Deformation and seismicity in the Coso geothermal area, Inyo County, California: Observations and modeling using satellite radar interferometry

Interferometric synthetic aperture radar (InSAR) data collected in the Coso geothermal area, eastern California, during 1993-1999 indicate ground subs idence over a similar to 50 km(2) region that approximately coincides with the production area of the Coso geothermal plant. The maximum subsidence ra te in the peak of the anomaly is similar to 3.5 cm yr(-1), and the average volumetric rate of subsidence is of the order of 10(6) m(3) yr(-1). The rad ar interferograms reveal a complex deformation pattern, with at least two i rregular subsidence peaks in the northern part of the anomaly and a region of relative uplift on the south. We invert the InSAR displacement data for the positions, geometry, and relative strengths of the deformation sources at depth using a nonlinear least squares minimization algorithm. We use ela stic solutions for a prolate uniformly pressurized spheroidal cavity in a s emi-infinite body as basis functions for our inversions. Source depths infe rred from our simulations range from 1 to 3 km, which corresponds to the pr oduction depths of the Coso geothermal plant. Underpressures in the geother mal reservoir inferred from the inversion are of the order of 0.1-1 MPa (ex cept a few abnormally high underpressures that are apparently biased toward the small source dimensions). Analysis of the InSAR data covering consecut ive time intervals indicates that the depths and/or horizontal extent of th e deformation sources may increase with time. This increase presumably refl ects increasing volumes of the subsurface reservoir affected by the geother mal exploitation. We show that clusters of microearthquakes associated with the geothermal power operation may result from perturbations in the pore f luid pressure, as well as normal and shear stresses caused by the deflation of the geothermal reservoir.