Collaborative Research: Towards improved imaging of the outermost core through determination of the effects of lowermost mantle heterogeneity and anisotropy

合作研究：通过确定最低地幔异质性和各向异性的影响来改善最外层地核的成像

• 批准号: 2307537
• 负责人: Daniel Frost
• 金额: $ 7.26万
• 依托单位: University of South Carolina at Columbia
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-10-01 至 2023-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2307537&HistoricalAwards=false
• 关键词: Collaborative; Research; Towards; improved; imaging

Earth’s outer core is molten iron, plus roughly 10% of a lighter alloying material. With a radius that is slightly larger than the planet Mars, the core sits roughly ~2900 km below Earth’s surface. The fluid outer core holds high importance for a number of reasons, including the generation of Earth’s magnetic field, and being an important mechanism to transfer heat to the mantle, which helps to drive the convective engine responsible for plate tectonics. In the past few decades, geophysicists have detected a thin shell at the top of the outer core where seismic waves appear to slow down, suggesting some form of stratification in the fluid. However, the seismic models for this shell do not all agree, which motivates further study. A principle challenge in seismically imaging the outer core is that seismic waves from earthquakes that are used to study the core have to travel through the entire mantle of the Earth twice (down and then back), and the mantle is very heterogeneous, with seismic velocities that vary with position. Thus, how can seismologists determine if patterns in measured signals are due to anomalous outermost core structure versus heterogeneities in Earth’s mantle? The purpose of this project is to document the degree to which the heterogeneous mantle affects the data which are used to map the core. The team of four seismologists will collect seismic data from earthquakes and seismic sensors from all over the world, predict the observations using state of the art seismic wavefield computations, and conduct refined measurements on the signals that are sensitive to deep mantle and outer core parts of the planet. The expected outcome is a better understanding of the degree to which Earth’s heterogeneous mantle affects measurements of data to study the core, and to produce an improved model of the outermost core by using the best data which are demonstrated to be minimally affected by the mantle. Better seismic mapping of Earth’s outermost core will inform research that aims to understand the enigmatic nature of the magnetic field, heat flow from the core to the mantle, as well as possible chemical exchange between the mantle and core which is important for understanding the chemical evolution of the planet. All four seismologists on the team will share results with the public in a variety of venues (including the classroom, public presentations, and science fairs) to promote awareness of the importance of Earth and planetary interiors in shaping phenomena experienced at the surface. The project will train several graduate and undergraduate students.Seismically imaged P wave velocity (Vp) reductions in the outermost 50-400 km of the core imply the presence of a stably stratified layer overlying the deeper, separately convecting interior. The precise thickness and nature of the reduced velocity (and density) layer holds critical significance for geodynamo models that address making Earth’s magnetic field, as well as the ability to understand core composition and mineralogy. However, in over ~30 years of seismic studies, no consensus has emerged among seismologists on either the thickness of the layer or the velocity structure within it. This is likely due to the effects of mantle heterogeneity and anisotropy on the seismic data used to probe the uppermost outer core. This project will investigate travel time, wave path, waveform, and shear wave splitting anomalies of seismic waves that travel in the outermost core, including multiple reflections from the underside of the core-mantle boundary (“SmKS” waves), which are more sensitive to outer core structure than any other seismic wave, in order to identify and mitigate the effects of mantle structure on outer core models. To accomplish this, a method that sums seismograms at geographically localized seismic sub-arrays will be used to improve the clarity of weak signals relative to noise, which can improve upon identification of contamination from mantle heterogeneity and anisotropy. 3D wavefield synthetics will be used to benchmark how 1D outer core imaging tools are affected by mantle heterogeneity. New outermost core seismic models will subsequently be determined in forward and inverse experiments based on highest-quality data that have been corrected for the effects of mantle heterogeneity and anisotropy. Thus, this project will thus directly test the longstanding (but likely imperfect) assumption that differential SmKS travel times can be used to reliably retrieve outer core structure without explicitly considering the effects of mantle heterogeneity and anisotropy. In addition to producing new models of outer core structure based on high-quality, corrected data, this project will produce complementary data products that contain new insights into lower mantle velocity heterogeneity and anisotropy.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

