The Oroville earthquake sequence of August 1975

abstract The characteristics of the Oroville, California earthquake sequence of 1975 are presented. Historically, the Oroville area is one of low seismicity; the largest earthquake in the region occurred with a magnitude (ML) 5.7 in 1940 some 50 km north of Oroville. The first foreshock of the sequence occurred on June 28, 1975. Twnety-one foreshocks (ML ≧ 1.6), the largest of magnitude 4.7, preceded the magnitude 5.7 main shock of August 1. All foreshocks and aftershocks of ML ≧ 3.0 were located using seismographs operated by the University of California at Berkeley, USGS, and the California Department of Water Resources. The aftershock region covers an area approximately 14 by 10 km southeast of the city of Oroville. The depth distribution of the earthquakes indicates a west dipping fault plane. The b value of 0.61 shows the sequence to be rich in larger magnitude aftershocks. Similar b values have been determined for other aftershock sequences in California, such as a sequence near Coalinga in August 1975. The aftershock occurrence rate follows an Omori relation with n(t∞t−0.70. Apparent variability in the earthquake mechanisms of the series makes interpretation of composite fault-plane solutions difficult, but the data indicate normal faulting striking NNW and a west dip of about 30°–40°, the Sierra Nevada moving up with respect to the Great Valley.