Volumetric components in the earthquake source related to fluid injection and stress state

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 ( M w 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 ( M w < 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 M w > 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.