Plume Structure and Mantle Layering Beneath the South Pacific: Modeling Teleseismic Waveforms from Traditional and Floating Sensors

除了对人类的巨大危害外，地震也是帮助我们想象地球内部的能量来源。地球是一个动态行星，其内部始终处于对流运动，为了将其理解为一个系统，地震学家挖掘了全球地震仪记录的地震波测量中包含的信息。一个特别令人感兴趣的区域是太平洋下面的地幔，那里散布着火山，大多数火山在水下，但许多火山以海洋岛屿的形式存在，如夏威夷、萨摩亚和塔希提岛。这些火山的发源区还不够清楚；特别是不清楚它们起源于地幔的什么深度。缺乏清晰度的部分原因是，很难获得大洋彼岸的地震观测。一种名为美人鱼的新型仪器是一种自由漂浮的水听器，本质上是一个水下麦克风，随着深海洋流漂移，可以捕捉地震波，几乎就像传统的地震仪一样。在这个项目中，研究人员将使用大约65台这样的新仪器的录音，根据我们目前对地震和地球内部的了解来更新地球模型。其结果将是新的、更好的地球内部图像，这些图像可以与在深处运行的过程联系起来，并表现为地表的火山活动。这些科学建模活动将有助于地球科学专业本科生、地球物理学研究生和博士后研究员的研究教育，研究人员通过存档、管理数据并通过国家数据中心与公众共享数据来参与社区和能力建设。MERMAID最初设计用于捕获纵波首达的P波到达。对使用这些数据的新技术的初步研究已经显示了可用于波形建模的整个波列的细节，使研究人员能够超越简单的旅行时间挑选。研究人员将进一步开发这项新技术，并将其应用于由当今全球海洋中活跃的所有美人鱼仪器返回的整个数据集中确定的遥远或“远震”事件，并将它们整合到波利尼西亚的地幔波速和阻抗对比模型中。层析成像的波形模拟技术侧重于平滑的波速变化，并依赖于利用传递函数方法通过频谱元素映射来模拟远震地幔波场的“海洋最后一英里”。波阻抗对比模拟技术侧重于波速和密度的强烈对比，并利用地表反射地震相的前兆来产生地幔过渡带和中地幔不连续的深层反射体的图像。这是一种波动方程成像方法，涉及使用全波形层析成像模型进行波场外推的逆时偏移。该模型考虑了地震图的所有三个组成部分，并伴随着详细的灵敏度和假设测试，以使人们有信心解释与地幔热柱相关的地幔不连续的物理性质，这些物理性质有望通过波速结构图揭示出来。该项目由地球科学部门的地球物理计划和地球信息学计划支持。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。

Besides formidable hazards to humans, earthquakes are sources of energy that help us image the Earth’s interior. Earth is a dynamic planet, its interior always in convective motion, and to understand it as a system, seismologists mine the information contained in the measurement of earthquake waves recorded by seismometers across the globe. An area of specific interest is Earth’s mantle below the Pacific, which is strewn with volcanoes, most of them underwater, but many present as oceanic islands such as Hawaii, Samoa, and Tahiti. The source region of these volcanoes is insufficiently known; in particular it is not clear at what depths in the mantle they originate. Part of the reason for the lack of clarity is that seismic observations across the oceans are very hard to obtain. A new type of instrument, MERMAID, is a free-floating “hydrophone”, essentially an underwater microphone, drifting with the deep ocean currents, that picks up earthquake waves, almost like traditional seismometers. In this project, the researchers will use recordings from about 65 of these new instruments to update Earth models based on our current state of knowledge about earthquakes and Earth’s interior. The results will be new and better images of the Earth’s interior, which can be linked back to the processes operating at depth and expressed as volcanism at the surface. These scientific modeling activities will contribute to the research education of undergraduate geosciences majors, a geophysics graduate student and a post-doctoral researcher, the researchers engage in community and capacity building by archiving, curating, and sharing the data with the public via national data centers.MERMAID was originally designed to capture compressional first-arriving “P”-wave arrivals. Preliminary research on new techniques for using this data has shown detail in the entire wave train that can be used for waveform modeling, allowing the researchers to go beyond simple travel-time picking. The researchers will further develop this new technique, and apply it to distant, or “teleseismic” events identified in the entire dataset returned by all the MERMAID instruments active in the global oceans today, and to integrate them into mantle wavespeed and impedance contrast models for Polynesia. The waveform modeling technique for tomography focuses on smooth wavespeed variations and relies on modeling the ``oceanic last mile'' of the teleseismic mantle wavefield via spectral-element mapping using a transfer-function approach. The impedance contrast modeling technique focuses on sharp contrasts in wave speed and density and uses precursors to surface-reflected seismic phases to produce images of deep reflectors of both mantle transition-zone and mid-mantle discontinuities. It is a wave-equation imaging method that involves reverse-time migration employing full-waveform tomographic models for wavefield extrapolation. The modeling takes into account all three components of the seismogram and is accompanied by detailed sensitivity and hypothesis tests to give confidence in interpretation of the physical nature of mantle discontinuities in relation to the mantle plumes that will hopefully be revealed from wavespeed structure mapping.This project is supported by the Geophysics Program and the Geoinformatics Program in the Division of Earth Sciences.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.