EARTHQUAKES AND SEAQUAKES 71 



try and encountered a mountain wall. Such seismic waves have 

 been especially common upon the Pacific shore of South America 

 and upon the Japanese littoral (Fig. 54). These waves proceed 

 from above the great deeps upon the ocean bottom, and clearly 

 result from the grander earth movements to which these depres- 

 sions owe their exceptional depth. The withdrawal of the water 

 from neighboring shores may be presumed to be connected with 

 a descent of the floor of the depression and the consequent draw- 

 ing-in of the ocean surface above. The later high wave would 

 thus represent the dispersion of the mountain of water which is 

 raised by the meeting of the waters from the different sides of the 

 depression. 



The grander and the lesser earth movements. Upon the 

 land the grander and so-called catastrophic earthquakes are 

 usually the accompaniment of important changes in the sur- 

 face of the ground that will be discussed in later sections. 

 Those shocks which do little damage to structures produce no 

 visible changes in the earth's surface, except, it may be, to shake 

 down some water-soaked masses of earth upon the steeper slopes. 

 Still other movements, and these too slight to be felt even in 

 the night when the animal world is at rest, may yet be distin- 

 guished by their sounds, the unmistakable rumblings which are 

 characteristic alike of the heaviest and the lightest of earth- 

 quake shocks. 



Changes in the earth's surface during earthquakes faults and 

 fissures. Each of the grander among historic earthquakes has 

 been accompanied by noteworthy changes in the configuration of 

 the earth's surface within the district 

 where the shocks were most intense. 

 A section of the ground is usually 

 found to have moved with reference to 

 another upon the other side of a verti- 

 cal plane which is usually to be seen; 

 we have here to do with the actual 

 making of a fault or displacement such 

 as we find the fossil examples of within FIG. 55. A fault of vertical 

 the rocks. The displacement, or throw, displacement, 



upon the fault plane may be either upward or downward or 

 laterally in one direction or the other, or these movements may be 



