664 S. TABER SOME CRITERIA USED IN RECOGNIZING FAULTS 



The displacement at that time extended for over 190 miles and was 

 chiefl}^ horizontal, amounting to a maximum of 21 feet. Vertical dis- 

 placement, where present, was small, and the low scarps faced west in 

 some places and east in others. 



Horizontal displacements along faults are scarcely mentioned in most 

 text-book.s, and that along the San Andreas fault in 1906 has been re- 

 garded as exceptional ; but I belicA^e that movements of this type have 

 been prevalent along the northern half of the fault since the Pleistocene, 

 and that such movements have probably played a very important part in 

 the development of the fault. The evidence bearing on this question is 

 outlined below. 



Along the northern half of the fault fresh scarps, other than those that 

 inclose the narrow troughs, are few and inconspicuous, and yet the 

 numerous troughs and sags furnish evidence of repeated displacements 

 in recent time. 



The fault-plane seems to be approximately vertical. It is true that 

 the fault, instead of being a single break, is a complex of j)arallel and 

 branching fractures, but the width of the zone of fracturing is seldom 

 more than a quarter of a mile, wliicli is small as compared with the 

 probable depth of the fault. The fault trace formed in 1906 was not 

 deflected where it crossed surfaces sloping in different directions. Al- 

 though large vertical displacements have resulted from past movements, 

 especially along the southern portion of the fault, there is no continuous 

 scarp on either side : a mountain block has been uplifted in one place to 

 form a scarp facing southwest, while in another the scarp faces north- 

 east. Between Tomales Bay and Bolinas Bay, Lawson found evidence 

 of large movements at different times in opposite directions, so far as 

 the vertical component was concerned.^ For long distances there is no 

 important scarp, and at a number of places the rocks exposed on both 

 sides of the fault are the same. The fault crosses from one side of the 

 Santa Cruz Mountains to the other, and in its long course it passes from 

 the west side of the Coast Ranges to the east side. 



Tangential thrust at right angles to the fault could not produce a 

 vertical fanlt-plane nor account for the alternating scarps. It is not 

 likely that the fault was the result of tension, for it maintains its long 

 course, without deflection, across different kinds of rock and earlier 

 structural features. It is not conceivable that a fault extending for such 

 a distance with only minor deviations could originate from purely ver- 



" A. C. Lawsou and others : The California earthquake of April 18, 190(5. Report of 

 the State Earthquake Investigation Commission, Carnegie lust, of Washington, Publica- 

 tion No. 87, vol. 1. pt. 1. 1908, p. 29. 



