Li, T; Gu, J; Wang, Z; Wang, R; Chen, Y; Song, T-R; Wang, R; (2019) Spatiotemporal Variations in Crustal Seismic Anisotropy Surrounding Induced Earthquakes Near Fox Creek, Alberta. Geophysical Research Letters , 46 (10) pp. 5180-5189. 10.1029/2018GL081766.

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Abstract

This study analyzes earthquake recordings from four near‐source (<10 km) stations near Fox Creek, Alberta, a region known for hydraulic fracturing‐induced seismicity. We examine the spatiotemporal variations of focal mechanisms and seismic anisotropy in the sedimentary strata. The focal mechanisms of surrounding earthquake swarms are generally consistent with the strike‐slip mechanism of the ML 4.6 earthquake, favoring a flower type of fault structure. The NE‐SW‐orientated fast splitting direction, determined from the shear wave splitting measurements, reflects the combined effects of (1) N‐S faults and (2) NE‐SW time‐dependent hydraulically stimulated fractures. The latter effect dominates the apparent anisotropy during the days leading to the mainshock, while its contributions are reduced by 60–70% after the mainshock. Loss of fluid into the fault damage zone, which causes the closure of fractures, is responsible for the observed spatiotemporal variation of seismic anisotropy near the hydraulic fracturing well. Plain Language Summary: The classic method that measures the travel time and polarization differences between two polarized shear waves, known as “shear wave splitting,” is widely used to determine directional‐dependent seismic wave speeds (i.e., anisotropy) at both global and exploration scales. This study takes advantage of industry data to measure and characterize the seismic anisotropy surrounding a hydraulic fracturing well near Fox Creek, Alberta. We analyze the induced seismicity and examine the earthquake source types and anisotropy in the sedimentary strata. The source mechanism confirms the previously reported N‐S striking, flower‐type fault system. The decreasing seismic anisotropy over time is most likely caused by the closures of hydraulic fractures surrounding the hydraulic fracturing well. This result is critical to the understanding of local seismic anisotropy, for both regulatory agencies and the industry.

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