| Summary: | SKS waveforms recorded at distances of about 110° 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 SP_dKS and SKP_dS. 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. © 2000 by the American Geophysical Union. Received July 12, 1999; revised March 27, 2000; accepted April 13, 2000. This research has been supported by National Science Foundation grant EAR97-2508. We thank Michael Gurnis for his helpful suggestions and the thoughtful comments by the referees, Rob van der Hilst, Barbara Romanowicz, and John Castle. Contribution 8633 of the Division of Geological and Planetary Sciences, California Institute of Technology. Published - 2000JB900143.pdf
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