The research, conducted by ASM Alauddin Al Azad and Reza Marsooli of Stevens Institute of Technology and published in the journal Ocean Dynamics, tackles a persistent blind spot in coastal oceanography. Satellite radar altimeters measure offshore sea surface height and wave height, but not the nearshore water-level rise caused by breaking waves. Tide gauges, some with records stretching back centuries, are typically housed in sheltered harbors where wave influence is minimal. Field campaigns with buried pressure sensors can capture wave setup directly, but only for days to weeks before currents, sediment transport, and wave energy destroy the instruments or interrupt the data. As a result, most existing estimates of long-term wave setup have relied on empirical formulas that depend on beach slope, deep-water wave energy flux, and limited field measurements — assumptions that can introduce substantial error when applied across diverse coastlines.

To move beyond these constraints, the team turned to dynamical modeling using a fully coupled hydrodynamic-wave system. The hydrodynamic component, ADCIRC, solves the depth-averaged barotropic shallow water equations to simulate tides and storm surges, while the spectral wave model SWAN solves the depth-integrated wave-action balance, incorporating wind input, whitecapping, bottom friction, nonlinear wave-wave interactions, and depth-limited breaking. On a shared unstructured mesh, the two models exchange information at every time step: ADCIRC passes water levels and currents to SWAN, which uses them to account for wave-current interaction and wave transformation through refraction, shoaling, and dissipation. SWAN then computes wave radiation stresses — the momentum flux transferred from breaking waves to the water column — and feeds the gradients of those stresses back into ADCIRC’s momentum equations. This two-way coupling allows wave-induced forcing on coastal water levels to emerge explicitly from the physics rather than from a formula.

The computational demands were considerable. The model domain covers the western North Atlantic between 6°N and 46°N and 98°W to 53°W, discretized into a mesh of more than 1.7 million nodes and 3.4 million triangular elements, with coastal resolution of 500 meters to 1 kilometer in waters shallower than 300 meters. SWAN’s spectral domain contained 36 directional bins and 31 frequencies spanning 0.04 to 0.667 hertz. Both models were forced with hourly surface pressure and 10-meter wind fields from the ERA5 reanalysis, along with open-ocean boundary water levels and direction-frequency wave spectra that account for swells generated far outside the domain. The team’s earlier validation work showed that the ST6 source-term package for wave physics gave the best agreement with National Data Buoy Center observations along both coasts. A single 31-day coupled simulation required roughly 26 hours on two compute nodes of Purdue University’s Anvil system, each carrying 128 AMD EPYC cores.

The core analytical trick was elegant in its simplicity: the researchers ran two parallel sets of decade-long simulations from 2006 to 2015, one with the full coupled system and one with stand-alone ADCIRC that excluded wave effects. At every coastal site and time step, wave setup was computed as the difference in simulated water level between the two runs, isolating the wave contribution under identical tidal and meteorological conditions. Thirty-two representative sites were selected where nearshore bathymetry is gently sloping, ensuring that the surf zone is wide enough to be resolved by at least two mesh nodes and that radiation stress gradients decrease smoothly toward shore. Steep, heterogeneous regions such as the Gulf of Maine — with its bedrock-framed, glaciated shelf — were deliberately excluded, because accurately capturing wave setup there would require ultra-high-resolution models that are computationally prohibitive at regional scale.

The results paint a clear picture of asymmetry between the two coasts. Wave setup along the U.S. East Coast is consistently larger than along the Gulf of Mexico, reflecting the Atlantic’s exposure to open-ocean fetch, frequent intense storms, and long-period swells. Across the Northeast and Mid-Atlantic sites, mean wave setup ranged from 0.8 to 1.47 centimeters, with extremes — defined as the 99th percentile — between 5.0 and 8.2 centimeters. The single largest extreme value, 8.19 centimeters, occurred near Virginia Beach, Virginia, a region exposed to some of the most energetic wave events on the eastern seaboard. The largest mean value, 1.7 centimeters, appeared in South Carolina. By contrast, Gulf Coast sites showed mean setups of just 0.2 to 1.0 centimeter and extremes of 1.5 to 4.7 centimeters. Averaged across all sites, Gulf Coast mean wave setup was only 44 percent of the East Coast average, and extreme wave setup just 48 percent — a gap rooted in the Gulf’s semi-enclosed geography and limited fetch, where large waves are almost entirely the product of hurricanes and winter cold fronts known as nortes.

