A. An earthquake fault opened in Formosa in 1906, with vertical and lateral displacements combined (after Omori).
B. Earthquake faults opened in Alaska in 1889, on which vertical slices of the earth’s shell have undergone individual adjustments (after Tarr and Martin).
Fig. 60.—Diagram to show how small faults in the rock basement may be masked at the surface through adjustments within the loose rock mantle.
The greater number of earthquake faults are found in the loose rock cover which so generally mantles the firmer rock basement, and it is almost certain that the throws within the solid rock are considerably larger than those which are here measured at the surface, owing to the adjustments which so readily take place in the looser materials. Those lighter shocks of earthquake which are accompanied by no visible displacements at the surface do, however, in some instances affect in a measure the flow of water upon the surface, and thus indicate that small changes of surface level have occurred without breaks sufficiently sharp to be perceived ([Fig. 60]). Intermediate between the steep escarpment and the masked displacement just described is the so-called “mole-hill” effect,—a rounded and variously cracked slope or ridge above the position of a buried fault ([Fig. 61]).
Fig. 61.—Diagram to show the appearance of a “mole hill” above a buried earthquake fault (after Kotô).
The escarpments due to earthquake faults in loose materials at the earth’s surface can obviously retain their steepness for a few years or decades at the most; for because of their verticality they must gradually disappear in rounded slopes under the action of the elements. Smaller displacements within a rock which rapidly disintegrates under the action of frost and sun will likewise before long be effaced. In those exceptional instances where a resistant rock type has had all altered upper layers planed away until a fresh and hard surface is exposed, and has further been protected from the frost and sun beneath a thin layer of soil, its original surface may be retained unaltered for many centuries. Upon such a surface the lightest of sensible shocks, or even the smaller earth movements which are not perceived at the time, may leave an almost indelible record. Such records particularly show that the movements which they register occur upon the planes of jointing within the rock, and that these ready formed cracks have probably been the seats of repeated and cumulative adjustments ([Fig. 62]).
