PHYSIOGEAPHIC EVIDENCE OF RECENT FAULTING 663 



ous limestone, where the two are in contact, prove that the recent dis- 

 placement was chiefly vertical. The form of the scarps where they inter- 

 sect narrow spurs is likewise indicative of the absence of an appreciable 

 horizontal component parallel to the fanlt-plane. Variations in the 

 height of the scarps where they cross certain post-Pleistocene features, 

 such as alluvial fans, prove that in places, at least, the present height of 

 the scarjDS is the result of more than one displacement. While no severe 

 earthquakes seem to have occurred along the Wasatch fault since the 

 region was settled, the evidence of relative instability in the surrounding 

 region is such that the earthquake hazard must be rated higher than in 

 the Mississippi Valley, which, because of the New Madrid earthquake 

 and its aftershocks, has a less favorable seismic history. 



The faulting at the time of the Owens Valley earthquake of 1872 was 

 accompanied by the formation of numerous depressed strips. The fault- 

 scarps have been described by Gilbert" and Whitney,^ and they are well 

 illustrated in the majos and jDhotographs by Johnson published by Hobbs.^ 

 The main scarp followed the base of the alluvial footslope of the Sierra 

 Nevada for 10 miles, varying from 5 to 20 feet in height, and, where 

 highest, it was paralleled by an opposite-facing, ten-foot scarp wdth a 

 depressed strip between. Elsewhere the displacement was distributed 

 over a belt of parallel, overlapping and sometimes branching faults, be- 

 tween which the narrow strips were depressed from 2 to 10 feet. These 

 strips varied in width up to 200 or 300 feet, and some were several hun- 

 dred yards in length. One of the depressed tracts, several thousand acres 

 in extent, is said to have moved northward about 15 feet. Some of the 

 fault-scarps seem to have been formed in part at a somewhat earlier 

 period. 



The San Andreas fault, extending through California for upward of 

 600 miles, is marked by hundreds of small depressions or sags, often 

 holding water, and by long, narrow valleys. By means of these features 

 the fault may be readily traced through most of its course. It is, in fact, 

 characterized by valleys rather than by scarps. Some of the valleys are 

 known to be diastrophic in origin, while others, such as the through 

 valleys, are also probably due chiefly to faulting. Small trenchlike de- 

 pressions were formed along the fault during the earthquake of 1906 and 

 some of the preexisting and usually larger depressions were deepened. 



^G. K. Gilbert: A theory of the earthquakes of the Great Basin; with a practical 

 application. Am. .Jour. Sci., 3d ser., vol. 27, 1884, pp. 49-53. 



■'J. D. Whitney: The Owens Valley earthcjuake. Overland IMonthlv, vol. 0, 1872, pp. 

 130-140 and 266-278. 



^W. H. Hobbs : The earthquake of 1872 in the Owens Valley, California. Beitragen 

 zur Geophysik, Bd. 10, 1910, pp. 352-384. 



