Mapping Fine-Scale Structure in Earth's Inner Core With a Global Array of Seismic Arrays

利用全球地震阵列阵列绘制地球内核的精细结构图

• 批准号: 1722542
• 负责人: Keith Koper
• 金额: $ 27万
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-07-01 至 2021-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1722542&HistoricalAwards=false
• 关键词: Mapping; Fine; Scale; Structure; Earth

After the Earth formed it quickly differentiated into a dense, fluid, metallic core surrounded by a magma ocean of less dense, molten rock. The magma ocean ultimately solidified into Earth's mantle and crust. These rocky outer layers act as an insulating blanket and have kept most of the core in a fluid state to the present. But at some point, long ago, the core became cold enough to solidify in the very center of Earth, where the pressure is highest. As the core continued to cool, the solid inner core slowly grew larger. Currently, the radius of the inner core is about 1/3 the radius of the core itself. The aim of the proposed work is to map out the subtle variations in the geologic structure of the inner core using scattered seismic energy that has been recorded by a global network of seismic arrays. These patterns will give insight into how the solid inner core is formed and deformed by dynamic processes in the fluid outer core. The proposed work will be conducted by the PI and a graduate student at the University of Utah in collaboration with early career scientists at Los Alamos National Laboratory (LANL). During the course of the project, the graduate student will visit LANL annually and make contacts within the large and diverse LANL geophysics program. It is likely that early career LANL scientists will also visit the University of Utah during the project, strengthening a lab-university collaboration. The proposed work will leverage the tens of millions of dollars that have been invested in developing and maintaining the International Monitoring System (IMS), which is operated in support of the Comprehensive Nuclear Test Ban Treaty. This project will highlight the value of IMS data for basic science and demonstrate a pathway that other academic researchers might use to gain meaningful access to IMS data.The PI proposes a three-year project to map out large-scale regional variations in the fine-scale structure of Earth's inner core using seismic data recorded by a global array of seismic arrays. The PI will apply array processing techniques to earthquake generated waveforms recorded at the small-aperture (~5-20 km) arrays of the International Monitoring System (IMS) to identify and isolate the subtle coda waves that follow pre-critical reflections from the inner core boundary (PKiKP). Pre-critical PKiKP coda waves are uniquely sensitive to the properties of small wavelength (~1-10 km) scatterers in the inner core that contribute to the well-known, but poorly understood, low Q of the inner core. To better understand the origin of long-wavelength variations in inner core Q, the PI will document regional variations in inner core scattering strength and rigorously compare them to existing quasi-hemispherical models of the inner core that are based on the apparent Q measured from waves refracted through the inner core (PKPDF). Forward modeling of pre-critical PKiKP coda envelopes will be conducted using a previously developed ray-based, single-scattering approach that has been validated against multiple-scattering phonon simulations. The proposed work will, for the first time, map out the scattering strength of the inner core over a complete range of longitudes, with dense sampling of both quasi-hemispheres and the longitudinal transition zones. The results will shed light on the fundamental nature of inner core heterogeneity, and constrain the range of geodynamic models that can explain its origin.

地球形成后，它迅速分化成一个致密的、流动的、金属的核心，周围是一个由密度较小的熔融岩石组成的岩浆海洋。岩浆海洋最终凝固成地幔和地壳。这些岩石外层就像一个绝缘层，使地核的大部分至今仍处于流体状态。但在很久以前的某个时刻，地核变得足够冷，在地球的中心凝固，那里的压力最高。随着地核不断冷却，坚硬的内核慢慢变大。目前，内核的半径大约是内核本身半径的1/3。拟议工作的目的是利用全球地震阵列网络记录的分散地震能量，绘制出内核地质结构的细微变化。这些模式将使我们深入了解固体内核是如何通过流体外核的动态过程形成和变形的。拟议的工作将由PI和犹他州大学的一名研究生与洛斯阿拉莫斯国家实验室（LANL）的早期职业科学家合作进行。在该项目的过程中，研究生将每年访问LANL，并在大型和多样化的LANL电子物理计划中进行接触。LANL的早期职业科学家也可能在项目期间访问犹他州大学，加强实验室与大学的合作。拟议的工作将利用为支持《全面禁止核试验条约》而运作的国际监测系统（IMS）的开发和维护所投入的数千万美元。该项目将突出国际监测系统数据对基础科学的价值，并展示一条其他学术研究人员可能用来获得有意义的国际监测系统数据的途径，PI提出了一个为期三年的项目，利用全球地震阵列记录的地震数据绘制地球内核精细结构的大规模区域变化。PI将对国际监测系统（IMS）小孔径（~5-20 km）阵列记录的地震生成波形应用阵列处理技术，以识别和隔离来自内核边界（PKiKP）的临界前反射后的微妙尾波。前临界PKiKP尾波对内核中的小波长（~1-10 km）散射体的特性非常敏感，这些特性导致了内核众所周知但知之甚少的低Q值。为了更好地理解内核Q长波长变化的起源，PI将记录内核散射强度的区域变化，并将其与现有的内核准半球模型进行严格比较，这些模型基于通过内核折射的波测量的表观Q（PKPDF）。前临界PKiKP尾波包络的正演建模将使用先前开发的基于射线的单散射方法进行，该方法已针对多散射声子模拟进行了验证。拟议中的工作将首次绘制出完整范围内的散射强度的内核，与两个准半球和纵向过渡区的密集采样。这些结果将揭示内核非均匀性的基本性质，并限制可以解释其起源的地球动力学模型的范围。