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).
[Illustration: FIG. 57.—A fault of lateral displacement.]
FIG. 58.—Fence parted and displaced fifteen feet by a transverse fault formed during the California earthquake of 1906 (after W. B. Scott).]
FIG. 59.—Fault with vertical and lateral displacements combined.]
A lateral throw, unaccompanied by appreciable vertical displacement (Fig. 57), is especially well illustrated by the fault in California which was formed during the earthquake of 1906 (Fig. 58). A combination of the two types of displacement in one (Fig. 59) is exemplified by the Baishiko fault of Formosa at the place shown in plate 3 A.
=The measure of displacement.=—To afford some measure of the displacements which have been observed upon earthquake faults, it may be stated that the maximum vertical throw measured upon the fault in the Neo valley of Japan (1891) was 18 feet, in the Chedrang valley of Assam (1897) 35 feet, and of the Alaskan coast (1899) 47 feet. Large sections of land were bodily uplifted in these cases within the space of a few seconds, or at most a few minutes, by the amounts given. The largest recorded lateral displacement measured upon an earthquake fault is about 21 feet upon the California rift after the earthquake of 1906; though an amount only slightly less than this is indicated in the shifting of roads and arroyas dating from the earthquake of 1872 in the Owens valley, California. Fault lines once established are planes of special weakness and become later the seat of repeated movements of the same kind.