John E. Vidale
Paul S. Earle
Harley M. Benz
Fei Xu
1998
<p>We investigate the reflection properties of upper-mantle discontinuities beneath southern Africa using precursors to the <i>df</i> branch of <i>PKPPKP</i> (<i>P′ P′</i>). The <i>P′ P′</i><i>df</i> branch is weaker than the <i>ab</i> and <i>bc</i> 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 (<i>M</i><sub>w</sub> = 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 <i>P′ P′<sub>df</sub></i>. This result contrasts with the observation of the “410” and the “660” reflecting comparable amounts of high-frequency energy under the Indian Ocean [<i>Benz and Vidale</i>, 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 <i>PcPPKP</i>.</p>
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10.1029/98GL00122
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AGU Publications
Mantle discontinuities under southern Africa from precursors to P′ P′<sub>df</sub>
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