THE CAUSES OF EARTHQUAKES 



Earthquake waves start out from the great area where 

 the cataclysm took place, and begin to disturb the earth 

 in all directions, just as if we were to put a row of 

 marbles on a table and were to strike the end marble of 

 the row. The marble farthest from it would presently 

 receive the shock as it travelled along the row of marbles. 

 Any one of our readers who has ever seen a train of 

 luggage wagons being shunted is familiar with the way in 

 which the shock of a sudden pull or push on the part of 

 the engine travels all down the line of wagons, and we 

 may think of the shock of an earthquake as travelling 

 along and through the earth in the same way. Observa- 

 tion, however, shows that these waves are propagated 

 farthest in one particular direction. For example, the 

 chief movement following the San Francisco earthquake, 

 which originated from fault lines running parallel to the 

 coast of California, was much more marked in countries 

 lying to the east or west of California than in countries 

 lying towards the south. England and Japan obtained 

 large records of the disturbance, while in Argentina the 

 records were extremely small. In the case of the Jamaica 

 earthquake, where the lines of origin ran east and west, 

 the phenomenon was reversed. Toronto received a large 

 quantity of motion, and England a very little. Another 

 peculiarity of this phase of earthquake motion is that it 

 may be propagated in one direction round the world 

 to a greater distance than in an opposite direction. The 

 suggestion is that the initial impulse was delivered in 

 the direction towards which motion was propagated 

 farthest. That which happens corresponds to what 



174 



