Earthquake

“ It was hard to tell which frightened one the most, the terrible rain-storms or the awful earthquakes. In the house there was a magnificent glass chandelier. The first time we had a severe earthquake that chandelier swayed back and forth in such a wild way that it seemed as if it must fall and crush every prism, tiny light, and bell. I felt sure whenever a quake began that I should not live through it. ”

“ No one can tell what harm an earthquake will, or will not, do. There are three kinds. One which raises the ground up perpendicularly, and sets it down again—which is the least hurtful; one which sets it rolling in waves, like the waves of the sea—which is more hurtful; and one, the most terrible of all, which gives the ground a spinning motion, so that things thrown down by it fall twisted from right to left, or left to right. But what kind of earthquake will take place, no one can tell. ”

“ On the morning of the 14th of January, 1824, we experienced a shock of the most unpleasant kind, which lasted about six seconds: the motion was perpendicular, not horizontal, and the various noises by which it was accompanied, the cracking of the doors, the rattling of the windows, and the melancholy howling of the dogs, who are usually the first to feel and to announce the approach of an earthquake, were well calculated to alarm even the least timid. ”

“ Again, under the name of earthquake we include two very different sets of phenomena, the one the rock-rift or fault, which is the disturbance itself, the other the spreading or interfering waves set in motion by the parting, shearing and grinding of the sundered walls of rocks in the earthquake fault. It is the jarring waves extending in widening and interfering circles which do the mischief to man and his affairs. It is the rift of rock which sends these waves forth on their blind mission of confusion or destruction. ”

“ To feel the ground shaking under the feet during an earthquake is far from agreeable, but it is a mere pleasant excitement compared to the feelings a person experiences, when he knows that at any moment he may be lifted off his legs and blown up into the sky in company with some dozen wagon-loads of stones and earth, and bricks and mortar, and beams and rubbish of all descriptions. ”

“ Hitherto no one appears to have formed any clear notion as to what an Earthquake is—that is to say, any clear idea of what is the nature of the movement constituting the shock, no matter what may be the nature or origin of the movement itself. ”

“ At half-past two o’clock of a moonlit morning in March, I was awakened by a tremendous earthquake, and though I had never before enjoyed a storm of this sort, the strange thrilling motion could not be mistaken, and I ran out of my cabin, both glad and frightened, shouting, “A noble earthquake! A noble earthquake!” feeling sure I was going to learn something. ”

“ The earthquake continued at intervals for a full year, the first five months in its original force, the remainder of the period with less violence. Sometimes the motion of the earth was like the pitching of a large vessel dragging heavily at its anchors ”

“ The occurrence of earthquakes establishes the unmistakable fact that there are forces at work from within disturbing the surface, while land-slides, and even little gullies washed out by the rain, show that other forces are working from without.
The vibration or trembling of the earth which we call an "earthquake" always arouses alarm, and frequently occasions great destruction and loss of life. Only a few of the various causes that may bring about earthquakes are as yet fully understood. ”

“ The shaking may be so violent that buildings may not be able to withstand it. Such is the phenomenon of an earthquake.
“When the earth is shaken by one of those occasional adjustments of the crust which I have described, the wave that spreads like a pulsation from the centre of agitation extends all over our globe and is transmitted right through it. At the surface lying immediately over the centre of disturbance there will be a violent shock. In the surrounding country, and often over great distances, the earthquake may also be powerful enough to produce destructive effects. ”

“ An earthquake is caused by a sudden displacement of the material which composes the earth's interior. The displacement gives rise to series of waves, which are propagated outwards in all directions, and which, [3] when they reach the surface, produce the sensations known to us as those of an earthquake.
The region within which the displacement occurs is sometimes called the hypocentre, but more frequently the seismic focus or simply the focus. ”

“ As the earthquake is so intimately associated with the volcano the causes of the latter are in great measure the causes of the former, and the forces at work frequently produce a more or less violent quaking of the earth’s surface before they succeed in opening a channel of escape through the mountain’s heart. One agency of great potency, and one whose work never ceases, has doubtless much to do with earthquake action. ”

“ We had laid down by our fire, and about ten o clock there came a most terrible earthquake, which shook the earth so that we rocked about like we had been in a cradle. We were very much alarmed; for though we were accustomed to feel earthquakes, we were now right in the region which had been torn to pieces by them in 1812, and we thought it might take a notion and swallow us up, like the big fish did Jonah. ”

“ An earthquake is ‘the transit of a wave or waves of elastic compression in any direction from vertically upwards to horizontally, in any azimuth, through the crust and surface of the earth, from any centre of impulse or from more than one, and which may be attended with sound and tidal waves, dependent upon the impulse and upon circumstances of position as to sea and land.’
In brief, so far as motion in the earth is concerned. Mallet defined an earthquake as being a motion due to the transit of waves of elastic compression. ”

“ Every half hour some considerable area of the earth’s surface is sensibly shaken by an earthquake, but earthquakes are by no means uniformly distributed over the globe. As we might infer from what we know as to their causes, earthquakes are most frequent in regions now undergoing deformation. Such are young rising mountain ranges, fault lines where readjustments recur from time to time, and the slopes of suboceanic depressions whose steepness suggests that subsidence may there be in progress. ”

