Seismic velocity structure and characteristics of induced seismicity at the Geysers Geothermal Field, eastern California

We present a newly developed three-dimensional seismic velocity model, high-precision earthquake relocations and focal mechanisms near the Geysers Geothermal Field in eastern California using seismic data recorded by the Northern California Earthquake Data Center from 1984 to 2015. The velocity model generally agrees with those in previous studies with the Vp model mainly corresponding to rock composition and the Vp/Vs model more correlated with fluid content. The dominating low Vp/Vs anomalies observed from 0 to 4 km depth below sea level is a reflection of the geothermal reservoir. The waveform cross-correlation relocated seismicity shows both spatial and temporal correlations with the geothermal operations in the study area, indicating that they are induced seismicity. Although the focal solutions are dominated by normal and strike-slip regimes throughout our entire study time period, there has been an increase in reverse faulting since 2008, which may be caused by the thermal contraction associated with the Northwest Geysers Enhanced Geothermal System project between 2008 and 2012. Our study provides a groundwork for future seismological studies in The Geysers.