Collaborative Research: NSFGEO-NERC: Advancing capabilities to model ultra-low velocity zone properties through full waveform Bayesian inversion and geodynamic modeling

该提案将改进对称为超低速度带（ULVZs）的特征的描述。ulvz是位于地核-地幔边界（CMB）上的薄块物质，深度为2891公里（1796英里）。顾名思义，ulvz可以使通过的地震波减慢50%。这使得它们成为地球内部最反常的特征之一。几个ulvz位于地球上一些最大的热点火山下面，比如夏威夷、冰岛和萨摩亚。地球内部的成像显示，炽热的上升物质，或地幔柱，将这些火山从ulvz连接到地表。但是这些ulvz的起源、寿命、大小和组成尚不清楚，它们在地幔柱形成中的作用也不清楚。这个项目将提供更多关于这些特征存在的地方、它们的形成和组成的细节。这项工作分析了两种类型的地震波，这些地震波对极薄极低频波层敏感，它们与极低频波层相互作用并反弹。这项研究的重点是西太平洋下方的一个区域，因为该地区有广泛的地震数据覆盖范围和已知的超低震区。该项目将根据韩国、台湾和日本的公共记录和网络建立一个新的地震数据库。然后，该数据库将使用现代统计能力对宇宙微波背景中的超低频结构进行成像。地幔流动的计算机模拟将有助于解释ulvz的形成、运动和在地幔过程中的作用。该项目包括为当地犹他太平洋岛社区的K-8学生举办的年度外展STEM活动。该活动将包括通过热点火山活动形成太平洋岛屿的地幔柱的教学活动。超低速度带（Ultra-Low Velocity Zones, ULVZs）是地球核幔边界（CMB）地震速度强烈降低的构造，与热点火山活动、大火成岩省、核幔相互作用、下潜俯冲和大低速省（Large- low Velocity province, llvp）有关或相关，因此是全球地幔动力学的重要组成部分。ULVZ的研究通常采用对ULVZ敏感的地震探头（如SPdKS、ScP、ScS）进行波形分析，但以往的研究由于建模权衡，ULVZ参数存在较大的不确定性，并且缺乏通过严格的参数估计对ULVZ进行地球物理推断。该项目将开发一种转换波形反演方法，并将其应用于ULVZ属性的表征，包括地震速度、密度、大小和形状。具体而言，该项目将(1)利用公开可用的地震阵列数据以及来自韩国、台湾、日本和国际监测系统（IMS）阵列的新数据，收集一个新的ULVZ敏感ScP和PcP波形数据库；(2)提高ULVZ属性的全波形贝叶斯反演能力，以定量区分有和没有ULVZ结构的区域（即贝叶斯模型选择），并进行ScP和PcP波形的联合反演。(3)量化二维/三维ULVZ结构的波形效应；(4)使用阵列处理方法在与三维ULVZ结构一致的ScP波场中寻找额外的、以前未发现的到达；(5)通过三维地球动力学模型将观测到的ULVZ位置和性质与最下层地幔流动和动力学联系起来。这是犹他大学、亚利桑那州立大学和利兹大学的科学家合作提出的一项建议，因此由美国国家科学基金会（NSF）和英国自然环境研究委员会（NERC）共同资助。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

This proposal will improve the description of features called Ultra-Low Velocity Zones (ULVZs). ULVZs are thin patches of material that sit on the Earth’s core-mantle boundary (CMB), which is at a depth of 2891 km (1796 miles). As their name implies, ULVZs slow passing seismic waves by as much as 50%. This makes which makes them some of the most anomalous features of the Earth’s interior. Several ULVZs are beneath some of Earth’s largest hot spot volcanoes, such as Hawaii, Iceland, and Samoa. Imaging of the Earth’s interior shows that hot rising materials, or mantle plumes, connect these volcanoes from ULVZs to the surface. But the origin, lifetime, size, and composition of these ULVZs are not known, nor is their role in the formation of the mantle plumes. This project will provide greater details as to where these features exist, their formation and composition. This work analyzes two types of seismic waves sensitive to thin ULVZ layering that interact with and bounce off these ULVZ features. The study focuses on an area beneath the western Pacific Ocean because of extensive seismic data coverage and known ULVZs in the region. The project will create a new seismic database built from public records and networks in South Korea, Taiwan, and Japan. This database will then enable imaging of ULVZ structures at the CMB using modern statistical capabilities. Computer simulations of mantle flow will help interpret the formation, motion, and role in mantle processes of the ULVZs. This project includes an annual outreach STEM event for K-8 students for the local Utah Pacific Island community. This event will include activities to teach about the mantle plumes that form Pacific Islands through hot spot volcanism.Ultra-Low Velocity Zones (ULVZs) are structures with strongly reduced seismic velocities at Earth’s core-mantle boundary (CMB) which have been associated with or linked to hot-spot volcanism, Large Igneous Provinces, core-mantle interaction, downwelling subduction and Large-Low Velocity Provinces (LLVPs) and thus are a critical component of global mantle dynamics. ULVZs are typically studied using waveform analysis of ULVZ-sensitive seismic probes (e.g., SPdKS, ScP, ScS), but previous studies suffer from large uncertainties in ULVZ parameters due to modeling trade-offs and lack a geophysical inference of ULVZs through rigorous parameter estimation. This project will develop a transformative waveform inversion approach and apply it to characterize ULVZ properties including their seismic velocities, density, size, and shape. Specifically, the project will (1) collect a new database of ULVZ-sensitive ScP and PcP waveforms utilizing publicly available seismic array data as well as new data from South Korea, Taiwan, Japan, and the International Monitoring System (IMS) arrays, (2) advance capabilities for full waveform Bayesian inversion for ULVZ properties to quantitatively distinguish regions with and without ULVZ structures (i.e., Bayesian model selection), and to perform joint inversion of ScP and PcP waveforms, (3) quantify the waveform effects from 2-D/3-D ULVZ structures, (4) search for additional, previously unlooked-for arrivals in the ScP wavefield consistent with 3-D ULVZ structure using array processing approaches, and (5) relate observed ULVZ localities and properties to lowermost mantle flow and dynamics through 3D geodynamic models. This is a collaborative proposal between scientists at the University of Utah, Arizona State University, and Leeds University, and is therefore co-funded by the National Science Foundation (NSF) and the United Kingdom’s Natural Environment Research Council (NERC).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.