Surface wave tomography is being used to map the seismic velocity and anisotropy of the upper mantle on a global basis [Nataf et al., Anisotropy and shear-velocity heterogeneities in the upper mantle, Geophys. Res. Lett., 11, 109-112, 1984].
The color figure shows cross sections of the upper 670 km of the mantle. (Note previous example published on the cover of Eos, April 17, 1984.) VSV is the velocity of vertically polarized shear waves, averaged with respect to azimuth. The orange regions are slow, presumably hot, regions of the mantle, although VSV can below due to crystal orientation, or anisotropy, as well as to high temperature. The ambiguity is removed by studying the anisotropy. The parameter XI is related to VSH-VSV, the difference in velocity between the two polarizations of shear waves. Olivine crystals oriented with the fast axis in the horizontal plane would give positive XI. This would be the expected situation for horizontal flow. Vertical flow is expected to give negative XI for an olivine-rich mantle. The cross sections for XI, with this interpretation, would be orange in regions of ascending and descending flow.