Depth migration of seasonally induced seismicity at The Geysers geothermal field

Seismicity from injected fluids provides insight into the hydraulically conductive fracture network at The Geysers (TG), California, geothermal reservoir. Induced earthquakes at TG result from both thermoelastic and poroelastic stresses as injected fluids cool the rocks and increase pore pressure. The spatiotemporal evolution of M >= 1.5 seismicity is characterized as a function of depth in the northwest and southeast regions of TG to develop time-dependent earthquake rates using an epidemic-type aftershock sequence model. The seismicity and injection follow an annual cycle that peaks in the winter months and is correlated by depth. The results indicate a time lag of <= 6 months for fluids to migrate >3 km below the injection depth. Water injection is the main cause of seismicity as fluids penetrate into the reservoir. Our results suggest that a steeply dipping fracture network of hydraulically conductive faults allows fluid migration to a few kilometers below the point of injection.