abstract

The USGS operated 16 short-period seismic stations near Oroville, California, in conjunction with our aftershock study of the Oroville earthquake of 1 August 1975 (ML = 5.7). Between June 1975 and August 1976, 35 earthquakes were recorded outside the aftershock zone with S-P times at Oroville less than 9 sec and magnitudes between 0.5 and 2.5. We have located those between 38°45′ and 40°15′N and 120°30′ and 122°15′W using the USGS Oroville network, supplemented with other regional stations. Maps are shown relating this regional seismicity to the Oroville aftershock zone, the M = 5.7 Chico event of 1940, and the historical seismicity. Of particular interest are two small but very well located events at a depth of 40 km down-dip from the Oroville aftershock zone.

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