Seismic Anisotropy in the Lower Mantle Beneath North America from SKS-SKKS and PKS-SKKS Splitting Discrepancie

Abstract:

Measurements of seismic anisotropy provide useful constraints on mantle deformation. In particular, shear wave splitting reflects anisotropic propagation of body waves within the Earth’s interior, and can be used to constrain patterns of deformation in the mantle. While shear wave splitting is a commonly used technique to study mantle anisotropy, it is most often applied to the upper mantle. While most of the lower mantle is thought to be isotropic, many studies have demonstrated that the D” layer at the base of the mantle exhibits significant anisotropy. Here we present measurements of the splitting of pairs of core-refracted shear phases (SKS-SKKS and PKS-SKKS) measured at stations in eastern North America. The goal of this study is twofold: first, we wish to place tighter constraints on the geometry of seismic anisotropy in the lowermost mantle beneath North America, and second, we wish to understand whether there is a contribution from lowermost mantle anisotropy to the splitting of SK(K)S phases beneath eastern North America. Previous measurements of SKS splitting at stations in this region (Long et al., 2010) showed complicated splitting patterns, with likely contributions from both the lithospheric and asthenospheric upper mantle. We analyzed seismograms measured at stations in the southeastern US that showed clear arrivals for pairs of phases (SKS-SKKS or PKS-SKKS) with similar paths in the upper mantle, but different paths in the lower mantle. Many regions of the lowermost mantle beneath North America do not show evidence for differential splitting of these phases. However, we find clear discrepancies between the measured splitting parameters of the different phases for the same event-station pairs for a group of raypaths that sample the lowermost mantle along the margin of a fast shear anomaly beneath the Midwestern US. This is a straightforward indicator of seismic anisotropy in the lowermost mantle in this region, consistent with previous studies.