EARTHQUAKES IN NORTH AMERICA GUTENBERG 309 



usual. Shallow shocks in the interior of Alaska represent an interior 

 structure. 



Another sector of the Pacific belt extends from southeastern Alaska 

 to Puget Sound and includes the rather active area of the Queen 

 Charlotte Islands (fig. 3, upper left), where a great earthquake 

 occurred in August 1949. There are neither well-developed ocean 

 deeps nor shocks at intermediate or greater depth in this area. The 

 seismic activity decreases considerably in the vicinity of the State of 

 Washington. There is a clear gap between this and the next seismic 

 zone, which begins about 200 miles off the coast of Oregon. Thence, 

 an uninterrupted belt of earthquake foci extends in a southeasterly 

 direction (fig. 3). It reaches the coast of northern California, then 

 follows the coastal area to the region of San Francisco and continues 

 inland following the well-known San Andreas fault zone. This zone 

 has been traced at the surface as far south as the Salton Sea, but the 

 earthquake belt continues along the Gulf of California at least as 

 far as the southern tip of Lower California. Volcanic activity is low 

 along this zone; the few volcanoes, such as Mount Lassen, and Tres 

 Virgenes in Lower California, appear to be in a late state of activity. 



The next sector to the southeast is one of noticeably higher activity. 

 It follows the Pacific coast from Colima in Mexico to Panama. There 

 are two lines of active volcanoes, one extending west-east across cen- 

 tral Mexico from Colima to Veracruz, the other beginning in Guate- 

 mala and extending southeastward through Central America. Ac- 

 companying the line of active volcanoes, once more earthquakes are 

 found at depths of somewhat less than 100 miles. Mexico City is in 

 the west-east belt of intermediate shocks and consequently experiences 

 rather frequent earthquakes; however, they usually cause relatively 

 little damage as a result of their considerable depth below the surface. 

 Ocean deeps off the Mexican coast are well developed and include the 

 Acapulco deep and the Guatemala trench. Unfortunately, gravity 

 measurements are very scarce off the whole Pacific coast of North 

 America but the few data available indicate appreciable negative grav- 

 ity anomalies, at least off the coast of Mexico in the neighborhood of 

 the ocean deeps. 



The earthquake belts mentioned thus far are responsible for most of 

 the seismic acivity in North America. In the United States, for ex- 

 ample, the California-Nevada region contains about 90 percent of the 

 whole seismic activity. This result is based mainly on instrumental 

 data covering the past 40 years, but is in good agreement with historical 

 information (Gutenberg and Richter, 1944). The remaining shocks 

 are partly situated in areas marginal to stable masses, partly in regions 

 which have undergone higher tectonic activity in the not too distant 

 geological past. The Rocky Mountains and related structures, which 



