Geomechanical Sensitivities of Injection-Induced Earthquakes

Wastewater reinjection increases pore fluid pressure and can make faults frictionally unstable, triggering earthquakes. Little is known, however, about how regional geomechanical properties influence the likelihood that disposal will induce seismicity. We investigate the impact of preinjection conditions on the proportion of possible fault orientations destabilized by a specified pore pressure increase. Stress state is the most important factor, with 7 times as many planes unstable in extension than contractionindeed, normal or strike-slip faulting environments characterize most injection-induced seismicityand additional fivefold increases in transitional stress states. Geomechanical susceptibility depends inversely on overburden density and depth, consistent with the shallow hypocenters of most induced events. The coefficient of friction and initial pore pressure have little impact: Faults initially below hydrostatic pressure can slip during gravity-fed injection. Combining these factors, site-specific geomechanical analyses could determine 300-fold variations in the likelihood of induced seismicity before disposal, offering a proactive way to manage hazard. Plain Language Summary We investigate what factors influence the likelihood that industrial activities will trigger earthquakes. The initial pore-fluid pressure is unimportant, meaning that gravity-fed injection can readily cause seismicity. By contrast, readily known properties of the region (stress state, depth to bedrock faults, and density of the overlying sedimentary rock units) produce 300-fold variations in the likelihood of injection-induced earthquakes. Conducting simple analyses prior to injection could mitigate seismic hazard.