Velocities and Chemical Stratification in the Outermost Core

Abstract

Travel times of long-period SKS and S2KS phases recorded at the GDSN and WWSSN stations were analyzed to retrieve information about the radial structure of the outer core. We propose a new velocity model for the outermost core named KTH. The velocity at the top of the core is 8.016 km s-1, and the velocity in the outermost 200 km of the core is 0.11 km s-1 faster than that. of HALES and ROBERTS (1971), that in the outer 200∼400 km of the core is identical within 1% to that of HALES and ROBERTS (1971). We find that the inhomogeneity index η is slightly larger than unity in the outermost 100∼200 km of the core where a preferable value of η just beneath the core-mantle boundary is 1.5 which corresponds to Brunt-Vaisala frequency N of 8.6 × 10-4 s-1. We suggest that the outermost 100∼200 km of the core is stably stratified. The thickness of the stable stratified layer in the present study is not far from those of 70∼80 km thickness derived from theoretical considerations. By comparing possible lateral heterogeneity in the outermost core with temperature distribution from a geomagnetic study, we infer that a chemically stratified layer is preferable in the outermost core.