“ An earthquake is transmitted from the focus through the elastic rocks of the crust, as a wave, or series of waves, of compression and rarefaction, much as a sound wave is transmitted through the elastic medium of the air. Each earth particle vibrates with exceeding swiftness, but over a very short path. The swing of a particle in firm rock seldom exceeds one tenth of an inch in ordinary earthquakes, and when it reaches one half an inch and an inch, the movement becomes dangerous and destructive. ”

“ The district over which an earthquake is perceptible to human beings without instrumental aid is its disturbed area. In like manner, that over which the earthquake-sound is heard is the sound-area.
A great earthquake never occurs alone. It is merely the most prominent member of a group of [4] shocks of greater or less intensity, and is known as the principal shock or earthquake, while the others are called minor or accessory shocks, and fore-shocks or after-shocks according as they occur before or after the principal earthquake. ”

“ 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. ”

“ Admitting that an earthquake, as it radiates from its centre, may act in such a manner, we see that a feeble disturbance might be the ultimate cause in the production of a destructive earthquake, just as the disturbance of a stone upon the face of a scarp might, by its impact upon other stones, cause many tons of material to be dislodged. ”

“ Almost every feature of the great earthquake points to an origin very different from that of the others described in this volume. The suddenness with which the shock began, its unusual duration, and the occurrence of many maxima of intensity, are inconsistent with a simple fault-displacement. ”

“ 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. ”

“ The gentleman to whom I have alluded, Don Tomas de la G——, describes the subterranean noise at Cua during the earthquake as something terrific, like the discharge of hundreds of cannon, while the earth rose simultaneously under his feet. There are two kinds of earthquakes—that of trepidacion, which comes directly from below, with an upward motion; the other, de oscilacion, where the earth sways to and fro like a pendulum, and which is generally less dangerous. ”

“ Last night, a few minutes before eleven o’clock, a shock of an earthquake was most distinctly felt here, which had the effect of seriously alarming many families; and many who were in bed were awakened by the concussion, Those who had newly retired to rest were most sensible of the shock, as they felt their beds move, first in a horizontal direction, and then back to the former situation ”

“ 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. ”

“ An earthquake is the vibration, sometimes violent, of the Earth’s surface that follows a release of energy in the Earth’s crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or even by manmade explosions. Most destructive quakes, however, are caused by dislocations of the crust. ”

“ Before concluding this chapter it will be well to define a few of the more ordinary terms which are used in describing earthquake phenomena. It may be observed that the English word earthquake, the German erdbeben, the French tremblement de terre, the Spanish terremoto, the Japanese jishin &c., all mean, when literally translated, earth-shaking, and are popularly understood to mean a sudden and more or less violent disturbance. ”

“ Without a moment’s warning, a great earthquake came, as fearful as the shock four hundred and eleven years before. The destruction was vastly greater. The tottering walls crushed thousands in the crowded{504} streets. Every avenue and alley became a death-trap. There is not, in all the pages of history, record of an earthquake of greater destructiveness. ”

“ Nearly all observers placed under favourable conditions agree that an earthquake begins with a deep rumbling sound, [329] accompanied, after the first second or two, by a faint tremor which gradually, and sometimes rapidly, increases in strength until it merges into the shock proper, which consists of several or many vibrations of larger amplitude and longer period, and during which the attendant sound is generally at its loudest; the earthquake dying away, as it began, with tremors and a low rumbling sound. ”

“ With this sketch of the mechanism of earthquake shocks we now turn to consider their effects upon the surface of the earth. From a geological point of view, the most important effect of earthquake shocks is found in the movement of rock masses down steep slopes, which is induced by the shaking. ”

“ I could not but see, that it was no less than a demonstration against the old notion of their cause.
6ly, Earthquakes are truly most, violent, in a rocky country: because the shock is proportionate to the solidity of the matter electrify’d. So that rocks, cliffs, quarries, old castle walls, and strong buildings, are most obnoxious to the concussion. The isle of Cerigo was more liable, and more rudely handled by the late earthquake; both because it was an isle, and because it was rocky. So we must say of the late earthquake in Switzerland, that split the mountain, and the old castle wall. ”

“ The transmission of an earthquake shock through the earth takes place with wonderful rapidity. The elastic wave varies in velocity from 800 to 1,000 feet per second in sand or clay to three miles per second in solid granite. ”

“ CHAPTER X EARTHQUAKES Any sudden movement of the rocks of the crust, as when they tear apart when a fissure is formed or extended, or slip from time to time along a growing fault, produces a jar called an earthquake, which spreads in all directions from the place of disturbance. ”

“ We now pass on to consider those more violent movements of the solid land known as earthquakes. This kind of disturbance is such as might be produced by a sudden shock or blow given below the ground, from which waves travel in all directions. First comes a rumbling noise like the roar of distant artillery; then come the earthquake waves one after another, causing the ground to rise and fall as a ship does on the waves of the sea; the ground 197 is frequently rent asunder, so that chasms are formed, into which in some cases men and animals have been hurled alive. ”

“ For example, let it be assumed that the first impulse of an earthquake is due to the sudden formation of a fissure, rent open from its centre, and that the waves leave the walls at all points normal to its surface. ”