Mapped Overland Distance of Paleotsunami High‐Velocity Inundation in Back‐Barrier Wetlands of the Central Cascadia Margin, U.S.A.

Abstract

Investigations of back-barrier, open-coastal plain settings have been used to establish minimum inundation distances of prehistoric tsunamis produced by great subduction zone earthquakes in the central Cascadia margin. Distinctive sand sheets were characterized at four localities within the central Cascadia margin, a shoreline distance of about 250 km. The sand sheets vary in thickness from 0.2 to 25 cm. They thin in the landward direction and consist of well-sorted beach sand that fines upsection. Many of the sand sheets include capping layers of organic-rich detritus, as well as assimilated mud rip-up clasts and soil litter. Marine diatoms and bromine (i.e., marine tracers) were used to confirm marine surge origins for the anomalous sand sheets. Radiocarbon dating of the sand sheets demonstrates correspondence with reported great Cascadia earthquake events at 0.3, ∼1.1, ∼1.3, ∼1.7, and ∼2.5 Ka. One sand sheet mapped at all four localities is dated at 600-950 calibrated radiocarbon years before present. This interpreted paleotsunami event does not correspond to a central Cascadia rupture, so it is tentatively assigned to a far-field source. Minimum overland inundation distances of the near field (Cascadia tsunami) at the four study localities range from 0.3 to 1.3 km, with a mean inundation for all sand sheets of 0.5 km.

Paleotsunami Inundation of a Beach Ridge Plain: Cobble Ridge Overtopping and Interridge Valley Flooding in Seaside, Oregon, USA

“…Dense sampling of target paleotsunami layers from multiple core sites at ∼50 m intervals has helped to resolve these record preservation problems in nearby beach plains [7]. In 2004 a pilot study of potential tsunami inundation in the Seaside study area was initiated by several US federal agencies [6].…”

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

“…The presence of marine diatoms has previously been used to establish paleotsunami inundation in some upland or inland freshwater settings of the Cascadia margin (Hemphill-Haley, 1996;Hutchinson et al, 1997;Kelsey et al, 2005). However, unexpected traces of marine diatoms in some control intervals, or non-tsunami deposits, from the central Cascadia beach plains and floodplains have suggested marine diatom transport by ocean wind/spray to distances of 2 km inland from the beach (Peterson et al, 2010a;Schlichting & Peterson, 2006). To further test these findings some target paleotsunami deposits and control intervals in the creek floodplains were examined for diatom taxa abundances following methods provided by Schilchting (2000).…”

“…Investigations of the associated nearfield paleotsunami inundations moved from the coastal marshes to overland inundation recorded in lakes, beach plains, back-barrier wetlands, and barrage lakes (Hutchinson et al, 1997;Kelsey et al, 2005;Schlichting & Peterson, 2006). The geologic records of paleotsunami sand sheet deposition that are located landward of foredunes, beach ridges, and other unstable coastal barriers provide minimum estimates of runup height .…”

“…Tsunami correlations (Tsunami #) in the central Cascadia margin are from Peterson et al, (2010a). The source of the 2 nd tsunami sand layer in the central Cascadia tsunami is not known (Peterson et al, 2008), but it might reflect a partial rupture in the northern part of the Cascadia margin (Clague et al, 2000;Schlichting & Peterson, 2006). Due to difficulties of discriminating the 2 nd and 3 rd tsunami events in some floodplain settings we assign them numbers of #2a and #2b, where they can be discriminated, and #2 where only one layer can be discriminated.…”

“…Diatoms occur in the silt size range, and are potentially very useful in discriminating marine surge sources of target paleotsunami layers in the Cascadia margin (Hemphill-Haley, 1996;Kelsey et al, 2005;Schlichting & Peterson, 2006). In this study a composite sample was taken from the top of the target tsunami layer where the fine to very-fine sand in the tsunami sand layer grades into sandy silt or sandy detrital organics in the tsunami debris layer.…”