EARTHQUAKES IN NORTH AMERICA — GUTENBERG 311 



additional major earthquakes (M^ 7. 0-7. 7) per century over the whole 

 earth, about 5 and 18 respectively can be expected to occur in the Pacific 

 United States, about 14 great shocks and 65 major shocks in Alaska 

 and the Aleutian Islands, and about 11 major shocks in the remainder 

 of central, eastern, and northern North America. It is not possible, 

 however, to predict the approximate location or time of larger earth- 

 quakes, since too little information is available on the sources of energy 

 and the processes involved in the building up of strain leading to an 

 earthquake. 



Some information on tectonic processes is being furnished by geo- 

 detic measurements. The United States Coast and Geodetic Survey 

 has installed a special system of triangulation stations and bench 

 marks in California, which are checked from time to time. In this 

 way, changes in elevation as well as horizontal movements over larger 

 areas are found. Such measurements have indicated, for example, 

 that during the past 60 years the region on the west side of the San 

 Andreas fault between San Francisco and San Jose has moved 

 roughly 10 feet north relative to the east side (Wliitten, 1948). This 

 is not a new type of movement; geological evidence indicates that this 

 type of movement has persisted during many centuries, at least. 

 Wherever rivers flow across the fault system the river bed has been 

 displaced in the same direction — the western side northward relative 

 to the eastern side. The total amount of these displacements is not 

 known. In the neighborhood of the San Andreas fault (pi. 1) some 

 offsets exceed 1 mile; however, no information concerning displace- 

 ments in excess of the distances between successive valleys can be 

 found in this way. Thus far no definite correlation has been found 

 between rocks corresponding to each other on the two sides of the San 

 Andreas fault. 



Records of earthquakes have been used to find the direction of the 

 movement at the source and to draw conclusions as to the fault move- 

 ment during a shock. It is possible to determine whether the first 

 motion of the longitudinal waves is from the source toward the sta- 

 tion or in the opposite direction^ Thus, for example, earthquakes 

 along the San Andreas fault to the north of Pasadena begin on the 

 Pasadena records with a dilatation toward the source, whereas earth- 

 quakes from the San Andreas fault to the east of Pasadena start with 

 a compression toward Pasadena (Gutenberg, 1941). The motion in 

 the shear waves can be investigated in a similar way. Studies of this 

 type, which have been undertaken in California during recent years, 

 have fully confirmed the persistence of the movements just described 

 (Dehlinger, 1950). They throw^ some light on the details of the 

 processes in earthquakes. 



It is of interest that similar investigations seem to indicate that 



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