The mb 5.9 earthquake of August 1, 1975, near Lake Oroville, California, was accompanied and followed by normal faulting at the surface. The distribution of aftershocks led directly to our discovery of the 3.8 km-long N- to NNW-trending zone of new fractures. In position, orientation, and sense of slip, the surface faulting agrees with the fault zone defined by mapped aftershocks and focal mechanisms, and is compatible with deformation shown by comparisons between pre- and postearthquake level surveys. The block to the east of the fault moved upward relative to that to the west, as shown by at least 55 mm of slip across the surface ruptures and 180 mm of vertical movement of benchmarks near the rupture zone. The faulting follows a zone of earlier displacement that may have been active in Quaternary time.

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