Microseismicity of the San Andreas fault in the Frazier Park-Fort Tejon region, California

The microseismicity of the Fort Tejon-Frazier Park region for 1984-1989 was analyzed from the California Institute of Technology and the United States Geological Survey (CIT-USGS) data files. In addition, a portable high-gain instrument was installed at Frazier Park in September, 1989 to monitor recent activity and to provide constraints on hypocentral depth. Two USGS programs, VELEST and HYPOELLIPSE, were used to invert the arrival time data for velocity model, earthquake locations, and station corrections. VELEST uses a modified damped least squares inversion technique to obtain velocity structures, earthquake locations, and station corrections from earthquake arrivals. Using the velocity model of Hadley and Kanamori (1977) as the initial model, the revised model consisted of six layers with lower layer boundaries at 5, 8, 15, 20 and 33 km depth, and with distributed velocities of 5.66, 6.08, 6.28, 6.57, 7.0 and 7.8 km/ sec for the layers. Then HYPOELLIPSE was used with the new model and station corrections to relocate the seismic events. The station corrections ranged from -0.17 to 0.44 seconds. The spatial distribution of activity correlates not only with the bend of the San Andreas, but also with local subsidiary faults. The earthquake location results of the joint earthquake location and station correction inversion indicate that the earthquake events seem to be distributed into five groups: the San Andreas right-lateral, Tejon thrust, North Frazier Mountain thrust, and South Frazier Mountain thrust systems, and the left-lateral tear fault located on the east side of Frazier Mountain. The analysis of focal mechanisms indicated both reverse and strike-slip faulting, which supported the interpretation of the five clusters of seismic events. The seismicity along the San Andreas fault itself ranges in depth from 3 to 20 km along a vertical fault plane, which is in contrast to the geological model of Namson and Davis (1988).