Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic travel time data

High precision P and S wave travel times for 2104 microearthquakes with focus <6 km are used in a non-linear inversion to derive high-resolution three-dimensional compressional and shear velocity structures at the Coso Geothermal Area in eastern California. Block size for the inversion is 0.2 km horizontally and 0.5 km vertically and inversions are investigated in the upper 5 km of the geothermal area. Spatial resolution, calculated by synthetic modeling of a cross model at critical locations, is estimated to be 0.35 km for Vp and 0.5 km for Vs. Model uncertainties are estimated by a jackknife approach and simulation of random and associated picking errors. Low-velocity zones for both P and S waves are identified at geothermal production depths (1-3 km). A large, low Vp (-6%) zone is found at depth 2-2.5 km 2 km southwest of Sugarloaf Mountain where high attenuation has been previously reported. However, a general high-Vp zone is seen under Coso Hot Springs with a slightly low Vs zone, which is characteristic of fluid saturation. The overall distributions of Vp and Vs perturbations do not correlate. An isolated high-Vs (+9%) feature, about 2 km in diameter, is unambiguously seen 2 km due west of Sugarloaf extending from surface to depth. This feature is surrounded by a circular, low-Fs belt of ∼1 km width. The surrounding belt is probably the cracked, high-porosity reservoir/conduit of geothermal fluid flow. In the 2 km southwest Sugarloaf region, we found low Vp and high Vs at geothermal production depths from 1 to 2.5 km. Combined with attenuation results, this may represent a hot, fluid-depleted center of magmatic activity