64 EARTHQUAKES 



The Causes of Earthquakes are better understood than those of 

 volcanoes, though much still remains which is difficult of explana- 

 tion. All earthquakes are not due to the same causes, for any 

 operation which will produce a blow or jar in the earth's interior 

 sufficiently strong to be propagated as a series of elastic waves to 

 the surface, will cause an earthquake. The most frequent of such 

 sources of disturbance are of two kinds. 



(i) In volcanic regions the explosions of steam are very often 

 the cause of earthquakes. This explains the association of earth- 

 quakes and volcanoes in the same region, and also the fact that a 

 great volcanic eruption is usually heralded by earthquakes, which 

 increase in violence up to the time of the outbreak, and then 

 cease. . Though more than two thousand miles distant, the erup- 

 tion of St. Vincent relieved the Mississippi valley earthquakes. It 

 must not be supposed, however, that all the earthquakes which 

 occur in volcanic areas can be brought into relation with eruptions, 

 for many of them cannot, as in the case of the countries around 

 the Mediterranean and of Japan. 



(2) A second and probably more important and widespread 

 cause of earthquakes is the sudden yielding of the earth's crust 

 to the strains which are set up within it. The contraction of the 

 earth must establish cumulative stresses in the crust, which, if 

 yielded to gradually, would not cause a jar, but when resisted, 

 increase until the strength of the rocks is overcome, and they give 

 way with a sudden shock, which produces an earthquake. The 

 association of volcanoes and earthquakes in the same regions is 

 thus not altogether, probably not even principally, a relation of 

 cause and effect, but is rather due to the fact that both are con- 

 nected with lines of fracture and weakness in the earth's crust. 

 In non-volcanic regions nearly all the earthquakes may be traced 

 to such lines of fracture, wherever the data have been collected 

 for an accurate determination of the foci. 



Changes of Level 



As was mentioned in the preceding section, permanent changes 

 of level frequently accompany earthquakes ; but these are par- 



