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

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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.