Summary

Seismological 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.

中文翻译：

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体波和正常模式数据中最外层分层的信号

概要

外核径向速度结构的地震学模型表明，最外核比PREM慢。对于从体波数据得出的模型，这些低速度被限制在外层岩心的顶部，而正常模式数据则倾向于在整个外层岩心中偏离PREM的速度梯度。这些不同的模型导致有关外部核心顶部是否存在分层的不同解释。虽然基于体波的模型已显示需要组成分层的最外层核，但从正常模式数据获得的速度和密度剖面对应于均匀的外层核。此外，在最外层岩心中观察到的低速很难与分层的组成模型相吻合，因为所需的轻元素富集通常会提高地震速度。在这里，我们研究了地震体波数据和法向数据如何适合约束外部岩心的速度和密度结构。为此，我们建模并比较了外核结构和D''结构对体波相位SmKS的微分传播时间和正常模式的中心频率的影响。我们发现两种数据类型都存在外核结构和D''结构之间的折衷，但是没有任何数据可以通过合理的D''速度和密度来容易地解释。因此，地震数据仍然要求最低外层速度。使用来自斯通利模式中心频率的其他信息（正常模式对外部核心顶部的速度和密度变化特别敏感），我们确认正常模式数据的确需要相对于最外部核心的PREM较低的速度，类似于最近的正常模式模型，并且总体上具有更高的外核心密度。因此，斯通利模式的中心频率不能立即支持最外层核中浮力分层的存在。正如我们从最直接的分层形成过程中所期望的那样，高地震速度的分层与我们的结果直接矛盾。以及总体上更高的外芯密度。因此，斯通利模式的中心频率不能立即支持最外层核中浮力分层的存在。正如我们从最直接的分层形成过程中所期望的那样，高地震速度的分层与我们的结果直接矛盾。以及总体上更高的外芯密度。因此，斯通利模式的中心频率不能立即支持最外层核中浮力分层的存在。正如我们从最直接的分层形成过程中所期望的那样，高地震速度的分层与我们的结果直接矛盾。