Geographical Distribution of Surface-Wave Velocities and 3-D Upper-Mantle Structure In Africa

Long-period records of surface waves from WWSSN, GDSN, IDA and GEOSCOPE stations are analysed to retrieve purely continental dispersion of the African continent in the period range of 20–200 s for Rayleigh waves and 20–125 s for Love waves. The phase and group velocities are obtained by two-station and single-station methods, with earthquakes within or near the continent. A regionalization without any a priori constraints is performed over the whole African continent to retrieve local phase and group velocities. Phase- and group-velocity geographical distributions show in the northeast of Africa, near the Red Sea, a region with lower velocities extending to the west, and high velocities in the northwestern and southern parts of Africa. At long periods, there is a general agreement with global models, but the resolution is improved in this regional study. In the resulting 3-D structure, cratonic areas show a fast lithosphere from the NW to the SE. Whereas the high SV velocities of the NW area extend deeper than 200 km, lower velocities appear below this depth in the centre. In the Red Sea region low velocities are locally deep-rooted, with a slow lithospheric layer 150 km thick extending to the west. In this region polarization anisotropy is evidenced by Love-Rayleigh incompatibility. Short-period lateral variations are used to model the crustal thickness.