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，地球一直通过对流慢慢冷却，在对流中，较热的物质上升到表面，寒冷的构造板块深入内部。了解这一过程对于一系列问题都很重要，例如确定驱动板块构造维持深水和碳循环的因素，从而在整个地球历史上稳定大气/水圈和气候。这与社会特别相关，既是因为板块构造过程是地震和火山等灾害背后的驱动力，也是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距离地震台站记录的地震资料，研究固体地球中的大尺度对流。众所周知，涌动是很难控制的。焦点将集中在三个终端成员的案例上：1)夏威夷，海洋岩石圈下发生深上涌的经典例子；2)黄石，大陆下发生的深上涌的经典例子；3)赤道中大西洋海脊，通常被认为是一个没有深上涌的地方。该项目将使用经典技术和一些新开发的方法，以更好地限制上升流的规模及其路径。一个外展计划将通过公众参与以及对学生和早期职业研究人员的教育和培训来增加地球科学的多样性。外展部分包括参观摩根州立大学的核心学科教室，这是一所历史悠久的黑人大学，与那里的两名教职员工建立了联系。访问和材料的目的是提高对职业可能性的认识，并提高地球科学的社会相关性。这一方法是仿照“谷歌驻校”项目的成功，该项目成功地将谷歌工程师安置在HBCU校园。最终，将包括更多的地球科学家，建立一个材料档案，可以根据需要和最佳实践建议进行调整和调整，这两个档案都将向更广泛的科学界公开提供。该项目还为本科生、研究生和博士后提供关于海底地震数据的尖端方法和使用的培训。地球的对流系统对于理解地球的演化很重要，包括从驱动和促成板块构造的因素到维持数十亿年来稳定大气/水圈和气候的深层水和碳循环的一切。人们普遍认为，冷地幔在俯冲带下沉回到地球，而热地幔在热点和大洋中脊下方上升。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，但成像上升流被证明是更具挑战性的，可能是因为地震波形难以分辨细而慢的管道。因此，这些热异常的确切尺寸、位置、特征、起源深度和大小，以及它们与周围地幔的化学和物理相互作用，如在过渡带和最上面的地幔中，人们知之甚少。同样，上升流在上升过程中大小和/或偏转的程度以及它们在不同构造环境中的变化也是不确定的。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，这些方法在热点特征和位置方面的研究存在一些差异，可能是因为它们在不同的位置使用了不同的方法和途径，具有不同的敏感性。这项研究将在不同的构造环境下系统地研究这一问题。计划中的方法将利用转换和反射地震震相的互补敏感性来成像岩石圈-软流圈边界和三个关键区域下的过渡带不连续，这三个区域代表了可能出现上升流特征变化的一系列构造环境。其中包括夏威夷靠近旧大洋板块中心的标志性热点，黄石公园大陆内陆下热点的经典例子，以及经典模型无法预测的大西洋中部山脊。分析将包括过渡带不连续的P-to-S成像和岩石圈-软流圈边界的S-to-P成像，这两种成像都对剪切波速度的垂直变化敏感，还将使用具有高深度分辨率的S反射。系统的方法将允许在各区域之间进行比较，将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。一旦从地震波形定义了所有的可能性，基于实验和从头计算约束的地球属性反演将确定可以解释观测结果的属性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.