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Earthquake locations for the time period March 1975 through March 1983 indicate diffuse seismicity in the Clear Lake area, with the area to the southeast of the lake having the highest level of activity. Swarms lasting 2 days, with events of magnitude ≥3.5, have been observed in the Konocti Bay fault zone. In contrast, almost no microearthquake activity is associated with the Collayomi fault zone. A 10-km-wide northeast-southwest-trending zone of seismicity is apparent although there is no known corresponding geologic feature. Earthquake depths in the Clear Lake area are shallower than in the surrounding major right-lateral fault zones. Focal mechanism solutions show predominant strike-slip with significant normal dip-slip movement. The maximum and minimum compressive stress orientations, north-northeast and east-southeast, respectively, are consistent with the San Andreas right-lateral transform boundary region. However, the component of extensional stress illustrated by normal fault-plane solutions and the shallowness of seismicity may be related to an inferred nearby crustal partial melt body.

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