EARTHQUAKES AND SEAQUAKES 



73 



It's- 



ment may be masked at the 



surface through adjustments 



within the loose rock mantle. 



at most a few minutes, by the amounts given. The largest re- 

 corded lateral displacement measured upon an earthquake fault 

 is about 21 feet upon the California 

 rift after the earthquake of 1906; 

 though an amount only slightly less 

 than this is indicated in the shifting 

 of roads and arroyas dating from the 

 earthquake of 1872 in the Owens valley, 

 California. Fault lines once established 

 are planes of special weakness and 

 become later the seat of repeated 

 movements of the same kind. 



The greater number of earthquake 

 faults are found in the loose rock cover 

 which so generally mantles the firmer FIG. 60. - Diagram to show how 



.... . small faults in the rock base- 



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 dis- 

 placements at the surface do, 

 however, in some instances affect 

 in a measure the flow of water 

 upon the surface, and thus indi- 

 cate that small changes of sur- 

 face level have occurred without 

 breaks sufficiently sharp to be 

 perceived (Fig. 60). Intermedi- 

 ate between the steep escarpment 

 and the masked displacement 



just described is the so-called 

 " mole-hill" effect, a rounded 

 FIG. 61. Diagram to show the appear- and variously cracked slope or 



ance of a "mole hill" above a buried ridge above the position of a 

 earthquake fault (after Kot6). buried ^^ ^ ^ 



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 



