62 EARTHQUAKES 



wider areas from the seat of disturbance than the effects of vol- 

 canoes, except in the case of the unusually terrible explosions. 

 In one respect, the earthquake bands deviate quite markedly from 

 the volcanic ; namely, in the presence of a seismic zone which 

 encircles the whole earth. This belt includes the Mediterranean 

 lands, the Azores, the West Indies and Central America, the Sand- 

 wich Islands, Japan, China, India, Persia, and Asia Minor. By 

 some authorities this is regarded as the main seismic band, espe- 

 cially liable to disturbance, from which the others are but branches. 

 Volcanoes occur in many parts of the belt, but they form no such 

 continuous band around the earth. 



The Phenomena of Earthquakes are of very great significance 

 from the standpoint of human interests, because of the appalling 

 destruction and loss of life which they often cause. Fearful re- 

 sults may be caused by a heaving of the ground in which there 

 is little actual displacement. The phenomena vary considerably, 

 according to the position of the place observed with reference 

 to the focus, or seat of disturbance, the violence of the shock, 

 and the character of the rocks through which the vibrations are 

 transmitted. Were the focus a point, and the rocks uniformly 

 elastic and homogeneous, the earthquake waves would be spheri- 

 cal, and their outcroppings at the surface (which produce the sen- 

 sible shocks) would be circular. But none of these conditions 

 exist, and so the waves are more or less irregular, and owing to 

 differences in the rocks, reflection and interference of the waves, 

 and the like, the result often seems most anomalous and capri- 

 cious. This, however, is not of sufficient importance for our pur- 

 pose to require description. 



When the shock occurs beneath the bed of the sea, the phe- 

 nomena are complicated by disturbances in the water. Of these, 

 the most important is the great sea wave (erroneously called tidal 

 wave), which, though not strikingly displayed in the open sea, in 

 shallow water piles up into enormous breakers and rushes upon the 

 coast, often doing far more damage than the earth waves themselves. 



One important result of the modern careful investigation of 

 earthquakes is to show that they are comparatively superficial phe- 



