310 AJsnsruAL report Smithsonian institution, 1950 



are of the same age as others belonging to the circum-Pacific belt, show 

 relatively low seismicity. 



Other regions with occasional earthquakes include the area between 

 Siberia and Alaska. This is transgressed by the Bering Sea, whose 

 coasts are probably of structural significance, since practically all the 

 known shocks of the regions are close to them. Activity marginal to 

 the Canadian shield includes a major earthquake off Newfoundland in 

 1929 ; to the northeast, marginal shocks have occurred in Davis Strait 

 and Baffin Bay. There is notable activitj^ along the St. Lawrence 

 River. 



The Appalachian belt is a region of fairly frequent minor activity. 

 The noithern part of it is shaken occasionally by marginal shocks 

 of the Canadian shield, and some moderate earthquakes originate 

 within the Appalachian area. Near the Atlantic coast is the epicenter 

 of the Charleston (S. C.) earthquake of 1886. Historically the 

 greatest shocks in the United States outside the Pacific area are the 

 earthquakes of 1811 and 1812 in the Mississippi Valley, which origi- 

 nated near New Madrid (Mo.) ; their magnitudes possibly surpassed 

 magnitude 8. It is of interest to note that shocks east of the Rocky 

 Mountains seem to originate occasionally at depths of about 30 or 

 40 miles below the surface, which is near the lower limit of "shallow" 

 earthquakes. As a consequence, these shocks are sometimes felt over a 

 wide area without doing any serious damage anywhere. An earth- 

 quake near Charleston (Mo.) in 1895 occasioned only minor damage 

 near its epicenter and yet was felt from the District of Columbia to 

 New Mexico, from Canada to Louisiana. Contrasting with these 

 shocks, California earthquakes usually originate at a depth of ap- 

 proximately 10 miles ; even when they cause considerable damage, they 

 have much smaller areas of perceptibility. 



The instrumental data furnish information as to the contemporary 

 seismicity of any given region. However, the historic records where 

 available indicate that in most areas the seismicity changes only rela- 

 tively little with time ; on the other hand, a few regions are known to 

 have shown a much higher seismic activity in earlier periods, and in 

 some instances major earthquakes have occurred in regions which have 

 been considered inactive. Of the roughly one million earthquakes per 

 year which are potentially strong enough to be felt somewhere on 

 earth (magnitude 2 and more) about 2 percent occur in the earthquake 

 belt of California and Nevada (including the shocks off the coast of 

 northern California and Oregon). For details see Gutenberg and 

 Richter (1949). 



It is possible to make certain statistical statements about the fre- 

 quency and the probability of the occurrence of earthquakes within 

 relatively large areas over long periods of time. For example, of the 

 present average of about 220 great shocks (il/^734) and about 1,200 



