The upper mantle discontinuities at the nominal depths of 410 and 660 km demarcate the upper and lower bounds of the mantle transitions zone and provide important constraints for constructing thermal and compositional models of the Earth. As seismic processing techniques and data density have improved, images of discontinuity structure have revealed more heterogeneous structure. These heterogeneities include variations in the depth, sharpness, and impedance contrast across the discontinuities over distances of just a few 100 km. Such variations require a revision of mantle compositional models and our understanding of factors responsible for discontinuity depths. This project is aimed at determining the depths and structure of the upper mantle discontinuities from the initial USArray data and developing techniques to study the degree to which anisotropy may be present at transition zone depths. The scientific goals of this work will be achieved by undertaking a systematic investigation into mantle structure in order to provide a seismic framework for future mantle models by estimating the sharpness of the upper mantle discontinuities and assessing the variability of discontinuity sharpness. Additionally, the azimuthal dependence of seismic properties in the mantle will be investigated in order to determine whether it can be due to anisotropy at the base of the upper mantle or transition zone. These observations will then be integrated with, and compared to, constraints from mineral physics experiments in order to improve models of mantle composition, and water content.
