On 17 August 2015, an $Mw$ 4.6 earthquake occurred northwest of Fort St. John, British Columbia, possibly induced by hydraulic fracturing (HF). We use data from eight broadband seismometers located $∼50km$ from the hypocenter to detect and estimate source parameters of more than 300 events proximal to the mainshock. Stress‐drop values estimated using seismic moment and corner frequency from single‐event spectra and spectral ratios range from $∼1$ to 35 MPa, within the typical range of tectonic earthquakes. We observe an $∼5$‐day delay between the onset of fluid injection and the mainshock, a $b$‐value of 0.78 for the sequence, and a maximum earthquake magnitude larger than the prediction based on the total injection volume, suggesting that the $Mw$ 4.6 sequence occurred on a pre‐existing fault and that the maximum magnitude is likely controlled by tectonic conditions. Results presented here show that pre‐existing fault structures should be taken into consideration to better estimate seismic hazard associated with HF operations and to develop schemes for risk mitigation in close proximity to HF wells.

Aseismic slip loading has recently been proposed as a complementary mechanism to induce moderate-sized earthquakes located within a few kilometers of the wellbore over the timescales of hydraulic stimulation. However, aseismic slip signals linked to injection-induced earthquakes remain largely undocumented to date. Here we report a new type of earthquake characterized by hybrid-frequency waveforms (EHWs). Distinguishing features from typical induced earthquakes include broader P and S-pulses and relatively lower-frequency coda content. Both features may be causally related to lower corner frequencies, implying longer source durations, thus, either slower rupture speeds, lower stress drop values, or a combination of both. The source characteristics of EHWs are identical to those of low-frequency earthquakes widely documented in plate boundary fault transition zones. The distribution of EHWs further suggests a possible role of aseismic slip in fault loading. EHWs could thus represent the manifestation of slow rupture transitioning from aseismic to seismic slip.

Here we search for dynamically triggered earthquakes in Canada following global main shocks between 2004 and 2014 with M_S > 6, depth < 100 km, and estimated peak ground velocity > 0.2 cm/s. We use the Natural Resources Canada (NRCan) earthquake catalog to calculate β statistical values in 1° × 1° bins in 10 day windows before and after the main shocks. The statistical analysis suggests that triggering may occur near Vancouver Island, along the border of the Yukon and Northwest Territories, in western Alberta, western Ontario, and the Charlevoix seismic zone. We also search for triggering in Alberta where denser seismic station coverage renders regional earthquake catalogs with lower completeness thresholds. We find remote triggering in Alberta associated with three main shocks using a matched‐filter approach on continuous waveform data. The increased number of local earthquakes following the passage of main shock surface waves suggests local faults may be in a critically stressed state.

We investigate the occurrence of remote dynamic triggering in three Canadian unconventional hydrocarbon regions where recent fluid injection activity is correlated with increasing numbers of earthquakes. We select mainshocks with an estimated local peak ground velocity exceeding $0.01cm/s$ occurring between 2013 and 2015, when station coverage was increased to monitor injection activity. A twofold approach, using continuous waveform data and an enhanced earthquake catalog created using a multiple‐station matched‐filter detection algorithm, suggests that remote dynamic triggering occurs at all three regions. The waveform‐based approach shows evidence for direct triggering in the surface wavetrain of the mainshock, as well as directly afterward in the coda. The enhanced catalog approach shows qualitative increases in earthquake rates at all three regions that are both immediate, and in some cases, sustained over 10‐day time windows and are corroborated with two types of statistical tests: a $p$‐value test to quantify the statistical significance of earthquake rate change following a stressing event and an interevent time test that provides a statistical measure of changes in seismicity rates. The occurrence of both direct and delayed triggering following transient stress perturbations of $<10kPa$ in all three regions suggests that local faults may remain critically stressed over periods similar to the time frame of our study (⁠$∼2$ yrs) or longer, potentially due to high pore pressures maintained in tight shale formations following injection. The results interpreted in the context of injection history and recent poroelastic modeling results may have implications for the mechanisms of remote triggering. Namely, triggering via poroelastic stresses may provide a unifying mechanism that can explain both delayed and immediate triggering observations.

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.