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
Tsunami Pilot Study Working Group, Tsunamis -- Oregon -- Seaside
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