AFTERSHOCKS AND FAULT ASEISMIC DEFORMATION IN SOUTHERN-CENTRAL ALBERTA ASSOCIATED WITH A HYDRAULIC-FRACTURING INDUCED EARTHQUAKE (Invited Presentation)

Seismic monitoring and imaging have been an effective tool to infer fluid migration within the upper crust. While much of the inferences rest upon the time-limited recordings of earthquakes, subtle signals from aseismic slip over a relatively long period of time have been successfully utilized in applications ranging from geothermal exploration to the understanding of cultural noise. A recent hydraulic-fracturing induced earthquake (with M_L 4.2) near Red Deer, Alberta, offers a rare opportunity to gauge both the seismic and aseismic deformations on a recently reactivated fault in the Western Canada Sedimentary Basin. In this study we analyze continuous data collected from a nodal array of 16 3-component geophones, deployed in quick response to the M_L 4.2 mainshock on March 4, 2019. We investigate the conditions of the fault zone and potential fluid migration through the combination of (1) high-precision earthquake relocations, (2) earthquake focal mechanisms, and (3) time history of micro-tremors over the three-week deployment period. Complemented by regional broadband data, we detected 417 events, and their spatiotemporal distribution and focal mechanisms suggest a NE-trending rupture with multiple strike-slip fault planes. Reactivation of pre-existing faults by pore pressure diffusion is likely responsible for the occurrence of the earthquake sequence.

Though waveform cross-correlation approaches and source migration, we were also able to detect aseismic deformation near the southern portion of the two related hydraulic-fracturing wells. These signals persisted over the three-week period and exhibited clear diurnal variation, where day-time detectability is partially hindered by ice-breaking events from the nearby Sylvan Lake (northwest), as well by cultural noises associated with the Red Deer city center (northeast) and Red Deer river (east and southeast). Spatiotemporal correlations between earthquake aftershocks and aseismic slip suggest that local stress perturbation, potentially in association with subsurface fluid migration, could last for weeks after the mainshock.