An array of moveable seismographic trailers was deployed at three sites along the northern section of the “Big Bend” in the San Andreas fault in southern California. The three sites monitored were the Carrizo Plains, Frazier Park, and Lake Hughes areas. Effective observation times at each site ranged from 38 to 69 days. The microearthquake activity rates observed were 0.3 events/day, 3.0 events/day, and 1.9 events/day, respectively, based on the number of located events plus the number of unlocated events with S-P ≦ 3.0 sec. The majority of the activity does not appear to be directly associated with the San Andreas fault. A comparison of the activity rates observed in this study with the results of a survey conducted in the same areas by Brune and Allen (1967), indicates more than an order of magnitude increase in activity rate in the Lake Hughes area and nearly the same levels of activity at the Carrizo Plains and Frazier Park sites.

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