Vol. 6] Reid: The Elastic-Rebound Theory of Earthquakes. 429 



occur; for if they did the clastic forces would be kept down 

 below the breaking point, and fractures would be averted. 



The objection might be made that rock at great depths and. 

 therefore, under heavy pressure could not fracture, but might 

 be suddenly deformed ; but if it could suddenly change shape 

 plastically, it could also change shape by a sudden slip along 

 a fault. Moreover, the sudden plastic yielding implies either 

 the sudden reduction of the forces resisting plastic deformation, 

 which is contrary to our knowledge of the properties of matter, 

 or the sudden increase in the deforming forces. We can readily 

 picture to ourselves fairly steady forces like gravity, or the forces 

 brought about by the disturbance of isostatie equilibrium; but 

 the only sudden forces, except those caused by blows or by the 

 release of elastic strain, are explosive forces, and no one would 

 imagine that the great foldings of the strata were due to suc- 

 cessive pressures brought about by a great number of explosions. 

 And as the observed foldings and contortions of the rock are 

 what we should expect from a slow plastic flow, it seems super- 

 fluous, to say the least, to ascribe it to sudden movements. And 

 if there is a slow plastic yielding there are slow movements of 

 the rock; for the movement of a given part of the rock must be 

 equal to the movement of the rock in front of, or behind, it. 



We can show that the elastic strains which caused the Cali- 

 fornia earthquake were not suddenly developed immediately 

 before the shock, but that the strain existed to some extent 

 twenty-five and fifty years earlier. It should be noticed that 

 in the first experiment with the jelly the elastic rebound of 

 points on the right side of the fracture brought them to exactly 

 the same position, relative to the right-hand block, that they 

 held before the strain was set up. In the second experiment, 

 where the relative positions were determined after the strain 

 was set up. the jelly on the right was displaced downward after 

 the slip. This is exactly what occurred in the movements on the 

 eastern side of the San Andreas fault. The points on that side 

 were displaced southward between the times of the second and 

 the third surveys, showing conclusively that at the time of the 

 second survey the ground was already in a state of elastic strain. 

 This is brought out also in another way. The third experiment 



