Abstract: The Cascadia subduction zone (CSZ) has previously hosted great earthquakes of moment magnitude > 8.0 and poses a major hazard to communities in the Pacific Northwest. The stratigraphic location of a megathrust fault impacts the local frictional properties of the fault, directly influencing the magnitude of an event. The CSZ has previously shown near-trench variability in the stratigraphic level of the megathrust, but few constraints exist at greater depth or throughout the margin. In this talk, I will present results from the CAscadia Seismic Imaging Experiment 2021 (CASIE21) that collected a regional-scale multichannel seismic reflection dataset to characterize the structure of the CSZ. Interpretations of the top of igneous oceanic crust, megathrust, and Moho allow for regional crustal constraints and on the stratigraphic location of the megathrust. While the mean values for both incoming and subducting regions of the plate show little variation, linear regressions show a generally unchanging crustal thickness for the incoming plate but thinning of the plate with distance from the trench as the plate subducts. We find significant variation in crustal thickness, as great as ~3.5 km, throughout the CSZ from either the three different plates, propagator wakes, or regions of thin crust. In particular, thin downgoing oceanic crust, >1 km thinner than the mean incoming plate, is found beneath the offshore extent of Siletz terrane off Oregon with no major bending-related features found in this region. This also marks a clear change in relative reflection amplitude of the interpreted megathrust. The stratigraphic location of the megathrust is usually found to be near the top of igneous oceanic crust or within the sediment column. Off Oregon, the megathrust step-downs from a sediment-hosted fault near the trench to near the top of igneous oceanic crust at depth. Thinning of oceanic crust under the Siletz terrane may be possible if the megathrust steps down from the sediments into the igneous oceanic crust itself, becoming a basalt-hosted shear zone. This process would alter the megathrust properties, directly impacting the down-dip limit of the seismogenic zone and its behavior.

*Refreshments:
10:30 AM - 11:00 AM (Atrium)