THE CAUSES OF EARTHQUAKES 



we see if we dip the blade of a spade in water and sud- 

 denly push the blade in some particular direction. The 

 water waves thus created travel farthest in the direction 

 of the impulse. 



Another curious phenomenon connected with the large 

 waves of certain earthquakes is that they may be very 

 marked for one thousand miles round their origin, and 

 may be perceived on the exactly opposite side of the 

 earth (though, of course, much reduced in size), but 

 cannot be recorded on the earthquake instruments of the 

 regions in between. For example, an earthquake 

 originating near New Zealand may be recorded in 

 that country, but not in India, Egypt, West Asia, 

 or east of Europe, though in Britain it may make itself 

 evident on the seismometer's record. The phenome- 

 non may be compared to a water wave running down 

 an expanding estuary. At the mouth of such an estuary 

 it may have become so flat that it is no longer recog- 

 nisable. Should it, however, run up a second estuary, we 

 can imagine concentration taking place, so that near the 

 top of the second estuary it would eventually become 

 recordable on instruments. In these antipodean survivors 

 we see the final efforts of a dying earthquake. It is only 

 occasionally that the precursors and the followers of these 

 large waves have sufficient energy to reach their anti- 

 podes. They die en route. 



From the earliest times philosophers have held that the 

 causes of earthquakes were associated with the contact 

 between fire and water. Plato, Aristotle, Strabo, and 

 Pliny all held that water and air penetrate into the earth 



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