Triggered Earthquakes Reveal Hydraulic Properties of the Subsurface

Seismicity both at The Geysers geothermal field (northern California) and in north-central Oklahoma is heavily influenced by industrial activities related to energy production, though the mechanism in which earthquakes are induced or triggered is different. At The Geysers, much of the seismicity is linked to thermoelastic stresses caused by injecting cold water into hot rocks, while in Oklahoma the seismicity is linked to a reduction of confining stress on faults due to increasing pore pressure resulting from wastewater injections. Here we show that these contrasting conditions are also evident in tidally-triggered earthquakes. At The Geysers, earthquakes preferentially occur during maximum extensional strain, which does not occur at the same time as maximum shear strain on optimally oriented faults in the regional stress field. In Oklahoma, earthquakes preferentially occur during maximum shear strain on optimally oriented faults, rather than maximum extensional strain. The magnitude of tidal extensional strain is naturally much greater than tidal shear strain. However, in a fluid saturated environment, pore pressure responds to changes in volume, which can counteract or reduce the effect of the applied stress. The difference in behavior at these two sites is indicative of the level of coupling between applied stress and pore pressure, corresponding to unsaturated conditions at The Geysers and high pore pressure in Oklahoma.