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: 2054964
• 负责人: Bruce Buffett
• 金额: $ 40.82万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Continuing 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=2054964&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.

地球的核心是主要是铁金属的球。它由液体外壳组成 - 外芯 - 包裹着固体内核。随着时间的流逝，液态铁冻结，增加内核并为外部芯产生地球磁场。内核的形状是球形的，但似乎并不统一。穿过它的地震波的速度取决于它们的方向，这是一种称为各向异性的特征。地震波沿着南北路径（与地球旋转​​轴接近平行）横穿内核的速度要比沿东西路径（在等方面的平面上）快的速度。内核地震各向异性携带有关铁冷冻时条件的信息。它归因于铁晶体在特定方向上的比对。但是导致这种对齐的过程尚不清楚。由于涉及复杂的过程以及核心中普遍存在的极端压力和温度，解释此功能受到了挑战。在这里，研究人员在实验中测试了铁的样品在核心条件下的表现。他们分析了在钻石砧室中进行的实验中的数据，在两种相对的钻石的尖端产生了极端条件。他们使用计算模型来表征实验过程中铁晶体对齐的表征，并计算产生的地震速度。他们使用地震分析方法来绘制内核的速度结构。与地球动力学建模相结合，多学科方法允许模拟内部核心生长并揭示导致其当今各向异性的过程。该项目支持早期的职业科学家。它在研究生和本科生的多学科背景下促进了培训，特别是来自地球科学中代表性不足的群体。它促进了对当地学校和社区大学的宣传。项目成果将广泛，自由地分发给社区。理解内部核心结晶对于理解地球诺（Geodynamo）至关重要。内核地震各向异性归因于本质上各向异性铁晶体的比对。在这里，研究人员研究了这种各向异性的原因和控制因素，以及内部核心过程如何影响外部核心过程。耦合地震分析，矿物质实验和地球动力学建模，他们研究了内核内晶体比对的动力学和矿物学控制。他们使用实验室数据和计算可塑性模型来限制铁在内部核心条件下的行为，从而对能够生成地震各向异性的矿物过程限制。同时，通过地球动力学模型，它们模拟了内部核心的生长，并确定流动的模式和强度，并迫使这种影响晶体取向。他们测试了对齐晶体在内部核心动态进化中的各向异性热和电运输特性的可能反馈。现有和新的地震测量结果提供了观察性约束，以测试内核的可能增长模型。团队解决的关键问题是：如何在内部核心增长过程中产生全球各向异性？是什么原因导致各向异性的方向？在一系列长度范围内，生长模型是否可以与空间地震结构进行对帐？结晶纹理对最终各向异性有什么影响？热和电动各向异性对地球其他地区的影响是什么？该奖项反映了NSF的法定任务，并且使用基金会的知识分子优点和更广泛的影响审查标准，通过评估被认为是宝贵的支持。