PROGRESSIVE INVERSION FOR HYPOCENTERS AND P-WAVE AND S-WAVE VELOCITY STRUCTURE - APPLICATION TO THE GEYSERS, CALIFORNIA, GEOTHERMAL-FIELD

Seismicity at The Geysers is induced by some aspect of steam production. Accurate earthquake locations, particularly hypocentral depth, are needed to determine the relationship between geothermal energy production and seismicity. Progressive P and S wave velocity-hypocenter inversions were done using data from 39 microearthquakes at The Geysers to estimate microearthquake locations and determine if the geothermal field has a distinctive seismic signature. Comparable final V(p), V(s), and V(p)/V(s) velocity models were obtained from four different starting velocity models. The elevation interval of maximum steam production coincides with minimum observed V(p)/V(s), and V(p)/V(s) increases below the primary production zone, suggesting that reservoir rock becomes more fluid saturated with increasing depth. A V(p)/V(s) peak at the condensation zone-production zone elevation delineates the top of the stream reservoir. Earthquake locations are confined to two depth intervals. A zone of shallow seismicity occurs immediately below the condensation zone and above maximum depths of steam production. A more arealy restricted zone of seismicity is located below maximum production depths and is abruptly terminated at an elevation of -3.7 km.