1906.] 



SEE— THE CAUSE OF EARTHQUAKES. 



385 



pressure, but the resistance is unequal in the different directions, 

 it is evident that as the imprisoned matter expands or explodes, the 

 reactions in the different directions will be inversely as the corre- 

 sponding resistances to the escaping fluid. In general the explosion 

 will give a rotation to the surrounding particles, since the moment 



Fig. 17. 



Seismographic record of the S,an Francisco Earthquake, April 18, 1906, 

 taken at the Chabot Observatory, Oakland, California, Professor Charles 

 Burkhalter, Director. 



of the resistances will not pass exactly through the center of the 

 exploding mass, and the rotation is as likely or more likely to be 

 m the horizontal plane than in any other. If the explosion or rapid 

 dift'usion of steam pressure takes place in stratified rock, the strata 

 are fairly sure to be approximately horizontal ; but no doubt most all 



