Testing Numerical Tsunami Simulations Against the Inland Extents of Prehistoric Cascadia Tsunami Deposits at Cannon Beach, Oregon

Cannon Beach, Oregon faces extreme flooding hazards from future tsunamis generated by seafloor deformation during Cascadia megathrust earthquakes. Trans-Pacific tsunamis also threaten the City as exemplified by the 1964 Alaska tsunami that caused $230,000 of damage. Field mapping of prehistoric tsunami deposits and historical accounts of the 1964 tsunami are used to delineate the inland extents of past tsunamis for comparison with simulations performed with hydrodynamic models. The comparisons offer an empirical test of the credibility of numerical tsunami simulations used to develop evacuation routes and emergency response tactics for coastal communities in Oregon. Although the mapped extents of tsunami deposits can only define minimum estimates of runup and inundation, their inland limits can be used to identify simulations of tsunami inundation that underestimate real-world conditions, while increasing confidence in simulated inundations that meet or surpass the distribution of deposits left by tsunamis. At least three Cascadia tsunamis flooded the lower 1.4-1.6 km of the Ecola Creek valley within the last 1000 years. Estimates of the minimum inundation come from the mapped extents of three sand layers, all of which satisfy 8-9 out of 10 criteria that favor a tsunami origin. The sand layers exhibit physical attributes of beach sand, the presence of fossil brackish-marine diatoms, sharp or eroded lower contacts, landward-thinning trends, and 14C ages that are consistent with regional Cascadia earthquake and tsunami chronologies. The youngest deposit records flooding by the AD 1700 Cascadia tsunami. Age ranges for two earlier tsunamis span 520-800 and 910-980 cal yr BP. A fourth sand layer deposited by a large flood of Ecola Creek about 1.3 ka was distinguished from the three younger tsunami deposits because it shares attributes of sand from the active creek channel. The deposit lacks a landward-thinning trend and its distribution along the central axis of the valley is consistent with overbank flood deposits. The 1964 Alaska tsunami damaged City infrastructure and private property in Cannon Beach. Estimates of the inundation extent and runup elevation are inferred from eyewitness accounts released days after the March 27 tsunami. Maximum runup probably reached 5.8-6.1 m (MLLW) derived from estimates of tsunami flow depth at two sites. The inferred extent of inundation comes from reports of damage to Highway 101, extensive flooding of the business district and from extrapolation of flow depth estimates from eye-witness accounts.