139 



JL, 



number of shocks are reported in the northern part of the bay. 

 For more than three years prior to this shock there were numerous 

 small quakes which were felt in the beach cities. The main 

 shock had a magnitude of 5 followed by 16 immediate aftershocks, 

 loseismal lines (lines of equal magnitude) indicate that the 

 shock was felt more than a hundred miles away» It was calculated 

 that the shock originated at a depth of 6 to 9 miles below the 

 earth's crust in bedrock, probably granite. 



Thickness of Overburden 



Thicknesses of unconsolidated sediment in several parts 

 of the bay are shown in Figure 35 „ Thicknesses of less than 

 10 feet are found in the rock and gravel area on the outer 

 shelf, off the Malibu and Palos Verdes coast. U. S. Coast and 

 Geodetic Survey charts and a chart prepared by Johnson (1940) 

 also show rock off the coast of MalibUo The overburden increases 

 eastward from the outer shelf, and reaches a thickness of 500 

 feet within one mile of the rock and gravel area. 



The scattered patches shown in Figure 36 are geophysical 

 anomalies and probably are due to irratic or scattered patches 

 of gravel, except some of the near shore ones which may be rock 

 a short distance below the surface. 



*This shock has not been shown on Figure 34 because it took 

 place before 1934. Prior to 1934, earthquakes were not system- 

 atically recorded. They were largely "eye-witness" accounts 

 and have little scientific value. 



