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

了解海底的结构和板块构造是如何在俯冲带引起板块运动、火山活动和巨型逆冲地震的，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，例如最近几年在泰国和日本发生的灾害。全球地震层析成像使用来自地震和其他地球运动的地震信号来成像地球深部结构，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。由于这一明显的非典型区域远离任何类型的已知热特征，如大洋中脊或热点(即从地球深处集中涌出的热量，从而形成夏威夷群岛的岩浆)，因此一定有不同的机制在起作用。由于东北太平洋目前缺乏地震记录站，该地区地壳和下伏地幔结构的分辨率不高，可能导致关于太平洋板块如何随时间演化的误导性理论。这项研究通过部署25个海底地震仪阵列15个月来解决这个问题，以捕获和记录来自地球运动的地震信号。这些数据将能够确定真实的结构，并将以太平洋板块4,000万-5,000万年前的部分为目标。这些新的地震数据将允许勘探当地的地震各向异性(不均匀的矿物/地壳结构方向)。当地幔矿物与地幔流动方向一致时，或者当板块(如太平洋板块)对沿其边缘的拉力做出反应时，各向异性就会形成。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在最先进的海上和陆上地震处理和建模方面的研究生和博士后培训。本科生将参与特殊项目。这项工作是对其他国际全球地震学努力的补充，涉及与英国、德国和法国科学家的合作。这项研究通过支持两名研究人员，扩大了未被充分代表的群体在科学领域的参与，他们的性别在科学界未被充分代表，他们是学生和其他地球科学家的榜样。这项研究提供了对地震和其他地球物理数据的综合分析，将阐明美国大陆和夏威夷之间太平洋板块上无月山海山以西600~公里宽区域的洋壳、岩石圈和软流圈的结构、非均质性和动力学。地震数据将使用25台海底地震仪阵列收集。研究目标包括地震仪阵列部署区域的区域研究，该区域能够分析面波、接收器功能、面波方位各向异性和该区域的横波分裂。这些结果提供了对4000万至5000万年前太平洋岩石圈局部地震结构的洞察，并将回答以下问题：该地区是否符合正常岩石圈板块冷却模型的预测，或者小尺度地幔对流等次级过程是否正在影响板块行为和壳/地幔结构。结果将与一套地幔流动条件和矿物排列/各向异性模式的预测进行对比检验。该项目还将使用新收集的面波数据来改进全球面波频散图，减少全球数据集中的成像偏差，这种偏差部分是由于研究区数据覆盖不均造成的。海底地震仪阵列复盖了全球地震学家确认的太平洋阵列数据不足的区域。这些数据将为进行全球层析成像和其他研究的全球地震学家社区提供服务，这些研究检查地球浅层和深层的非均质性和过程，帮助我们更好地了解地球是如何工作的，以及深地幔驱动的过程可能对地球表面产生的影响。这项工作是对一个由美国、英国、德国和法国科学家组成的国际合作小组最近在大西洋开展的工作的补充，该小组使两个洋盆之间的结果比较成为可能。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。

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.