Shear Wave Splitting and Pore Fluid Pressure in Northern Oklahoma Using the Community Wavefields Experiment Array

Abstract:

Measurable shear wave splitting has been reported in natural intraplate seismic zones and in regions of induced earthquakes within sedimentary basins in the central US. The fast S-wave orientation (Φ_f) and differential times (δt) following shear wave splitting are sensitive to pore fluid pressure (P_f) in a crack-critical system, but anisotropy can also result due to layering of sedimentary rocks. We seek to understand if shear-wave splitting can be used to track P_f changes at the earthquake source or along the local distance raypath, and hence be developed into a useful tool to forecast seismicity in areas of active industry development. Here, we present shear wave splitting results collected from local earthquakes recorded by a mix of broadband and nodal stations as part of the IRIS Community Wavefields Experiment. We adopt the Splitwavepy python package that utilizes an eigenvalue decomposition method best suited for direct S-waves. The preliminary shear wave splitting results collected from local distance M2.0+ events recorded by the 18 broadband stations describe complex Φ_f orientation distributions with a mixture of azimuth values paralleling the regional maximum and minimum horizontal stress orientations. Additionally, some broadband station Φ_f values appear to parallel the back-azimuth values rather than measuring any structural or stress-controlled anisotropy. We will extend the shear wave splitting analysis by incorporating the 363 3-component nodal stations from the array in order to gain further insight into the spatial variation of and effects of instrumentation on the shear wave splitting results. To correct true north of seismometers, orientation angles are estimated based on the ambient seismic noise analysis using the low frequency horizonal components of the waveforms. The earthquake catalog used for this project is composed of ~1300 earthquakes (M-0.5 - M3.0) with epicentral distances of <15 km from the array and it along with an associated focal mechanism catalog were developed by the authors for future publication.