A shear-velocity model of the mantle

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 +/-6% anomalies in shear velocity) in the topmost 400 km of the mantle and in the lowermost 500 km (up to +/-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 dis-continuity supporting the idea that an endothermic phase transition is not a barrier to large-scale flow in the mantle.