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

Abstract

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 ( M w = 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 .