Inferences on Flow at the Base of Earth's Mantle Based on Seismic Anisotropy

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Science  16 Jan 2004:
Vol. 303, Issue 5656, pp. 351-353
DOI: 10.1126/science.1091524

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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 ∼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.

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