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

SKS waveforms recorded at distances of about 110 degrees are extremely usef ul to constrain seismic velocity structure at the base of the mantle. SKS w aves 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 st ations 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 h undred kilometers and in which the shear velocity is up to 30% lower than i n the Preliminary Reference Earth Model (PREM). The spatial extent of these structures, their position with respect to the SKS core exit points, and t heir seismic characteristics can not be uniquely determined. However, the p resence of a dipping or a concaved upper interface is a key attribute of su ccessful models. Models that invoke flat layers are insufficiently complex to explain the most erratic waveform behavior. The most anomalous data corr espond to sampling regions at the base of the mantle beneath the East Afric an Rift and beneath the Iceland, where possibly, whole mantle upwellings fo rm.