Seismicity of the Coso Range, California

Abstract

A 16‐station seismographic network, approximately 40 km north‐south by 30 km east‐west, was installed in the Coso Range, California, in September 1975 as part of a geological and geophysical assessment of the geothermal resource potential of the range. During the first 2 years of network operations, 4216 local earthquakes (0.5 ≤ m ≤ 3.9) defined zones of seismicity that strike radially outward from a Pleistocene rhyolite field located near the center of the Coso Range. Most earthquakes were located in zones showing a general northwest trend across the range. Six earthquake swarms occurred within the area that includes the rhyolite field. Fault plane solutions show regional north‐south compression: earthquakes located in northwest striking zones generally had right lateral strike slip focal mechanisms, those in northeast striking zones left lateral strike slip focal mechanisms, and those in north‐south striking zones both normal and strike slip focal mechanisms. Earthquake depths showed little variation across the Coso Range; the depth distribution is similar to that of several carefully studied segments of the central San Andreas fault. The b value calculated for the entire range is 0.99±0.08. The rhyolite field has a significantly higher b value of 1.26±0.16; if only the shallow events (depth <5 km) are used in the calculation, the b value for this area becomes even higher, 1.34±0.24. The higher b values were interpreted as reflecting the existence of short average fault lengths (<5 km) within the rhyolite field. The seismic data and other data suggest that the fault system lying between the rhyolite field and the adjacent Coso Basin is an important tectonic boundary. Present information is insufficient to determine the geothermal production capability of this fault system, but it does suggest that the system is a good target for further exploration.