SEQUENTIAL FAULT REACTIVATION AND SECONDARY TRIGGERING IN THE MARCH 2019 INDUCED EARTHQUAKE SWARM NEAR RED DEER, ALBERTA

Recently, emerging cases of seismicity in the Duvernay East Shale Basin have garnered major public attention and scientific interest. These events show high spatiotemporal correlations with adjacent hydraulic fracturing (HF) operations. In this study, we focus on the March 2019 induced earthquake swarm near Red Deer, Alberta, and investigate the seismicity in the surrounding area. We utilize a combined data set from a temporary nodal array and broadband seismic stations and detect a cluster of 417 events using a machine learning phase picker and an automated earthquake association and location algorithm. The near-field nodal data enable us to determine the fault-plane solutions for low-magnitude earthquakes based on double-couple components using first-motion polarities and amplitude ratios. The spatial distribution and focal mechanisms unveil a NE-trending rupture area with two strike-slip fault planes. Reactivation of pre-existing faults by pore pressure diffusion is likely responsible for the occurrence of the earthquake sequence following the M_L 4.18 mainshock. The temporal sequence of reactivated fault orientations suggests apparent changes in the local stress field following the mainshock. We also observe a remotely triggered cluster one month after the mainshock, which offers further evidence for stress perturbations associated with pressure redistribution due to aseismic slip and/or the HF operation. This secondary triggering process highlights the need to consider trailing seismicity during risk assessment. In summary, quick-response nodal deployment provides an improved understanding of the relationship between HF-induced seismicity and background geological framework in western Canada.