EARTHQUAKE MOTION DEDUCED FROM EXPERIMENT. 61 



The Intensity of Artificial Disturbances, — The data 

 which we have at our disposal for determining the inten- 

 sity of an earth particle which has been caused to vibrate 

 by the explosion of a charge of dynamite are a series of 

 records similar to that given on p. 60. These disturb- 

 ances are practically surface movements, and may be com- 

 pared with the movements of an earthquake which spreads 

 over an area the radius of which is great as compared with 

 its depth. 



To find the mean acceleration of an earth particle, 

 which quantity has been taken to represent intensity, 

 during any simple backward or forward motion of the 

 earth, it will be first necessary to determine the amplitude 

 of this motion and its maximum velocity, the mean accele- 



ration being equal to . 



The second and third movements in a shock invariably 

 exhibited the greatest intensity, and to a distance of 400 

 feet from the origin, where about three pounds of dynamite 

 had been exploded in 

 a bore-hole about six 

 feet deep, these in- 

 tensities decreased 

 directly as the dis- 

 tance from the origin. 

 The less intense 

 movements also de- 

 creased directly as 

 the distance from the 

 origin to a certain 

 point, but after that ^^^- ^^• 



they decreased more slowly. A mean result of the more 

 prominent vibrations in four sets of experiments is shown 

 in the curve, fig. 12, where the horizontal measurements 









— 



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— 











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1 





- 



- 



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K4-800 





























5 













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s 













































N 































4 

















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Z2P- 



































































_ 



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u 



Origin 



too' 



200' 300' 



Scale of Feet. 



400 



