Imaging deep mantle structure beneath Alaska using full waveform tomography

使用全波形断层扫描对阿拉斯加下方的深部地幔结构进行成像

• 批准号: 2050011
• 负责人: Daniel Frost
• 金额: $ 22.13万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-02-15 至 2023-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2050011&HistoricalAwards=false
• 关键词: Imaging; deep; mantle; structure; beneath

The Earth’s surface plates are constantly moving and are being consumed into the Earth at plate boundaries. In North America, the oceanic Pacific plate is subducting beneath the south coast of Alaska, generating powerful earthquakes and active volcanoes on Earth’s surface. But where inside the Earth does this subducted plate then go? Tracking this plate into the Earth tells us about the more recent history of plate movement on the surface over millions of years, as well as how the Earth has evolved since it formed over billions of years. To understand the present, we must investigate the past. Much like ultrasound waves are used to look at organs inside a body, seismologists use seismic waves generated by earthquakes to investigate the inner workings of the Earth. The sharpness or blurriness of the images is controlled, among others, by the pitch (or frequency) of the waves that are used. Higher frequencies improve the focus but require heavy computations in the whole Earth. Assumptions can be used to simplify the calculations, reducing the heavy computations to the part of the Earth that we want to image. This simplification means that we can improve our focus and better track the history of subduction under Alaska. The study will support the training of a graduate student and provide support for an early-career investigator. The PIs will share codes and models with the community, and will be involved in local outreach in the Bay Area.The evolution of Alaska over the past 200 Ma features multiple episodes of subduction, collision and accretion. The remnants of this long subduction history should be present down to the lowermost mantle, but past regional and global tomographic models resolve inconsistent structures, likely owing to methodological limitations and limited sampling. Our understanding of the plate tectonics history of the Northern Pacific is currently incomplete. Remaining questions include: how deep do slabs penetrate beneath Alaska, what is the slab geometry and thickness, and how does it interact with the transition zone? The primary objective of this proposal is to improve the resolution of whole mantle regional seismic images beneath Alaska using a Full Waveform Inversion method applied within a restricted region, referred to as “box” tomography. Full Waveform Inversion is required to account for the effects of multipathing and wavefront healing that otherwise mask strong and local heterogeneity, such as slabs and surrounding mantle wedges. Moreover, the spatially restricted “box” approach couples a fast 1D and slower 3D wavefield solver thus reducing computation time, which enables the team to use higher frequency regional and teleseismic body waves. Using a combination of 3-component surface wave, overtone and body waveforms, a shear velocity model will first be constructed. Increasing the maximum frequency of the computations as iterations progress, and with additional body waveforms sensitive to compressional velocity, compressional velocity images will be obtained. This analysis will significantly sharpen existing images of seismic wavespeeds and radial anisotropy, particularly at transition zone depths and the mid and lower mantle. These higher resolution tomographic images of the mantle beneath Alaska will help to (1) constrain the history of subduction and mantle dynamics in this region, and (2) compute more accurate mantle corrections for core phases observed on polar paths from the south Sandwich Islands to stations in Alaska, which present a particularly large spread of travel time anomalies, at least part of which is likely due to Alaska slab structure. The results of the work will be of interest to geodynamicists for modeling flow in the mantle beneath subduction zones, and in plate motion reconstructions, by allowing better identification of subducted slabs. It will also be of interest to geodynamicists and mineral physicists investigating the pattern and origin of inner core anisotropy. Moreover, a robust, high resolution tomographic model of Alaska and a method for providing corrections for its effects will improve the utility of the USArray stations in Alaska for other deep Earth studies.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.

地球表面板块不断移动，并在板块边界处被地球吞噬，太平洋板块在阿拉斯加南海岸下方俯冲，在地球表面产生强烈地震和活火山。那么这个俯冲板块会进入地球吗？追踪这个板块可以告诉我们数百万年以来地表板块运动的最新历史，以及地球自形成以来数十亿年来是如何演化的。目前，我们必须就像超声波用于观察体内器官一样，地震学家利用地震产生的地震波来研究地球的内部运作，图像的清晰度或模糊度由音调等因素控制。所使用的波（或频率）较高，但需要在整个地球上进行大量计算。可以使用假设来简化计算，从而减少我们想要成像的地球部分的大量计算。这简化意味着我们可以提高我们的关注度并更好地追踪阿拉斯加下方的俯冲历史。这项研究将支持研究生的培训，并为早期职业调查员提供支持。PI 将与社区共享代码和模型。过去 200 Ma 中阿拉斯加的演化以多次俯冲、碰撞和增生为特征。这段漫长的俯冲历史的残余物应该存在于最底层的地幔中，但过去的区域性和增生性的现象仍然存在。全球的层析成像模型解决了不一致的结构，这可能是由于我们对北太平洋板块构造历史的了解尚不完整，剩下的问题包括：板块在阿拉斯加下方的穿透深度是多少，板块的几何形状和厚度是多少。它如何与过渡带相互作用？该提案的主要目标是使用在有限区域内应用的全波形反演方法（称为“盒”全断层扫描）来提高阿拉斯加下方整个地幔区域地震图像的分辨率。波形需要反演来解释多路径和波前愈合的影响，否则会掩盖强烈的局部异质性，例如板片和周围的地幔楔。此外，空间受限的“盒子”方法结合了快速的 1D 和较慢的 3D 波场求解器，从而减少了计算量。时间，这使得团队能够使用更高频率的区域和远震体波，首先将使用三分量表面波、泛音和体波形的组合来构建剪切速度模型。随着迭代的进行，增加计算的最大频率，并且通过对压缩速度敏感的附加体波形，将获得压缩速度图像，该分析将显着锐化现有的地震波速和径向各向异性图像，特别是在过渡带深度和中部。这些阿拉斯加下方地幔的高分辨率断层扫描图像将有助于（1）限制该区域的俯冲历史和地幔动力学，以及（2）计算更准确的地幔。对从南桑威奇群岛到阿拉斯加站的极地路径上观测到的核心相位进行修正，这些路径呈现出特别大的传播时间异常，至少部分可能是由于阿拉斯加板块结构造成的。这项工作的结果将是。通过更好地识别俯冲板片，地球动力学家对俯冲带下地幔的流动和板块运动重建感兴趣。研究内核的模式和起源的地球动力学家和矿物物理学家也感兴趣。此外，一个强大的、高分辨率的阿拉斯加层析成像模型及其影响校正方法将提高阿拉斯加 USArray 站在其他地球深部研究中的实用性。该奖项反映了 NSF 的法定使命，并被认为是值得的。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来提供支持。