This study presents observations using new data from a deployment of eight broadband seismometers surrounding a horizontal well pad at distances of~1-3 km for the period before, during, and after a hydraulic fracturing treatment in the Montney Basin, British Columbia, Canada. We use a multistation-matched filter detection and double-difference earthquake relocation to develop a catalog of 350 events associated with hydraulic fracturing stimulation, with magnitudes ranging from −2.8 to 1.8 and estimated catalog completeness of approximately −0.2. The seismicity distribution suggests a statistically significant association with injection, and event migration can be described by a hydraulic diffusivity of~0.2 m 2 /s. A comparison between daily seismicity rate and analytical stress evolution inferred from daily injection volumes implies that pore pressure diffusion largely controls earthquake nucleation at distances less than 1 km, whereas poroelastic stress transfer likely dominates at intermediate distances of~1-4 km at time scales shorter than diffusion. Both mechanisms likely have a limited effect on stress perturbation at distances over 5 km.

We use broadband seismic data collected within 3 km of a hydraulic fracturing (HF) well in northeast British Columbia, Canada, to estimate the stress drop values of HF-induced earthquakes and their spatial variation. Applying both spectral ratio and clustered single-spectra fitting methods to 484 induced earthquakes (M-1.0 to 3.0), we find that earthquakes close to the injection well have lower stress drop values than those at greater distance. Stress drop values are generally invariant within clusters either proximal (~0.1-1 MPa) or distal (~1-10 MPa) to the well, suggesting that dynamic ruptures in rocks with similar rheological properties tend to have relatively constant stress drop values. Clustered single spectrum fitting also suggests that the seismic quality factor (Q) is lower proximal to the well. We interpret the lower stress drop values and higher seismic attenuation proximal to the well as a result of higher fracture density and/or elevated pore pressure in the rock matrix due to hydraulic stimulation. Plain Language Summary Earthquake static stress drop is the average difference in stress across the fault surface before and after an earthquake rupture. It is a measure of the stress released by fault slip. Investigating the stress drop of induced earthquakes helps us understand the causal relation between hydraulic fracturing (HF) and earthquake source properties. Here we study the source parameters of 484 HF induced earthquakes in the Montney Play of northeast British Columbia. We find that stress drop increases with distance to the injection well but is roughly constant within the respective proximal or distal groups of events. We also find that seismic energy loss during wave propagation (seismic attenuation) is higher near the well. The observations lead us to interpret that either the higher fracture density and/or elevated pore pressures near the well prevent the crustal rocks from storing and releasing larger magnitudes of stress at distances approximately <1 km from the well.

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.

Injection-induced earthquakes (IIE) are part of the inconvenient consequences of the unconventional oil and gas production. Although only a very small fraction of injecting wells are seismogenic (<1%; Ghofrani & Atkinson, 2020;Schultz et al., 2020), injections related to wastewater disposal (WD) and hydraulic fracturing (HF) have been reported to cause a surge of seismicity rate near the injection sites and the occasional occurrence of damaging earthquakes (M4+; e.g.,