Abstract

A shallow‐borehole array was employed to record induced seismicity sequences that occurred during multistage hydraulic‐fracturing operations near Fox Creek, Alberta. Using continuous three‐component (3C) waveforms, we performed moment‐tensor inversion for 530 high‐quality induced events with Mw>0.2. The source mechanisms are dominantly strike slip with subvertical nodal planes. A distinct cluster of events is characterized by more complex mechanisms, with slip on a shallow‐dipping plane accompanied by significant (>30%) non‐double‐couple (non‐DC) components; the non‐DC components may be due to tensile crack opening and/or coslipping on several fault strands that have been mapped using 3D seismic data. Stress inversion using two methods, assuming DC and shear‐tensile source mechanisms, respectively, yielded consistent results. These stress inversions made use of linear alignments of events to determine the correct nodal plane, yielding an inferred strike‐slip regime with SHmax in a direction of N60.4°E. This orientation is consistent with the nearest available borehole measurement from the World Stress Map but differs from the median regional SHmax direction by 15°. Our analysis suggests that north–south‐trending strike‐slip faults, which hosted most of the Mw>1.5 events, are misoriented for slip and therefore would require a considerable change in effective stress state (e.g., due to a pore‐pressure increase) to be brought to a state of incipient failure. Assuming a critically stressed state and using Monte Carlo analysis based on our stress inversion results, constrained by other sources of stress data, we estimate that 12±4  MPa change in pore pressure would be required relative to ambient pore‐pressure conditions.

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