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

Abstract

Summary 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.

Sharp and seismically transparent inner core boundary region revealed by an entire network observation of near-vertical PKiKP

“…Figure 4(b) shows that the strongest energy of the two phases can be observed between 0.8 and 2.0 Hz, indicating that the CMB and the ICB are relatively sharp (∼ few km). It is a common practice to use the amplitude ratio of PKiKP/PcP to infer the density jump at the ICB (e.g., Souriau and Souriau, 1989). The observed ratio is 0.125 which is comparable within the error bounds to the value (0.118) obtained for DSM synthetic seismograms (Takeuchi et al, 1996) calculated for PREM (Dziewonski and Anderson, 1981).…”

“…They also noted that the ICB must be sharp enough to reflect seismic energy at ∼1 Hz (i.e., the ICB thickness <5 km; Cummins and Johnson, 1988). Since their work, there have been attempts (not too many) to further constrain the physical properties of the ICB (e.g., Souriau and Souriau, 1989;Shearer and Masters, 1990). …”

Irregular topography at the Earth’s inner core boundary

“…In the precritical distances, PKiKP travel times and amplitudes are sensitive to topography, geometry, and property contrast across the ICB (11)(12)(13)(14)(15)(16). We adopted PcP waves [a compressional wave that is reflected off the core-mantle boundary (CMB)] as a reference phase and used PKiKP-PcP differential travel time and relative amplitude to study the ICB property.…”

Short wavelength topography on the inner-core boundary

“…Significant long-wavelength topography of the ICB is ruled out by dynamical considerations (4). Although hemispherical variations in the seismic properties at the top of the inner core have been documented (5-7), seismological investigations indicate that the ICB is, to a good approximation, quite spherical (8). However, the observation of significant PKiKP coda, likely due to multiple scattering (9,10), indicates that the structure of the ICB is more complex at short wavelengths.…”