Shear-wave splitting and fracture alignments at the Northwest Geysers, California

Abstract

Shear-wave splitting is commonly observed on three-component microearthquake seismograms recorded at the Northwest Geysers geothermal field, California. The polarization of leading shear waves around each recording station is predominantly N10°-40°E, suggesting that the shear-wave splitting is caused mainly by pervasive, north-northeast orientated anisotropy. Time delays between the leading and second shear waves can be as large as 10ms/km to 30ms/km. We propose that the direction and magnitude of this shear wave splitting is due to the presence of pervasive, stress-aligned secondary fractures related to several northwest-trending, right-lateral faults in the Geysers area. Copyright 1997 by the American Geophysical Union.