Fig. 54.—Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R. Scidmore).
Fig. 55.—A fault of vertical displacement.
The grander and the lesser earth movements.—Upon the land the grander and so-called catastrophic earthquakes are usually the accompaniment of important changes in the surface of the ground that will be discussed in later sections. Those shocks which do little damage to structures produce no visible changes in the earth’s surface, except, it may be, to shake down some water-soaked masses of earth upon the steeper slopes. Still other movements, and these too slight to be felt even in the night when the animal world is at rest, may yet be distinguished by their sounds, the unmistakable rumblings which are characteristic alike of the heaviest and the lightest of earthquake shocks.
Fig. 56.—Escarpment produced by an earthquake fault of vertical displacement which cut across the Chedrang River and thus produced a waterfall, Assam earthquake of 1897 (after R. D. Oldham).
Changes in the earth’s surface during earthquakes—faults and fissures.—Each of the grander among historic earthquakes has been accompanied by noteworthy changes in the configuration of the earth’s surface within the district where the shocks were most intense. A section of the ground is usually found to have moved with reference to another upon the other side of a vertical plane which is usually to be seen; we have here to do with the actual making of a fault or displacement such as we find the fossil examples of within the rocks. The displacement, or throw, upon the fault plane may be either upward or downward or laterally in one direction or the other, or these movements may be combined. A movement of adjacent sections of the ground upward or downward with reference to each other ([Fig. 55]) has been often observed, notably at Midori after the great Japanese earthquake of 1891, and in the Chedrang valley of Assam after the earthquake of 1897 ([Fig. 56]).
Fig. 57.—A fault of lateral displacement.