Foreshock Triggers in Injection-Induced Quakes

More than 90% of magnitude-3-or-greater earthquakes triggered by fluid injection in Western Canada were preceded by detectable foreshocks, researchers report – a finding that could strengthen efforts to forecast and mitigate induced seismic hazards. Injection-induced earthquakes (IIEs), which can arise from activities such as hydraulic fracturing and wastewater disposal, are becoming an increasing concern; some have reached damaging magnitudes. Because of this, regulators commonly use a "traffic-light" system that calls for injection to be reduced or stopped when smaller earthquakes occur. This assumes that foreshocks can signal a larger earthquake and that shutting down operations can prevent this outcome. However, it has been unclear why some IIEs have many foreshocks while others have few or none, and how foreshock activity relates to the processes that ultimately produce a quake. Using a decade of high-quality seismic recordings from western Canada, Bei Wang and colleagues examined foreshock activity before 77 IIEs of magnitude 3 or greater. Wang et al. found that roughly 92% of the earthquakes (71 of 77) were preceded by identifiable foreshocks. Foreshock productivity and spatial and temporal patterns reflected the interplay among fluid injection, the seismogenic index, and the fault stress state. Moreover, the authors identified three rupture nucleation models: fluid-driven cascade, fluid-driven pre-slip with an intact source asperity, and injection-weakened pre-slip. This illustrates how fluids can promote aseismic slip and stress transfer before a mainshock. Together, the findings suggest that IEE risk mitigation should be informed by regional and local conditions and based on combined seismic and geodetic monitoring.