Heterogeneity in the lower mantle beneath Africa from S and ScS waveforms

The African plate has 25 hotspots on land, eight at see and ten near the surrounding region. This is the greatest concentration of hotspot on the Earth. The African continent is also characterized by the large scale topography such as basins and swels, and rifting. According to the HINEP model, the Nyiragongo/Nyamuragira hotspot in the central Africa is antipoes to the world's largest hotspot Hawaii in the central Pacific. A knowledge of the condition at the base of the mantle is necessary to understand not only the dynamics occurring beneath Africa but also the mantle plume activity in the Earth. We have mapped S wave velocity variations and apparent Q values near the base of the mantle beneath Africa and the bordering region, and have discussed the results with reference to hotspot-mantle plume dynamics.
Sixteen intermediate-depth and deep focus earthquakes recorded by the long-period seismograph at the 46 seismological stations of WWSSN and Lwiro were digitized by the image processing system. Transversely polarized S and ScS phases are analyzed with respect to the differential travel times and the differences of anelastic parameter t* between two phases. The residuals of the ScS-S differential travel times have strongly positive correlation with those of the ScS travel times. The residuals decrease with increasing the distance from the central Africa, and become negative values at the horizontal distance of 60 degrees. The pattern of this decreased shows a concentric circle centered at the central Africa. The ScS bounce point of the largest positive residuals are located beneath the central Africa. These positive residuals can be explained by the 2% slower S velocity near the base of the mantle below 2000 km depth than thoat of PREM, or by the negative velocity gradient (1.89 x 10 ^-3 s^-1) in the D" region with thickness more than 200 km.
Anelastice parameters t* of S waves propagating beneath Africa is larger about 1 than those of ScS waves, suggesting that apparent Q values in the mid-mantle (1000-2000 km depth) is lower than those near the base of the mantle below 2000 km depth.
The position of the largest positive residual is close by the Nyiragongo/Nyamuragira hotspot, and also by the various geophysical poles such as the symmetrical axis of lower mantle heterogeneity, the geoid height, the moment of inertia of the crust, and the geotectonic cneter. These results, in cooperation with the geophysical and geological evidences, support an existence of the mantle plume near the base of the mantle below the central Africa.