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University of California Publications. 



[Geology 



shows that the total slip at the fault-plane, at the time of the 

 rupture, is exactly equal to the total relative displacement of the 

 blocks of wood; therefore, we must infer, since the total slip on 

 the San Andreas fault amounted to about 21 feet, that the shift 

 of the distant regions must have been as great ; but it was found 

 that between surveys II and III the shift was only 5.8 feet and 

 between I and II 4.6 feet ; that is, in all, only about 10.4 feet 

 since the earliest surveys, some fifty years before the shock. We 

 can therefore say definitely that the shift which set up the elastic 

 strains which finally resulted in the earthquake had already 

 accumulated to about half its final amount fifty years earlier; 

 that between surveys I and II it increased to about three-quarters 

 of its full amount, and that the last quarter was added between 

 surveys II and III. In the experiments the elastic rebounds of 

 the two sides Avere equal and in opposite directions. The sur- 

 face rocks on opposite sides of the fault are not identical as is 

 the jelly, but it is probable that at no great distance below the 

 surface they are similar, which would lead us to expect approxi- 

 mately equal rebounds on the two sides. We have no determi- 

 nations of the displacements just before the earthquake, and 

 therefore no measures of the actual rebounds, but Professor 

 Lawson has pointed out that if we assume the displacements to 

 have been continuous and uniform during the interval between 

 surveys I and II, and to have continued at the same rate up to 

 thi' time of the earthquake, then the region about the fault would 

 have been so far north at that time that the rebounds on the 

 two sides would have been practically equal, just as with the 

 jelly. It is hardly possible in view of the above history, of the 

 relations just mentioned and of the difficulty of imagining forces 

 capable of suddenly moving and stopping large areas, not to be 

 convinced that the shift accumulated gradually. 



To summarize, we may say. that for many years, perhaps 

 for a century, a slow relative movement to the north of the 

 region under the Pacific Ocean, just west of California and com- 

 prising a part of the coast, was taking place and setting up a 

 shearing strain in the coast region, which finally became too great 

 for the rock to endure; that a fracture occurred along an old 

 fault-line and that the two sides sprang back towards positions 



