Collaborative Research: Illuminating the Cascadia plate boundary zone and accretionary wedge with a regional-scale ultra-long offset multi-channel seismic study

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At the Cascadia Subduction Zone, the slow ongoing descent of the Juan de Fuca plate beneath the northwestern coast of North America has generated large earthquakes and associated tsunami in the past. Geologic records suggest that some sections of the subduction zone fault or "megathrust", which extends ~35-90 miles seaward from the coasts of northern California all the way to southern British Columbia, slipped less than other sections during the last large earthquake (1700 AD), and that in some prior EQ only parts of the subduction zone ruptured. Whether these along-margin variations may persist in future EQ has important implications for quantifying earthquake and tsunami hazards within the heavily populated Pacific Northwest margin. Geologic structure such as seamounts and other topographic features in the descending Juan de Fuca plate, the structure and properties of the thick folded and faulted package of sediments that forms above the subduction zone fault, or the properties of megathrust fault rocks, could contribute to these along-margin variations. However the current observations are limited and allow for a wide range of possible future earthquake scenarios. Modern marine seismic reflection imaging techniques provide the best tools available for illuminating a subduction zone to the depths of the earthquake source region and below. The overall goal of this project is to acquire a regional grid of modern marine seismic reflection data spanning the entire Cascadia Subduction zone to image how the geologic structure and properties of this subduction zone vary both along and across the margin. This project will serve the national interest in that the acquired data will form a foundational dataset for future initiatives focused on hazard assessment in the U.S. Pacific Northwest, including for an earthquake early warning network and deep drilling to sample the fault zone rocks, and it will promote the advancement of science by providing a new regional framework relevant for the wide range of multi-disciplinary onshore and offshore studies conducted in this region over the past several decades.Specifically, this study will use ultra-long-offset multi-channel seismic (MCS) data to characterize subducting plate and accretionary wedge structure, and properties of the megathrust, to address the following specific questions: 1. Are there any systematics in the structure and properties of the incoming Juan de Fuca plate, the megathrust zone, and accretionary wedge associated with inferred paleo-rupture segmentation? 2. Are there down-dip variations in megathrust geometry and reflectivity indicative of transitions in fault properties, and what are the properties of the potentially tsunamigenic shallow portion of the megathrust? Long 15-km-offset MCS data will be acquired along twenty 2-D profiles at 50-100 km spacing oriented perpendicular to the margin and located to provide coverage in areas inferred to be paleo-rupture patches and their boundary zones. The survey will also include one continuous strike line along the continental shelf centered roughly over gravity-inferred fore-arc basins to investigate possible segmentation near the down-dip limit of the seismogenic zone. The margin normal lines will extend approximately 50 km seaward of the deformation front to image the region of subduction bend faulting in the incoming oceanic plate, and landward of the deformation front to as close to the shoreline as can be safely maneuvered. The acquired data will be designed to characterize 1) the deformation and topography of the incoming plate, 2) the depth, topography and reflectivity of the megathrust, 3) sediment properties and amount of sediment subduction, 4) the structure and evolution of the accretionary wedge, including geometry and reflectivity of fault networks, and how these properties vary along strike, spanning the full length of the margin and down dip across what may be the full width of the seismogenic zone at Cascadia. The data will be processed to pre-stack depth migration using state-of-the art seismic processing techniques and will be made openly available to the community, providing a high-quality data set illuminating the regional subsurface architecture all along the Cascadia Subduction Zone.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.