Role of Critical Stress in Quantifying the Magnitude of Fluid-Injection Triggered Earthquakes

Abstract

Abstract Understanding and mitigating fluid-injection-induced-seismicity is a key need in the routine implementation of net-zero-carbon energy (CCS, geothermal) and other industrial operations (hydraulic fracturing, subsurface wastewater disposal). We directly constrain the impact of pre-existing critical stresses on the relation linking seismic moment to injection volume. We report shear reactivation experiments on laboratory faults triggered by fluid pressurization. Experiments are conducted under both zero-displacement and constant shear stress boundary conditions – differentiating the role of stress relaxation during fault slip. Both are shown capable of linking event magnitude (\(M\)) with injected volume (\({\Delta }V\)) and fault pre-stress. Injection response defines two discrete and linear stages in \(M-{\Delta }V\)space linked by a discrete up-step. The first limb (stage) represents the elastic deformation of the fault, the vertical up-step its reactivation and the second limb the rupture response as incremented sliding. Faults loaded to different pre-stress identify and quantify the controlling role of pre-existing shear stress in conditioning event magnitude. Laboratory results are readily explained by a model that considers the pre-existing stress state in the rupture of a rigid fault with slip limited to the zone of pressurization. This cumulative moment is defined as \(M=\frac{c}{1-c}G{\Delta }V\) with c defining the proportion of the static stress drop already applied by tectonic stressing, alternatively viewed as the proximity to failure. The model and confirmatory laboratory observations explain the occurrence of triggered earthquakes at EGS sites significantly larger than expected based on previous models relative to injection volumes.