Anisotropy of the African continent inferred from surface waves

Abstract

A no-block regionalization is performed over the African continent to retrieve simultaneously local surface wave velocities and the 2ψ azimuthal variation of Rayleigh waves. The geographical distribution of phase and group velocities shows general agreement with global models, but with enhanced lateral resolution. A correlation with surface tectonics is found even at long periods: low velocities occur in the tectonically active regions, and high velocities in the shield areas. The azimuthal anisotropy analysis results in a general north-south fastest direction for Rayleigh waves, which could be correlated with the African plate's absolute motion. The amplitude is of the order of 2%. An isotropic inversion with data corrected for effects of surficial layers is performed. Very slight differences appear between the corrected and uncorrected results. Lateral heterogeneities of Rayleigh and Love waves are then inverted simultaneously with the 2ψ azimuthal terms of Rayleigh waves by vectorial tomography to retrieve the 3D structures of S-wave velocity and anisotropy throughout the African continent. Good correlation between low velocities and small anisotropies is found and their location at depth suggests an origin for African hotspots.