Collaborative Research: Investigating Structure and Seismicity Within the Southern M9.2 1964 Great Alaska Earthquake Rupture Area Using a Dense Node Array

俯冲带是大洋板块沿着浅倾斜断层滑到大陆板块之下的区域。其中一个例子是阿拉斯加-阿留申俯冲带，它比世界上任何其他俯冲带都要负责更多的8级或更大的地震，其中包括1964年的9.2级阿拉斯加大地震。科迪亚克岛位于阿拉斯加-阿留申俯冲带断层的一部分上方，该断层导致了1964年的地震，2019年，作为阿拉斯加两栖社区地震实验（AACSE）的一部分，400台地震仪器在那里部署了25天。沃辛顿和她的团队将利用机器学习和其他技术，重新分析这些仪器收集的数据，以识别许多非常小的地震，并确定它们的位置。这将为地震如何沿着俯冲带断层及其附近分布提供前所未有的快照。这些地震和其他地震的记录将一起用于生成地下图像，包括俯冲带断层本身，并用于估计断层及其周围环境的物理特性。对浅层俯冲前弧区域地震活动性和构造的详细描述，有助于深入了解发震大逆冲构造的板块界面性质，以及活动变形和过去变形的分布、位置和运动学。作为阿拉斯加两栖社区地震实验（AACSE）的一部分，该项目于2019年5月利用沿约50公里横断面部署的密集间隔地震节点阵列的现有数据，描述了Qikertaq （Kodiak岛）地下的地震活动和结构。Kodiak节点阵列位于美国历史上最大的地震，1964年Mw9.2大阿拉斯加地震的南部陡坡内，那次地震使阿拉斯加俯冲带断裂了600-800公里。该项目的主要目标是：1)通过增加微震活动性（M1.0）的探测和重新定位，确定地震间旋回期间上、下板块与界面地震活动性的分布；2) 1964年南部地震破裂陡坡浅弧前的板块界面性质及其控制地震破裂、传播和起爆的性质；3) Kodiak区域内的上板块速度结构，为弧前向海部分的材料强度和弹性特性提供了约束。利用机器学习和基于聚结的自动检测算法，该项目将在25天的部署期间生成增强的地震活动目录。现有的和新的探测位置将被改进，以提供在大逆冲地震之后的震间间隔内地震活动性分布的快照。来自活跃源海洋拍摄和当地地震（包括新探测到的事件）的旅行时间将用于射线追踪和反演，以开发板块界面上方浅层前弧的高分辨率速度模型。这个速度模型将反过来用于进一步细化事件位置。来自局部地震的能量将用于对节点阵列下方的地壳和板块界面进行成像，利用散射波和反射相位进行垂直反射剖面和尾波自相关。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。

A subduction zone is a region where an oceanic plate slips below a continental plate, along a shallowly inclined fault. One example is the Alaska-Aleutian subduction zone, which is responsible for more Magnitude 8 or greater earthquakes than any other subduction zone in the world, among them the 1964 Magnitude 9.2 Great Alaska Earthquake. Kodiak Island lies just above part of the Alaska-Aleutian subduction zone fault that was responsible for the 1964 earthquake, and in 2019, 400 seismic instruments were deployed there for 25 days as part of the Alaska Amphibious Community Seismic Experiment (AACSE). Using machine learning and other techniques, Worthington and her team will re-analyze data collected by these instruments to identify numerous very small earthquakes, and pinpoint their locations. This will provide an unprecedented snapshot of how earthquakes are distributed along the subduction zone fault and its vicinity. Recordings of these and other earthquakes will be used together to generate images of the subsurface, including the subduction zone fault itself, and to estimate physical properties of the fault and its surroundings.Detailed characterization of seismicity and structure within shallow subduction forearc regions offers insight into plate interface properties along the seismogenic megathrust, and the distribution, location and kinematics of active and past deformation. This project characterizes seismicity and structure beneath Qikertaq (Kodiak Island) using existing data from a densely-spaced seismic node array deployed along a ~50 km transect as part of the Alaska Amphibious Community Seismic Experiment (AACSE) in May, 2019. The Kodiak node array is located within the southern asperity of the largest historical US earthquake, the Mw9.2 1964 Great Alaska event, which ruptured a 600-800 km section of the Alaskan subduction zone. The main goals of the project are to determine: 1) The distribution of upper plate vs lower plate vs interface seismicity during the interseismic cycle through increased microseismicity (M1.0) detection and relocation; 2) The nature of the plate interface in the shallow forearc of the southern 1964 earthquake rupture asperity, properties of which control earthquake rupture, propagation and initiation; 3) Upper plate velocity structure within the Kodiak region, providing constraints on material strength and elastic properties of the seaward portion of the forearc. Using both machine-learning and coalescence-based automatic detection algorithms, this project will produce an enhanced seismicity catalog for the duration of the 25-day deployment. The locations of existing and new detections will be refined to provide a snapshot of the distribution of seismicity during the interseismic interval following great megathrust earthquakes. Travel-times from active-source marine shots and local earthquakes (including newly detected events) will be used for raytracing and inversion to develop a high-resolution velocity model of the shallow forearc above the plate interface. This velocity model will, in turn, be used to further refine event locations. Energy from local earthquakes will be used for imaging the crust and plate interface below the node array using scattered waves and reflected phases for vertical reflection profiling and coda auto-correlation.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.