Analysis of seismic moment tensors and in situ stress during Enhanced Geothermal System development at The Geysers geothermal field, California

Seismicity in the vicinity of an Enhanced Geothermal System (EGS) demonstration project in the Northwest Geysers at the Prati 32 (P-32) injection well and the Prati State 31 (PS-31) production well is investigated to determine earthquake focal mechanisms, moment magnitudes and in situ stress of events that occur before and during reservoir stimulation starting 2011 October 6. We present a catalogue of 167 waveform-based seismic moment tensors ranging in moment magnitude from 0.6 to 3.9. The moment tensor catalogue is subsequently used to invert for the stress tensor and to investigate possible temporal stress changes resulting from fluid injection. We find an approximate 15-degree counter-clockwise rotation of the least compressive stress sigma(3) during injection. More remarkable is a change in orientation of the maximum compressive stress sigma(1) from subhorizontal to vertical when injection operations temporarily cease. The orientation of sigma(1) returns to subhorizontal when injection operations resume. It is found that there was a systematic reduction in the stress shape factor, R, as the injected water volume increases, indicating an evolution towards a more transtensional stress state.