SEEKING CONSENSUS – THE SOUTHERN CASCADIA WORKING INTEREST GROUP SCWIG

The southern Cascadia subduction zone (CSZ) is formed by the subduction of the Gorda and Juan de Fuca plates beneath the North America plate and extends from Cape Mendocino in northern California to southern Oregon. However, there exists a difference between how California and Oregon treat tsunami hazards. Oregon uses both a distant tsunami scenario and a 10,000-year worst case quasi-probabilistic local tsunami scenario as a basis for tsunami hazard mapping. California uses a 1,000-year “plus” probabilistic tsunami model for local and distant sources as a basis for tsunami hazard mapping. Both states share a goal, to protect people from tsunami hazard exposure.

This contrast in hazard evaluation is based partially on physical differences in CSZ tsunamigenesis in these two regions and partially on a philosophical difference for how these hazards are treated. We formed the Southern Cascadia Working Interest Group (SCWIG) to evaluate these differences, especially those related to the hazard evaluation. The first SCWIG workshop in June 2021 had two goals: (1) to provide a scientifically based consensus statement about the southern CSZ, and (2) to discuss the initial basis for the expert judgement used to assign probabilities to logic tree branches in the next generation PTHA analysis for Cascadia tsunami modeling, especially in southern Cascadia. The consensus statement is intended to assist emergency managers, planners, and educators in communicating tsunami hazard information and messaging for the Cascadia tsunami hazard/threat for northern California and southern Oregon.

The attendees of this first workshop discussed the state of the science along the entire margin, emphasizing physical processes that directly affect tsunamigenesis, including patterns of seismicity and faulting, convergence rate variation, continental slope steepness, paleotsunami/paleoearthquake prehistory, and seismogenic coupling models. As a result, we prepared a preliminary consensus statement, with only one dissenting opinion: We agree that the tsunami source characterization of the southern Cascadia subduction zone (the Gorda segment), where it meets the unstable Mendocino triple junction, differs from the central and northern CSZ (Juan de Fuca segment) in several ways. A report detailing these differences is forthcoming.