Full Moment Tensor Inversions of Microseismic Events Revealing Fault Activation of the 17 August 2015 Earthquake in the Northern Montney Formation, British Columbia, Canada

Full moment tensor source mechanisms of seismic events during fault reactivation of induced earthquakes provide critical information for microseismic monitoring, and the uncertainty quantification of these full moment tensors (MTs), particularly when station coverage is uneven and limited, is fundamentally important for understanding triggering mechanisms of these earthquakes. In this study, utilizing the open-source package MTUQ (Moment Tensor Uncertainty Quantification), we invert high-frequency P-phase amplitude ratios recorded by a dense 198-geophone surface monitoring array for source mechanisms of (micro)seismic events that occurred during hydraulic fracturing operations between August and September 2015 in the northern Montney formation, British Columbia. Specifically, we obtain full MTs and their uncertainties for 567 (micro)seismic events with Mw≥0.4 in the vicinity of the main fault plane along the Blueberry fault that resulted in an Mw∼4.6 induced earthquake on 17 August and another Mw∼3.17 event ∼3 hr and 28 min later. The full MTs of these two events show a dominant double-couple component, whereas in comparison, MTs of seven foreshocks starting from three and a half days before the Mw∼4.6 event show a noticeable positive compensated linear vector dipole component (+CLVD), suggesting a tensile opening component in the mechanisms possibly associated with poroelastic stress transfer and/or aseismic slip resulting from fluid injections. The aftershocks exhibit positive isotropic components in addition to the significant +CLVD, which may be attributed to the combination of aseismic slip and fluid migration through newly formed pathways after fault reactivation.