Seismic evidence for ultralow-velocity zones beneath Africa and eastern Atlantic

SKS waveforms recorded at distances of about 110 degrees are extremely useful to constrain seismic velocity structure at the base of the mantle. SKS waves near this distance develop a complicated interference pattern with the phases SPdKS and SKPdS. We report anomalous behavior of this interference in a number of recordings of deep earthquakes beneath South America from stations in Europe and Africa. We model these data with two-dimensional dome-like structures at the base of the mantle which extend laterally by a few hundred kilometers and in which the shear velocity is up to 30% lower than in the Preliminary Reference Earth Model (PREM). The spatial extent of these structures, their position with respect to the SKS core exit points, and their seismic characteristics can not be uniquely determined. However, the presence of a dipping or a concaved upper interface is a key attribute of successful models. Models that invoke flat layers are insufficiently complex to explain the most erratic waveform behavior. The most anomalous data correspond to sampling regions at the base of the mantle beneath the East African Rift and beneath the Iceland, where possibly, whole mantle upwellings form.