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 foc us <6 km are used in a non-linear inversion to derive high-resolution three -dimensional compressional and shear velocity structures at the Coso Geothe rmal Area in eastern California. Block size for the inversion is 0.2 km hor izontally and 0.5 km vertically and inversions are investigated in the uppe r 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 V-p and 0.5 km for V-s. Model uncertainties are estimated by a jackkni fe approach and simulation of random and associated picking errors. Low-vel ocity zones for both P and S waves are identified at geothermal production depths (1-3 km). A large, low V-p (-6%) zone is found at depth 2-2.5 km 2 k m southwest of Sugarloaf Mountain where high attenuation has been previousl y reported. However, a general high-V-p zone is seen under Coso Hot Springs with a slightly low V-s zone, which is characteristic of fluid saturation. The overall distributions of V-p and V-s perturbations do not correlate. A n isolated high-V-s (+9%) feature, about 2 km in diameter, is unambiguously seen 2 km due west of Sugarloaf extending from surface to depth. This feat ure is surrounded by a circular, low-V-s belt of similar to 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 V-p and high V-s at geothermal production depths from 1 to 2.5 km. Comb ined with attenuation results, this may represent a hot, fluid-depleted cen ter of magmatic activity.