Seasonality emerged as a dominant signal. Winter months, defined as October through March, produced substantially higher mean and extreme wave setup than summer months at every region analyzed. Along the Northeast Atlantic coast, winter mean wave setup averaged 1.4 centimeters against a summer average of 0.9 centimeters, a difference the authors attribute to the frequent passage of slow-moving extratropical cyclones — nor’easters — that batter the coast with northeast winds for days at a time. Month-by-month analysis showed that Atlantic sites peak in November, when late-season tropical cyclones overlap with the onset of the winter storm season, while Gulf sites peak slightly later, in December, consistent with the dominance of winter frontal systems. July registered the lowest values everywhere, reflecting mid-summer quiescence. Interestingly, along the Southeast Atlantic coast the seasonal gap in extreme values narrows considerably, because powerful swells generated by distant Atlantic hurricanes propagate toward the coast even in summer and elevate water levels far from any local storm.

Year-to-year variability told a similar story of Atlantic dominance. The interannual variability of annual mean wave setup was about 57 percent larger along the East Coast than the Gulf, and that of extreme values about 28 percent larger. Hotspots of variability aligned with physical geography: central Florida sites fronted by narrow continental shelves showed the highest fluctuations, because narrow shelves allow waves to retain energy until breaking close to shore, so small changes in incident wave energy translate into comparable changes in setup. Conversely, the broad, shallow shelves off Georgia and South Carolina dissipate incoming swells and shelter the coast behind a concave shoreline, damping variability to the lowest values recorded. The authors link the Atlantic’s interannual swings to large-scale climate drivers — the El Niño–Southern Oscillation, which modulates both Atlantic hurricane activity and mid-latitude storm tracks, and the Pacific North American pattern, which covaries with winter wave power along the western North Atlantic boundary. Along the Gulf, variability is governed mainly by hurricane landfalls, winter fronts, and coastally trapped Kelvin waves.

The spatial statistics added a further layer of insight. Monthly wave-setup anomalies were strongly correlated between nearby sites on both coasts — mean Pearson correlations of 0.69 and 0.76 within 75 kilometers along the East and Gulf coasts, respectively — but coherence decayed far more slowly along the Atlantic. A fitted spherical variogram yielded a decorrelation range of 642 kilometers for the East Coast against just 292 kilometers for the Gulf, indicating that the Atlantic coastline responds coherently to basin-scale storm systems and swells over vast stretches, while Gulf Coast behavior transitions quickly to locally differentiated patterns shaped by variable shelf width, bathymetry, and coastal orientation.

Trend analysis over the decade revealed a mixed and geographically patchy picture. East Coast sites showed both positive and negative trends, often with adjacent sites displaying opposite signs and no consistent north–south gradient; the largest positive trend, +0.30 millimeters per year, occurred at Virginia Beach, while a site in New York recorded −0.35 millimeters per year. The Southeast Atlantic trended predominantly upward, averaging +0.1 millimeters per year. The Gulf Coast, by contrast, was dominated by negative trends, with the steepest decline of −0.324 millimeters per year in the Florida Panhandle. Nearly all trends were statistically significant at the 95 percent confidence level, though the authors caution that a ten-year window is short, and the detected patterns may partly reflect internal climate oscillations rather than persistent, climate-driven change. The patterns do, however, mirror observed multidecadal trends in significant wave height at nearby buoys.

The practical implications extend beyond academic climatology. Wave setup is a spatially variable addition to coastal water levels that current sea-level assessments largely ignore, and even modest wave-induced increases can push high tides above flooding thresholds, sharply raising the frequency of minor high-tide flooding. Previous research has shown that wave setup contributed up to 17 percent of peak storm tides from historical tropical cyclones along these very coasts, and up to half of the 100-year surge on narrow-shelf segments. By identifying hotspot segments and quantifying the natural variability against which future changes must be judged, this study provides a dynamic baseline that could improve flood forecasting, sharpen sea-level rise projections, and ultimately prevent the systematic underestimation of coastal water levels in one of the world’s most densely developed shoreline regions.

