Seismic Anisotropy Layering Beneath the Northwestern United States Revealed by Shear Wave Splitting and Receiver Function Analyses

Abstract:

Splitting of P-to-S converted waves at the core-mantle boundary, including PKS, SKKS, and SKS (hereafter referred to as XKS collectively), is a commonly employed technique in structural seismology to delineate crustal and mantle azimuthal anisotropy. While XKS splitting measurements have an excellent lateral resolution, the vertical resolution is low due to the steep angle of incidence. In this study, we combine XKS splitting measurements with results of azimuthal anisotropy from fitting the systematic azimuthal moveout of the P-to-S conversions from the Moho and the 410‐km and 660‐km discontinuities to investigate anisotropy layering beneath the northwestern United States. We also use splitting of direct S waves from local events to determine anisotropy in the seismogenic zone of continental crust and the mantle wedge beneath the study area. Preliminary results obtained at approximately 1200 broadband seismic stations suggest that while the lower mantle is mostly azimuthally isotropic, weak anisotropy with E-W fast orientations in the mantle transition zone beneath the subducting Juan de Fuca slab is observed, which is consistent with results from some of the recent seismic tomographic studies. The results indicate that the bulk of the observed azimuthal anisotropy from XKS originates from the upper mantle.