VALIDATION OF PREDICTED TSUNAMI INUNDATION FOR THE INLAND COAST OF THE SALISH SEA ASSOCIATED WITH CASCADIA SUBDUCTION ZONE EARTHQUAKES

Large (Mw 9.0) earthquakes along the Cascadia subduction zone (CSZ) margin occur every 300-700 years, with the most recent event occurring January 26, 1700. These earthquakes act as the catalyzing force for large tsunamis that inundated the outer coast of Washington, Oregon and northern California. In recently published simulations based on the 1700 event parameters, Cascadia tsunamis have been modeled to travel through the Strait of Juan de Fuca and inundate the inner coast of the Salish Sea. Tsunamis can deposit sand and other sediments during inundation from near shore in low-elevation marshes and lakes, and historic marsh environments along the outer coast of Washington and along the coast of the Salish Sea have been identified as locations that likely preserve tsunami deposits from past CSZ events. Tsunami models for the Salish Sea are continually evolving, integrating new seismic scenario modeling, lidar and bathymetric data sets. Spatial variations of inundation predictions among different models presents a question of accuracy for hazard planning. Because a deposit is physical evidence for tsunami inundation, a tsunami deposit can be used as a proxy for tsunami model validation. Field work in August and September 2018 will focus on coring and core classification at three locations: Ship Harbor, Anacortes WA with up to 3.8m of predicted inundation, Cannery Lake, Anacortes WA with 2.15m of predicted inundation, and Eliza Island marsh, WA, with 1.6m predicted inundation. Ship Harbor and Cannery lake are predicted to be inundated in newer models from Gica et al. (2016), and not predicted in older models from Walsh et al. (2004), while Eliza Island marsh is predicted to be inundated in both. Presence, or not, of deposits at these different locations allows us to ground truth these different inundation models. Initial results from field work will be presented. Data collected from the cores such as sediment morphology and magnetic properties will be used to make an analysis of wave run-up during inundation, factoring historic sea level change and tides.