The Cascadia subduction zone is a geological boundary where the massive Pacific tectonic plate slides beneath the lighter North American plate. This interaction generates tremendous strain that accumulates over centuries, eventually releasing in catastrophic megathrust earthquakes. When such an event occurs, vertical land movement is a key factor influencing coastal hazard. This latest study highlights how rapid subsidence—where the land sinks abruptly by as much as 6.5 feet—following an earthquake can dramatically alter coastlines, expanding federally designated floodplains by between 35 to 116 square miles. Such drastic changes have never been fully quantified before at this scale, marking a significant advancement in understanding earthquake-induced coastal hazards.

Tina Dura, assistant professor of geosciences at Virginia Tech and lead author of the study, explains that the consequences of this land subsidence are profound and multifaceted. Using an extensive suite of tens of thousands of complex earthquake simulations, her team estimated the range of possible land sinking scenarios that might follow the next large Cascadia event. These models were integrated with detailed geospatial analyses of 24 estuaries along the Cascadia coast to calculate how far flood risk zones could expand immediately after an earthquake strikes—both under current sea level conditions and projected future scenarios for 2100. In doing so, the research offers a sobering forecast of future risk compounded by climate change.

One of the most alarming outcomes of the study is the expected increase in flood exposure for human populations and infrastructure. Following a hypothetical earthquake today, over 14,000 additional residents would suddenly fall within the expanded floodplain. The surge in risk also covers more than 22,000 buildings and nearly 800 miles of roadways—doubling the current extent of flood exposure. Vital community resources such as five airports and 18 critical facilities, including schools and hospitals, would be jeopardized. The floodplain expansion also threatens key utilities and sites that could generate environmental contamination, underscoring the complex interplay of natural hazard and public health challenges when such disasters intersect.

Dura emphasizes that the severity of flood exposure will escalate dramatically under future sea-level rise scenarios. The Intergovernmental Panel on Climate Change’s localized projections for the Cascadia region anticipate a relative sea-level rise of up to three feet by the end of this century. When coupled with earthquake-induced subsidence, this rise amplifies flood risk, potentially tripling the number of residents, structures, and transportation networks vulnerable to inundation by 2100. This finding suggests that coastal communities might face an unprecedented scale of risk that will complicate disaster response, recovery efforts, and long-term resilience.

Beyond the immediate threat to human settlements and infrastructure, the study draws attention to the catastrophic impacts on natural ecosystems that serve critical protective functions. Coastal estuaries, intertidal wetlands, dunes, and beaches act as natural buffers, absorbing storm surges and dissipating erosional forces. These landscapes are especially vulnerable to worsening tidal inundation and salinization of soils, which could lead to irreversible ecological degradation. Agricultural lands currently protected by dikes and drainage systems may become economically unviable due to saltwater intrusion, leading to significant losses for local economies that depend on farming and cattle grazing in low-lying coastal areas.

The ecological implications extend to the loss of ecosystem services such as water filtration, fish habitat, and carbon sequestration. Intertidal wetlands, in particular, function as important carbon sinks, capturing and storing atmospheric carbon dioxide. The transformation of these wetlands into tidal flats through erosion and inundation undermines their capacity to sequester carbon, potentially exacerbating climate change feedbacks. Moreover, the displacement of these habitats leaves coastal biodiversity at risk, with knock-on effects for fisheries and migratory bird populations. Tina Dura highlights this alarming environmental dimension, emphasizing that ecosystem loss may be irreversible, with inland migration limited by human development and topographical barriers.

The historical perspective provided by coastal geological records paints a vivid picture of the Cascadia subduction zone’s seismic history. Although no great earthquake—with a magnitude exceeding 8.0—has occurred here since 1700, geological evidence reveals at least eleven similar earthquakes over the past six to seven thousand years. These events, recurring every few centuries, triggered land subsidence ranging from 1.5 to 6.5 feet along the coastline, dramatically altering the coastal landscape and posing repeated risks for human and ecological systems. Understanding this recurrence interval is crucial to anticipate future hazards and prioritize mitigation efforts in the region.

