G6S S. TABER SOME CRITERIA USED IX RECOGNIZING FAULTS 



Troughlike depressions and nndrained basins may be formed on slop- 

 ing surfaces by a warping or selective uplift of the lower side of a fault, 

 but in most cases the uplift is on the opposite side and therefore tends 

 to accentuate the slope. 



A careful study of the small fault troughs and other depressions found 

 along several active faults has led the writer to the conclusion that most 

 of them are due to the downthrow of narrow blocks or wedges within a 

 belt of parallel and interlacing fractures. The settling of the blocks is 

 believed to take place while the rocks are momentarily separated by the 

 passage of earthquake vibrations. The formation of these small troughs 

 and sags is, therefore, regarded as a result of earthquakes and not as a 

 cause, though it is probable that some aftershocks are due to minor dis- 

 placements of blocks which have not reached a position of complete 

 stability during the earthquakes. 



During the California earthquake of 1906 a fault fissure was opened 

 so wide as to admit a cow, which fell in head first and was thus entombed, 

 the closure leaving only the tail visible. At this point a trench was 

 formed 6 to 8 feet wide and 1 or 2 feet in depth. ^^ 



When a small fault trough is formed in rock that is buried under thick 

 deposits of unconsolidated sediments, the surface manifestations may be 

 no more than a shallow depression without definite boundaries. It was 

 largely by means of such depressions that the writer traced the Inglewood 

 fault of southern California.^* 



Ridges are sometimes left between two downfaulted blocks, but most 

 of the low ridges formed during the California earthquake of 1906 were 

 due to the breaking up of surface soil into a loose aggregate of irregular 

 clods or blocks, with a consequent high percentage of voids, and they 

 gradually disappeared as the material again became compacted. 



The hypothesis advocated above is not applicable to large fault troughs 

 having a width of several miles, which seem to be formed as a result of 

 repeated displacements, sometimes on one side and sometimes on the 

 other. The displacements which result in the formation and growth of 

 large fault troughs are, therefore, regarded as a cause of earthquakes and 

 not as effects. 



" G. K. Gilbert : "The earth movement on the fault of April 18, 1906 — Tomales Bay 

 to Bolinas Lagoon," in the California earthquake of April 18, 1906, vol. 1, pt. 1, 1908, 

 p. 72. 



" Stephen Taber : The Inglewood earthquake in southern California, June 21, 1920. 

 Bull. Seis. Soc. Amer., vol. 10, 1920, pp. 139-142. 



