Coastal flooding and beach retreat from coseismic subsidence in the central Cascadia margin, USA

Prehistoric great earthquakes (Mw 8-9) in the central Cascadia margin have produced coastal subsidence (0-2 m) that has resulted in chronic lowland fl ooding and catastrophic beach erosion. Geologic records of wetland burial i n 13 bays from Washington and Oregon are used with simple calculations to e stimate likely coastal flooding and beach retreat following a great Cascadi a earthquake, Plant macrofossils and peat-to-mud ratios recorded in core logs discriminat e between forest, marsh, colonizing marsh, and mud-flat tidal settings. Tra nsitions between these settings, i.e., across abrupt burial contacts, demon strate either 0+/-0.5, 1+/-0.5, or 2+/-0.5 m of paleosubsidence. Paleosubsi dence from the last Cascadia event (AD 1700) decreases from 2+/-0.5 m in th e eastern reaches of southwest Washington bays to 0+/-0.5 m in the western reaches of central Oregon bays. First-order estimates of post-subsidence fl ooding hazards are based on the predicted regional subsidence added to curr ent 10- and 100-year flood elevations, At least 525 km of bay shorelines ar e threatened by chronic flooding following coseismic subsidence. Catastroph ic beach retreat is estimated from Bruun's Rule to range from 50 to 300 m d epending on assumed depth of closure, measured beach-berm heights, and pred icted coseismic subsidence along the margin, Shorelines that are susceptibl e to catastrophic beach retreat from coseismic subsidence total at least 25 0 km in longshore distance.