Radiocarbon dating support was provided by NOAA, through grants to Oregon Graduate Institute and through the Tsunami Pilot Study Working Group, 2006.
Journal of Geological Research
Tsunamis -- Oregon -- Seaside -- History, Tsunamis -- Environmental aspects, Sediment transport, Coastal sediments, Coast changes -- Oregon -- History
The Seaside beach ridge plain was inundated by six paleotsunamis during the last ∼2500 years. Large runups (adjusted >10m in height) overtopped seawardmost cobble beach ridges (7m elevation) at ∼1.3 and ∼2.6 ka before present. Smaller paleotsunami (6−8m in height) likely entered the beach plain interior (4-5m elevation) through the paleo-Necanicum bay mouth. The AD 1700 Cascadia paleotsunami had a modest runup (6-7mheight), yet it locally inundated to 1.5 km landward distance. Bed shear stresses (100−3,300 dyne cm−2) are estimated for paleotsunami surges (0.5−2m depths) that flowed down slopes (0.002−0.017 gradient) on the landward side of the cobble beach ridges. Critical entrainment shear stresses of 1,130−1,260 dyne cm−2 were needed to dislodge the largest clasts (26−32 cm diameter) in paleotsunami coulees that were cut (100−200m width) into the landward side of the cobble ridges.
Curt D. Peterson, Harry M. Jol, Tom Horning, and Kenneth M. Cruikshank, “Paleotsunami Inundation of a Beach Ridge Plain: Cobble Ridge Overtopping and Interridge Valley Flooding in Seaside, Oregon, USA,” Journal of Geological Research, vol. 2010, Article ID 276989, 22 pages, 2010. doi:10.1155/2010/276989