COMMENT ON "WAVE PROPAGATION EFFECTS AND THE EARTH'S STRUCTURE IN THE LOWER MANTLE"

Abstract. In a recent contribution, Haddon and Buchbinder introduced a new shear velocity model for the lower mantle which, combined with effects of velocity inhomogeneity at the base of the mantle, they concluded to be consistent with observed SH arrivals preceding the core reflected phase ScSH at distances beyond 70 ø. Using Kirchhoff wave theory, they computed synthetic waveforms for the radially symmetric version of their model (which has a 1.4 % discontinuous velocity decrease at 2300 km depth) that seem to match some SH data, even without allowing for lateral heterogeneity. However, synthetic waveforms for this model computed using generalized ray theory and reflectivity do not match the Kirchhoff synthetics or the data, casting doubt upon the conclusions drawn in their study. Earlier models by Lay and Helmberger, involving a 2.75 % discontinuous velocity increase at a depth of about 2600 km, require only slight modification in order to match all of the SH wave observations