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Climate-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. 

Beginning ~3,500 to 3,300 y B.P., humans voyaged into Remote Oceania. Radiocarbon-dated archaeological evidence coupled with cultural, linguistic, and genetic traits indicates two primary migration routes: a Southern Hemisphere and a Northern Hemisphere route. These routes are separated by low-lying, equatorial atolls that were settled during secondary migrations ~1,000 y later after their exposure by relative sea-level fall from a mid-Holocene highstand. High volcanic islands in the Federated States of Micronesia (Pohnpei and Kosrae) also lie between the migration routes and settlement is thought to have occurred during the secondary migrations despite having been above sea level during the initial settlement of Remote Oceania. We reconstruct relative sea level on Pohnpei and Kosrae using radiocarbon-dated mangrove sediment and show that, rather than falling, there was a ~4.3-m rise over the past ~5,700 y. This rise, likely driven by subsidence, implies that evidence for early settlement could lie undiscovered below present sea level. The potential for earlier settlement invites reinterpretation of migration pathways into Remote Oceania and monument building. The UNESCO World Heritage sites of Nan Madol (Pohnpei) and Leluh (Kosrae) were constructed when relative sea level was ~0.94 m (~770 to 750 y B.P.) and ~0.77 m (~640 to 560 y B.P.) lower than present, respectively. Therefore, it is unlikely that they were originally constructed as islets separated by canals filled with ocean water, which is their prevailing interpretation. Due to subsidence, we propose that these islands and monuments are more vulnerable to future relative sea-level rise than previously identified. 

Overwash deposits from tropical cyclone-induced storm surges are commonly used as modern analogues for paleo-storm studies. However, the evolution of these deposits between their time of deposition and their incorporation into the geologic record is poorly understood. To understand how the characteristics of an overwash deposit can change over time, we analyzed overwash deposits from four mangrove islands in southern Florida two to three months and twenty-two months after Hurricane Irma's landfall in the region on 10 September 2017. We analyzed the stratigraphy, mean grain size, organic and carbonate contents, stable carbon isotopic signatures, and microfossil (foraminifera and diatom) assemblages of pre-Irma and Irma overwash sediments. Hurricane Irma's storm surge deposited light gray carbonate muds and sands up to 11 cm thick over red organic-rich mangrove peats throughout mangrove islands in southern Florida. Stratigraphy, grain size, loss-on-ignition, and foraminifera analyses provided the strongest evidence for differentiating Irma's overwash deposit from underlying mangrove peats and, if preserved, are expected to identify Hurricane Irma's overwash event within the geologic record. Mean grain size showed the overwash deposit (5.0 ± 0.8 ɸ) was coarser than underlying mangrove peats (6.7 ± 0.7 ɸ), and loss-on-ignition showed the overwash deposit had a lower organic content (19.8 ± 9.1%) and a higher carbonate content (67.8 ± 20.7%) than the underlying peats (59.4 ± 14.6% and 33.7 ± 11.0%, respectively). The overwash deposit was dominated by a diverse, abundant assemblage of sub-tidal benthic calcareous foraminifera compared to a uniform, sparse assemblage of agglutinated foraminifera in the pre-Irma mangrove peats. Geochemical indicators were not able to provide evidence of an overwash event by differentiating organic δ13C or C/N of the overwash deposit from those of the mangrove peats. The complex relationship between diatoms and local environmental factors prevented diatom assemblages from providing a statistically clear distinction between Irma's overwash sediments and underlying mangrove peats. By visiting Hurricane Irma's overwash deposit immediately following landfall and nearly two years post-storm, we were able to document how the overwash deposit's characteristics changed over time. Continued monitoring on the scale of five to ten years would provide further insights into the preservation of overwash deposits for paleo-storm studies. 

Abstract Stratigraphic, lithologic, foraminiferal, and radiocarbon analyses indicate that at least four abrupt mud-over-peat contacts are recorded across three sites (Jacoby Creek, McDaniel Creek, and Mad River Slough) in northern Humboldt Bay, California, USA (∼44.8°N, −124.2°W). The stratigraphy records subsidence during past megathrust earthquakes at the southern Cascadia subduction zone ∼40 km north of the Mendocino Triple Junction. Maximum and minimum radiocarbon ages on plant macrofossils from above and below laterally extensive (>6 km) contacts suggest regional synchroneity of subsidence. The shallowest contact has radiocarbon ages that are consistent with the most recent great earthquake at Cascadia, which occurred at 250 cal yr B.P. (1700 CE). Using Bchron and OxCal software, we model ages for the three older contacts of ca. 875 cal yr B.P., ca. 1120 cal yr B.P., and ca. 1620 cal yr B.P. For each of the four earthquakes, we analyze foraminifera across representative mud-over-peat contacts selected from McDaniel Creek. Changes in fossil foraminiferal assemblages across all four contacts reveal sudden relative sea-level (RSL) rise (land subsidence) with submergence lasting from decades to centuries. To estimate subsidence during each earthquake, we reconstructed RSL rise across the contacts using the fossil foraminiferal assemblages in a Bayesian transfer function. The coseismic subsidence estimates are 0.85 ± 0.46 m for the 1700 CE earthquake, 0.42 ± 0.37 m for the ca. 875 cal yr B.P. earthquake, 0.79 ± 0.47 m for the ca. 1120 cal yr B.P. earthquake, and ≥0.93 m for the ca. 1620 cal yr B.P. earthquake. The subsidence estimate for the ca. 1620 cal yr B.P. earthquake is a minimum because the pre-subsidence paleoenvironment likely was above the upper limit of foraminiferal habitation. The subsidence estimate for the ca. 875 cal yr B.P. earthquake is less than (<50%) the subsidence estimates for other contacts and suggests that subsidence magnitude varied over the past four earthquake cycles in southern Cascadia.