Spatially Distinct Tectonic Zones across Oklahoma Inferred from Shear-Wave Splitting

To better understand relationships among crustal anisotropy, fracture orientations, and the stress field in Oklahoma and southern Kansas, we conduct shear-wave splitting analysis on the last 9 yr of data (2010-2019) of local earthquake observations. Rather than a predominant fast direction (o), we find that most stations have a primary fast direction of polarization (opri) and a secondary fast direction of polarization (osec). At most stations, either the primary fast direction of polarization (opri) or the secondary fast direction of polarization (osec) is consistent with the closest estimated maximum horizontal stress (sigma H max) orientation in the vicinity of the observation. The general agreement between fast directions of polarization (o) and the maximum horizontal stress orientations (sigma H max) at the regional level implies that the fast polarization directions (o) are extremely sensitive to the regional stress field. However, in some regions, such as the Fairview area in western Oklahoma, we observe discrepancies between fast polarization directions (o) and maximum horizontal stress orientations (sigma H max), in which the fast directions are more consistent with local fault structures. Overall, the primary fast direction of polarization (opri) is mostly controlled and influenced by the stress field, and the secondary fast direction of polarization (osec) likely has some geologic structural control because the secondary direction is qualitatively parallel to some mapped north-striking fault zones. No significant changes in fast directions over time were detected with this technique over the 5 yr (2013-2018) of measurements, suggesting that pore pressure may not cause a significant enough or detectable change above the magnitude of the background stress field.