title: Inferences on flow at the base of Earth's mantle based on seismic anisotropy.
abstract: We applied global waveform tomography to model radial anisotropy in the whole mantle. We found that in the last few hundred kilometers near the core-mantle boundary, horizontally polarized S-wave velocities (VSH) are, on average, faster (by approximately 1%) than vertically polarized S-wave velocities (VSV), suggesting a large-scale predominance of horizontal shear. This confirms that the D" region at the base of the mantle is also a mechanical boundary layer for mantle convection. A notable exception to this average signature can be found at the base of the two broad low-velocity regions under the Pacific Ocean and under Africa, often referred to as "superplumes," where the anisotropic pattern indicates the onset of vertical flow.
authors.affiliation: Berkeley Seismological Laboratory, 215 McCone Hall, University of California, Berkeley, CA 94720, USA. mpanning@seismo.berkeley.edu
authors.forename: Mark
authors.initials: M
authors.lastname: Panning
authors.forename: Barbara
authors.initials: B
authors.lastname: Romanowicz