ON THE EARTHQUAKE AND VOLCANIC PHENOMENA OF JAPAN. 



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the convexity of its upjiei- part that after a decrease in velocity in the 

 epicentral regions at great distances the veh)city again increases to 

 become infinite. Dr. Schmidt has Ukewise shown that actual observations 

 which have been made upon earthquakes are best satisfied by a velocity 

 curve drawn on the supposition that the actual velocity within the earth 



Fig. 17. 



is not a constant, but varies with a change in elasticity and density of the 

 rocks through which the waves are propagated. 



As we descend in depth on account of an increasing temperature it is 

 probable that with other changes there is a change in the elastic moduli of 

 rocks. This being admitted, it then follows that a series of waves start- 

 ing from a centrum would be propagated at a greater rate downwards than 

 upwards towards the surface, while the normals to such a series of waves 

 would by refraction gradually be bent upwards. 



As illustrative of what would occur under the supposed conditions, 

 Dr. Schmidt gives a diagram like fig. 18, 



in which coseismals have been drawn, on ^ '^^- ^^■ 



the assumption that the velocity has in- __+ > 

 creased proportionately with the depth. In :::.1.t.] 

 this case the earthquake rays which are -'-f j-'c . ^ . _ 

 perpendicular to successive coseismals are -'''''-'-—■-■'■- ■'^'' 



by refraction turned upwards, and no 

 longer radiate in straight lines. The co- 

 seismals meet the surface at intervals, 

 which first decrease from the epicentre 

 and then increase, indicating a decrease 

 and then an increase in the apparent ve- 

 locity. The value for v is never less than 

 the velocity at the centre, but after rapidly 

 decreasing until it equals this value, it again 

 increases. The velocity curve or earthquake hodograph which shows these 

 changes is drawn through points determined as they were determined for 

 Seebach's hyperbola. If there is an increase in the velocity of propaga- 

 tion of earth waves or in the quantity t/p as we descend beneath the 

 surface, whether we take the centrum near to the surface or at a great 



