The block movements of the disturbed district.—The displacements upon earthquake faults are thus seen to be subdivided into sections, each of which differs from its neighbors upon either side and is sharply separated from them, at least in many instances. These points of abrupt change of displacement are, in many cases at least, the intersection points with transverse faults ([Fig. 69]). Such points of abrupt change in the degree or in the direction of the displacement may be, when looked at from above, abrupt turning points in the direction of extension of the fault, whose course upon the map appears as a zigzag line made up of straight sections connected by sharp elbows ([Fig. 70]).
Fig. 70.—Map of the faults within an area of the Owens valley, California, formed in part during the earthquake of 1872, and in part due to early disturbances, In the western portions the displacements cut across firm rock and alluvial deposits alike without deviation of direction (after a map by W. D. Johnson).
Such a grouping of surface faults as are represented upon the map is evidence that the area of the earth’s shell, which is included, has at the time of the earthquake been subject to adjustments as a series of separate units or blocks, certain of the boundaries of which are the fault lines represented. The changes in displacement measured upon the larger faults make it clear that the observed faults can represent but a fraction of the total number of lines of displacement, the others being masked by variations in the compactness of the loose mantling deposits. Could we but have this mantle removed, we should doubtless find a rock floor separated into parts like an ancient Pompeiian pavement, the individual blocks in which have been thrown, some upward and some downward, by varying amounts. Less than a hundred miles away to the eastward from the Owens Valley, a portion of this pavement has been uncovered in the extensive operations of the Tonapah Mining District, so that there we may study in all its detail the elaborate pattern of earth marquetry ([Fig. 71]) which for the floor of the Owens valley is as yet denied us.
Fig. 71.—Marquetry of the rock floor of the Tonapah Mining District, Nevada (after Spurr).
Fig. 72.—Map of a portion of the Alaskan coast to show the adjustments in level during the earthquake of 1899 (after Tarr and Martin).
The earth blocks adjusted during the Alaskan earthquake of 1899.—For a study of the adjustments which take place between neighboring earth blocks during a great earthquake, the recent Alaskan disturbance has offered the advantage that the most affected district was upon the seacoast, where changes of level could be referred to the datum of the sea’s surface. Here a great island and large sections of the neighboring shore underwent movements both as a whole in large blocks and in adjustments of their subordinate parts among themselves ([Fig. 72]). Some sections of the coast were here elevated by as much as 47 feet, while neighboring sections were uplifted by smaller amounts ([Fig. 73]), and certain smaller sections were even dropped below the level of the sea.
