Spatiotemporal Variations of Faulting Regimes and Source Parameters of Induced Seismicity: A Case Study from The Geysers Geothermal Field

Spatial, kinematical and magnitude characteristics of induced seismicity occurring during different fluid-injection stages are investigated using a high-resolution hypocenter catalog of a prominent cluster of induced seismicity at the northwestern part of The Geysers geothermal field. There, changes of the stress field orientation and relative stress magnitude were identified during two peak-injections in a previous study. These observations were interpreted as an effect of the increase in pore pressure and horizontal stress magnitudes at reservoir depth during the peak-injections.

Studying the characteristics of this seismicity cluster in detail we find that during peak-injections seismicity occurs at greater distances from the injection point, mainly towards the north-east, i.e. parallel to the maximum horizontal stress orientation. In contrast, during lower injection rates seismicity migrates dominantly to the south and concentrates closer to the injection point. Fault plane solutions show that during peak-injection intervals the percentage of strike-slip and/or thrust faulting events increases and the seismic moment released by the strike-slip events is higher on average than that of normal faulting events. The b values decrease during peak-injections, suggesting the increase in differential stresses at the reservoir during these periods.

The observed differences in the seismicity characteristics during the different injection stages could be related to variable influence of physical mechanisms inducing seismicity. Prior to peak-injections, the seismicity might be predominantly connected with the thermal fracturing of the reservoir rock. However, during peak injections, the limited pore pressure increase (~1MPa) may play a significant role, even though pore pressure remains under hydrostatic values. By estimating the reservoir permeability and the thermal and hydraulic diffusivities, we confirm that the pore pressure propagation front reaches a greater distance from the injection point than the corresponding thermal propagation front.