Microearthquake activity adjacent to the Rocklin pluton near Auburn, California

The occurrence of the Oroville earthquake (M_L = 5.7) of August 1, 1975 heightened interest in the seismotectonics of the Sierra foothills region and particularly the Foothills fault system. Clark (1960) recognized the Foothills fault system as being a major structural feature of the western Sierra Nevada. An 11-week reconnalssance microearthquake study was conducted by Woodward-Clyde Consultants (WCC) for the U.S. Bureau of Reclamation (USBR) to assess the seismic activity of the Sierra foothills 75 km S-SE of Oroville near Auburn, California. Microearthquake activity was detected in the Rocklin-Auburn region with eight events recorded during July 1 to July 24 and seven events during August 15 to October 8, 1976 (see Figure 1). The mobile seismic array consisted of eight Sprengnether MEQ-800 portable seismographs with Mark Products L-4C vertical seismometers. Data recordings were normally 48 hours in length, with a record speed of 1 mm/sec.