Lowermost mantle anisotropy beneath the northwestern Pacific: Evidence from PcS, ScS, SKS, and SKKS phases

In this study, we utilize data from broadband seismic stations of the Japanese F-net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern Pacific. The comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle, such as PcS/ScS or SKS/SKKS, can yield constraints on anisotropy in the D '' region. We measured splitting for SKS, SKKS, PcS, and ScS phases at F-net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region. We observed many examples of discrepant SKS/SKKS splitting and splitting of ScS, PcS, SKS, and SKKS phases that does not agree well with the known upper mantle anisotropy beneath individual stations. The most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle. In particular, for SKS/SKKS phases that sample the lowermost mantle just to the south and east of the Kamchatka peninsula, we observed generally N-S fast directions with delay times between 0.5 and 1.5 s. These data suggest the presence of a fairly large, coherent region of deformation in the lowermost mantle beneath the northwestern Pacific. Our preferred model for these observations is that solid-state flow at the base of the mantle induces a lattice-preferred orientation of lowermost mantle minerals, generating a seismically anisotropic fabric.