Earthquakes and their Habits in California

Abstract

A certain reluctance in earlier years to discuss earthquakes publicly in California has given way in large measure to the conviction that the best way to combat an alleged evil is to know all about it and to be prepared for it. Earthquakes are of course vibrations of the ground. The movement is complicated in nature. The ground at a given place may move horizontally back and forth, or vertically up and down, or oscillate in any oblique direction between the two, and the direction of movement may change suddenly. Moreover, the ground may tilt first in one direction, and then immediately in another, as surface earthquake waves pass underneath the spot. Appalling as these motions are to the senses, they are in reality but small movements of the ground. The backward and forward motion is probably never more than a few inches, except in alluvial filled areas where it may perhaps attain several feet, and in the great majority of rather severe earthquakes the solid ground or rock appears to have oscillated less than an inch. But in such shocks the vibration is repeated many times, continuing sometimes for a minute or two, and the force is irresistibly great; buildings may begin to sway more and more violently and finally rack to pieces, and the phenomenon may become quite terrifying.

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