On the statistical distribution of seismic velocities in Earth's deep mantle

Abstract

The existence of uncoupled shear ( S) and compression ( P) wave velocity variations in Earth's mantle is a characteristic that might only be explained by the presence of significant chemical and/or phase heterogeneity, with important implications for the dynamics and evolution of Earth's interior. While making a one-to-one comparison between tomographic models for P and S velocity ( V P and V S ) variations for a particular geographic region is ill-posed, their global statistical distributions reveal several robust characteristics indicative of the nature of uncoupled V P and V S in the deep mantle. We find that all of the V P and V S model distributions at a given depth are Gaussian-like throughout the lowermost mantle. However, a distinct low velocity feature is present in V S distributions below ≈ 2200 km depth that is not present or is relatively weak in V P models. The presence of anomalously low V S material cannot be explained as an artifact, nor can the absence of a similarly strong feature in P models be ascribed to under-resolution. We propose that this feature can be partly explained by laterally variable occurrences of post-perovskite (pPv) lenses in the D″ layer, however, the persistence of significantly slow V S regions at heights up to ≈ 700 km or more above the core–mantle boundary is likely to be incompatible with a pPv origin and might only be explained by the presence of a laterally discontinuous layer of chemically distinct material and/or some other kind of phase heterogeneity. There also exist significant discrepancies between tomographic models with respect to the width of the distributions as well as differences between the modeled peak values. We propose a scheme for comparison between different seismic models in which the widths of the dominant features in their statistical distributions is exploited.