Proximal Records of Paleotsunami Runup in Barrage Creek Floodplains from Late-Holocene Great Earthquakes in the Central Cascadia Suduction Zone, Oregon, USA

Abstract

A 100 km section of the central Oregon coast (Fig. 1) was surveyed for proximal creek floodplains, located at less than 500 m distance from the ocean shoreline, that could host sand sheet records of paleotsunami inundation (Peterson & Cruikshank, 2007). The study region is located near the center of the Cascadia margin, an active subduction zone that spans about 1000 km distance in the central west coast of North America (Atwater et al., 1995; Darienzo et al., 1994). Previous work in two distal floodplain localities within the study region, Neskowin and Beaver Creek, showed multiple paleotsunami inundations of one to several kilometers distance landward during the last ~ 2,500 years (see section 2.2) (Peterson et al., 2010a). It was not known how those distal records might relate to proximal or shoreline runup heights of the same paleotsunami events in the region. Such proximal or near-shoreline runup heights are needed to 1) demonstrate flooding hazards in coastal areas that have not suffered catastrophic flooding in historic time (Dengler, 2006), and 2) independently test flooding predictions based on assumed fault displacements and numerical tsunami runup models (Gonzalez et al., 2009). In this paper we document anomalous sand sheet layers in 8 small creek floodplains that exceed 6 m elevation thresholds for tsunami inundation. Target sand sheets are examined for evidence of marine shell fragments, tracers of marine surge origins, and the landward limits of sand sheet extent. The time span of continuous deposition in one representative floodplain locality is dated by radiocarbon. An adjacent floodplain locality is examined for landward trends of sand sheet composition, sand sheet thickness, microscopic tracers of marine deposits, and sand sheet radiocarbon age. Maximum sand sheet extent in the proximal floodplain locality is compared to maximum sand sheet extent in a distal floodplain locality (Peterson et al., 2010a) to yield a landward runup height gradient for the most recent event of large magnitude runup. The runup attenuation gradient is tested against previously reported runup elevations in another central Cascadia study area, Cannon Beach, Oregon (Fig. 1) (Peterson et al., 2008). The methods reported here should be applicable in similar settings to the documentation of paleotsunami runup heights in other susceptible coastlines around the world.