Detection of a D" discontinuity in the south Atlantic using PKKP

A strong arrival in the early coda of major-arc PKKP_ab is observed on recordings of earthquakes in the Banda Sea region from the short-period, small-aperture Yellowknife array. The arrival has a move-out close to PKKP_ab with an average delay of ∼12 s. We show that the phase is most likely an underside reflection from a D discontinuity above the PKKP reflection point at the core-mantle boundary beneath the southern Atlantic on the edge of the African superplume. Differential travel times between PKKP_ab and PK^dKP (the underside reflection at the D discontinuity) is used to constrain potential D velocity models. The high amplitude of PK^dKP requires proximity to the b-caustic adding further velocity control. We find that a P-velocity model with a sharp positive velocity jump e1280 km above the core-mantle boundary, and a negative velocity gradient closer to the core-mantle boundary, fits the data well.