CHAPTER III 



EARTHQUAKES — CHANGES OF LEVEL 



An earthquake consists of a series of elastic waves, similar in 

 principle to sound waves, propagated through the crust of the 

 earth and due to some disturbance in its interior. The number 

 and frequency of earthquakes are exceedingly great, so much so, 

 that the crust of the earth is in a state of constant disturbance at 

 one or more points. Besides the trembling and shocks that may 

 be plainly felt, minute tremors, which can be detected only by the 

 aid of very delicate instruments, are in continual progress, even in 

 countries rarely visited by sensible shocks. Slight shocks and 

 movements generally pass unnoticed, but when pains are taken to 

 collect them, it is surprising to find how numerous they are. These 

 facts show that the crust of the earth is not the stable, rigid structure 

 which our ordinary experience would lead us to consider it. 



The Distribution of Earthquakes is very similar indeed to that 

 of volcanoes, and volcanic regions are preeminently those in which 

 the most frequent and violent earthquakes occur. They may 

 manifest themselves at any part of the earth's surface, but they 

 increase notably both in frequency and violence as the volcanic 

 areas are approached. The great earthquakes which shook the 

 Mississippi valley in 1811-12, are among the very few instances 

 of violent and long-continued shocks in a region far from any vol- 

 cano. Yet, even these would appear to have had some connection 

 with the volcano of St. Vincent in the West Indies, and ceased 

 when that vent became active. 



The earthquake, or seismic} bands thus follow the coast-lines 

 and mountain chains, and coincide with the volcanic bands, but 

 are much wider than the latter, earthquakes being propagated over 



1 Greek seismos, earthquake. 

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