The signal of outermost-core stratification in body-wave and normal-mode data
The signal of outermost-core stratification in body-wave and normal-mode data
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DOI：
10.1093/gji/ggaa368
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发表时间：
2020-11-01
影响因子：
2.8
通讯作者：
Thomas, Christine
中科院分区：
文献类型：
--
作者：
van Tent, Runa;Deuss, Arwen;Thomas, Christine
外核径向速度结构的地震学模型表明，最外层的核比PREM慢。对于从体波数据导出的模型，这些低速度仅限于外核的顶部，而正常模式数据更喜欢偏离PREM的整个外核的速度梯度。这些不同的模型导致了相互矛盾的解释，在外层核心的顶部分层的存在。虽然基于体波的模型已被证明需要一个成分分层的最外层的核心，从正常模式数据获得的速度和密度分布对应于一个均匀的外核。此外，观测到的最外层岩芯的低速很难与分层的成分模型相一致，因为所需的轻元素富集通常会增加地震速度。在这里，我们调查如何很好地适合地震体波和正常模式的数据来约束外核的速度和密度结构。为此，我们模拟和比较外核结构和D "结构的体波相位SmKS的微分走时和正常模式的中心频率的影响。我们发现，外核结构和D "结构之间的权衡存在两种数据类型，但没有数据可以很容易地解释合理的D"速度和密度。因此，地震学数据仍然需要较低的最外核速度。使用额外的信息，从中心频率的斯通利模式正常模式，特别是敏感的速度和密度的变化在顶部的外核-我们确认，正常模式的数据确实需要低速度相对于PREM在最外层的核心，类似于最近的正常模式模型，和整体较高的外核密度。因此，斯通利波模的中心频率并不直接支持最外层核中存在浮力分层。高地震速度的分层，正如人们所期望的最简单的分层形成过程，是直接矛盾的，我们的结果。
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.