Complex anisotropy in D″ beneath the eastern Pacific from SKS–SKKS splitting discrepancies

Although observations of seismic anisotropy in the lowermost mantle are abundant, the mechanism which generates anisotropy in the D″ layer is not well understood. Most observational constraints on D″ anisotropy come from the splitting of shear waves that propagate (nearly) horizontally through the lowermost mantle. However, anisotropy in the D″ layer can also give rise to discrepancies in shear wave splitting for SKS and SKKS phases for the same event-station pair, as these phases sample different regions of the lowermost mantle. Here I report observations of strongly discrepant SKS–SKKS splitting observed at broadband stations in western Mexico and California. In particular, strong SKKS splitting with fast polarization directions near ~ 60° and delay times up to ~ 3 s is observed for a group of raypaths that sample a region of the D″ layer beneath the eastern Pacific Ocean. A comparison of SKKS splitting with SKS splitting observed from the same events, as well as with SKS splitting observed at a variety of backazimuths at each station, suggests that the anomalous anisotropic structure is in the lowermost mantle. My preferred model for the unusual anisotropic geometry in this region is large shear deformation in D″ at the edge of a region where slab material impinges upon the core–mantle boundary, resulting in lattice preferred orientation of lower mantle minerals.