TEMPORAL VARIATIONS OF FRACTURE DIRECTIONS AND FRACTURE DENSITIES IN THE COSO GEOTHERMAL FIELD FROM ANALYSES OF SHEAR-WAVE SPLITTING

Abstract

This project aims to improve understanding of the subsurface fracture system in the Coso geothermal field, located in the east central California. We applied shear-wave splitting technique on a set of high quality, locally recorded microearthquake (MEQ) data. Four major fracture directions have been identified from the seismograms recorded by the permanent sixteen-station down-hole array: N10-20W, NS, N20E, and N40-45E, of which the first and the third are the most prominent. All orientations are consistent with the known strike of local sets of faults and fractures at depth and at the surface, as well as with previous analyses of seismic anisotropy in the region. Significant changes in shear-wave time delays, which are governed by crack density, have been detected from data recorded during five consecutive years (1996-2000). Variations in shear-wave time delays were simulated using synthetic seismograms and tentatively interpreted as due to a local ~3% decrease in shear-wave anisotropy in the southwestern part of the field during 1999.