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

Abstract

S U M M A R YThe density jump ( ρ ICB ) 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 ρ ICB 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 ρ ICB = 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 ρ ICB . 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 ρ ICB in the range 0.6-0.9 g cm −3 and β ICB in the range 2-3 km s −1 . Our estimate of ρ ICB 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 ρ ICB for geodynamicists. Our study also provides evidence for a shear velocity gradient at the top of the inner core.

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

“…To make a 10% reduction of the impedance contrast at the ICB, up to about a 3% increase in melt fraction (translates into 0.37 km/s S-wave velocity reduction) may be required. To account for a 20-30% reduction, the S-wave velocity just below the ICB may be as low as ∼2.5 km/s, which is in a similar range as some of recent models (Koper and Pyle, 2004;Cao and Romanowicz, 2004). Thus, it appears possible to discriminate different views of the ICB once we know the overall feature of the ICB region.…”

Core formation and core composition from coupled geochemical and geophysical constraints

“…On the other hand, the high oxygen concentration in the outer core elegantly solves for one of the core's most perplexing observations: that of the very strong density jump (4.8-7.4%) at the ICB (36,37), between the liquid outer core and solid inner core. The density change due to melting is on the order of 1.7% (38), and the observation naturally calls for strong chemical buoyancy and a stark chemical contrast between the inner core and outer core; this can only be achieved by the presence of a very .…”

Constraints from material properties on the dynamics and evolution of Earth’s core

“…Body waves have much better resolution of a few kilometers, but the estimates vary widely because PKiKP is a noisy phase [34][35][36] ; they give an upper bound 36 of 1.1 gm cc −1 . There is also evidence for an anomalously dense layer in the lowermost 150 km of the outer core 37 , which probably has a chemical origin 38 .…”