Dura’s role as the Paleoseismology Working Group Lead within the Cascadia Region Earthquake Science Center (CRESCENT) at the University of Oregon illustrates the collaborative, multidisciplinary approach needed to study this hazard. CRESCENT integrates geological, seismological, and community-based data to inform earthquake preparedness. Documenting past subsidence events at estuaries through sediment core sampling has been indispensable in reconstructing Cascadia’s seismic behavior and improving predictive models. Dura’s team has played a pivotal role in generating these insights that provide the empirical foundation upon which the current risk modeling is built.

Importantly, this research situates the Cascadia subduction zone’s threat in a broader global context. Subduction zones, where one tectonic plate slips beneath another, are found globally—from Alaska and Russia to Japan, Indonesia, New Zealand, and South America. Each of these zones experiences a cycle of strain accumulation and release, causing megathrust earthquakes accompanied by ground deformation and tsunamis. Similar patterns of uplift and subsidence have been observed in places such as Chile, Alaska, Indonesia, and Japan, where seismic events have led to dramatic environmental and societal upheaval. These parallels illustrate the widespread relevance of Dura’s findings for global subduction zone hazard assessments.

The sequence of events during a great subduction zone earthquake is intense and swift yet triggers enduring changes. Earthquake shaking itself lasts only minutes, during which land subsides and flooding may begin almost immediately, influenced by tidal stage. Tsunami waves follow within 15 to 20 minutes, delivering another wave of inundation and destruction. While the earthquake and tsunami cause immediate damage, the sinking of land persists long afterward—decades or even centuries—altering drainage patterns, damaging infrastructure, and compromising recovery efforts. The prolonged nature of subsidence highlights the importance of integrating long-term geological changes into disaster planning frameworks.

Historic earthquakes further underscore these profound effects. The 1960 Chile earthquake permanently submerged forests and farmlands, converting them into tidal marshes, and led to the abandonment of affected coastal towns. Similarly, the 1964 Alaska earthquake necessitated relocating entire communities and airstrips to higher ground. The 2004 Sumatra-Andaman earthquake inflicted widespread coastal erosion and destroyed aquaculture operations, while the 2011 Tohoku earthquake in Japan caused extensive port damage and was linked to a nuclear disaster. These case studies reinforce the urgency of understanding and preparing for subsidence impacts in subduction zones worldwide.

The implications of this Virginia Tech study are clear: coastal communities along the Cascadia subduction zone face a converging crisis of seismic hazard and climate change-induced sea-level rise. The expanded floodplains, compromised infrastructure, ecological degradation, and social vulnerability demand a new paradigm for risk management that accounts for dynamic geological processes and long-term environmental change. As Tina Dura suggests, the subsidence effects here may eclipse those seen during other recent large earthquakes globally, challenging the resilience of coastal populations and ecosystems in unprecedented ways.

This research serves as a clarion call for policymakers, emergency planners, and scientists to collaborate in developing adaptive strategies that minimize flood risk, protect critical infrastructure, and preserve valuable ecosystems. It also underscores the importance of integrating geological history into contemporary hazard models to capture the full scope of potential impacts. With megathrust earthquakes inevitable on a geological timescale, proactive measures guided by robust scientific understanding will be essential for safeguarding the Pacific Northwest and informing global efforts in other tectonically active regions.

Subject of Research: Earthquake-driven land subsidence and coastal flood risk expansion in the Cascadia subduction zone under current and future sea-level scenarios.

Article Title: Expansion of Coastal Floodplains after a Great Earthquake in Cascadia: Implications of Seismic Subsidence and Sea-Level Rise.

News Publication Date: 28-Apr-2025

Image Credits: Image and photo courtesy of Tina Dura.

Keywords: Earthquakes, Floods, Subduction, Tectonic plates, Subsidence, Earth sciences, Natural disasters, Sea level change, Sea level, Geophysics, Climatology