Spatiotemporal Variations in Earthquake Triggering Mechanisms During Multistage Hydraulic Fracturing in Western Canada

Dense arrays deployed near the hydraulic fracturing (HF) wells greatly enhance the understanding of injection-induced seismicity. In this study, we revisit the continuous recordings that are acquired by 69 three-component nodes at an HF site in Alberta, Canada, taking advantage of a machine learning-based seismic detection and location workflow. The obtained new earthquake catalog contains 21,619 events with relative location errors of <1 m, which exceeds similar to 20% of the number of earthquakes (18,040) reported previously (Igonin et al., 2021). This high-resolution catalog reveals the distribution of earthquake sequences at much improved spatiotemporal resolution and illustrates several previously unmapped faults/fractures. Earthquake frequency-magnitude distribution reveals that the average b value increases with depth from similar to 1.1 above 3.5 km to similar to 2.5 at greater depths. Further spatial analysis of seismic clusters indicates that the b value varies laterally (similar to 1-1.7) at shallow depths and is inversely related to the proximity to injection wells in conjunction with the change in structural types (i.e., reactivated fault and pre-existing fracture). The seismic sequence on the north-south oriented faults also shows a distinctive occurrence pattern and temporal affinity to the fracturing network reactivation between the two stages of HF operations. The change in faulting behavior could reflect a shift of dominating triggering mechanisms and physical processes from (a) the rapid diffusion of pore fluid pressure along pre-existing fracture corridors to (b) the cascade migration of earthquake sequences in response to the cumulative Coulomb stress perturbation on the fluid-lubricated, critically stressed faults.