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Minimum Runup Heights of Paleotsunami from Evidence of Sand Ridge Overtopping at Cannon Beach, Oregon, Central Cascadia Margin, U.S.A.

¹ Portland State University, Portland, Oregon, 97207, U.S.A.; Petersonc{at}pdx.edu
² Portland State University, Portland, Oregon, 97207, U.S.A.
³ University of Wisconsin Eau Claire, Eau Claire, Wisconsin, 54702-4004, U.S.A.
⁴ Cleaveland High School, Portland, Oregon, 97202, U.S.A.

The back-barrier wetlands of Cannon Beach, Oregon, record up to six target paleotsunami events in the last three thousand years. The four youngest recorded inundation events are tied to central Cascadia Margin paleotsunami at 1700 AD (event #1), possible farfield paleotsuanmi at 0.8–0.9 ka (event #2), nearfield Cascadia earthquake subsidence at ~ 1.1 ka (event #3), and nearfield Cascadia earthquake subsidence at ~ 1.3 ka (event #4). Stratigraphically-correlated deposits from events #1–4 demonstrate strong-flow inundation distances from between 500 m (event #1) and 1,500 m (event #4) from the present beachfront. Based on the spatial distribution and thickness of tsunami sand deposits in the back-barrier wetlands the event #1 paleotsunami did not substantially overtop a stable barrier ridge at 6 m elevation NGVD88 or MLLW at Cannon Beach. The three previous paleotsunami events #2–4 did overtop the barrier, and they deposited sand (1–40 cm thickness) in the back-barrier wetlands. Only paleotsunami event #4 deposited sand across the Ecola Creek upland flood plain, at an inundation distance of at least 1,500 m. Shore-parallel GPR profiles image thin cut-and-fill sequences (1–2 m vertical relief) in the overtopped barrier ridge. A shore-normal GPR profile establishes scour in the wetlands (at least 1.5 m thick cut) at the landward edge of the barrier ridge. The most recent scour is tied to event #2, though earlier paleotsunami overtopping flows might have also caused scour in the back-barrier wetlands. Assuming a 1.0 m water column height and a 1.0 m lower sea level at 1 ka, the three overtopping paleotsunami are estimated to have had minimum runup heights of 8 m relative to mean lower low water.