Structure and Hydration of the Explorer-Juan de Fuca-Gorda Plate System at the Onset of Subduction Beneath Cascadia

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Earthquakes, tsunamis, and explosive volcanism are geohazards associated with subduction zones. Understanding the variability along the subduction zone is important for what controls geohazards in this populated region of North America. This project will analyze marine seismic profiles along the Cascadia margin from offshore Vancouver Island to southern Oregon. Measurements and analyses will constrain variability in subducting sediment thickness, amount of water in the downgoing plate, and topography of the sediment-basement interface. This project will contribute to the education of two graduate students by providing them support, data and training in geophysical imaging.In 2021, a suite of modern and extensive controlled-source seismic datasets was collected along profiles spanning most of the length of the Cascadia margin. The datasets along ten of the profiles consist of co-located ultralong-streamer multichannel seismic reflection and wide-angle seismic reflection/refraction data recorded in ocean bottom seismometers (OBSs). The project will use these datasets to model the seismic properties and structure of the sediments, crust and mantle of the incoming and down-going plates as well as those of the accretionary prism. Fracturing and alteration of crustal and mantle rocks give rise to detectable changes in seismic velocities, and seismic methods provide an important remote sensing tool for assessing plate hydration. The researchers will produce models of the P-wave and S-wave seismic velocity structure along each OBS transect using state-of-the-art wide-angle seismic data processing and imaging methods, including downward-continuation of ultra-long streamer data, super-virtual refraction interferometry, and joint travel-time refraction/reflection tomography of streamer and OBS data. The seismic velocity models will be used together with effective medium theory to constrain crust and mantle porosity, and the amount and distribution of fluid and chemically-bound water of the incoming and down-going plate. The findings from this project will be of great interest to researchers investigating Cascadia subduction zone processes, from the deformation front to sub-arc depths. The project results will also be a key component for updating and improving the U.S. Geological Survey’s margin-scale 3-D seismic velocity model for Cascadia.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.