This quake was felt over a wide area, and, but for the sparse settlement of the country, would probably have caused great havoc. Many people think of a severe earthquake as consisting of a violent heaving or lurching of the earth's surface, but such movements, when they occur, are only incidental. The essential feature of an earthquake is a series of rapid vibrations in the elastic rocks of the earth's crust, caused by a sudden jar, which occurs, as a rule, some miles underground.
The jar is due to the breaking or slipping of the rocks under heavy strains. There are several possible causes for such strains, and there has been much controversy among geologists as to which of them predominate. One quite obvious cause is the transfer of material from place to place on land and from land to sea by the process of erosion and deposition. The point in the depths of the earth where the quake originates is called the "focus," and the spot or area on the surface of the ground directly over the focus is called the "epicenter."
The latter is also the place where the strongest shocks are felt. A typical earthquake begins with a low rumbling sound, which gradually grows louder, and is sometimes mingled with heavy, explosive crashes. The first tremors are felt just after the sounds begin. They are like those experienced during the passage of a heavy train or wagon, and occur at the rate of four or five a second. These preliminary tremors merge into the main shocks of the earthquake, which occur at a rate of only two or three a second; sometimes each lasts more than a second. Then comes another series of less violent and more rapid tremors, fading into a faint quivering motion. The entire duration of these events ranges from a few seconds to three or four minutes.
The Charleston earthquake of 1925 lasted seventy seconds; the California earthquake of 1906, three and a half minutes, which was long enough! The principal earthquake is often preceded, at intervals of hours or days, by lighter "fore-shocks," and every severe earthquake is followed by a large number of "after-shocks." In the quake that destroyed Tokyo and other Japanese cities on September 1, 1923-the worst disaster of its kind in history-more than a thousand after-shocks were felt during the five days following the main quake. Sometimes the after-shocks continue for years.
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