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，并携带来自海底的流体和物质。当这种物质被加热时，水等流体被释放出来，润滑浅层断层，并使温暖的地球更深的地幔产生岩浆。因此，俯冲带是地球上最大的地震和大多数猛烈的火山喷发的发源地。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--阿拉斯加发生了北美最大的地震，也是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。同时，这个东端的地质很复杂，使得流体的路径很难理解，并使火山活动的基本理论复杂化。这个项目的目的是使用一个革命性的新数据集，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到了大量高质量的地震信号-自2015-16年以来，整个州都由EarthScope可移动阵列覆盖，该阵列每85公里放置一台最先进的仪器，同时在感兴趣的地区部署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吻合。这个具体的项目旨在利用这些数据来解决和测试几个假说，这些假说将有助于更好地了解大火山形成的方式，以及更广泛地说，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。该项目侧重于关于俯冲带地球动力学过程的三个一般性假设：1)从俯冲板块释放的流体的可变性与上覆地幔楔形融化程度的可变性相关-通过各种地震指标进行了测试。2)地震各向异性揭示的岩石组构在三维流动控制的情况下，如预期的那样受控于距板边的距离。3)地幔楔体从冷弧前向热弧下转变的深度是全局恒定的。地幔深度的地震衰减测量为所有这些地区的温度提供了一个替代指标，而局部地震横波分裂将补充新的远震(SKS)分裂测量，以推断各向异性。对地震活动和与板块表面相互作用的高频阶段的平行观测，使得关于地幔楔形的推断可以与板块脱水进行比较。分裂横波的高频波场模拟将评估板面各向异性慢层的最大深度，这可能是板幔耦合深度的特征。与此同时，岩石学驱动的模型提供了一个框架，用于进行预测，以检验每一种假设。将通过比较阿拉斯加境内三个不同的走廊来检验这些假设，EarthScope和相关项目为这些走廊提供了异常良好的采样：(A)库克海湾走廊，在那里正常的太平洋岩石圈俯冲，弧体健壮；(B)雅库塔特大洋高原俯冲并产生中等深度地震的接近岩浆的德纳利走廊；(C)兰格尔火山场走廊，在那里几乎没有板岩地震活动，但存在非常高的火山活动。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、SAMMON、MOOS、WVLF)相结合的优势，这些实验是由PI进行的先前项目，并对每一条走廊进行采样。该项目针对EarthScope的科学目标，强调岩石学、地震学和地球动力学之间的交叉学科工作。它通过EarthScope国家办公室利用教育和外联机会，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--包括在两个机构获得资助的研究生--将与地球范围国家办公室合作，最大限度地扩大该项目的科学影响。该项目将改进对阿拉斯加中南部地区，包括安克雷奇大都市区内地震波振幅的预测；因此，该项目可以为地震危险性评估和地面运动预测做出贡献。该奖项反映了NSF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。

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.