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，并携带海底的流体和物质。当这种物质加热时，像水这样的流体被释放出来，润滑了浅层的断层，并使温暖的地球深部地幔流动，形成岩浆。因此，俯冲带是地球上最大的地震和大多数猛烈火山爆发的所在地。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-阿拉斯加曾发生过北美最大的地震，以及地球上世纪最大的火山爆发。这个俯冲带的东端直接位于该州大部分人口的下方，因此造成了重大的自然灾害。与此同时，火山的东端地质复杂，使得流体的路径难以理解，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，显着推进我们对地幔在这一复杂过渡中的性质的理解。在过去的几年里，从阿拉斯加的新地震仪中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公里放置一台最先进的仪器，并在感兴趣的区域进行了一些较小的密集部署。所有这些项目都在这个高度活跃和复杂的地区用地震信号穿透地球内部。这个具体项目旨在利用这些数据来解决和测试几个假设，这将有助于更好地了解大型火山形成的方式，以及更普遍的地幔温度变化。这些结果将为解释地质记录中来自类似环境的岩石提供一个框架。该项目重点研究了俯冲带地球动力学过程的三个一般假设：1）俯冲板块流体释放的变化与上覆地幔楔熔融程度的变化相关-通过各种地震代理测试。2)地震各向异性揭示的岩石组构与三维流动控制的岩石组构一样，受到板块边缘的距离的控制。3）地幔楔从冷弧前向热弧下过渡的深度是全球恒定的。在地幔深处的地震衰减的测量提供了一个代理的温度在所有这些地区，和本地地震剪切波分裂将补充新的地壳（SKS）分裂测量，以推断各向异性。与板块表面相互作用的地震活动性和高频相位的平行观测，然后允许地幔楔的推论与板块脱水进行比较。分裂剪切波的高频波场模拟将评估板上各向异性慢层的最大深度，这是板幔耦合深度的可能特征。与此同时，岩石学驱动的模型提供了一个框架，用于进行测试每个假设的预测。这些假设将通过比较阿拉斯加境内的三个不同走廊来检验，EarthScope和相关项目为这三个走廊提供了非常好的取样：（a）库克湾走廊，那里是正常的太平洋岩石圈俯冲带，弧是坚固的;（B）亚库塔海洋高原俯冲带和产生中深度地震的近无岩浆的德纳里走廊;和（c）兰格尔火山场走廊，那里几乎没有板状地震活动，但有非常高的火山活动量。这些比较利用阿拉斯加可移动阵列结合几个密集的便携式宽带实验（BEAAR，SALMON，MOOS，WVLF），以前的项目进行的PI和采样每个这些走廊。该项目致力于EarthScope的科学目标，强调岩石学、地震学和地球动力学之间的跨学科工作。它通过地球范围国家办事处，特别是通过地球范围网站和社交媒体提供的机会，利用教育和外联机会。所有项目参与者-包括在两个机构得到支助的研究生-将与地球镜国家办事处合作，最大限度地扩大该项目的科学范围。该项目将改进阿拉斯加中南部地震波振幅的预测，包括安克雷奇大都市地区;因此，该项目可以为地震危险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.