Although hydraulic fracturing-induced earthquakes have been widely reported in Alberta, Canada, only one seismic cluster (the Cordel Field) has thus far been linked to wastewater disposal (WD). In this study, we report a statistically significant spatiotemporal correlation between recent earthquakes and nearby WD wells near Musreau Lake—the second disposal-induced earthquake swarm in Alberta. This newly occurred swarm contains five events with local magnitudes ML>3 from January 2018 to March 2020, forming into three tightly spaced clusters. The refined locations and focal mechanisms suggest a ∼10 km long northwest–southeast-trending rupture along the northern Rocky Mountains that developed over time, during which both poroelastic effects and static stress transfer played key roles. Through a statistical analysis of all reported induced earthquake clusters in the western Canada sedimentary basin (WCSB), we propose a linear predictive relationship (i.e., the “Interpolated Strike Orientation” model) between fault rupture direction and fault distance to the Rocky Mountains. This observation-based model, which is supported by both the focal mechanisms of the natural earthquakes and the nearby northwest-striking geological faults, is a new and useful reference for future assessments of seismic hazard in the WCSB.
Aftershocks are commonly removed from observed earthquake catalogs in probabilistic seismic hazard analyses, using declustering techniques. We use stationary and temporal Epistemic‐Type Aftershock Sequence (ETAS) models to generate aftershocks from background seismicity and preceding aftershocks. We assume that the mainshocks equal the background seismicity, to divide the synthetic earthquake catalogs into mainshock (declustered) and complete (nondeclustered) versions. Only mainshocks follow a Poissonian distribution. We then evaluate how accurately we can forecast the recurrence of the largest events based on the simulated catalogs. A single b value is derived from the simulated catalogs and used in the magnitude‐frequency forecast. When the b value of the mainshocks is considerably smaller than the b value of the aftershocks, the information derived from the mainshock catalog leads to accurate predictions of the occurrence of the largest events. Conversely, when the mainshock and aftershocks have comparable b values, only the complete catalog produces representative estimates for the occurrence statistics of the largest events. We also show that using Poisson statistics leads to representative assessment of long‐term recurrences, even if aftershocks have a non‐Poissonian distribution. Finally, we analyze a recent case of induced seismicity, Oklahoma, USA, where the complete catalog displays a kink in the magnitude‐frequency distribution. Declustering removes this kink, leading to better b value estimations for the largest magnitude events. We conclude that temporal declustering for seismic hazard assessment is only recommended in catalogs with a large number of earthquakes and in catalogs where the b values of the mainshocks are significantly different from the b values of the complete catalog.
We analyze the temporal evolution of the induced seismicity related to hydraulic fracturing activities in the Duvernay Formation, near Fox Creek, Alberta, Canada. For this analysis, we estimate annual Gutenberg‐Richter parameters, a(t) $a(t)$‐ and b(t) $b(t)$‐ values, and then calculate the annual likelihood of earthquakes greater than magnitude M>4 $M > 4$ from 2014 to 2020. The seismic hazard near Fox Creek has consistently decreased since 2015, from a 95% probability of an earthquake greater than magnitude M>4 $M > 4$ in 2015 to 4% in 2019 and less than 1% probability in 2020. The induced seismicity in Fox Creek is characterized by two actively seismic regions with distinctive features: (a) an Eastern region (∼220 events M>2 $M > 2$) with lower b‐values and higher hazard; (b) a Western region (∼210 events M>2 $M > 2$) with higher b‐values and lower seismic hazard. In contrast, extensive regions where hydraulic fracturing is performed, particularly East of the seismic cluster, remain non‐seismogenic. The overall decreasing seismic hazard, which contrasts with increasing operator activity, can be associated with (a) the intensification of hydraulic fracturing operations toward areas less susceptible to induced seismicity and (b) the reduction of seismic activity in the Eastern region, which exhibits the highest seismic hazard. We also find evidence of a minimum annual injection volume required to trigger induced seismicity in both the Western and Eastern regions. The minimum injection threshold increases over the years, implying increasingly successful mitigation strategies, likely due to regulatory implementations in the area, which has led the operators to exercise precaution in regions with significant seismic hazard and adapt treatment strategies to avoid triggering moderate magnitude size events during hydraulic fracturing stimulations.
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