A shear - velocity model of the mantle

Abstract

We present a new model of shear velocity structure in the mantle which is designed to fit data-sets of absolute and differential body-wave travel times, surface-wave phase velocities over a broad range of frequencies, and structure coefficients of modes of free oscillation. The model is parametrized laterally by spherical harmonics (truncated at degree 16) and by 30 natural cubic B-splines in radius. Our best model features large-amplitude structure (up to $pm $6% anomalies in shear velocity) in the topmost 400 km of the mantle and in the lowermost 500 km (up to $pm $2.5%) with most of the power in the low harmonics (l < 6). The middle of the mantle is characterized by low-amplitude anomalies with a much whiter spectrum. Our models generally show no distinctive changes in structure (in either shape or amplitude) at the 660 km discontinuity supporting the idea that an endothermic phase transition is not a barrier to large-scale flow in the mantle.