STRUCTURE OF THE EARTH HODGSON 



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ity at a known depth. For, as we have seen, we can determine 

 emergence angles at every epicentral distance on the stripped earth. 

 That, very briefly, is exactly what has been done. It is known 

 as the Herglotz-Wiechert method. 



Applying the Herglotz-Wiechert method once for one arbitrarily 

 chosen epicentral distance, we get one vertex depth and the one 

 corresponding true velocity. Applying it repeatedly, we get other 



,in Foctts 



FiGDKE 9. — Cross sectiou of the earth showing rays and wave fronts of earthquake waves 

 and the "blind zone.'" The last direct P wave to emerge at the surface is the one 

 grazing the core whos^e wave front is marked 14'" indicating that the transmission time 

 from the focus to this point is 14 minutes. 



depths and their corresponding velocities and thus bit by bit we get 

 the complete graph, which shows the variation of velocity with depth 

 at all distances down to the core. The velocity depth curve deduced 

 by Witte in 1932 is reproduced in figure 8. 



Down to the core, why no farther? Because the ray which barely 

 grazes the core is the last direct P wave to reach the surface. All 

 others are deflected into the core in such a way that, beyond a 

 certain distance range, no direct ray emerges at the surface. There 



