Abstract

Rayleigh-wave phase velocities have been determined from an array of stations located near Johannesburg, South Africa, and from the World Wide Standard Seismic Network stations at Pretoria, Bulawayo and Windhoek. Multi-mode Rayleigh- and Love-wave group velocities have also been found from the records of a number of earthquakes originating near the Kariba Dam, and in southern Malawi. The phase velocities cover the period range 20-100 sec and the group velocities extend from 2-40 sec for the fundamental mode and 2-12 sec for the higher modes. The multi-mode dispersion obtained indicates that the crustal and upper mantle velocities to the east of a line through Pretoria and Bulawayo are higher than those to the west. The phase velocities for the array area confirm the high sub-crustal shear velocities previously determined from refraction studies. The phase- and group-velocity curves obtained for the Kariba-Pretoria path are similar to those for other shield areas. The central United States, eastern Australia and the western part of southern Africa have very similar Rayleigh-wave phase velocities, which are lower than those for the shield areas.

An iterative inversion technique has been developed and used to derive a shear-velocity model, which is in accord with the measurements for one phase-velocity and five group-velocity curves, for the path between Pretoria and Kariba. The possible existence of crustal low-velocity channels and thin, high contrast velocity reversals in the upper mantle are discussed.

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