Finally we published a paper related to investigation of seismic moment tensors of microseismic events at The Geysers geothermal field.
Martínez-Garzón, P., Kwiatek, G., Bohnhoff, M., and G. Dresen (2017). Volumetric components in the earthquake source related to fluid injection and stress state. Geophys. Res. Lett. 44, DOI: 10.1002/2016GL071963. [ Article Page ]
We investigate source processes of fluid-induced seismicity from The Geysers geothermal
reservoir in California to determine their relation with hydraulic operations and improve the corresponding seismic hazard estimates. Analysis of 869 well-constrained full moment tensors (Mw 0.8–3.5) reveals significant non-double-couple components (>25%) for about 65% of the events. Volumetric deformation is governed by cumulative injection rates with larger non-double-couple components observed near the wells and during high injection periods. Source mechanisms are magnitude dependent and vary significantly between faulting regimes. Normal faulting events (Mw<2) reveal substantial volumetric components indicating dilatancy in contrast to strike-slip events that have a dominant double-couple source. Volumetric components indicating closure of cracks in the source region are mostly found for reverse faulting events with Mw>2.5. Our results imply that source processes and magnitudes of fluid-induced seismic events are strongly affected by the hydraulic operations, the reservoir stress state, and the faulting regime.