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.

地球表面板不断移动，并在板边界处被消耗到地球上。在北美，海洋太平洋板块在阿拉斯加南海岸下方俯冲，在地球表面产生了强大的地震和活火山。但是，这个俯冲板在地球内部的何处？追踪该板进入地球，向我们讲述了数百万年以来地面上板块运动的最新历史，以及地球自数十亿年以来的发展方式。要了解现在，我们必须调查过去。就像超声波一样，被用来查看体内的器官，地震学家使用地震产生的地震波来研究地球的内部作用。除其他外，图像的清晰度或模糊性通过所使用的波的音高（或频率）控制。较高的频率改善了焦点，但需要在整个地球上进行大量计算。假设可用于简化计算，将重型计算减少到我们要成像的地球部分。这种简化意味着我们可以提高注意力，并更好地跟踪阿拉斯加领导下的俯冲历史。这项研究将支持研究生的培训，并为早期研究员提供支持。 PI将与社区共享代码和模型，并将参与湾区的当地宣传。在过去的200 MA中，阿拉斯加的演变具有俯冲，碰撞和积聚的多个情节。长俯冲历史记录的残余物应存在于最低的地幔，但是过去的区域和全球层析成像模型可以解决不一致的结构，这可能是由于方法论上的局限性和有限的采样。我们对北太平洋板块构造历史的理解目前不完整。其余的问题包括：平板在阿拉斯加下方穿透多深，什么是平板几何形状和厚度，以及它如何与过渡区相互作用？该提案的主要目的是通过在受限区域内应用的完整波形反转方法（称为“盒子”断层扫描），改善阿拉斯加下面地幔区域地震图像的分辨率。需要全波形反转以说明多路径和波前愈合的影响，否则掩盖了强烈和局部异质性，例如板和周围的地幔楔。此外，在空间上受到限制的“盒子”方法夫妻夫妻夫妻夫妻夫妻夫妻夫妻夫妻夫妻夫妇较慢，较慢的3D波场求解器，从而减少了计算时间，这使团队能够使用较高的频率区域和远距离震动体波。使用3组分的表面波，泛音和身体波形的组合，将首先构建剪切速度模型。随着迭代的进展，增加计算的最大频率，并且随着对压缩速度敏感的其他身体波形，将获得压缩速度图像。该分析将显着锐化地震波和径向各向异性的现有图像，尤其是在过渡区深度以及中和下地幔的现有图像。 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.通过允许更好地识别俯冲板，将作品的结果吸引地球动力学家对俯冲带下的地幔和板运动重建的地幔进行建模的结果。地球动力学家和矿物理学家研究内核各向异性的模式和起源也将很感兴趣。此外，阿拉斯加的强大，高分辨率的层析成像模型以及为其影响提供更正的方法将改善阿拉斯加的USARRAY站的实用性，用于其他深入的地球研究。这项奖项反映了NSF的法定任务，并被认为是通过基金会的知识分子和更广泛影响的评估来审查的审查标准来通过评估来获得的支持。