Seismic detection of a thin laterally varying boundary layer at the base of the mantle beneath the central-Pacific

We explore lowermost mantle structure beneath the Pacific with long-period recordings of the seismic core phases SKS, SP_dKS, and SKKS from 25 deep earthquakes. SP_dKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SP_dKS arrivals are explained by a laterally varying mantle-side boundary layer at the CMB, having P-velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large-scale low velocity feature in the lower mantle beneath the central-Pacific. SKS, SP_dKS, and SKKS data for the generally faster-than-average circum-Pacific lower mantle are well-fit by models lacking any such low-velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions.