Ellipticity and density at the inner core boundary from subcritical PKiKP and PcP data

Abstract

The first part of this article gives new travel‐time data for the phase PKiKP at subcritical distances (Δ < 110°) obtained at the Warramunga array in Northern Australia. A compilation of the PKiKP subcritical travel times obtained from previous studies has also been carried out. In the second part, differential PKiKP‐PcP travel times at short distances (Δ<45°) are used to determine the inner core ellipticity and mean radius, subject to hypotheses on the equilibrium figure of the liquid core and the topography of the core‐mantle boundary (CMB). The coherence of the PKiKP‐PcP travel‐time residuals is increased when Morelli & Dziewonski's CMB topography is taken into account. According to various hypotheses, the values obtained for the ellipticity range from (1.9±2.2)×10−3 to (5.6±1.7)×10−3. This denotes a small inner core flattening corresponding to a decrease of the polar radius from (1.6±1.8) to (5.0±1.5) km with respect to a spherical inner core. The preferred solutions which involve the CMB topography, are consistent with the ellipticity 2.43times10−3 deduced from hydrostatic equilibrium of a rotating inner core. The last part of the paper concerns the amplitude variations of PKiKP. The amplitude ratio PKiKP/PcP at short distances (Δ<45°) is used to constrain the density jump δρ at the inner core boundary. δρ values of 1.35–1.66 g cm−3 are obtained for a quality factor in the liquid core ranging from 10 000 to infinity. These values, as well as a possible stratification in the inner core, are in favour of an inner core composition which is not pure iron. Copyright © 1989, Wiley Blackwell. All rights reserved