On August 1, 1975, a magnitude 5.9 (mb) earthquake occurred approximately 8 km SSE of the town of Oroville, California. This earthquake and its associated foreshock-aftershock sequence are of particular interest because of their possible relation to the impounding of the 4.3 billion m3 Lake Oroville. Hypocenter locations for 336 aftershocks that occurred during August define a fault plane striking N3°E and dipping 60° to the west to a depth of 10 km. Dimensions of the epicentral area are approximately 7 km in an east-west direction by 15 km in a north-south direction. The fault plane passes beneath Oroville Dam at 5-km depth, and if projected up dip, would crop out beneath the reservoir to the east. The distribution in space and time of foreshocks and aftershocks suggests that rupture began at depth and progressed up dip and to the north and south.

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