22 



HISTORY OF THE OCEANS 



2 5 X 10 



I960 



Fig. 8. Plot of strain-rebound, 5 in (ergs)^'-, vs date, t in years, for a sequence 

 of earthqualces with focal depths h > 300 km and magnitude M > 7.0 since 1905 

 (incomplete for magnitudes 7 to 7^ for 1905 to 1917). These shocks occur below 

 the low-velocity region (after original by H. Benioff, from Gutenberg, 1959). 



x'\s will be seen from the results of surface wave dispersion, it 

 now seems more likely that the lower boundary of the continents 

 is at a lesser depth, roughly 100+ km, where these studies placed 

 the lower boundary of the high-velocity, topmost mantle layer. 



There is also the strong possibility that Benioff's and Guten- 

 berg's results do not apply to normal mantle structure, since the 

 data are inherently restricted to anomalous regions such as deep 

 trenches and continental margins. 



It seemed likely that, despite the variability of body wave 

 results, a standard upper mantle structure for continental areas 

 did exist, because the velocity-depth curves for depths greater 

 than 400 km were well determined and in excellent agreement for 

 many areas. But there seemed to be little possibility of making- 

 detailed upper mantle investigations of a significant number of 

 representative continental areas to determine the standard 

 continental structure, and there was no possibility of studying 



