514 



WITHIN THE EARTH'S CRUST 



are due to breaks or slips of a few inches or a few feet in the 

 rock structure. From the place at which the break or slip 

 takes place the motion is transmitted through the rock mass 

 to the surface, where it causes sudden and often tremendous 

 shocks. These slippings may occur occasionally for ages 



along the same fault line. 

 Sometimes they are in- 

 tense enough to cause 

 great damage; at other 

 times only a slight tremor 

 is felt. 



The rapidity of the 

 transmission of the shock 

 differs with the kind of 

 material through which 

 it is transmitted, varying 

 from a few hundred feet 

 to several thousand feet 

 per second. The nearer 

 a place is to the break 

 or slip the greater is the 

 intensity of the shock. 

 Sometimes the crack or 

 fault along which the movement occurs reaches to the 

 surface and makes the displacement apparent. 



If an earthquake originates under the sea, a great wave may 

 be developed which rushes inland from the coast, causing 

 great destruction. One of the most fearful of these waves 

 occurred at Lisbon, Portugal, in 1755, sweeping away thou- 

 sands of people who had rushed into an open part of the city 

 to get away from the falling buildings caused by the earth- 

 quake shock. 



FENCE BROKEN BY THE SLIPPING OF THE 

 EARTH ALONG A FAULT LINE 



