Upper mantle structure of southern Africa from Pnl waves

We modeled Pnl waveforms from two moderate‐sized normal‐fault earthquakes in Zambia to determine an upper mantle P wave velocity model for southern Africa. The event parameters are May 15, 1968, mb = 5.7, depth = 28 km; and December 12, 1968, mb = 5.9, depth = 6 km. Teleseismic body wave inversions by previous researchers constrain the focal parameters for these events. We compute synthetic seismograms for six upper mantle velocity models using a wave number integration algorithm until an acceptable fit to the data is obtained. These models have different mantle P wave velocity gradients and low‐velocity zone (LVZ) depths. Quality of fit is measured primarily by the Pn/PL amplitude ratio. Source‐station geometry allows for the independent sampling of the upper mantle beneath the Kaapvaal‐Zimbabwe craton and mobile belt structural provinces. Synthetics from a constant velocity mantle model do not contain prominent precursor arrivals seen in the data; these are interpreted as P waves turning in the upper mantle. These synthetics also produce a too low Pn/PL amplitude ratio. Synthetics for a model with a mantle P wave velocity gradient of 0.0033 s−1 and a LVZ no shallower than 170 km fit the cratonic path data adequately. A slightly lower gradient, with a LVZ no shallower than 150 km, is indicated for the mobile belt regions. All the data preclude a mantle lid velocity gradient as high as 0.0066 s−1. Different velocity gradients between the two provinces implies different thermal gradients, which supports the hypothesis that a deep, cool lithospheric root exists beneath the Kaapvaal‐Zimbabwe craton. Cratonic thermal gradients derived from our velocity gradients agree closely with those derived from thermal modeling by Ballard and Pollack [1987].