Dura, TinaChilton, WilliamSmall, DavidGarner, Andra J.Hawkes, AndreaMelgar, DiegoEngelhart, Simon E.Staisch, Lydia M.Witter, Robert C.Nelson, Alan R.Kelsey, Harvey M.Allan, Jonathan C.Bruce, DavidDePaolis, JessicaPriddy, MichaelBriggs, Richard W.Weiss, RobertLa Selle, SeanPaulWillis, MichaelHorton, Benjamin P.2025-05-012025-05-012025-050027-8424https://hdl.handle.net/10919/128268Climate-driven sea-level rise is increasing the frequency of coastal flooding worldwide, exacerbated locally by factors like land subsidence from groundwater and resource extraction. However, a process rarely considered in future sea-level rise scenarios is sudden (over minutes) land subsidence associated with great (>M8) earthquakes, which can exceed 1 m. Along the Washington, Oregon, and northern California coasts, the next great Cascadia subduction zone earthquake could cause up to 2 m of sudden coastal subsidence, dramatically raising sea level, expanding floodplains, and increasing the flood risk to local communities. Here, we quantify the potential expansion of the 1% floodplain (i.e., the area with an annual flood risk of 1%) under low (~0.5 m), medium (~1 m), and high (~2 m) earthquake-driven subsidence scenarios at 24 Cascadia estuaries. If a great earthquake occurred today, floodplains could expand by 90 km² (low), 160 km² (medium), or 300 km² (high subsidence), more than doubling the flooding exposure of residents, structures, and roads under the high subsidence scenario. By 2100, when climate-driven sea-level rise will compound the hazard, a great earthquake could expand floodplains by 170 km² (low), 240 km² (medium), or 370 km² (high subsidence), more than tripling the flooding exposure of residents, structures, and roads under the high subsidence scenario compared to the 2023 floodplain. Our findings can support decision-makers and coastal communities along the Cascadia subduction zone as they prepare for compound hazards from the earthquake cycle and climate-driven sea-level rise and provide critical insights for tectonically active coastlines globally.application/pdfenCreative Commons Attribution-NonCommercial-NoDerivatives 4.0 Internationalcoastal subsidencecompound hazardsearthquake hazardssea-level risesubduction zone hazardsArticle - Refereedhttps://doi.org/10.1073/pnas.242465912212218Weiss, Robert [0000-0002-7168-5401]Dura, Cristina [0000-0003-0830-7894]402942621091-6490