EARTHQUAKES IN NORTH AJMERICA — GUTENBERG 315 



andesite line indicates tectonic processes extending to rather large 

 depth. The deep-focus earthquakes are connected with these proc- 

 esses, and the fact that the very deep shocks occur nowhere on earth 

 except near and inland of the andesite line is another indication of the 

 unique structure of the Pacific basin. 



All information available for the Atlantic side of North America 

 indicates that the transition from the continent to the bottom of the 

 Atlantic is rather gradual. Although granitic material is probably 

 missing in the deeper parts of the Atlantic basin, as shown by recent 

 seismic explorations by W. Maurice Ewing and his collaborators, 

 deeper continental material may be present throughout the bottom 

 of the Atlantic Ocean (Ewing et al., 1937, 1950). In contrast with 

 the Pacific coasts, there are no earthquake belts surrounding the 

 Atlantic or the Indian Ocean. However, earthquakes and volcanoes 

 occur along the mid-Atlantic ridge. In contrast with this ridge and 

 similar ridges with seismic activity in the Indian Ocean, no ridges 

 of the Pacific show any earthquake activity, with the exception of the 

 area near the Hawaiian Islands. 



There is no agreement on any hypothesis as to the ultimate source 

 that furnishes the energy for earthquakes. Perhaps they are con- 

 nected with differences in heat production in the various units of the 

 earth's crust. Laboratory experiments indicate that much more 

 radioactive heat is generated in granitic material than in the more basic 

 material (simatic rocks) of the deeper layers and much less in the 

 ultrabasic material which probably extends relatively close to the 

 surface in the Pacific area. There is, in addition, the effect of the 

 temperature difference between ocean bottom (which is kept at a 

 temperature near 0° C. by the deep water in the oceans) and the 

 temperature of roughly 200° C. at the corresponding depth under the 

 continents. Subcrustal currents may be a consequence of this hori- 

 zontal temperature gradient. This may be combined with the fact 

 that the structural arcs along the Pacific boundary are usually in- 

 terpreted as due to forces either pushing or drawing subcrustal mate- 

 rial downward toward the foredeeps, with compensating movements 

 elsewhere. However, we know too little about the details of these 

 processes. 



During recent years it has been a very common experience in geo- 

 physics that hypotheses concerning the structure and the processes 

 in the earth's crust have become less and less certain as data accumu- 

 late; frequently the fact is revealed that the approximations used were 

 not as good as was believed. There is little doubt that the number of 

 recognized unsolved problems is increasing rather than decreasing in 

 practically all fields of geophysics. The hope of explaining and 

 predicting earthquakes seems to be more remote now than at any 

 previous time. 



