MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 527 



Earthquakes. 



When the tremors spring from sources within the earth itself and 

 are of appreciable violence, they are recognized as earthquakes. The 

 sources of earthquake tremors are various. The most prevalent is 

 probably the fracture of rocks and the slipping of strata on each other 

 in the process of faulting. The interpretation of movements of this 

 class has now been so far perfected that the length and depth of the 

 fault, the amount of the slip, and the direction of the hade are capable 

 of approximate estimation.^ To the same class belong the movements 

 due to slumping. They are illustrated by the sliding and arrest of 

 great masses of sediment along the steep fronts of deltas, and of the 

 accumulations of deep-sea oozes on steep submarine slopes. Such 

 slumping is, in reahty, superficial faulting. Seismic tremors often 

 attend volcanic eruptions, and are then probably attributable to the 

 sudden fracture and displacement of rock by the penetration of lava, 

 or by rapid and unequal heating. They are perhaps also due some- 

 times to the sudden generation or cooling of steam in underground 

 conduits, crevices, and caverns, the action possibly being in some cases 

 of the '^ water-hammer " type. In rare instances, probably, the bursting 

 of beds overlying pent-up non-volcanic gases may give origin to earth- 

 quakes. A more superficial source of earthquake vibrations is the 

 collapse of the roofs of subterranean caverns. 



Seismic vibrations seem to be in part compressional, in part distor- 

 tional, in part (on the surface) undulatory, and in part irregular. The 

 distortional are especially significant, as they seem to imply a soHd 

 medium of transmission. 



Points of origin, foci. — It is probable that nearly or quite all earth- 

 quake movements start within the upper ten miles of the crust, and 

 most of them' within the upper five. Some of the earlier estimates 

 indeed placed the points of origin as deep as 20 or 30 miles, but in these 

 cases the necessary corrections, discussed below, were neglected. Most 

 of the recent and more accurate estimates fall within the limits given. 



The method of estimating the depth of the centers of disturbance 

 consists in observing the directions of throw or thrust of bodies at the 

 surface, and in regarding these as representing the lines of emergence 

 of the earthquake-wa^TS. By plotting these lines of emergence, and 



1 Davison. Jour, of Geol., Vol. VIII, p. 301. 



