Evaluation of the use of paleotsunami deposits to reconstruct inundation distance and runup heights associated with prehistoric inundation events, Crescent City, southern Cascadia margin

Historic-and prehistoric-tsunami sand deposits are used to independently establish runup records for tsunami hazard mitigation and modeled runup verification in Crescent City, California, located in the southern Cascadia Subduction Zone. Inundation from historic (1964) farfield tsunami (similar to 5-6 m runup height) left sand sheet deposits (100-200 m width) in wetlands located behind a low beach ridge [3-4 m elevation of the National Geodetic Vertical Datum of 1988 (NAVD88)]. The most landward flooding lines (4.5-5 m elevation) in high-gradient alluvial wetlands exceed the 1964 sand sheet records of inundation by 1-2 m in elevation. The most landward flooding in low-gradient alluvial wetlands exceed the corresponding sand sheet record of inundation distance by 1000 m. Nevertheless, the sand sheet record is an important proxy for high-velocity inundation. Sand sheet deposition from the 1964 historic tsunami closely corresponds to the landward extent of large debris transport and structural damage in the Crescent City waterfront. The sand sheet deposits provide a proxy for maximum hazard or 'kill zone' in the study area. Six paleotsunami sand sheets (0.3-3 ka) are recorded in the back-ridge marshes in Crescent City, yielding a similar to 450 year mean recurrence interval for nearfield Cascadia tsunami. Two paleotsunami sand deposit records, likely correlated to Cascadia ruptures between 1.0 and 1.5 ka, are traced to 1.2 km distance and 9-10 m elevation, as adjusted for paleo-sea level. The paleotsunami sand deposits demonstrate at least twice the runup height, and four times the inundation distance of the farfield 1964 tsunami sand sheet in the same marsh system. The preserved paleotsunami deposits in Crescent City are compared to the most landward flooding, as modeled by other investigators from a predicted Cascadia (similar to Mw 9) rupture. The short geologic record (similar to 1.5 ka) yields slightly lower runup records than those predicted for the modeled Mw 9 rupture scenario in the same marsh, but it generally verifies predicted maximum tsunami runup for use in the planning of emergency response and rapid evacuation. Copyright (C) 2011 John Wiley & Sons, Ltd.