Probing the nature of 410- and 660-km discontinuities beneath hotspots using the SS-precursors

The seismic wave SS is accompanied by precursors resulting from underside reflections off the upper mantle discontinuities, most notably off the 410 and 660 km phase boundaries (S410S and S660S, respectively). These precursors potentially provide important seismic constraints on the depth, topography, and sharpness of upper mantle discontinuities, which translates to useful information on mantle dynamics, rheology, and composition; for instance, a thinned transition zone is expected in the vicinity of a hot plume, based upon the anti-correlation of the Clapeyron slopes at the 410- and 660-km discontinuities. In this work, we use the SS precursors to explore the regional seismic structure of 410- and 660-km discontinuities beneath major mantle hotspots (e.g. Iceland, Hawaii), by selecting station-earthquake geometries in the 100-160 degree distance range with SS bounce points at or near each hotspot. For example, we used over 500 broadband records (collected from the IRIS and the CNSN data centers) with bounce points located within several degrees of the Hawaiian hotspot to study the underlying discontinuity structure. We stacked data using a variety of geographical binning methods, and corrected the data for time perturbations predicted by tomographically derived mantle heterogeneity prior to stacking. S410S and S660S energy is detectable in some broadband stacks without low-pass filtering. We will present these data compared to finite-difference synthetics computed using axi-symmetric geometry, for models of upper mantle discontinuity perturbations, including predictions for discontinuity perturbations for upper versus whole mantle plumes.