Global mantle shear velocity model developed using nonlinear asymptotic coupling theory

Abstract

We present a three‐dimensional shear velocity model of the whole mantle developed using S H waveform data. The model is expressed horizontally in terms of spherical harmonics up to degree 12, and vertically in terms of Legendre polynomials up to degrees 5 and 7 in the upper and lower mantle, respectively. What distinguishes this model from other tomographic models published to date is (1) the theoretical normal mode‐based wave propagation approach, where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately; (2) the wave‐packet weighting scheme which allows to balance contributions from high‐amplitude and low‐amplitude phases, increasing the resolution in some parts of the mantle. We also relax the constraints on the Moho depth, which is allowed to vary in the inversion, thus absorbing some uncertainties in crustal structure. The resulting model is generally in good agreement with other recent global mantle S velocity models and with some regional models. The rms profile with depth has more power than other models in the upper mantle/lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth. This model provides a particularly good fit to the non‐hydrostatic geoid through harmonic degree 12 (79% variance reduction), as well as good fits to observed splitting functions of S velocity sensitive mantle modes, indicating that both large‐scale and small‐scale features are really well constrained.