Long‐term Fluid Injection Can Expedite Fault Reactivation and Development: Riedel Shear Structures Illuminated by Induced Earthquakes in Alberta, Canada

Riedel shear structures (RSS) are often observed in the embryonic stage of strike-slip fault development, which can be depicted in the field through outcrops and coseismic surface ruptures. It is a critical concept linking the geomechanical behavior of individual earthquakes to structural geology at both local and regional scales. However, the influence of long-term fluid injections on the developing process of RSS, as manifested by the common occurrences of injection-induced earthquakes, has been rarely addressed. Here we document for the first-time subsurface RSS expedited by long-term wastewater disposal injections in western Canada. We study an earthquake sequence consisting of 187 events (M-L ranging 1.3-3.9) between 1 January 2018 and 15 July 2021 in an area without any previous seismic history. According to 31 well-constrained focal mechanism solutions, the injection-related earthquake sequence exhibits various faulting types with the vast majority (87%) being compatible with the background stress regime (S-Hmax azimuth = N38 degrees E). The orientation of derived nodal planes collectively indicates a model of RSS that consists of four primary strike-slip structures striking 19 degrees (R '), 79 degrees (R), 94 degrees (PDZ), and 109 degrees (P), respectively. Moreover, six fault segments delineated from the relocated local seismicity are parallel to the substructures of RSS. Mohr-Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa. Our observations suggest that long-term fluid injection can expedite the development of local fault systems. Therefore, it is probably important to consider the dimension of local/regional RSS in the assessment of the overall seismic hazard due to fluid injections.