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: 2329499
• 负责人: Daniel Frost
• 金额: $ 22.13万
• 依托单位: University of South Carolina at Columbia
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-10-01 至 2025-03-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2329499&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.

地球表面的板块在不断移动，并在板块边界处被消耗到地球中。在北美，太平洋板块正在向阿拉斯加南海岸俯冲，在地球表面产生强烈的地震和活火山。但是，这个俯冲的板块在地球内部会去哪里呢？跟踪这个板块进入地球告诉我们最近的历史板块运动在表面上数百万年，以及如何地球已经演变，因为它形成了数十亿年。为了了解现在，我们必须调查过去。 就像超声波被用来观察身体内部的器官一样，地震学家使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所使用的波的音高（或频率）控制。更高的频率改善了焦点，但需要在整个地球上进行大量的计算。可以使用假设来简化计算，将繁重的计算减少到我们想要成像的地球部分。这种简化意味着我们可以提高我们的重点，更好地跟踪阿拉斯加俯冲的历史。该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将与社区分享代码和模型，并将参与湾区的当地推广活动。阿拉斯加在过去200 Ma的演变以俯冲，碰撞和增生的多个事件为特征。这一漫长的俯冲历史的残余物应该存在于地幔的最低层，但过去的区域和全球层析成像模型解决了不一致的结构，可能是由于方法的局限性和有限的采样。我们对北方板块构造历史的了解目前还不完整。其余问题包括：板块在阿拉斯加的地下渗透有多深，板块的几何形状和厚度是什么，它是如何与过渡区相互作用的？ 本提案的主要目的是使用在有限区域内应用的全波形反演方法（称为“盒”层析成像）提高阿拉斯加下方全地幔区域地震图像的分辨率。全波形反演需要考虑多路径和波前愈合的影响，否则会掩盖强烈的局部非均匀性，如板片和周围的地幔楔。此外，空间受限的“盒子”方法耦合了快速的1D和较慢的3D波场求解器，从而减少了计算时间，这使团队能够使用更高频率的区域和非线性体波。使用三分量面波、泛音和体波波形的组合，首先将构建剪切速度模型。随着迭代的进行，增加计算的最大频率，并且利用对纵波速度敏感的附加体波形，将获得纵波速度图像。这项分析将大大提高现有的地震波速度和径向各向异性的图像，特别是在过渡带深度和中下地幔。 这些更高分辨率的阿拉斯加地幔断层图像将有助于（1）限制该地区俯冲和地幔动力学的历史，以及（2）计算从南桑威奇群岛到阿拉斯加站的极地路径上观察到的核心相位的更准确的地幔校正，这表现出特别大的旅行时间异常，其中至少有一部分可能是由于阿拉斯加板状结构。这项工作的结果将是感兴趣的地球动力学模拟流动的地幔下俯冲带，并在板块运动重建，允许更好地识别俯冲板。这也将是感兴趣的地球动力学家和矿物物理学家调查的模式和起源内核各向异性。此外，一个强大的，高分辨率的阿拉斯加层析成像模型和一种方法，提供其影响的校正将提高在阿拉斯加的USAray站的其他深地球研究的效用。这一奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。