Collaborative research: Structure and dynamics of the Alaska mantle wedge

俯冲带是地球上最活跃的地质特征——海洋板块以地质速度下降到地球内部，并从海底携带流体和物质。当这些物质加热时，水之类的流体被释放出来，润滑了浅层的断层，并使温暖的地球深层地幔流动，形成岩浆。因此，地球上最大的地震和最猛烈的火山爆发都发生在俯冲带。在北美，阿拉斯加-阿留申俯冲系统是迄今为止最大的俯冲系统——阿拉斯加曾发生过北美最大的地震，以及20世纪地球上最大的火山喷发。这个俯冲带的东端位于该州大部分人口的正下方，因此造成了重大的自然灾害。与此同时，东端的地质结构复杂，使得流体的流动路径难以理解，并使火山作用的基本理论复杂化。该项目旨在使用革命性的新数据集，显著提高我们对这一复杂转变中地幔性质的理解。在过去的几年里，从阿拉斯加的新地震仪收集了大量高质量的地震信号——自2015-16年以来，整个州都被EarthScope可移动阵列覆盖，该阵列每85公里放置最先进的仪器，并在感兴趣的地区进行了一些较小的密集部署。在这个高度活跃和复杂的地区，所有这些项目都与地球内部的地震信号相一致。这个特殊的项目旨在利用这些数据来解决和测试几个假设，这些假设将有助于更好地了解大型火山形成的方式，以及更广泛地了解地幔温度的变化。这些结果将为解释地质记录中来自相似环境的岩石提供一个框架。本项目重点研究了关于俯冲带地球动力学过程的三种一般假设：1)俯冲板块释放的流体变异性与上覆地幔楔体熔融程度的变异性相关——通过各种地震代理进行了测试。2)地震各向异性所揭示的岩构在三维流动的控制下，与预期的一样，受离岩板边缘距离的控制。3)地幔楔从冷前弧向热次弧过渡的深度在全球范围内是恒定的。地幔深度的地震衰减测量为所有这些区域的温度提供了一个代理，而本地地震剪切波分裂将补充新的远震（SKS）分裂测量来推断各向异性。对地震活动性和与板块表面相互作用的高频相位的平行观测，然后可以将地幔楔的推断与板块脱水进行比较。劈裂横波的高频波场模拟将评估超板块各向异性慢层的最大深度，这可能是板块-地幔耦合深度的标志。与此同时，岩石学驱动的模型提供了一个框架来进行预测，以检验每个假设。这些假设将通过比较阿拉斯加境内三个不同的走廊来检验，地球观测和相关项目提供了非常好的样本：(a)库克湾走廊，正常的太平洋岩石圈俯冲，弧是坚固的；(b)近岩浆的迪纳里走廊，雅库塔海洋高原俯冲并产生中深度地震；(c) Wrangell火山场走廊，这里几乎没有板块地震活动，但有非常大的火山活动。这些比较利用了阿拉斯加可移动阵列与几个密集的便携式宽带实验（BEAAR， SALMON， MOOS， WVLF），这些实验是由pi先前进行的项目，并对这些走廊进行了采样。该项目旨在解决EarthScope的科学目标，并强调岩石学、地震学和地球动力学之间的跨学科工作。它利用EarthScope国家办公室提供的教育和推广机会，特别是通过EarthScope网站和社交媒体提供的机会。所有项目参与者——包括两个研究所支持的研究生——将与地球观测国家办公室合作，最大限度地扩大该项目的科学推广。该项目将改进阿拉斯加中南部的地震波振幅预测，包括安克雷奇大都市区；因此，该项目可为地震危险性评估和地震动预测做出贡献。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知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones are the most active geologic features on the planet - oceanic plates descend into the earth's interior at geological speeds and carry with them fluids and materials from the sea floor. As this material heats, fluids such as water are released, lubricating faults at shallow depths and fluxing the warm Earth's deeper mantle to make magmas. As a result, subduction zones host the planet's largest earthquakes and most of the violent volcanic eruptions. In North America, the Alaska-Aleutian subduction system is by far the largest such system - Alaska has hosted the largest earthquake in North America, and the planet's largest 20th century eruption. The eastern end of this subduction zone lies directly beneath most of the population of the state so creates a major natural hazard. At the same time this eastern end is geologically complex, making the pathways of fluid difficult to understand and complicating the underlying theories of volcanism. This project aims to significantly advance our understanding of the nature of the mantle in this complex transition, using a revolutionary new data set. Over the last several years a vast amount of high-quality earthquake signals has been collected from new seismometers in Alaska - the entire state has been covered since 2015-16 by the EarthScope Transportable Array that places state-of-the-art instrumentation every 85 km, accompanied by a number of smaller, dense deployments over areas of interest. All these projects ensonify with earthquake signals the interior of the planet in this highly active and complex region. This specific project aims to capitalize on these data to address and test several hypotheses that will help better understand the ways in which large volcanoes form and more generally the variations in temperature of the Earth's mantle. The results will provide a framework for interpreting rocks that come from similar environments in the geologic record. This project focuses on three generic hypotheses regarding geodynamic process in subduction zones: 1) Variability in fluid release from subducting plates correlates with variability in the degree of melting in the overlying mantle wedge - tested through a variety of seismic proxies. 2) Rock fabric as revealed by seismic anisotropy is controlled by distance from the edge of the slab as expected if three-dimensional flow controls it. 3) The depth at which the mantle wedge transitions from cold forearc to hot subarc is globally constant. Measurements of seismic attenuation at mantle depths provides a proxy for temperature in all of these regions, and local-earthquake shear-wave splitting will complement new teleseismic (SKS) splitting measurements to infer anisotropy. Parallel observations of seismicity and high-frequency phases that interact with the slab surface then allow inferences about the mantle wedge to be compared with slab dehydration. High-frequency wavefield simulations of split shear waves will assess the maximum depth of a supra-slab anisotropic slow layer, a probable signature of slab-mantle coupling depth. At the same time, petrologically-driven models provide a framework for making predictions that test each hypothesis. These hypotheses will be tested via comparison of three distinct corridors within Alaska for which EarthScope and related projects provide unusually good sampling: (a) the Cook Inlet corridor where normal Pacific lithosphere subducts and the arc is robust; (b) the nearly amagmatic Denali corridor where the Yakutat oceanic plateau subducts and generates intermediate-depth earthquakes; and (c) the Wrangell Volcanic Field corridor where slab seismicity is nearly absent but there is very high volume volcanism. These comparisons take advantage of Alaska Transportable Array combined with several dense portable broadband experiments (BEAAR, SALMON, MOOS, WVLF), previous projects conducted by the PIs and which sample each of these corridors. This project addresses EarthScope science objectives and emphasizes interdisciplinary work at the interface between petrology, seismology, and geodynamics. It leverages education and outreach opportunities through the EarthScope National Office, notably those available through the EarthScope website and social media. All project participants - including graduate students supported at two institutions- will work with the EarthScope National Office to maximize scientific outreach of the project. The project will generate improved predictions of amplitudes of seismic waves in south-central Alaska, including within the Anchorage metropolitan region; therefore the project can contribute toward seismic hazard assessments and ground motion prediction.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.