CONSTRAINTS ON DENSITY AND SHEAR VELOCITY CONTRASTS AT THE INNER CORE BOUNDARY

Abstract

The density jump $(ΔρICB )$ at the inner core boundary (ICB) is an important constraint on the dynamics and history of the Earth's core. Two types of seismological data sensitive to $ΔρICB $ have been studied since the 1970s: free oscillation eigenfrequencies and the amplitudes of core reflected phases (PKiKP /PcP ). The preliminary reference earth model (PREM) of Dziewonski & Anderson, based largely on normal mode data, has a relatively low value of $ΔρICB = 0.60 g cm-3 $, whereas most studies based on PKiKP /PcP amplitude ratios find significantly larger values, sometimes in excess of 1.0 g cm-3. It has been argued that, because PKiKP is rarely observed in the distance range considered (10-70°), the latter type of measurement provides only upper bounds on $ΔρICB $. We have analysed 10 yr of high-quality global broad-band data accumulated since the work of Shearer & Masters. We systematically analysed over 4500 seismograms from intermediate/deep events (depth >70 km) and nuclear explosions in the distance range 10-70°. The data were filtered in the bandpass 0.7-3 Hz. We performed rigorous data selection and identified five pairs of very clear (quality A), and 15 possible (quality A- ) PKiKP and PcP arrivals. In addition, 58 records showed no PKiKP but a clear PcP. Together, we obtain a much less dispersed data set than previously available, with the quality A data at the lower end of the ensemble of amplitude ratios versus distance. We combine our high-quality measurements with two measurements from the literature that fall within our rigorous selection criteria and obtain estimates of $ΔρICB $ in the range 0.6-0.9 g cm-3 and $ΔβICB $ in the range 2-3 km s-1. Our estimate of $ΔρICB $ is in agreement with a recent re-evaluation of normal mode data, thus reconciling results from body wave and mode studies and providing a tighter constraint on $ΔρICB $ for geodynamicists. Our study also provides evidence for a shear velocity gradient at the top of the inner core.