To characterize the subsurface geomechanical response to hydraulic fracturing activities, we study the spatiotemporal changes of seismic velocity during the completion of four injection 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.

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 co-seismic 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 2018/01/01 and 2021/07/15 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. The orientation of derived nodal planes collectively indicates a model of RSS that consists of four primary strike-slip structures striking 19º (R’), 79º (R), 94º (PDZ) and 109º (P), respectively. Moreover, six fault segments delineated from the relocated local seismicity are parallel to the sub-structures 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.

Earthquakes resulting from hydraulic fracturing (HF) can have delayed triggering relative to injection commencement over a varied range of time scales, with many cases exhibiting the largest events 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. Our results reveal that poroelastic effects might contribute to induced seismicity, but are insufficient to activate a non-critically stressed fault of sufficient size. The rapid pore-pressure build-up from HF can be very localized and capable of producing large, felt earthquakes on non-critically stressed fault segments. 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 deepen our understanding of the current stress state of crustal faults and also explain why so few injection operations are seismogenic.