STRUCTURE OF THE EARTH — HODGSON 



287 



begin above the graph for a normal earthquake. But, at the anti- 

 podes of the epicenter, the shock must arrive earlier for a deep focus 

 than for a normal one and the second curve must here lie below 

 the first. The curves must cross. If the focal depth were 200 miles, 

 the new time-distance graph would start even later and arrive earlier 

 than before — and so for successively greater depths of focus. A 

 typical pair of graphs is shown in figure 4. If the arrival times of 

 the P wave at a sufficient number of stations can be obtained, it is 

 possible to determine from such a set of curves not only the distance 

 to the epicenter but also the focal depth. In passing, let us note that 

 Canada with her great expanse of territory would leave a wide gap 

 in many of these curves did she not maintain a certain number of 





DISTANCE 



FiGUHB 4. — Characteristic form of the time-distance curve for normal focus and for 



deep focus. 



stations at strategically spaced positions. Her seven stations are little 

 enough coverage for a country of such dimensions. 



Depths of focus as great as 500 miles and more have been established 

 and are a distinct contribution to the knowledge of the structure of 

 the earth. Tliey show that the earth can sustain built-up strain at 

 depths previously considered impossible. We can no longer maintain 

 that the crust alone is crystalline and that the mantle is plastic. At 

 least the mantle is not plastic in the sense it was thought to be until 

 deep-focus earthquakes were established as a fact. Probably no con- 

 tribution of the science has aroused a greater interest among those 

 concerned with learning something of the structure of the earth. 



But such depths are taking us into the mantle of the earth, and 

 we are not yet finished with our consideration of the problem of 



