Local sharpness and shear wave speed jump across the 660-km discontinuity

Abstract

We examine vertical component short-period teleseismic seismograms from earthquakes in the Izu-Bonin subduction zone recorded by networks in the western United States for phases associated with conversions from mantle discontinuities. The dominant phase in the stacked P coda is the result of near source S-to-P conversions from a subhorizontal discontinuity at a depth ranging from 650 km to 745 km. We previously used 88 timings of this phase, called S660P, to determine the topography of the 660-km discontinuity. Employing the 17 best recorded S660P phases, we modeled the S660P amplitude accounting for attenuation and correcting for three-dimensional discontinuity topography. Just to the east of the Izu-Bonin subduction zone, the 660-km discontinuity is sharp (< 10 km) and the S-wave velocity contrast across the discontinuity is 0.60 ± 0.11 km s-1. This value is 60% larger than the preliminary reference Earth model (PREM) value (70% larger than in the iasp91 and ak135 models) and, unlike estimates from reflected SH waves, is independent of the estimated density contrast at 660 km. The large S-wave velocity contrast inferred here is consistent with recent mineral physics experiments and extrapolations if the mantle at 660 km depth and a few hundred kilometers east of the slab is at near normal mantle temperatures and contains between 3% and 5% cation aluminum, as expected in a pyrolitic mantle.