Change in microseismic anisotropy lag time reveals stress changes around a fault Nadine Igonin* 1 , James P. Verdon 2 and David W. Eaton 1 1. Department of Geoscience, University of Calgary, Calgary, Canada 2. School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, UK Summary Using hydraulic fracturing induced seismicity data, we explore the possibility of using anisotropy analysis to determine stress changes due to induced earthquake rupture on a fault. A dataset from the Fox Creek, Alberta area is used, where the largest observed magnitude was Mw 4.1. It is found that the orientation of the fast S-wave direction remained constant over time, but the strength of the anisotropy varied – some stations experienced a systematic increase or decrease in the delay time. To interpret the results, we modelled the static stress transfer produced by slip on the faults that had been imaged by microseismic analysis. Stations that experienced increases in seismic anisotropy corresponded to zones of rock that would have experienced an overall reduction in hydrostatic stress, while stations that experienced decreases in seismic anisotropy corresponded to zones of rock that would have experienced an overall increase in hydrostatic stress. This behavior is what we would expect if static stress changes around a reactivated fault were modulating seismic anisotropy via the opening or closing of fractures or cracks. These observations show that monitoring of seismic anisotropy during hydraulic fracturing can be used to image changes in stress conditions in the target reservoir. Introduction Understanding induced seismicity due to fluid injection requires knowledge of the dynamics of fault activation. Fault activation may occur as a result of pressure and/or stress perturbation due to fluid injection. As faults are reactivated, movement on these features may also change the stress field in the surrounding rocks. In this study, shear-wave anisotropy from microseismic events is used to gain insight into the dynamically-changing stress field around a fault that has been activated due to hydraulic fracturing. The stress state around a fault is a critical factor for determining whether or not that fault will be prone to reactivation by fluid injection (Townend and Zoback, 2000). One way to characterize the subsurface stress state is by analyzing shear-wave anisotropy over space and time. Shear-wave anisotropy measures the polarization of the fast S-wave, and the difference in arrival time between the fast and slow S-waves. Seismic anisotropy occurs in materials that have a preferential fabric or orientation (structural- induced anisotropy), and in materials that are subject to non- isotropic stresses (stress-induced anisotropy) (Zoback, 2010). In the case of structure-induced anisotropy, the direction of anisotropy may be used to determine the orientation of key fabrics between the source and the receivers. Stress-induced anisotropy is controlled by the subsurface stress conditions. Since structure-induced anisotropy is determined by geological factors, it will remain constant over short timescales. However, since stress conditions may change quickly in the subsurface as industrial operations are carried out, changes in the strength and polarization of anisotropy can be used to infer changes in subsurface stress field (Andrews, 2016; Gajek et al., 2018). Li et al. (2019) carried out anisotropy analysis for events due to hydraulic fracturing fault activation. The authors used regional array data over a pad from Fox Creek, Alberta, and found a change in orientation and strength of anisotropy before and after the largest earthquake in the sequence. We have access to the local array data from this dataset, and one of the aims of this work is to validate the results in the paper and use the high density of the local array to better characterize the observations. The dense spatial coverage promises to provide higher resolution of the anisotropy near this well-studied fault, as well as temporal changes after the earthquake. We begin by introducing the dataset and methods, and then move into the analysis of the anisotropy measurements over space and time. Finally, we conclude with a discussion of a few possible physical mechanisms for the observations. Data In this study we analyze the Waskahigan dataset, from hydraulic fracturing of the Duvernay Formation in Fox Creek, Alberta. This pad was one of the first in the Fox Creek area to experience an earthquake above magnitude 4.0 (Bao and Eaton, 2016). This dataset has been the focus of several publications (e.g. Eyre et al., 2019a; Eyre et al., 2019b) and is a prime example of high quality microseismic observations being used to image fault reactivation by hydraulic fracturing. Figure 1a shows the geometry of the seismic monitoring stations in relation to the single well that was completed. There are a total of 98 shallow borehole array stations that blanket the entire area around the injection well. A 4.5 Hz 3- component geophone is at the bottom of each 27 m deep borehole. The average station separation is less than 500 m, allowing for high resolution epicentral locations to be obtained. This dense station coverage is advantageous for detailed spatial mapping of anisotropy. A study of anisotropy has already been performed at this site by Li et al. (2019). However, Li et al. used only the four broadband seismometer stations (WSK01-04), which are labelled in Figure 1a, severely limiting the spatial resolution of the results that they were able to obtain. The shallow borehole array was active for 17 days, from December 29, 2015 to January 16, 2016. A catalog processed 10.1190/segam2021-3583081.1 Page 1206 © 2021 Society of Exploration Geophysicists First International Meeting for Applied Geoscience & Energy Downloaded 09/22/21 to 3.235.143.148. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/segam2021-3583081.1