Mantle discontinuities under southern Africa from precursors to P′ P′df

We investigate the reflection properties of upper‐mantle discontinuities beneath southern Africa using precursors to the df branch of PKPPKP (P′ P′). The P′ P′df branch is weaker than the ab and bc branches, but it does not have the complication of a caustic and appears across a wider distance range. Stacks from hundreds of short‐period seismograms recorded in California from the March 9, 1994 Tonga earthquake (Mw = 7.6) show an ∼5% reflection (at 3.5 s dominate period) from 660‐km depth indicating a sharp “660” under southern Africa. A 3.5 s period reflection from 410‐km depth is also visible in these stacks, but only ∼2% the strength of P′ P′df. This result contrasts with the observation of the “410” and the “660” reflecting comparable amounts of high‐frequency energy under the Indian Ocean [Benz and Vidale, 1993a], indicating either a diffuse “410” boundary under southern Africa or global variations in the impedance change across the “410”. A 1.5 s period reflection may indicate the existence of fine‐scale heterogeneity near 320‐km depth. Reflectivity synthetic seismograms also show that a previously claimed reflection from 785‐km depth has the more likely explanation as PcPPKP.

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