Subsurface Characterization of the Coso Geothermal Field and Surroundings by Ambient Noise Tomography

We apply seismic ambient noise tomography to image and investigate the shallow shear velocity structure beneath the Coso Geothermal Field and surrounding areas. Data from a PASSCAL experiment operated within the Coso Geothermal region between 1998 and 2000 and surrounding broadband stations from the Southern California Seismic Network (SCSN) are acquired and processed. Daily cross-correlations of ambient noise between all pairs of stations that overlapped in time of deployment were calculated and then stacked over the duration of deployment. Phase velocities of Rayleigh waves between 3 to 10 sec period are measured from the resulting cross-correlations. Depending on period, between about 300 to 600 reliable phase velocity measurements are inverted for phase velocity maps from 3 to 10 sec period, which in turn are inverted for a 3D shear velocity model beneath the region. The resulting 3D model reveals features throughout the region that correlate with surface geology. Beneath the Coso Geothermal Area shear velocities are generally depressed, a prominent low velocity anomaly is resolved clearly within the top 2 km, no significant anomaly is seen below about 14 km depth, and a weakly resolved anomaly is observed between 6 and 12 km depth. The anomaly in the top 2 km probably results from geothermal alteration in the shallow subsurface, no magmatic body is imaged beneath 14 km depth, but the shear velocity anomaly between 6 and 12 km may be attributable to partial melt. The thickness and amplitude of the magma body trade-off in the inversion and are ill-determined. Low velocities in the regions surrounding Coso at depths near 7 km underlie areas with Miocene to recent volcanism, suggesting that some magmatic processing of the crust could be focused near this depth. Submitted to Geochemistry, Geophysics, and Geosystems, September 2010.