EARTHQUAKES IN NORTH AMERICA — GUTENBERG 



307 



extinct, and shocks about 200 to 300 kilometers deep; (F) a belt of 

 deep shocks (below 300 kilometers). In some arcs only a few of these 

 features can be identified ; this is true of the similar structural arcs 

 along the southern Alpid tvont of the trans-Asiatic zone. In parts of 

 the Pacific belt (for example, along the coast of the continental United 

 States (fig. 3) and British Columbia) structural arcs and the accom- 

 panying features are absent. In many such sectoi-s (as in California) 

 there is strong evidence of block faulting in place of the folding 

 cliaracteristic of the arcs. 



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PiGXjEE 2. — The structural arc in northern Japan. (After Gutenberg and Richter, 



1949.) (See also fig. 1.) 



The seismicity of North America is mainly associated with the Paci- 

 fic belt. Relatively high activity occurs in the area of the Aleutian 

 Islands. The Aleutian arc is a typical Pacific arc; it extends from 

 the Commander Islands into central Alaska. Seismic and volcanic 

 activity is relatively high. In general, shallow seismic activity fol- 

 lows the northern concave side of the Aleutian trench. Intermediate 

 shocks at depths down to about 100 miles occur along the north side of 

 the island arc. No shocks originating deeper than 20 miles are known 

 in the area of the North American Continent. The shocks having 

 depths of approximately 60 miles occur near the line of volcanoes, as 



