Abstract
SummarySeismological models of the outer core’s radial velocity structure show that the outermost core is slower than PREM. For models derived from body-wave data these low velocities are confined to the top of the outer core, while normal-mode data prefer a velocity gradient that deviates from PREM throughout the entire outer core. These different models have led to conflicting interpretations regarding the presence of stratification at the top of the outer core. While body-wave based models have been shown to require a compositionally stratified outermost core, the velocity and density profiles obtained from normal-mode data correspond to a homogeneous outer core. In addition, the observed low velocities in the outermost core are difficult to reconcile with compositional models of stratification, as the required enrichment in light elements would generally increase seismic velocities. Here, we investigate how well-suited both seismic body-wave and normal-mode data are to constrain the velocity and density structure of the outer core. To this end, we model and compare the effects of outer-core structure and D″ structure on the differential traveltimes of body-wave phases SmKS and on the centre frequencies of normal modes. We find that a trade-off between outer-core structure and D″ structure exists for both data types, but neither data can be readily explained by reasonable D″ velocities and densities. Low outermost-core velocities are therefore still required by seismological data. Using additional information from the centre frequencies of Stoneley modes—normal modes that are particularly sensitive to variations in velocity and density at the top of the outer core—we confirm that normal-mode data indeed require low velocities with respect to PREM in the outermost core, similar to a recent normal-mode model, and an overall higher outer-core density. The presence of buoyant stratification in the outermost core is therefore not immediately supported by the centre frequencies of Stoneley modes. Stratification with high seismic velocity, as one would expect from most straightforward stratification-forming processes, is directly contradicted by our results.
Highlights
Earth’s liquid outer core is considered approximately homogeneous in composition, as a result of the vigorous convection that is required to sustain the geodynamo that generates Earth’s magnetic field
Using synthetic seismograms calculated at every 0.5◦ between 50◦ and 170◦ for PREM and PREM with KHOMC (Kaneshima & Helffrich 2013) in the outer core, we calculate vespagrams for groups of seismograms per 10◦ distance
We have shown that SmKS differential traveltimes require a velocity gradient in the outermost core that is steeper than EPOCVinet and similar to KHOMC
Summary
Earth’s liquid outer core is considered approximately homogeneous in composition, as a result of the vigorous convection that is required to sustain the geodynamo that generates Earth’s magnetic field. In a well-mixed outer core, elastic properties change with increasing pressure along an adiabatic gradient as defined by an equation of state (EoS). Deviations of the velocity and density gradients from the self-compression profiles defined by the EoS may be used as indicators of compositional heterogeneity (Birch 1952) in the outermost core Seismic data indicate a stratified outer core: other observations that appear to require outermost-core stratification are fluctuations in the Earth’s magnetic field with a period of approximately 60 yr that can best be explained by weaker, thermal stratification, as opposed to stronger, compositional stratification (Buffett et al 2016).
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