to pieces. Were this the case, however, the earth ought to appear in many places subsiding. Why then is it that hitherto many places have been very subject to these convulsions which do not present any such remarkable differences from others? Yet such ought to be the case. And, moreover, those who think thus must assert that earthquakes constantly become less and less, and at last cease altogether. For the continual condensation of the earth would cause this. Wherefore, if this be not the fact, it is plain that this is not the correct explanation."

Besides the above, there are numerous references to earthquakes in the works of other writers. Thales, Seneca, and Pliny all speak of these phenomena and appear to describe correctly the movement of the earth in waves both in the solid land, as well as on the sea.

Coming down to less ancient writers, Mallet refers to a book by Fromondi, published in Antwerp, in 1527, that contains much valuable and interesting information. Among other things Fromondi declares that in the year 369, in the reign of Valentinian, there was a great earthquake that shook nearly the entire world and that another earthquake of almost equal severity occurred in 1116. He also states that in 1601 an earthquake continued for nearly forty days; that a great earthquake in Italy, in 1538, lasted fifteen days, and that another, in Spain, lasted for nearly three years.

This does not mean that these earthquakes actually continued to shake the earth violently for the times mentioned. These are only the times during which, at intervals of greater or less length, successive shocks were felt in these localities.

Another of the less ancient writers referred to by Mallet is Travagini, who published a book in Venice in 1683. This book contains a description of a terrible earthquake occurring in Italy on the 6th of April, 1667, which affected large portions of the country adjacent to Ragusa.

Without attempting at present to discuss the various theories of earthquakes, it will suffice to say that earthquakes can be divided, according to their origin, into two classes: volcanic earthquakes, or earthquakes that are caused by practically the same forces that cause volcanoes, and tectonic[3] earthquakes, or those produced by the slipping of a large mass of rock lying along the lines of old or new fractures.

Earthquakes of the first class are found especially in volcanic districts, while those of the second class are found in all parts of the world, whether in volcanic districts or elsewhere. According to Dana, earthquakes of the second class generally start in the neighborhood of mountains, where old lines of fractures are especially abundant.

As regards the direction of the shaking movements of the earth, earthquakes can be divided into three different classes: explosive earthquakes, or those in which the force acts vertically upwards; horizontal earthquakes, or those in which the force moves in a more or less horizontal direction, or parallel to the general surface of the earth, and rotary earthquakes, or those in which the earth rotates or moves in great eddies or whirls.

When the earthquake wave is started below the earth's surface, it spreads through the crust in all directions. The direction these waves will have on emerging, or coming out of the surface, will depend on the distance of this point from the place the waves started. When a place is situated directly over where the wave started, the waves will emerge so as to move vertically upwards, so that the earth at this point will be shaken by an explosive earthquake. As the point where the waves pass out is situated further and further from the place where the waves start, the waves will emerge more nearly horizontally, the greater the distance from the source.

In explosive earthquakes, which, as just explained, occur at areas almost immediately above the point where the disturbance starts, the force is, generally speaking, the greatest. In earthquakes of this character the force is sometimes sufficiently great to throw large bodies high up into the air. In the case of the great Riobamba earthquake of 1797, the force was not only sufficiently great to fracture the earth in various places, but also to throw bodies lying on the surface great distances into the air. Bodies of men were thrown several hundred feet into the air and were afterwards found on the other side of a broad river or high up on the side of a hill.

It is possible that Humboldt did not inquire with as much care as he should have done into these reports. They were probably greatly exaggerated, since it is difficult to understand how a force great as this would have failed to detach the soil at these places, and hurl it after the people. This much, however, can be accepted, that the upward force was very great.

In the great Calabria earthquake of March, 1783, Dolomieu states that the tops of the granite hills of Calabria were distinctly seen to rise and fall. In some cases houses were suddenly raised a great distance in the air, and were afterwards brought down again to a position of rest, at a higher level without any damage occurring to them. In a similar manner during the Caracas earthquake of March, 1812, the ground was seen to rise and fall in a nearly vertical direction. But, perhaps, one of the most terrible earthquakes of this character was the earthquake that destroyed the greater part of Jamaica in June, 1793. During this earthquake the entire surface of the ground at Port Royal assumed the appearance of a rolling sea. Houses were shifted from their old sites. Many of the inhabitants who had succeeded in escaping from the city to the neighboring country were thrown great distances into the air. Some of these, by good fortune, fell into the harbor, from which, in some cases, they escaped with their lives. Here again the projectile force was probably greatly exaggerated.

Vertical movements characterized the great earthquake of Lisbon, on November 1st, 1755, the city appearing to have been not far from the point of origin.

The commonest type of earthquakes is the horizontal, where the waves emerge at the surface in a direction either horizontal or parallel to the general surface, or at least inclined to it at a very small angle. Where the materials of the earth's crust, through which the waves spread, are of the same kind and of the same density in all directions, the area shaken is approximately circular, but where the materials of the crust are more or less dense in some directions than in others, the area of disturbance is of course oblong or elliptical.

In some cases earthquakes of the horizontal type are limited almost entirely to a single direction. This is especially the case with earthquakes that occur in mountainous districts. These earthquakes are known as linear earthquakes, since they spread almost in a single line.

When earthquake waves pass from one medium to another, that is, from one kind of rock to another, the greater portion of the waves is refracted or bent out of their straight direction as they pass into the new medium; a part of the waves, however, are reflected. It is these reflected waves that probably cause rotary earthquakes.

The speed with which the surface waves move outwards in all directions, varies not only with the force of the wave, but also with the kind of material through which they pass. This velocity may be in the neighborhood of twenty miles per second, while in others the velocity is as great as 140 miles per second.

Naturally, one would suppose that the most severe earthquakes are those in which the waves move the most rapidly. On the contrary, however, the comparatively feeble shocks are sent through the earth with greater velocity.

In rotary earthquakes, as the name indicates, the ground is whirled or twisted in the manner of a violent eddy, and is often left in this twisted condition. In the great Calabria earthquake, huge blocks of stone forming obelisks were twisted on one another in a manner represented in Fig. 39. In this case the pedestals remained unaffected, but the separate blocks of stone were partially turned around, as shown. During this earthquake the earth was so twisted that trees, which had been planted in straight lines before the earthquake, were left standing in zigzags. During the great Charleston earthquake, South Carolina, the chimney-tops of the houses were separated at places where they joined the roof and were twisted around these places without being overthrown. In some of the houses wardrobes or bureaus were turned at right angles to their former positions, and in some cases were even found with their faces turned towards the wall.

Fig. 39. Heavy Stone Obelisks Twisted by Calabrian Earthquake of 1783

Mallet suggests that in some cases the rotary motion is more apparent than real, being due only to a to-and-fro motion without any twisting, the apparent turning being due to the greater freedom of motion of the object in one direction than in another. A twisting motion, however, has actually taken place in some earthquakes.

While separate shocks, in a given locality, may follow one another at intervals for fairly long times, yet the principal shock or shake that produces the greatest damage is generally of exceedingly short duration. In the Caracas earthquake the greatest destruction was accomplished in about one minute. There were three distinct shocks, each of which lasted but three or four seconds. The great Calabria earthquake, of 1783, lasted but two minutes. The earthquake of Lisbon, in 1755, lasted five minutes, but the first, and worst, shock, was only from five to six seconds.

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