Migration of SS precursor Data to Image Fine-scale Structure on the Upper Mantle Discontinuities Beneath Hawaii

Using a wavefield migration technique, we use the precursors to the seismic phase SS to study the reflectance and topography of the 410 and 660- km discontinuities in the region of the Hawaiian hotspot. Our dataset consists of broadband waveforms from 9 shallow focus events (0-75 km depth) in the southwestern Pacific recorded by seismic arrays in the Canadian National Seismic Network and by POLARIS, with central bouncepoints of the SS phase located within and outside of the proposed location of Hawaiian hotspot mantle plume. Migration allows us to constrain the topography, depth, and sharpness of these discontinuities in a region predicted to have a high degree of heterogeneity, and is not restricted to the assumption of horizontal reflectors used in slowness stacking approaches. Measurement of these parameters provides useful information for understanding the dynamics, thermal structure, and composition of a plume rising through the Transition Zone. Seismograms are instrument deconvolved, low-pass filtered at ∼ 6 seconds, and aligned on the SS phase to study the coherency of the precursors at depth. The wavefield is then migrated to each node in a 3-D grid of potential reflectors over a region spanning 170 to -140 deg E and 0-30 deg N, in increments of 1 deg laterally, and 10 km vertically. Initial migration results indicate a strong reflector at a depth of 630 km near the projected location of the hotspot, and at a depth of 660 (i.e. close to the global average) away from the hotspot. A 30 km elevation of the 660 km phase boundary is consistent with an excess plume temperature of ∼ 350 K for the negative Clapeyron slope of the ringwoodite to perovskite phase transition. The migrations show strong lateral amplitude anomalies in SS precursor Fresnel zones, indicating significant off great circle path contributions to observed waveforms. The migrated results are compared with slowness stacking of a much larger SS dataset consisting of 4500 broadband and filtered records, with extended coverage over most of the central Pacific. Stacks for these regions show a correlation with the migration results, though the effects of Fresnel zone sampling and azimuthal dependence bias the slowness stacks. Additional upper and lower mantle reflectors are imaged using our migration technique and will be discussed.