The looming threat of a major earthquake in the Pacific Northwest has long been recognized, but new research from Virginia Tech reveals that the seismic hazard is only part of a larger, more complex risk scenario. According to a groundbreaking study published in the Proceedings of the National Academy of Sciences, an impending Cascadia subduction zone earthquake, coupled with rising sea levels, could dramatically expand flood-prone areas, potentially putting thousands of residents, critical infrastructure, and ecosystems along the northern California, Oregon, and Washington coastlines at heightened risk. This dual-threat scenario underscores the urgent need for integrated disaster planning and climate adaptation strategies.
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
