Volcanic transcrustal magmatic systems imaged with teleseismic converted phases

阿拉斯加的阿留申弧由几十座活火山组成。它们的喷发危及当地社区以及国内和国际空中交通。这样的弧之所以具有科学意义，还因为它们是新大陆地壳形成的地质环境。我们对火山地下管道的理解最近有所发展。从单一岩浆房的简单图像，到地壳中各个层次的相互连接的岩浆房的复杂系统的图像已经出现。详细研究这些深层结构对于更好地了解火山及其相关危害至关重要。然而，由于技术上的限制，这种调查一直具有挑战性。在这里，研究人员使用了一种新方法。他们与阿拉斯加火山观测站合作，利用全球各地发生的地震记录。通过分析记录波形的整体形状，而不仅仅是到达时间，他们绘制了火山下的三维岩浆结构的图像。它们揭示了重要的特征，例如，凝固岩浆中晶体的方向和邻近岩石中融化的裂缝。新的观测显示，复杂的地下结构在火山周围呈同心状，在地壳的所有深度都可以探测到。该项目有助于改善对当地社区、基础设施和空中交通路线的危害评估。它通过乌纳拉斯加州的阿留申博物馆（Museum of the aleutian），尤其是当地的电视广播，向K-12科学教师和公众推广。它还通过面向地球科学领域代表性不足的群体的项目，促进对本科生的培训。岛弧作为大陆地壳产生的地点，在构造学中引起了人们的兴趣。阿留申群岛和阿拉斯加的火山为弧火山活动提供了极好的横截面。该项目利用了对阿拉斯加火山现有监测网络的最新更新。它测试了一种通过所有地壳深度对弧火山岩浆系统进行被动源成像的新方法。提出的工作是开发一种新的基于波形的方法，用于深入地壳的火山结构的详细成像和表征。它使用少量的宽带站，补充了更昂贵的有源实验和密集部署。该方法还使用了来自弧火山附近的远震接收函数的大到达，这些远震接收函数由周期性的反方位角分量（极性反转与入射方位角）主导。极性反转的位置落在与火山大厦同心的方位角上。观测到的到达意味着各向同性层和/或以火山为中心的各向异性结构之间的倾斜对比至少达到地壳中部深度。最近阿拉斯加火山观测站对监测网络的升级使可供研究的火山数量增加了一倍多。建模工作将有助于区分跨地壳岩浆结构的几种端元模型，这些模型已被提出用于解释其他方法（如岩石学、层析成像、大地测量学、大地电磁学和地震活动重定位）的结果。这些端元模式包括与高速或低速地壳体的倾斜界面、岩浆房上方的倾斜断层、脉岩杂岩的倾斜。所提出的方法提供的附加约束将使整个地壳的岩浆结构比目前可能的分辨率更高。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。

The Aleutian arc in Alaska consists of dozens of active volcanoes. Their eruptions imperil local communities as well as domestic and international air traffic. Such arcs are also of scientific interest because they are the geological setting in which new continental crust forms. Our understanding of the subterranean plumbing of volcanoes has recently evolved. From the simple picture of a single magma chamber, the image of a complex system of interconnected chambers at all levels in the Earth’s crust has emerged. Investigating these deep structures in detail is critical to better understand volcanoes and their associated hazards. However, such investigation has been challenging because of technical limitations. Here the researchers use a new method. In collaboration with the Alaska Volcano Observatory, they use seismic records of earthquakes occurring around the globe. By analyzing the whole shape of recorded waveforms – instead of just the arrival times - they image the 3D magmatic structure underneath volcanoes. They reveal important features, e.g., the orientation of crystals in solidified magma and of melt-filled cracks in the adjacent rocks. New observations show complex subterranean structures that appear concentric around volcanoes, detected at all depth levels of the crust. The project helps improving hazard assessment for local communities, infrastructures as well as air traffic routes. It fosters outreach to K-12 science teachers, and to the public through the Museum of the Aleutians in Unalaska with, notably, a local television broadcast. It also promotes training for undergraduate students via programs geared towards groups underrepresented in Earth Sciences.Island arcs are of interest in tectonics as sites of continental crust production. Volcanoes in the Aleutians and Alaska offer an excellent cross section of types of arc volcanism. This project leverages recent updates to existing monitoring networks at Alaskan volcanoes. It tests a new approach for passive source imaging of magmatic systems under arc volcanoes through all crustal depths. The proposed work is to develop a new waveform-based method for detailed imaging and characterization of volcanic structures into the deep crust. It uses sparse numbers of broadband stations, complementing more expensive active source experiments and dense deployments. The approach also uses large arrivals in teleseismic receiver functions from the vicinity of arc volcanoes that are dominated by a periodic backazimuthal component (polarity reversals with azimuth of incidence). The positions of the polarity reversals fall on azimuths that are concentric to the volcanic edifice. The observed arrivals imply dipping contrasts between isotropic layers and/or anisotropic fabric centered on the volcano to at least midcrustal depths. Recent station upgrades to monitoring networks by the Alaska Volcano Observatory more than double the number of volcanoes available for study. A modeling effort will help distinguish between several endmember models of transcrustal magmatic structure that have been proposed to explain results from other methods such as petrology, tomography, geodesy, magnetotellurics, and seismicity relocations. These endmember models include dipping interfaces to high- or low-velocity crustal bodies, dipping faults above magma chambers, and dip in dike and sill complexes. The added constraints offered by the proposed method will allow higher resolution magmatic structures throughout the crust than currently possible.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.