POTENTIAL CASCADIA PALEOTSUNAMI DEPOSIT IN SHIP HARBOR MARSH, ANACORTES, WASHINGTON

Large earthquakes (Mw ~9.0) along the Cascadia subduction zone (CSZ) margin are predicted to occur every 300-700 years, with the most recent event occurring January 26, 1700. These earthquakes act as the catalyzing force for large tsunamis predicted to inundate the outer coast of Washington, Oregon and northern California. In recently published simulations, Cascadia tsunamis have been modeled to travel through the Strait of Juan de Fuca and inundate portions of the inner coast of the Puget Sound. During inundation, tsunamis can transport sand and other sediments from near-shore and the beach, depositing them in adjacent low-elevation marshes or lakes. Marsh environments along the coast of the Puget Sound have been identified as locations which may have preserved tsunami deposits from past CSZ events. Field work in August and September 2018 focused on gouge-auger coring, vibracore collection, and subsurface classification at two locations: Ship Harbor marsh, Anacortes, WA and Eliza Island marsh, Eliza Island, WA. These locations were chosen due to predicted inundation from models by Walsh et al. (2004), Gica and Arcas (2016), and Eungard et al. (2018). Analysis of core data through stratigraphic interpretations, particle size analysis, C¹⁴ dating, magnetic fabrics analysis and magnetic paleosecular variation dating have helped determine the likely presence of a paleotsunami deposit in Ship Harbor marsh. Preliminary sequence modeling of C¹⁴ dates suggest deposition between 1802-1730 years B.P. The presence of a Cascadia paleotsunami deposit at this location has implications regarding the accuracy of the L1 scenario (Witter et al. 2013), and helps confirm that tsunami models produced from the L1 are a good basis for future tsunami hazard planning in the Puget Sound.