Constraints on density and shear velocity contrasts at the inner core boundary

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.