Fig. 1. Theoretical mantle structure model compared with corresponding tomographic images for Africa’s Afar region, where the African plate is splitting in two. (top left) A theoretical model that consists of two regions of high wave speed flanking the rift in the upper mantle. (top right) A relative arrival time tomographic result obtained by analyzing seismic waves that would pass through such a theoretical model. A strong low-velocity feature, which is an artifact of the analysis method, appears in the well-resolved region between the two high-velocity flanks. Conversely, an input model consisting of only a slow wave speed anomaly beneath the rift would result in erroneously fast wave speed rift flanks. (bottom) Cross-sectional views of (left) the theoretical model and (right) the tomographic result. Credit: Ian Bastow