JENNIFER FLETCHER
Imaging Mantle Discontinuities Beneath the Western Pacific Using ScS Reverberations
We use shear waves, due to their low attenuation and high reflectivity off the free surface and core-mantle boundary, to image mantle discontinuities. The ScS reverberative interval is particularly useful because constructive interference increases signal to noise ratio. Using the technique described in Revenaugh and Jordan (1989), we use SH-polarized seismograms to derive ScS wave travel times and radial velocity changes. Zeroth-order reverberations constrain the quality factor Q and whole-mantle travel time; higher order reverberations provide internal mantle velocity and density changes. Using these, we calculate mantle reflectivity under Japan and the Kurile islands. We then employ a 1-D model of mantle discontinuities to produce synthetic data to match our calculated reflectivity. Under the Kurile Islands, we find evidence for discontinuities at 410 km and 660 km depth, at D", and, interestingly, a strong velocity increase at 520 km. Under Japan, we again find the 410, 520, and 660 km discontinuities as well as D". Interestingly, we also find strong evidence for a shallow Lehman discontinuity and a velocity increase at 811 km. One hypothesis is that this discontinuity results from a previously subducted slab prior to westward migration of the subduction zone. This motivates further research into the mechanics that formed this feature.