CHAPTER VIII 



THE INTERRUPTED CHARACTER OF EARTH MOVE- 

 MENTS: EARTHQUAKES AND SEAQUAKES (Concluded) 



Experimental demonstration of earth movements. The study 

 of the Alaskan earthquake of 1899 showed that during this adjust- 

 ment within the earth's shell some of the local blocks moved up- 

 ward and by larger amounts than their neighbors, and that still 

 others were actually depressed so that the sea flowed over them. 

 It must be evident that such differential vertical movements of 

 neighboring blocks at the earth's surface can only take place 

 if lateral transfers of material are made beneath it. From under 

 those strips of coast land which were depressed, material must 

 have been moved so as to fill the void which would otherwise have 

 formed beneath the sections that were uplifted. If we take into 

 consideration much larger fractions upon the surface of our planet, 

 we are taught by the great seaquakes which are now registered 

 upon earthquake instruments at distant stations that large down- 

 ward movements are to-day in progress beneath the sea much more 

 than sufficient to compensate all extensions of the earth's surface 

 within those districts where the land is rising in mountains. From 

 under the offshore deeps of the ocean to beneath the growing 

 mountains upon the shore, a transfer of earth material must be 

 assumed to take place when disturbances are registered. 



Within the time interval that separates the sudden adjustments 

 of the surface which are manifested in earthquakes, the condition 

 of strain which brings them about is steadily accumulating, due, 

 as we generally assume, to earth contraction through loss of its 

 heat. It seems probable that the resistance to an immediate ad- 

 justment is found in the rigidity of the shell because of the com- 

 pression to which it is subjected. To illustrate : a row of blocks 

 well fitted to each other may be held firmly as a bridge between 

 the jaws of a vice, because so soon as each block starts to fall a 

 large resistance from friction upon its surface is called into exist- 

 ence, a force which increases with the degree of compression. 

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