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

摘要

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 V_p and 0.5 km for V_s. 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 V_p (-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-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. An 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 feature is surrounded by a circular, low-V_s belt of approx 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. Combined with attenuation results, this may represent a hot, fluid-depleted center of magmatic activity.