| The Geysers is the site of a large geothermal energy project (>1 GW) and also one of the most seismically active regions in northern California. In addition there has been substantial surface deformation in the region. Significant correlations (>99.9th %ile) are found between industrial operations and seismicity, i.e. between steam production and shallow seismicity (z ≤ 1 km), as well as between both water injection and deep seismicity (z ≥ 1 km), and steam production and deep seismicity. The distribution of shallow earthquakes closely matches mapped low pressures in the reservoir and the areas of maximum volume strain inferred from surface deformation data, suggesting that these events are caused by poroelastic stressing. The deep injection correlated seismicity appears inconsistent with simple models based on changes in fracture fluid pressure. Localised thermoelastic stress perturbations due to liquid injection predicts results that are more consistent with the observations. Deep production induced seismicity is consistent with highly localised thermoelastic stressing caused by evaporative cooling. The inverted volume strain closely matches the mapped pressure drops within the reservoir; the reservoir scale strain magnitudes are inconsistent with a dominant thermal contraction mechanism, but are consistent with a poroelastic contraction mechanism and elastic models of fractured rock. At the scale of single conductive fractures though, simple models of the injection process show that local to a typical injection well, thermal stresses are often the dominant process. Injection induced seismicity is therefore likely to be thermoelastically controlled in many geothermal fields. |
