Shear Wave Splitting Analyses in West Texas
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We have analyzed shear wave splitting data from local earthquakes in the Permian Basin in West Texas to understand crustal stress and induced seismicity. The selected events of ML2.0 are recorded by the TexNet and USArray/Transportable Array (TA) stations. SWS parameters, the fast polarization direction and the delay time, are computed using a semi-automatic algorithm. Only a single measurement is obtained at several TA stations near the west and east boundary of the Midland Basin. More SWS parameters are determined in the Delaware Basin and the Snyder area. Fast directions show a dependence on source location and the ray paths, indicating a heterogeneous anisotropic structure. In the Snyder area, varying fast directions are observed at eight closely located stations. The NE-SW fast direction is roughly consistent with the maximum horizontal stress direction as well as the strikes of active normal faults, and the NW-SE fast direction is attributed to the NW trending strike-slip faults. The different combinations of the multiple faults may be the cause of other different fast directions. SWS results are unique in the Delaware Basin. First, the delay time tends to be small at long distances, reflecting varying anisotropy along ray paths. On the other hand, the large delay times are from close and shallow events, implying a highly anisotropic upper crust. Second, the stations in the high seismicity region have significantly diverse fast directions. Such diversity could be explained by the presence of multiple sets of cracks that have different orientations. This situation is possible in the crust with high pore pressure. In addition, the complex anisotropy is also attributed to the cracks generated by hydraulic fracturing in the Delaware Basin. There is no systematic change in the SWS measurements over time and no clear correlation between the splitting parameters and the rate of seismicity. We propose that the diversity of fast SWS directions could be a typical phenomenon in the regions with a high rate of induced seismicity. However, more SWS measurements are necessary to support the interpretations made from the data.