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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



(3) Narrow belts of shallow shocks, which follow the principal ridges 

 in the Atlantic, Arctic, and Indian Oceans. (4) Moderate activity 

 associated with rift structures such as those of East Africa and the 

 Hawaiian Islands. 



The most extensive inactive block is the Pacific basin (excluding 

 the Hawaiian Islands) . On the continents, most of the ancient shields 

 are quite inactive. Between the stable shields and the active belts are 

 regions of minor to moderate activity having occasional large shocks. 

 Small shocks (magnitude 5 and less) apparently occur everywhere. 



FiGUKE 1. — The structural arc from northern Japan to Kamchatka. (After 

 Gutenberg and Richter, 1949.) (See also fig. 2.) 



A structural arc of the Pacific region — for example the Tonga 

 arc, the Marianas arc, or the northern Japan arc (figs. 1 and 2) — ex- 

 hibits the following typical features in order, beginning at the con- 

 vex side: (A) a foredeep; (B) shallow earthquakes and negative 

 gravity anomalies along anticlines; (C) positive gravity anomalies 

 and slightly deeper shocks; (D) the principal mountain arc (Ter- 

 tiary or older) , with active volcanoes and shocks about 100 kilometers 

 deep; (E) an older structural arc with volcanism in a late stage or 



