Jr. Evans et al., SHEAR-WAVE SPLITTING FROM LOCAL EARTHQUAKES AT THE GEYSERS GEOTHERMAL-FIELD, CALIFORNIA, Geophysical research letters, 22(4), 1995, pp. 501-504
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.