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

Portland State University, Geology Department, Portland, OR, United States

Prehistoric great earthquakes (Mw 8-9) in the central Cascadia margin have produced coastal subsidence (0-2 m) that has resulted in chronic lowland flooding and catastrophic beach erosion. Geologic records of wetland burial in 13 bays from Washington and Oregon are used with simple calculations to estimate likely coastal flooding and beach retreat following a great Cascadia earthquake. Plant macrofossils and peat-to-mud ratios recorded in core logs discriminate between forest, marsh, colonizing marsh, and mud-flat tidal settings. Transitions between these settings, i.e., across abrupt burial contacts, demonstrate either 0+ or -0.5, 1+ or -0.5, or 2+ or -0.5 m of paleosubsidence. Paleosubsidence from the last Cascadia event (AD 1700) decreases from 2+ or -0.5 m in the eastern reaches of south-west Washington bays to 0+ or -0.5 m in the western reaches of central Oregon bays. First-order estimates of post-subsidence flooding hazards are based on the predicted regional subsidence added to current 10- and 100-year flood elevations. At least 525 km of bay shorelines are threatened by chronic flooding following coseismic subsidence. Catastrophic beach retreat is estimated from Bruun's Rule to range from 50 to 300 m depending on assumed depth of closure, measured beach-berm heights, and predicted coseismic subsidence along the margin. Shorelines that are susceptible to catastrophic beach retreat from coseismic subsidence total at least 250 km in longshore distance.

This record provided courtesy of AGI/GeoRef.

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