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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 (ML 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 (SHmax azimuth = N38°E). The orientation of derived nodal planes collectively indicates a model of RSS that consists of four primary strike‐slip structures striking 19° (R′), 79° (R), 94° (PDZ), and 109° (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. Plain Language Summary: Under a shear stress regime, randomly distributed small fault segments would evolve into a mature strike‐slip fault system. During the early stage, a network of shear structures with favored orientations for rupturing are often observed, called Riedel shear structures (RSS). The depiction of RSS can help understand the geomechanical behavior of individual earthquakes. It is well accepted that long‐term fluid injections can cause earthquakes, yet their influence on the developing process of RSS is rarely discussed. Here we document a clear case in western Canada where the development of a local RSS system is expedited by 25 years of wastewater injection. The RSS system is manifested by an earthquake sequence consisting of 187 small‐to‐moderate‐sized events. Focal mechanisms of these events exhibit various faulting types with the majority being compatible with the background stress regime. The orientation of derived nodal planes and six fault segments depicted from the refined earthquake distribution collectively define the overall geometrical characteristics of the RSS. Mohr‐Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa. The dimension of RSS could be important in the assessment of the overall seismic hazard due to fluid injections. Key Points: A seismically quiet area becomes active after 25 years of fluid injectionInduced earthquake source characteristics indicate Riedel shear structuresLong‐term fluid injection can notably expedite the reactivation and development of local fault system [ABSTRACT FROM AUTHOR]

Earthquakes resulting from hydraulic fracturing (HF) can have delayed triggering relative to injection commencement over a varied range of time scales, with the majority of M ≥ 4 mainshocks occurring near/after well completion. This poses serious challenges for risk mitigation and hazard assessment. Here, we document a high‐resolution, three‐dimensional source migration process with delayed mainshock triggering that is controlled by local hydrogeological conditions near Fox Creek, Alberta, Canada. Our results reveal that poroelastic effects might contribute to induced seismicity, but are probably insufficient to activate a large fault segment not critically stressed. The rapid pore‐pressure build‐up from HF can be very localized and capable of producing large, felt earthquakes if adequate hydrological paths exist. We interpret the delayed triggering as a manifestation of pore‐pressure build‐up along pre‐existing faults needed to facilitate seismic failure. Our findings can explain why so few injection operations are seismogenic. Plain Language Summary: Fluid injection‐induced earthquakes (IIE), especially those M ≥ 4 mainshocks, are often observed to occur near or after well completion. Such delayed triggering relative to injection commencement poses serious challenges for both regulators and the energy industry to establish an effective mitigation strategy for the potential seismic risk. In this study, we reveal a high‐resolution, complex three‐dimensional pattern of IIE migration near Fox Creek, Alberta, Canada. The observed first‐outward‐then‐inward IIE sequence highlights the significance of hydrogeological networks in facilitating fluid pressure migration and the associated seismic failure. The detailed spatiotemporal distribution of IIE suggests that the effect of pore‐pressure build‐up from hydraulic fracturing (HF) can be very localized. The delayed triggering is a combined result of the fluid pressure migration and the current stress state of the hosting fault system away from the HF wells. The findings from this study also provide plausible explanations on why only a very limited number of fluid injections are seismogenic. Key Points: We document a complex 3D source migration process with delayed mainshock triggering that is controlled by a local hydrogeological settingPoroelastic effects contribute to induced events but are probably insufficient to activate a large fault segment not critically stressedRapid pore‐pressure build‐up can be very localized and lead to large earthquakes if adequate hydrological paths exist [ABSTRACT FROM AUTHOR]

Injection‐induced earthquakes associated with wastewater disposal (WD) in the Western Canadian Sedimentary Basin are much fewer than those linked to hydraulic fracturing. Recently, 43 WD‐related earthquakes (M1.3–3.9) occurred in a historically seismically quiet area where long‐term injections became intensive since 2018, foreshadowing a critical stress state accumulated from long‐term injections. Our improved earthquake locations show that the injection well targeting the deeper middle‐upper Devonian aquifer system causes more vigorous seismicity with a wide range of stress drop values (0.8–230 MPa) compared to other shallower wells. Three physical mechanisms may collectively lead to the observed seismic pattern: (a) the underlying Devonian reef system makes it easier for injected fluids to migrate horizontally, (b) fluids can be channeled into the widespread critical faults and migrate vertically to cause earthquakes along it, and (c) aseismic slip probably triggered within the Duvernay formation by fluid migrating can further facilitate seismicity at shallower depths. Plain Language Summary: Earthquakes can be triggered during the injections of both hydraulic fracturing and wastewater disposal. In the Western Canadian Sedimentary Basin, the latter injection type appears to have caused much fewer earthquakes than the former. Recently, a sequence of 43 small‐to‐moderate‐sized earthquakes occurred in a seismically quiet area, where injections have been safely operated for more than two decades prior to becoming intensive in 2018. The refined earthquake distribution confirms that these earthquakes are related to disposal injections. The inception of local seismicity implies that long‐term injections probably have loaded nearby fault structures to rupture. Among the four active wells, the one with a deeper injection depth caused more earthquakes than other shallower wells. As the injection of the deeper well is close to the ancient reef system, horizontal fluid migration is facilitated and more likely to reach nearby faults. Fluid migration along subvertical faults can cause deeper earthquakes in the basement and aseismic creep in the overlying shale formation. The induced aseismic slip can further load the adjacent fault segment and foster earthquakes at shallow depths. The fact that stress released by individual earthquakes varies widely between 0.8 and 230 MPa also hints the existence of aseismic creep. Key Points: Emerging seismicity in a historically seismically quiet area in western Canada is linked to the intensive wastewater disposal injectionsInjection targeting the deeper upper‐middle Devonian aquifer has caused more vigorous seismicity with varying stress drop valuesFluid migration facilitated by the Devonian reef system and aseismic slip in the Duvernay are proposed to boost the seismic triggering [ABSTRACT FROM AUTHOR]

To characterize the subsurface geomechanical response to hydraulic fracturing (HF) activities, we study the spatiotemporal changes of seismic velocity during the completion of four HF wells in the Fox Creek area, Alberta, Canada. We estimate temporal velocity changes (dv/v) from ambient seismic noise recorded during the Tony Creek Dual Microseismic Experiment (ToC2ME) by comparing a 5-day stacked noise correlation function with a reference noise correlation function stacked over the deployment period. In the frequency band (0.1–0.4 Hz) most sensitive to the injection depths (~3.4 km), we observe daily dv/v that revealed alternating gradual velocity decreases and increases with magnitudes in the range of ±0.9%. We found a strong temporal correlation between the onset of velocity decreases and periods of intense seismicity, suggesting that the observed dv/v reductions are likely caused by stress-induced subsurface deformation due to elevated pore pressures, increased crack density, and ground shaking. A period of dv/v increase observed between the beginning and end of different well stimulation is attributed to crustal healing. Comparing the dv/v time series with injection parameters, we observed a 272.66% increase in induced seismicity and 50% more reduction in dv/v during the second injection phase that are correlated with 90.53%, 169.64%, and 4.34% increase in the injection volume, rate, and pressure, respectively. Our study provides valuable new information on the changes in reservoir elastic properties within the Western Canadian Sedimentary Basin. It also demonstrates that coda wave interferometry using data from dense seismic arrays near injection sites can be an additional tool for monitoring hydraulic fracturing operations. • We observe spatiotemporal changes in seismic velocity related to well stimulation. • Velocity reductions temporally correlate with induced earthquake swarms. • Increased injection parameters lead to increased seismicity and reduced velocity. [ABSTRACT FROM AUTHOR]