Speaker: Dr. Shuoshuo Han (University of Texas Institute for Geophysics)

The Cascadia subduction zone has hosted numerous great earthquakes, up to Mw ~9.0, during the Holocene and poses a significant earthquake and tsunami threat to the heavily populated Pacific Northwest. However, with little instrumentally recorded seismicity, the geometry and physical properties of the megathrust in the seismogenic zone are still poorly known. In June-July 2021, we conducted the CAscadia Seismic Imaging Experiment (CASIE21), a regional active source seismic survey on board R/V Langseth to characterize the structure and physical properties of the incoming plate, overriding plate, and the plate interface of the Cascadia subduction zone. Multichannel seismic data were acquired using an air gun source of 6600 in3 and a streamer of 12km/15km length. The new pre-stack depth migrated seismic images and accompanying high-resolution velocity models along 18 dip lines and 7 strike lines span the offshore region from Vancouver Island to southern Oregon and provide new constraints on megathrust geometry, structure and strength of the accretionary wedge, and submarine landslides along this margin. Offshore Vancouver Island, Washington, and northern Oregon, incoming sediments are fully accreted to form an outer wedge of high seismic velocity. Offshore central and southern Oregon, the decollement forms in the middle of the incoming sediment section; the region of thick sediment underthrusting coincides with a narrow outer wedge of low seismic velocity, and numerous massive paleo-landslide deposits seaward of the deformation front. In this talk, I will present new seismic images and high-resolution velocity models from the CASIE21 experiment. I will also discuss the causes of along strike variations of outer wedge structure and strength in relation to sediment input and backstop geometry, and their impact on plate interface slip behavior.