SHEAR-WAVE SPLITTING FROM LOCAL EARTHQUAKES AT THE GEYSERS GEOTHERMAL-FIELD, CALIFORNIA

Shear-wave splitting from local microearthquakes recorded in The Geyse rs geothermal field shows that seismic anisotropy is distributed in a complex geographic pattern. At stations within about 2 km of northwest -striking regional faults, the fast polarization direction is parallel to those faults. The geothermal field, lying between two such faults, has both northwest and northeast fast polarization directions, often at the same station. This pattern suggests at least two causes of spli tting: (1) extensive dilatancy anisotropy (EDA) and (2) fault-produced fractures or rock fabric. The observed anisotropy may derive from the upper 1.5 km of the crust, averaging 4% there, or it may be heterogen eously distributed throughout the upper 5 km. Fast polarization direct ions coincide with fracture directions inferred from borehole data for one of the youngest rock types in the region, a felsite pluton of abo ut 1 Ma, and with injectate pathways inferred from microseismicity and geochemistry. Including in reservoir models a permeability anisotropy with a pattern similar to seismic anisotropy may help in optimizing f luid injection and steam recovery.