The heterogeneity spectrum of Earth’s upper mantle constrained by global observations of scattered P-waves

Abstract:

Placing observational constraints on the spectrum of seismic-wavespeed fluctuations throughout Earth’s mantle improves our understanding of processes that generate, preserve, and destroy heterogeneity. Constraints on the heterogeneity spectrum from tomography have been limited to very large (>1000 km) length scales, but scattered energy in high-frequency waveforms suggests that significant structure exists at smaller scales (~10 km). Here we present constraints on intermediate-scale heterogeneity in the upper mantle from globally-averaged P-coda amplitudes. Using a particle-based “phonon” method to generate synthetics, we find that the heterogeneity power is inversely proportional to wavenumber at length scales between 5 and 500 km. The best-fitting r.m.s. velocity perturbation over this range of length scales is 6%, assuming that the heterogeneous layer is 600 km thick. Calculations using a spectral-element method verify this result for periods longer than 17 seconds.

It has been proposed that mantle convection produces strains that stretch, fold, and stir subducted lithosphere to smaller scales. To test if this process provides a viable explanation for the observed heterogeneity spectrum, we plan to simulate 5 billion years of heterogeneity production, subduction, and stirring. Using relationships from mineral physics, the final compositional field can be converted to a three-dimensional velocity model, for which synthetic seismograms can be computed and compared with actual data.