Seismic investigation of the crust and upper mantle of east Africa

Abstract

Interstation Rayleigh - and Love wave phase and group velocities have been determined for seismic recording stations in East Africa. Single station group velocities have also been determined for events in the African continent. The phase velocities cover the period range of 15 - 60 sec and the group velocities extend from 12 - 50 sec. The most characteristic feature of this set of Rayleigh wave phase velocity curves is that there is a definite merging of phase velocities from longer to shorter periods. The spread at 60 sec period is 0.33 km/sec whereas at 20 sec period this is reduced to 0,1 km/Fourier analysis method is used to determine the phase velocities and Multiple filter techniques to determine the group velocities. A number of non-linear optimization techniques have been developed to derive the shear velocity - depth structure of the crust and upper mantle from surface wave dispersion data. These techniques have a number of advantages over Least Square Inversion techniques. The non uniqueness problem is investigated in detail; and a method is proposed to determine the order of magnitude of the thickness of the layers to overcome such a problem. The structural models derived show that the total thickness of the crustal layers is roughly the same throughout East Africa, and is between 34 to 42 km, except along the centre of the eastern rift valley where crustal thinning is indicated by refraction and gravity studies (Griffiths et al, 1971). There is some indication of modification of the lower crustal layer towards shear velocity along the flanks of the eastern rift valley system. The upper mantle structure beneath the eastern rift valley system is similar to that beneath Iceland; further this structure extends to a greater distance south of Nairobi. It seems the eastern and the western rift valley systems are separated by a shield type structure Lake Victoria. P - delay at Addis Ababa and Nairobi is found to be nearly the same as that for stations in the western United States and Iceland. P - delay at Lwiro is about half as that at Nairobi. The rift valley system in East Africa has not yet developed to a stage comparable to Mid-ocean ridges.