FIG. 53.—Map showing the localities at which shocks have been reported at sea off Cape Mendocino, California.]
Far grander disturbances upon the floor of the ocean have been revealed by the great sea waves—the so-called “tidal waves”, properly referred to as _tsunamis_—which recur in those sea districts which adjoin the special earthquake zones upon the continents (p. 86). The forerunner of such a sea wave approaching the shore is usually a sudden withdrawal of the water so as to lay bare a portion of the bottom, but this is well-recognized to be the premonition of a gigantic oncoming wave which sweeps all before it and is only halted when it has rolled over all the low-lying country and encountered a mountain wall. Such seismic waves have been especially common upon the Pacific shore of South America and upon the Japanese littoral (Fig. 54). These waves proceed from above the great deeps upon the ocean bottom, and clearly result from the grander earth movements to which these depressions owe their exceptional depth. The withdrawal of the water from neighboring shores may be presumed to be connected with a descent of the floor of the depression and the consequent drawing-in of the ocean surface above. The later high wave would thus represent the dispersion of the mountain of water which is raised by the meeting of the waters from the different sides of the depression.
[Illustration: FIG. 54.—Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R. Scidmore).]
[Illustration: 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.