TOWARDS A SYSTEMATIC CHARACTERIZATION OF CASCADIA UPPER PLATE MORPHOLOGY, STRUCTURE, AND QUATERNARY DEFORMATION HISTORY: AN INTEGRATED ONSHORE-OFFSHORE APPROACH (Invited Presentation)

Uncertainty related to rupture extent, slip distribution, and recurrence of past subduction megathrust earthquakes leads to ambiguity in earthquake and tsunami hazard assessments in Cascadia. Observations from recent subduction zone earthquakes in Japan and elsewhere highlight significant earthquake, tsunami, and landslide hazards posed by co-seismic or triggered deformation of the upper plate, particularly offshore above the shallow megathrust. Therefore, there should be a record of megathrust rupture behavior in the coastal and marine forearc in Cascadia that, if methodically interrogated, could improve hazard forecasting. To address this important knowledge gap, we are pursuing an integrative onshore-offshore study to systematically characterize upper plate Quaternary structure, deformation patterns, and evidence for interaction with the megathrust. Because megathrust co-seismic and interseismic deformation is focused offshore and along the coastline, coordinated onshore-offshore structural synthesis is vital to understanding the record of subduction earthquakes in the upper plate.

We summarize the current state of knowledge based on existing offshore data, discuss important offshore data gaps, and present a path forward involving integrative onshore-offshore collaborative research. High-resolution imaging (multibeam, seismic-reflection) represents an important data gap offshore that limits our ability to characterize Quaternary deformation. We present a systematic quantitative analysis of morphological and structural variation along the margin based on a compilation of available multibeam and seismic-reflection data. Results of this analysis form the basis of a comprehensive effort to quantify along-strike variation in margin morphology, structure, and Quaternary deformation that may be linked to long-term manifestation of megathrust behavior. Ultimately, this integrative onshore-offshore characterization can provide insights and boundary conditions for models of subduction processes and hazards.