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University of California Publications. 



[Geology 



by half the relative slip. If we take the length of the fault at 

 270 miles, its depth at twelve and a half miles, and the average 

 slip at thirteen feet, we find that the work done was 130,000,- 

 000.000.000,000 foot-pounds. It is not surprising that the liber- 

 ation within a few seconds of this enormous amount of energy 

 should be followed by such great destruction in the megaseismic 

 district, and that seismographs in all parts of the world should 

 record the disturbance. 



Energy is never created ; it merely changes its form. Whence, 

 then, came the energy of elastic strain which existed in the great 

 rock-spring before the earthquake? We have ascribed it to the 

 slow movements of the crust and of the underlying material ; 

 but the question still persists, Whence came the energy of these 

 slow movements? It may have been gravitational, as we shall 

 see a little later when we consider general suggestions that have 

 been advanced to account for earthquakes, but we cannot answer 

 with confidence. If we could follow this energy, step by step, 

 back into the infinite past, we could solve the riddle of the physi- 

 cal universe. 



Let us now turn our attention to the manner in which rock 

 breaks along the fault-line. 



Since the fracture results from elastic strains due to slow 

 displacements of neighboring regions, it is practically certain, 

 on account of irregularities in the displacement and in the 

 character of the rock, that the stresses will not reach the limit- 

 ing strength of the rock over the whole area of the fault, or 

 even over a large fraction of it at once, but will begin in a very 

 limited area, and extend from there at a rate not greater than 

 the velocity of compressional elastic waves in the rock. When 

 the rupture occurs at a point, or a small area, the elastic stresses 

 are no longer supported there, and are therefore transferred to 

 neighboring points, which in turn give way, and thus the 

 rupture spreads along the fault-surface until the elastic strains 

 are so reduced that they can carry it no further. 



But the transfer of stress is not instantaneous; time is 

 required for the successive parts of the rock to be sufficiently 

 displaced to bring the strain in their neighborhood to the actual 

 breaking limit, and the amount of time necessary to accomplish 



