J362D28: a new joint model of compressional and shear velocity in the Earth's mantle

Abstract

The discrimination between chemical and thermal heterogeneity in the Earth's mantle remains one of the most important and challenging questions to be answered by observational and theoretical geophysics. To answer this question requires a thorough knowledge of the ratio between compressional and shear velocity anomalies. We describe results of a joint inversion for compressional and shear velocity in the mantle using a large and diverse data set consisting of traveltimes, complete waveforms and surface wave dispersion measurements. A horizontal tessellation consisting of 362 spherical splines is used to parametrize the model, which is approximately equivalent to a spherical harmonic of degree 18 in resolution. The model contains peak variations (from PREM) of up to ±7 per cent in S velocity and ±2.5 per cent in P velocity in the upper mantle. These variations decrease to ±1.5 and ±0.6 per cent, respectively, at 1000 km depth and reach ±2.5 and ±1.0 per cent, respectively, in the D″ region. The rms ratio of S to P velocity perturbations is fairly constant between 2.0 and 2.5 in the lower mantle, but a local minimum in this ratio occurs at a depth of approximately 1700 km. Resolution tests show that the recovery of P and S velocity is not geographically uniform, but also show that this amplitude ratio is well resolved between 670 and 2700 km depth. We also observe a persistent negative correlation between bulk-sound and shear velocity throughout the same region of the mantle. In addition, the model contains a minimum correlation between P and S velocity between 670 and ∼1100 km. This feature is supported by both the favourable outcome of resolution experiments and the poor fit provided by the starting model from the inversion (in which the P and S velocities are perfectly correlated) to our data set of P-wave traveltimes. The power spectra of both P and S velocity heterogeneity are similar, although we note a slightly larger dominance of degree two in the spectra for P velocity in the mid-mantle where resolution is highest.