Abstract

More than 60 small earthquakes (ML 0.7–3.0) were detected from December 2011 to March 2012 north of Cardston, Alberta, an area with little evidence for previous seismic activity. The timing of these events closely correlates (>99.7% confidence) with hydraulic fracturing completions of the Devonian–Mississippian-age Exshaw Formation at a nearby horizontal well. Unanimous waveform multiplicity within the swarm suggests that the events share a similar origin and source mechanism. This observation is corroborated by the point-like collocation of hypocenters within the crystalline basement during robust, double-difference relocations. Furthermore, the presence of a pre-existing fault is confirmed via formation-top offset mapping and interpreted to be a Late Cretaceous extensional fault. The confirmation of this fault at depth provides a plausible pathway for rapid hydraulic communication from the fracturing interval into the crystalline basement. Consistent with structural interpretations and available stress information, moment tensor inversion of the largest magnitude event (Mw 3.0) indicates reactivation of a basement fault with normal slip. We conclude that the genesis of this earthquake swarm was likely caused by increased pore pressure, within the basement fault, as a result of fracturing stimulation.

Online Material: Figure showing frequency–magnitude distribution of earthquakes, and table of the velocity models used in the study.

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