Collaborative Research: Mantle dynamics and plate tectonics constrained by converted and reflected seismic wave imaging beneath hotspots

自数十亿年前形成以来，地球一直在通过对流缓慢冷却，在对流中，较热的物质上升到表面，而较冷的构造板块深入到内部。理解这一过程对于许多问题都很重要，例如确定驱动板块构造以维持深水和碳循环的因素，这些因素稳定了整个地球历史上的大气/水圈和气候。这对社会来说尤其重要，因为板块构造过程是地震和火山等灾害背后的驱动力，也因为我们的气候和大气使地球适合居住。本项目利用上升流区远震台站的地震资料，研究固体地球的大尺度对流。众所周知，上升流很难控制。重点将放在三个端元案例上：1)夏威夷，深海上升流发生在海洋岩石圈下面的典型例子；2)黄石，深海上升流发生在大陆下面的典型例子；3)赤道大西洋中脊，通常被认为是一个没有深海上升流的地方。该项目将使用经典技术和一些新开发的方法来更好地限制上升流的规模及其路径。一项外展计划将通过公众参与、教育和培训学生和早期职业研究人员来增加地球科学的多样性。拓展部分包括参观摩根州立大学（Morgan State University）的核心学科教室，这是一所历史悠久的黑人大学，与该校的两名教员建立了联系。访问和材料的目的是提高对职业可能性的认识以及地球科学的社会相关性。这种方法借鉴了“谷歌驻校”计划的成功，该计划成功地将谷歌工程师安置在HBCU校园。最终将有更广泛的地球科学家参与其中，方法是建立一个可以根据需要进行缩放和调整的材料档案，并提供最佳实践建议，这两者都将向更广泛的科学界公开提供。该项目还为本科生、一名研究生和一名博士后提供海底地震数据前沿方法和使用方面的培训。地球的对流系统对于理解地球的进化非常重要，包括从驱动和实现板块构造的因素，到维持数十亿年来稳定大气/水圈和气候的深水和碳循环。人们普遍认为，冷地幔下沉回地球，例如，在俯冲带，热地幔上升，例如，在热点和洋中脊之下。尽管地幔层析成像已经解决了许多与俯冲有关的地震快速异常，但事实证明，上升流成像更具挑战性，这可能是因为地震波形难以解决薄而缓慢的管道。因此，这些热异常的确切尺寸、位置、特征、成因深度和震级，以及它们与周围地幔（如过渡带和上地幔）的化学和物理相互作用，都是鲜为人知的。同样，上升流在上升过程中大小变化和/或偏转的程度以及它们在不同构造环境中的变化程度也是不确定的。过渡带不连续面和岩石圈-软流圈边界的转换波和反射波成像应提供更严格的约束。然而，这些方法的研究在热点特征和位置上存在一些差异，这可能是因为它们在不同的地点使用了不同的方法和方法，灵敏度也不同。本研究将在不同的构造环境中系统地考察这一问题。计划中的方法将使用转换和反射地震相位的互补灵敏度来成像岩石圈-软流圈边界和过渡带不连续的三个关键区域，这些区域代表了可能出现上升流特征变化的构造环境范围。其中包括夏威夷靠近古老海洋板块中心的标志性热点，大陆内部下方的黄石热点的经典例子，以及经典模型无法预测的深层上升流的中大西洋脊。分析将包括过渡带不连续面的P-to-S成像和岩石圈-软流圈边界的S-to-P成像，两者都对横波速度的垂直变化很敏感，并且还使用s反射，这具有高深度分辨率的额外优势。系统的方法将允许在各区域之间进行比较，并进行各向异性测试和F-K全波形建模，以确定各向异性和/或聚焦/散焦对任何明显差异的影响。一旦从地震波形中定义了所有的可能性，基于实验和从头算约束的地球性质反演将确定可以解释观测结果的性质。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。

Since its formation billions of years ago, Earth has been slowly cooling via convection, where warmer material rises to the surface and cold tectonic plates plunge deep into the interior. Understanding this process is important for a wide range of problems, such as determining the factors that drive plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere/hydrosphere and climate throughout Earth history. This is particularly relevant for society, both because plate tectonic processes are the driving forces behind hazards such as earthquakes and volcanoes and also because our climate and atmosphere make Earth habitable. This project studies large scale convection in the solid Earth by using earthquake data recorded at distant seismic stations in regions of upwelling. Upwellings are notoriously difficult to constrain. The focus will be on three end-member cases: 1) Hawaii, the classic example where deep upwelling occurs beneath ocean lithosphere, 2) Yellowstone, the classic example of deep upwelling that occurs beneath a continent, and 3) the equatorial mid-Atlantic Ridge, which is typically assumed to be a location without deep upwellings. The project will use classic techniques and also some newly developed approaches to better constrain the scale of the upwellings and their pathways. An outreach program will increase diversity in the Earth Sciences via public engagement and education and training of students and early career researchers. The outreach portion includes visits to core discipline classrooms at Morgan State University, a historically black university, with an established connection with two faculty members there. The goal of the visits and the materials is to increase awareness of career possibilities and also the societal relevance of the Earth Sciences. The approach is modeled on the success of the "Google in Residence" program that successfully placed Google engineers on HBCU campuses. Eventually a wider range of Earth scientists will be included by developing an archive of materials that can be scaled and adapted according to needs and also best-practice advice, both of which will be made publicly available to the broader scientific community. The project also provides training for undergraduate students, a graduate student, and a post doc in cutting-edge methodologies and use of seafloor seismic data. Earth’s convective system is important for understanding the evolution of the planet, including everything from the factors that drive and enable plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere/hydrosphere and climate over billions of years. It is generally accepted that cool mantle sinks back into the Earth, e.g., at subduction zones, and hot mantle rises, e.g., beneath hotspots and mid ocean ridges. Although mantle tomography has resolved many seismically fast anomalies associated with subduction, imaging upwellings has proven more challenging, potentially because seismic waveforms have difficulty resolving thin slow conduits. Thus, the exact dimensions, locations, characteristics, origin depths and magnitudes of these thermal anomalies are poorly known, as well as their chemical and physical interaction with the surrounding mantle, such as in the transition zone and uppermost mantle. Similarly, the degree to which upwellings change in size and/or are deflected during their ascent and how they vary among tectonic environments is uncertain. Converted and reflected wave imaging of the transition zone discontinuities and the lithosphere-asthenosphere boundary should provide tighter constraints. However, there are some discrepancies in studies using these methods regarding hotspot character and location, perhaps because they have used different methodologies and approaches with different sensitivities in different locations. This study will examine this issue systematically at a varied suite of tectonic environments. The planned approach will use the complementary sensitives of converted and reflected seismic phases to image the lithosphere-asthenosphere boundary and transition zone discontinuities beneath three key regions that are representative of the range of tectonic environments where variability in upwelling characteristics might be expected. These include the iconic hotspot of Hawaii near the center of an old oceanic plate, the classic example of Yellowstone hotspot beneath a continental interior, and finally the mid-Atlantic Ridge where deep upwellings are not predicted by classic models. Analyses will include P-to-S imaging of the transition zone discontinuities and S-to-P imaging of the lithosphere-asthenosphere boundary, both sensitive to vertical changes in shear wave velocity, and also use S-reflections, which have the added advantage of high depth resolution. A systematic approach will allow comparisons among the regions and anisotropic testing and F-K full-waveform modelling will be performed to determine the influence of anisotropy and/or focussing/defocussing for any apparent discrepancies. Once a full range of possibilities is defined from the seismic waveforms, inversions for Earth properties based on experimental and ab initio constraints will determine the properties that can explain the observations.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.