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 2.43×10−3 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.