Small-Scale Scattering Heterogeneities in the Lowermost Mantle From a Global Analysis of PKP Precursors

Small-scale heterogeneities contain important information on the composition of the Earth's interior and are vital in understanding the dynamics and evolution of the Earth. Here we explore a global data set of PKP waves in both array and network data and search for scattered waves arriving as precursors to PKPdf. We classify the observed precursors into incoherent and coherent types and find that scattering regions for these two types overlap at the base of the mantle. In addition, a global map of scattering strength displays strong regional variations with strong scattering mixed with weak scattering. The complex distribution of scattering strength indicates lateral variations of heterogeneities on short length scales in the lowermost mantle. A mixture model of localized strong scatterers embedded in a random medium could explain the observed precursors in this study.