Page:Popular Science Monthly Volume 69.djvu/108

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104
THE POPULAR SCIENCE MONTHLY.

vertical crack. The principal displacement was not vertical, but horizontal. If one thinks of the land to the east of the crack as stationary, then the change may be described as a northward movement of the land west of the crack. If the land to the west be thought of as stationary then the land to the eastward moved toward the south. It is probable that both tracts shared in the movement, the eastern shifting toward the south and the western toward the north. Perhaps the nature of the change can be more readily understood by reference to Fig. 4, which represents an ideal block of the earth's crust, 100 feet square on the surface and 25 feet deep, before and after its division and dislocation by the earthquake-causing fault.

Wherever a fence, road, row of trees, or other artificial feature following a straight line was intersected by the fault its separated parts were offset, and an opportunity thus afforded for measuring the amount of change. The measurements range in the main from 6 to 15

PSM V69 D108 Faulted road near tomales bay.png

Fig. 5. A Faulted Road near the Head of Tomales Bay. The nearer and more distant parts of the road were originally in one line—a continuous, straight road. The present offset is twenty feet.