436 



University of California Publications. 



[Geology 



move as a whole; but even if they did, vibrations would not 

 originate in their volume any more than vibrations would 

 originate in the volume of any other body falling under gravity ; 

 for vibrations are started, not by simple velocity, or acceleration, 

 but by the differential velocity in contiguous elements. Friction 

 starts vibrations by causing rapid changes of velocity at the 

 surface of the slipping mass. If a block were suddenly started, 

 or stopped by elastic forces, the vibrations must start from the 

 boundary, where alone the forces could be applied. 



We may now sum up our general results. The observations 

 of the California earthquake and the deductions drawn from 

 them based as they are upon the elastic properties of rock and 

 upon the well-known relative movements of different parts of 

 the earth's crust, have led to certain general conceptions of the 

 mass movements which take place before and at the time of 

 tectonic earthquakes, which may be expressed as follows : 



1. The fracture of the rock, which causes a tectonic earth- 

 quake, is the result of elastic strains, greater than the strength 

 of the rock can withstand, produced by the relative displacements 

 of neighboring portions of the earth,' 's crust. 



2. These relative displacements are not produced suddenly at 

 the time of the fracture, but attain their maximum amounts 

 gradually during a more or less long period of time. 



3. The only mass movements that occur at the time of the 

 earthquake are the sudden elastic rebounds of the sides of the 

 fracture towards positions of no elastic strain; and these move- 

 ments extend to distances of only a few miles from the fracture. 



4. The earthquake vibrations originate in the surface of frac- 

 ture ; the surface from which they start has at first a very small 

 area, which may quickly become very large, but at a rate not 

 greater than the velocity of compressional elastic waves in the 

 rock. 



5. The energy liberated at the time of an earthquake was, 

 immediately before the rupture, in the form of energy of elastic 

 strai)i of the rock. 



These statements, which may be called the elastic rebound 

 theory of tectonic earthquakes, do not broach the original cause 

 of earthquakes, which lies in the source of the slow movements 



