Upper mantle structure of southern Africa from Pnl waves


Pnl waveforms from two moderate-sized normal-fault earthquakes in Zambia were modelled 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. Synthetic seismograms are computed 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. 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. -from Authors

Publication Title

Journal of Geophysical Research