Field-seismic investigation of the August 1975 Oroville, California, earthquake sequence

Several thousand aftershocks of the August 1, 1975 Oroville, California, earthquake (M_L = 5.7) were recorded by an 8-station field-seismic network. Focal coordinates of 104 of these events were fitted by least-squares to a plane striking N07°W and dipping 59°W; the strike (but not the dip) of this plane is in good agreement with that (N09°W) obtained from a fault-plane solution for a large foreshock 8 sec before the main shock, and it agrees fairly well with the trend (N15°W) of structural lineaments in the vicinity of Lake Oroville. The surface trace of the plane of foci passes through the Oroville Dam, as well as through surface cracking 12 km south of the dam. The main shock occurred 7 years after the filling of Lake Oroville, but only a month after the most rapid filling since 1968. The rate of aftershock occurrence during the first month decayed approximately as1/t. Event duration was measured for more than 2,000 aftershocks during August and September; average log-duration, taken over samples of 100 events, decreased gradually during this period. Close-in spectra obtained from strong-motion recordings of several of the larger aftershocks have corner frequencies that are quite high compared to other western U.S. earthquakes of similar magnitude. The Oroville earthquakes had several features in common with another Sierra Nevada earthquake sequence, near Truckee, California, in September, 1966.