Maximum-recorded overland run-ups of major nearfield paleotsunamis during the past 3000 years along the Cascadia margin, USA, and Canada

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Maximum-recorded run-up estimates of six major nearfield paleotsunamis, dating from 0.3 to 2.8 ka, are compiled from reported studies at 12 reliable localities distributed over a north–south distance of 1000 km in the Cascadia subduction zone. The run-up estimates are based on surveyed elevations and positions of terminal sand sheet layers that were deposited by the dated paleotsunamis. Maximum terminal deposit elevations from open-coastal sites range from 3 to 12 m NAVD88. Paired proximal and distal run-up sites at four localities demonstrate landward vertical attenuation gradients (−2.5 to −4.2 m km−1) of decreasing terminal sand deposit elevation with increasing distance inland. An averaged attenuation gradient is reversed (3.0 m km−1) to project paleotsunami run-up elevations to adjacent ocean shorelines. The run-up projections are further adjusted by paleotsunami age and relative sea level curves to estimate shoreline inundation elevations under modern sea level conditions. The tsunami shoreline inundation elevations range from 3 ± 2 to 15 ± 2 m NAVD88, with the largest values occurring along the central Cascadia margin and the smallest values occurring in the eastern Juan de Fuca Strait. Contradictory to some numerical tsunami modeling assumptions, there is no apparent correlation between duration of interseismic strain accumulation or estimated upper-plate elastic flexure and corresponding paleotsunami run-up heights on the central Cascadia margin. The short duration since the last Cascadia megathrust rupture (0.3 ka) cannot be used to imply smaller run-up values for a near-future Cascadia tsunami. Coastal communities should plan for the maximum paleotsunami run-ups as recorded at the nearest reliable run-up localities.