地球的外核是熔融铁，加上大约 10% 的较轻合金材料，其半径略大于火星，地核位于地球表面以下约 2900 公里处，对于许多人来说，流体外核具有很高的重要性。原因有很多，包括地球磁场的产生，以及将热量传递到地幔的重要机制，这有助于驱动负责板块构造的对流引擎。在过去的几十年里，地球物理学家已经发现。外核顶部的薄壳，地震波似乎减慢，表明流体中存在某种形式的分层。然而，该壳的地震模型并不完全一致，这激发了地震学方面的进一步研究。外核成像的一个特点是，用于研究地核的地震地震波必须穿过整个地幔两次（向下然后返回），而且地幔非常不均匀，地震速度各不相同因此，如果测量信号中的模式是由异常的最外层核心结构与地幔的不均匀性引起的，那么地震学家如何才能确定地幔的不均匀性对用于绘制地图的数据的影响程度？由四名地震学家组成的核心团队将从世界各地的地震和地震传感器收集地震数据，使用最先进的地震波场计算预测观测结果，并对信号进行精确测量。对地球深部地幔和外核部分敏感的预期结果是更好地了解地球异质地幔影响数据测量的程度，以研究地核，并通过使用最外层地核产生改进的模型。被证明受地幔影响最小的最佳数据将为旨在了解磁场的神秘性质、从地核到地幔的热流以及可能的化学物质的研究提供信息。交换地幔和地核之间的化学演化对于理解地球的化学演化非常重要，团队中的所有四位地震学家将在各种场所（包括课堂、公开演讲和科学博览会）与公众分享结果，以提高人们的认识。该项目将培训几名研究生和本科生。核心最外层 50-400 公里的地震成像 P 波速度 (Vp) 降低意味着存在稳定的地表现象。覆盖在更深的、单独对流内部的分层层，降低速度（和密度）层的精确厚度和性质对于解决地球磁场的地球发电机模型以及理解核心成分和矿物学的能力具有至关重要的意义。在超过 30 年的地震研究中，地震学家们对于地层的厚度或其内部的速度结构没有达成共识，这可能是由于地幔不均匀性和各向异性的影响。用于探测最外层核心的地震数据该项目将研究在最外层核心传播的地震波的行进时间、波路、波形和剪切波分裂异常，包括来自核心-地幔边界下侧的多次反射。 （“SmKS”波），它比任何其他地震波对外核结构更敏感，为了识别和减轻地幔结构对外核模型的影响，一种总结方法。地理上局部地震子阵列的地震图将用于提高弱信号相对于噪声的清晰度，这可以改进对地幔异质性和各向异性污染的识别，3D 波场合成将用于对 1D 外核成像工具进行基准测试。随后将根据已针对地幔影响进行校正的最高质量数据，在正向和反演实验中确定新的最外层岩心地震模型。因此，该项目将直接检验长期以来（但可能不完善）的假设，即除了产生新的外核结构外，可以使用差分 SmKS 旅行时间可靠地恢复外核结构，而无需明确考虑地幔不均质性和各向异性的影响。基于高质量、校正数据的外核结构模型，该项目将反映产生补充数据产品，其中包含对低地幔速度异质性和各向异性的新见解。该奖项是 NSF 的法定使命，并已被通过使用基金会的智力优点和更广泛的影响审查标准进行评估，认为值得支持。

项目成果

Ultralow velocity zone and deep mantle flow beneath the Himalayas linked to subducted slab

喜马拉雅山下方的超低速区和深层地幔流与俯冲板块有关

• DOI: 10.1038/s41561-024-01386-5
• 发表时间: 2024-02-26
• 期刊: Nature Geoscience
• 影响因子: 18.3
• 作者: Jonathan Wolf;Maureen D. Long;Daniel A. Frost
• 通讯作者: Daniel A. Frost