MANTLE DISCONTINUITIES UNDER SOUTHERN AFRICA FROM PRECURSORS TO P'P'(DF)

We investigate the reflection properties of upper-mantle discontinuiti es beneath southern Africa using precursors to the df branch of PKPPKP (P'P'). The P'P' df branch is weaker than the ab and be branches, but it does not have the complication of a caustic and appears across a w ider 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 similar to 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 similar to 2% the strength of P'P'(df). This result contrasts with the observation of the ''410'' and the ''660'' reflecting compar able amounts of high-frequency energy under the Indian Ocean [Bent and Vidale, 1993a], indicating either a diffuse ''410'' boundary under so uthern 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 seismogr ams also show that a previously claimed reflection from 785-km depth h as the more likely explanation as PcPPKP.