Collaborative Research: Mapping and Understanding Seismic Anisotropy in the Northeast Pacific Ocean

了解海底的结构以及板块构造是如何引起板块运动、火山活动和俯冲带的大型逆冲地震的，对于了解各种类型的地质灾害及其触发因素非常重要。这些知识对于规划和了解如何减轻和/或尽量减少灾害的影响至关重要，例如近年来在泰国和日本发生的灾害。全球地震层析成像是利用地震和其他地球运动的地震信号来成像地球的深层结构，长期以来一直是成像和了解地球内部结构及其变形方式的首选工具。为此，目前的层析成像图像表明，位于美国大陆和夏威夷之间的东北太平洋板块比目前海洋板块模型预测的要复杂得多。由于这一明显非典型的区域远离任何已知的热特征，例如洋中脊或热点（即，来自地球深处的热量和岩浆的集中上涌，就像形成夏威夷群岛一样），因此一定有一种不同的机制在起作用。由于目前东北太平洋缺乏地震记录站，该地区地壳和下地幔结构的分辨率不够高，这可能导致了太平洋板块如何随时间演变的误导性理论。这项研究解决了这个问题，部署了一个由25个海底地震仪组成的阵列，为期15个月，以捕捉和记录地球运动产生的地震信号。这些数据将有助于确定真实的结构，并将以4000万至5000万年前的太平洋板块为目标。这些新的地震数据将允许勘探当地地震各向异性（不均匀的矿物/地壳结构方向）。当地幔矿物与地幔流动方向对齐时，或者当一个板块，如太平洋板块，对沿其边缘拉扯的力作出反应时，各向异性就形成了。因此，对地震各向异性的分析使科学家能够探索形成、移动并最终导致地球构造板块俯冲的地幔过程。该项目的更广泛影响包括研究生和博士后在海上和陆上地球物理处理和建模方面的培训。本科生将参与特殊项目。这项工作补充了其他国际全球地震学工作，并涉及与英国、德国和法国科学家的合作。这项工作通过支持两名在科学领域性别代表性不足的研究人员，扩大了代表性不足群体在科学领域的参与，这两名研究人员是学生和其他地球科学家的榜样。本研究对地震和其他地球物理资料进行了综合分析，阐明了位于美国大陆和夏威夷之间的太平洋板块无月山以西600公里宽区域内的海洋地壳、岩石圈和软流圈的结构、非均质性和动力学。地震数据将由25个海底地震仪组成的阵列收集。研究目标包括一个区域研究，覆盖地震仪阵列部署区域，允许分析该区域的表面波、接收函数、表面波方位各向异性和剪切波分裂。结果提供了对4000万至5000万年前太平洋岩石圈局部地震结构的见解，并将回答有关该地区是否符合正常岩石圈板块冷却模型的预测，或者次要过程（如小规模地幔对流）是否正在影响板块行为和地壳/地幔结构的问题。结果将与一系列地幔流动条件和矿物排列/各向异性模式的预测相对照进行测试。该项目还将使用新收集的表面波数据来改进全球表面波色散图，减少全球数据集中的成像偏差，部分原因是研究区域的数据覆盖不均匀。海底地震仪阵所覆盖的区域是全球地震学家认为太平洋地震仪阵数据不足的区域。这些数据将为大型的全球地震学家社区提供服务，这些地震学家进行全球层析成像和其他研究，检查浅层和深层地球的异质性和过程，帮助我们更好地了解地球是如何工作的，以及深层地幔驱动过程对地球表面的影响。这项工作补充了最近由美国、英国、德国和法国科学家组成的一个国际合作小组在大西洋开展的工作，这使得比较两个海洋盆地的结果成为可能。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。

Understanding the structure of the seafloor and how plate tectonics, which causes plate motion, volcanism, and megathrust earthquakes in subduction zones works, is important for understanding various types of geohazards and what triggers them. This knowledge is critical in order to plan for and understand how to mitigate and/or minimize the impact of disasters, such as those that have occurred in recent years in Thailand and Japan. Global seismic tomography, which uses seismic signals from earthquakes and other earth movements to image Earth's deep structure, has long been the tool of choice to image and understand the structure of Earth's interior and how it deforms. To this end, present tomographic images indicate that the northeast Pacific plate in the area between the continental U.S. and Hawaii is more complex than predicted by present oceanic plate models. Because this apparently atypical area is far from any type of known thermal feature, such as a mid-ocean ridge or a hotspot (i.e., a concentrated upwelling of heat and thus magma from deep inside the Earth like that which formed the Hawaiian Islands) a different mechanism must be acting. Due to the current lack of seismic recording stations in the northeast Pacific, the resolution of crustal and underlying mantle structure in this area is under-resolved and may have resulted in misleading theories of how the Pacific plate has evolved over time. This research addresses this problem by deploying an array of 25 ocean bottom seismometers for 15 months to capture and record seismic signals from earth motions. These data will allow the determination of real structures and will target the part of the Pacific plate that is 40-50 million years old. These new seismic data will allow exploration of the local seismic anisotropy (the nonuniform mineral/crustal structural orientation). Anisotropy forms when mantle minerals align with mantle flow directions or when a plate, such as the Pacific plate, responds to forces tugging along its edges. The analysis of seismic anisotropy therefore allows scientists to explore mantle processes that form, move, and ultimately cause the subduction of Earth's tectonic plates. Broader impacts of the project include graduate student and postdoctoral training in state-of-the-art geophysical at-sea and onshore seismic processing and modeling. Undergraduate students will be involved with special projects. The work complements other international global seismology efforts and involves collaboration with U.K., German, and French scientists. The work broadens participation of underrepresented groups in the sciences by supporting two investigators whose gender is underrepresented in the sciences and who serve as role models for students and other geoscientists.This research provides an integrated analyses of seismic and other geophysical data that will elucidate the structure, heterogeneity, and dynamics of the ocean crust, lithosphere and asthenosphere in a 600~kilometer wide region west of the Moonless Mountain Seamounts on the Pacific plate halfway between the US mainland and Hawaii. Seismic data will be collected using an array of 25 ocean bottom seismometers. Research targets include a regional study, covering of the seismometer array deployment area which permits analysis of surface waves, receiver functions, surface wave azimuthal anisotropy, and shear-wave splitting in the area. Results provide insights into the local seismic structure of 40 to 50 million-year-old Pacific lithosphere and will answer questions about whether this area conforms to predictions from models of normal lithospheric plate cooling or if secondary processes, such as small-scale mantle convection, are impacting plate behavior and crustal/mantle structure. Results will be tested against predictions for a suite of mantle flow conditions and mineral alignment/anisotropy modes. The project will also use the newly collected surface wave data to improve global surface wave dispersion maps, reducing imaging biases in the global dataset that result, in part, from uneven data coverage in the study area. The ocean bottom seismometer array covers an area for which there is inadequate data for the Pacific Array as identified by the global community of seismologists. These data will serve a large global community of seismologists that conduct global tomographic and other studies that examine shallow and deep-Earth heterogeneity and processes that help us better understand how the Earth works and the impacts deep seated mantle-driven processes can have on Earth's surface. The work complements that recently carried out by an international, collaborative, group of US, UK, German, and French scientists in the Atlantic Ocean, which makes possible a comparison of results between both ocean basins.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.