EARTHQUAKES OF MAY 19 60 IN CHILE — SAINT-AMAND 355 



a right-handed strike-slip system oriented roughly parallel to the coast, 

 the second a left-handed system oriented about 60 degrees to the east- 

 ward of the other. These faults are presently active. 



Another observation is that the Chilean coast is, in general, sub- 

 merging along these great faults. The faults inland, in the coastal 

 mountains and in the central valley, move primarily horizontally; 

 those in the Andes also slip horizonally, but there is, as well, a strong 

 component of vertical uplift on these with the mountains continuing 

 to grow with the passage of time. Those faults at the edge of the 

 coast show a tendency to permit the oceanward block to drop obliquely 

 beneath the sea, sliding northward as it goes down. There is ample 

 evidence that certain portions of the coast and even some extensive 

 areas such as the Mejillones Peninsula are rising. However, lack of 

 Eecent or Tertiary marine sediments along the flanks of the coast 

 range counterindicate post-Cretaceous submergence and subsequent 

 uplift. The faulting along the coast must be nearly vertical in order 

 to produce the extremely straight coast line. 



The observations, while valid for northern Chile, have not been 

 proved for southern Chile. In the discussion that follows it is im- 

 plicitly assumed that the same facts hold true, although it must be 

 remembered that this may not be so. For example, the peninsula of 

 Arauco and a portion of the offshore platform are obviously rising 

 while the coastal strip itself is sinking, perhaps forming a new sub- 

 marine central valley. 



There is always a concentration of activity at either end of the 

 active fault zone with, usually, a section of reduced activity along the 

 main fault. The active area is bomided by a rough quadrilateral some 

 350 by 1,000 km. The maximum energy released by the earthquakes 

 subsequent to the main shock was near the Arauco Peninsula. The 

 length of the fault that actually moved must have been about 1,200 

 km. This is analogous to the great Kamchatka earthquake of 1952 

 [IT]. 



From the strain distribution, it is obvious that the most energy 

 came from the eastern block. This and the greater extent of the de- 

 formed zone on the eastern side indicate a greater energy storage in 

 the eastern block, or a fault that dips under the blocks to the east. 

 The changes in elevation of the coast line and islands are such that 

 the fault seems certainly to lie between Isla Guafo, Isla Mocha (fig. 

 1), and the coast. Also the arguments adduced on the basis of the 

 tsuami indicate an origin near the coast. 



The energy radiated by the large earthquake was distributed along 

 this fault and not solely at the epicenter. This means that towns to 

 the south of the epicenter felt the quake more severely than those to 

 the north. The distribution of intensity, as shown by a high intensity 



