Subducted Structure in Central Cascadia from Margin-Crossing Seismic Tomography

Abstract

Studies indicate that the Cascadia Subduction Zone is capable of producing great earthquakes, and, like many other subduction zones, it exhibits along-strike differences in rupture history and plate coupling. Paleoseismic data, locking models, and microseismicity point to a change in seismic behavior around 44.5°N, offshore central Oregon. In this study, we use controlled-source seismic data from four shoreline-crossing experiments to invert for the P-wave velocity structure and Moho interface between 43.6°N and 45.2°N and to investigate changes in seismic structure within the forearc. The model reveals changes in the backstop geometry and an interpreted mafic dike embedded within an accreted terrane. WNW trending low velocity anomalies along the Alvin Canyon Fault suggest the fault cuts through both the accretionary wedge and subducting oceanic plate. At the top of the subducting slab, nine circular high-velocity anomalies are present that we interpret to be seamounts at various stages of subduction. The final Moho interface shows significant changes in depth related to the density of accreted terranes within the overriding plate. We propose that the subducted topography is partially responsible for large slides and scars at the deformation front and that subducting seamounts may have uplifted broad regions along the margin. Evidence for ongoing deformation is provided by seismicity clustered near two of the proposed seamounts. Finally, we suggest that this rough plate interface may lead to lower coupling and/or have acted as a rupture barrier in past megathrust earthquakes.