Collaborative Research: Behavior and structure on and around the megathrust revealed by the Alaska Amphibious Seismic Community Experiment

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The planet’s largest earthquakes take place on subduction plate boundaries. These “megathrust” earthquakes, and the tsunamis they sometimes trigger, can cause extreme damage and loss of life in local and far-field communities. The Alaska Peninsula segment in particular hosted the second largest earthquake recorded anywhere and has been identified as a high-risk tsunami source for the west coast of North America. The Alaska megathrust also generates the most recorded earthquakes of any fault system in the U.S. In 2018-2019, the geophysical community deployed the first major shoreline-crossing seismic array off Alaska, placing 75 high-quality seismometers on the sea floor to complement several onshore synchronized deployments. This dataset, now openly available, provides the first comprehensive nearby sampling of signals from small to moderate earthquakes along the megathrust in Alaska. This project will analyze a range of earthquake-generated signals from this new dataset, to address questions such as: why are earthquakes more common in some places than others despite similar geology? where are the active faults in the region? and what is the nature of fault materials that allows earthquakes to be generated? The results will greatly improve the working models by which we understand the physics of earthquakes, of tsunami generation, and other related hazards. This project supports the training of graduate and undergraduate students.One of the best-known examples of along-strike variability of a megathrust is observed along the Alaska Peninsula. In a few hundred km, the plate interface varies from fully locked and capable of generating M9.2 earthquakes near Kodiak, to fully creeping near the Shumagin Islands. The fundamental controls on the rupture variability of megathrusts are poorly known, relevant to both along-strike variations as seen in Alaska, as well as updip/downdip changes that are seen along all subduction zones. A major new publicly available community seismic dataset has just been acquired along this segment of the Aleutian megathrust. The 15-month broadband dataset, termed the Alaska Amphibious Seismic Experiment, AACSE, provides extensive coverage far offshore and onshore of seismicity and structure, with uniform sampling. This project focuses on the megathrust sampled by the AACSE by characterizing and understanding the earthquake sources on it, the tremor produced along it, and by imaging the fine-scale structure using local-earthquake and teleseismic signals. Specifically, waveform-based methods will be used to greatly increase the quantity and precision of earthquake hypocenters along the megathrust, enabling characterization of along-strike variability. These will be complemented by studies of earthquake source properties to better understand spatial variation in rupture behavior, and to separate megathrust earthquakes showing upper plate and lower plate deformation. Modern tremor detection methods will be applied to the full dataset, to characterize any regions of downdip and potential updip tremor, and their along-strike variation. High-frequency mode-converted signals and autocorrelation of local-earthquake coda will be used to image the megathrust targeting spatial variations in its reflectivity strength and thickness; these analyses will be integrated with collocated receiver functions to generate a wide-band image of the plate boundary. This research will address several fundamental questions: Where is the plate boundary (as opposed to nearby regions in the upper and lower plate) seismogenic? How does the slip behavior and earthquake dynamics vary down-dip, and between locked and creeping sections? Do sharp boundaries in coupling correlate with strong variations in seismic behavior? How do earthquakes on the plate boundary relate to structure, and how does structure relate to dehydration or lithology? What explains the along-strike variability between locked and creeping sections? Do major potentially seismogenic splay faults exist in the upper plate? How is non-volcanic tremor related to regular earthquakes? The extensive amphibious array here allows an unprecedented opportunity to accurately resolve seismicity, tremor and structure in exactly the same place, with a variety of techniques, and in doing so provides an excellent opportunity to make progress on these issues.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.