Royal Society Publishing

The Constitution of the Core: Seismological Evidence

B. A. Bolt


The present best estimates of seismic velocities in the core are compared with the 1939 solution of Jeffreys, with emphasis on the remaining uncertainties and present resolution capability. The relative contributions of measurements of seismic body waves and terrestrial eigenspectra to the inverse problem of the determination of elastic parameters, density and damping in the core are compared. Linear perturbation algorithms and smoothing functions used with the spectral data reduce their capacity for fine structural definition. Between radii of 1400 and 3300 km (shell E), the outer core appears to be substantially homogeneous and non-stratified, with small or zero rigidity and a dimensionless seismic quality factor, Q, of order 10$^{4}$. It is a sufficient but still not a necessary condition that density $\rho $ follows precisely the Adams-Williamson equation in E; for an averaging interval of 400 km, estimates of $\rho $ have a standard error there of about 0.2 g cm$^{-3}$. There is as yet no unequivocal seismological evidence for or against a boundary shell (thickness less than about 200 km) at the top of the liquid outer core. At the bottom of the outer core, the evidence is becoming stronger that any reduction in the rate of increase with depth of P wave velocity $\alpha $ is confined to a minor transition layer little more than 100 km thick. The inner core has a sharp outer boundary at about 1216 km radius, but below it only average physical properties are estimated with any confidence. The average seismic compressional and shear velocities are about $\alpha $ = 11.2 and $\beta $ = 3.5 km s$^{-1}$ and 12.5 < $\rho $ < 13.6 g cm$^{-3}$, yielding a peculiar mean Poisson ratio of 0.44 or greater. At the inner core boundary, jumps in parameters are: $\Delta \alpha \approx $ 0.65, $\Delta \beta $ = 2.0-3.0 km s$^{-1}$ and $\Delta \rho \approx $ 1.0 g cm$^{-3}$. Recent travel-time and waveform synthetics suggest a strong increase of P (and perhaps S) velocity in the upper 300 km of the inner core, which could be interpreted as a mixing or melting effect. Damping properties in the inner core may have an unusual dependence on wave frequency with an order of magnitude increase in Q from 1 Hz to 4 mHz vibrations.

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