XII 



The Crust of the Earth 



227 



the strain. Every large fault found in rocks must have 

 given rise to earthquakes. Professor Milne points out that 

 most shocks originate along the lower part of the slopes of 

 the world ridges. This coincides with the lines along which 

 the process of elevation is going on most rapidly, and where 

 the strata are consequently subject to accumulating stresses. 

 The regions in which earthquakes are common are 

 coloured light blue on Plate II. and those where they 

 are very severe and frequent are coloured in a darker shade. 

 Many geologists believe that sea-water filtering through the 

 bed of the ocean, or buried to a great depth in the lower 

 layers of terrigenous deposits, causes explosions in the in- 

 tensely heated region below, and that all great earthquakes 

 originate from this cause and are essentially volcanic ; 

 the upheavals accompanying earthquakes would thus be 

 reckoned as ..heir consequences, not their causes. 



300. Propagation of Earthquakes. If the crust of the 

 Earth were perfectly uniform in substance, and a shock were 

 communicated to it at any point by a sudden yielding to 

 stress, a wave would spread in concentric spherical shells 

 from that centre like the sound-wave from a vibrating bell 

 in air ( 58). In the rock the wave travels more rapidly 

 than in air, and the to-and-fro movement of each particle 

 passing it on is very small. If the shock is given at A 

 (Fig. 44) the circles 

 I. II. III. show the 

 position of the crest of 

 the wave at intervals 

 of i, 2, 3 seconds. 

 The wave is shown 

 reaching the surface 

 at B, directly over the 

 centre of disturbance, 

 in 3 seconds ; there 

 it strikes perpendicu- 

 larly from beneath, although the force of the shock 

 is greatest at a little distance from B. A second later 

 the wave reaches the surface along a circular path 

 (IV.-IV.) and strikes obliquely upward; at the posi- 



FIG. 44. Earthquake wave, illustrating Mallet's 

 method of finding the depth at which an 

 earthquake originates. 



