Three-dimensional structure of the African superplume from waveform modelling

Previous 2-D modelling of seismic waveforms and traveltimes has revealed a large-scale ridge-like velocity anomaly beneath Africa, which is usually referred to as the African superplume. The structure starts from the southern Indian Ocean and extends northwestwards into the Atlantic Ocean, with its base on the core-mantle boundary. The structure has relatively sharp lateral boundaries with the shear velocity inside 3 per cent lower than the ambient mantle, while the compressional velocity is almost normal. The 3-D structure is best illustrated by seismic waveforms recorded by the South Africa Array generated by earthquakes in the western Pacific Ocean. The diffracted S phase travels along the axis of the structure for over 60 degrees, with the northern- and southernmost paths sampling its edges. The S waveforms are simple but delayed by up to 20 s for paths sampling the middle of the structure, whereas they display two arrivals for paths along the boundaries. These complex waveforms can be explained by 3-D multipathing due to rapid lateral variations in shear-wave velocity along the edges of the structure. These sharp features are confirmed by modelling broadband records associated with the proposed ridge structure with two independent methods: the spectral element method and a ray-based 3-D code (DWKM).