CSEDI Collaborative Research: The Origins and Implications of Inner Core Seismic Anisotropy

CSEDI合作研究：内核地震各向异性的起源和意义

• 批准号: 2054993
• 负责人: Lowell Miyagi
• 金额: $ 24.71万
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-07-01 至 2024-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2054993&HistoricalAwards=false
• 关键词: CSEDI; Collaborative; Research; Origins; Implications

The Earth's core is a ball of mostly iron metal. It consists of a liquid outer shell - the outer core - enveloping the solid inner core. As the Earth cools down over time, liquid iron freezes, growing the inner core and providing energy to the outer core to generate Earth’s magnetic field. The inner core is spherical in shape but appears not to be uniform. The speed of seismic waves traveling through it depends on their direction, a feature known as anisotropy. Seismic waves traversing the inner core along a north-south path (near-parallel to the Earth’s rotation axis) go faster than those propagating along an east-west path (in the plane of the equator). Inner-core seismic anisotropy carries information about the conditions at the time of iron freezing. It has been attributed to alignment of iron crystals in specific directions; but the processes causing this alignment is unclear. Interpreting this feature has been challenging because of the complex processes involved and the extreme pressures and temperatures prevailing in the core. Here, the researchers test experimentally how samples of iron behave at core conditions. They analyse data from experiments carried out in the diamond anvil cell where extreme conditions are generated at the tips of two opposing diamonds. They use computational models to characterpize iron crystal alignment during the experiments and calculate the resulting seismic velocities. They use seismic analytical methods to map the velocity structure of the inner core. Combined with geodynamic modeling, the multidisciplinary approach allows simulating inner-core growth and unveiling the processes causing its present-day anisotropy. The project supports an early career scientist. It promotes the training in a multidisciplinary context of graduate and undergraduate students, notably from underrepresented groups in geosciences. It fosters outreach towards local schools and community colleges. The project outcomes will be broadly and freely distributed to the community. Understanding inner-core crystallization is central to understanding the geodynamo. Inner-core seismic anisotropy is attributed to alignments of intrinsically anisotropic iron crystals. Here, the researchers investigate the causes and controlling factors of this anisotropy, and how inner-core processes influence outer core processes. Coupling seismic analysis, mineral physics experiments, and geodynamic modeling, they investigate the dynamics and mineralogical control of crystal alignments within the inner core. They use both laboratory data and computational plasticity models to constrain the behavior of iron at inner-core conditions, placing limits on the mineralogical processes able to generate seismic anisotropy. Concurrently, with geodynamic models they simulate inner-core growth and determine the pattern and strength of flows and forcing that impact crystal orientation. They test possible feedback of anisotropic thermal and electrical transport properties of aligned crystals on inner-core dynamic evolution. Existing and new seismic measurements provide observational constraints to test possible growth models for the inner core. Key questions addressed by the team are: how is global anisotropy generated during inner-core growth? What caused the present-day orientation of anisotropy? Can growth models be reconciled with spatial seismic structure across a range of length scales? What effect does crystallization texture have on final anisotropy? What are the implications of thermal and electrical anisotropy for the rest of the Earth, notably for the geodynamo?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.

地球的核心是一个主要由铁金属组成的球。它由一个液体外壳-外核-包裹固体内核组成。随着时间的推移，地球冷却下来，液态铁冻结，增长内核，并为外核提供能量，以产生地球磁场。内核是球形的，但看起来并不均匀。地震波在其中传播的速度取决于它们的方向，这一特征被称为各向异性。地震波沿沿着南北路径（几乎平行于地球自转轴）穿越内核比那些沿沿着东西路径（在赤道平面内）传播的地震波要快。堆芯内部的地震各向异性携带了铁冻结时的条件信息。 它被归因于铁晶体在特定方向上的排列;但导致这种排列的过程尚不清楚。解释这一特征一直具有挑战性，因为涉及复杂的过程和极端的压力和温度在核心占主导地位。在这里，研究人员通过实验测试了铁样品在核心条件下的行为。他们分析了在金刚石压砧单元中进行的实验数据，其中极端条件在两个相对的金刚石尖端产生。他们使用计算模型来表征实验过程中的铁晶体排列，并计算由此产生的地震速度。他们使用地震分析方法来绘制内核的速度结构。结合地球动力学建模，多学科的方法可以模拟内核的增长和揭示的过程，导致其目前的各向异性。 该项目支持早期职业科学家。它促进在多学科背景下对研究生和本科生进行培训，特别是来自地球科学领域代表性不足的群体。它促进与当地学校和社区学院的联系。项目成果将广泛和自由地分发给社区。理解内核结晶是理解地球发电机的核心。内核地震各向异性归因于固有的各向异性铁晶体的排列。在这里，研究人员研究了这种各向异性的原因和控制因素，以及内核过程如何影响外核过程。结合地震分析、矿物物理实验和地球动力学建模，他们研究内核内晶体排列的动力学和矿物学控制。他们使用实验室数据和计算塑性模型来约束铁在内核条件下的行为，限制能够产生地震各向异性的矿物学过程。与此同时，他们用地球动力学模型模拟内核的生长，并确定流动的模式和强度，以及影响晶体取向的作用力。他们测试可能反馈的各向异性的热和电输运性能的对齐晶体内核动态演化。现有的和新的地震测量提供了观测的限制，以测试可能的增长模型的内核。该团队解决的关键问题是：在内核生长过程中如何产生全局各向异性？是什么导致了今天的各向异性取向？增长模型能与跨越一系列长度尺度的空间地震结构相协调吗？结晶织构对最终各向异性有什么影响？热各向异性和电各向异性对地球的其他部分，特别是对地球发电机有什么影响？该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估的支持。