The Cascadia subduction zone, a vast fault line extending from Northern California to British Columbia, has remained unsettlingly silent for 300 years. Should it finally rupture, a substantial earthquake is anticipated to shake the Pacific Northwest for several minutes.

Even more alarming, the resulting tsunami could generate waves reaching up to 100 feet (30 meters), wreaking havoc along the coast.

New research published in Proceedings of the National Academy of Sciences indicates that this event would only mark the onset of Cascadia’s cataclysm.

As if waves equivalent in size to those of the 2004 Boxing Day tsunami weren’t frightening enough, the latest analysis reveals land along the coast could subside by more than 2.5 meters (8 feet) almost instantaneously.

“We often discuss climate-induced sea level rises at a rate of 3-4 millimeters annually,” noted Professor Tina Dura, the lead author of the study, in an interview with BBC Science Focus. “But here, we could witness a 2-meter sea level rise in just a few minutes. Why aren’t we talking about that more?”

What Happens When Cascadia Erupts?

The Cascadia subduction zone represents the boundary where the oceanic Juan de Fuca plate descends beneath the North American plate. However, these tectonic plates tend to get stuck rather than moving freely. When they finally lock in place, immense pressure builds over centuries.

Once this pressure is released, it results in a formidable earthquake.

Cascadia is capable of producing earthquakes reaching magnitudes of 9.0 or higher, historically experienced once every 450-500 years. The last event occurred on January 26, 1700, with estimates suggesting its magnitude was likely between 8.7 and 9.2.

According to the US National Earthquake Hazard Model, there is a 15% chance of experiencing earthquakes of magnitude 8.0 or greater in this zone over the next 50 years.

Planning exercises conducted by the Federal Emergency Management Agency in 2022 predicted that these tremors could directly result in 5,800 fatalities, with an additional 8,000 lives lost to the tsunami.

Over 100,000 individuals could be injured, and more than 618,000 buildings could be damaged or destroyed, impacting over 2,000 schools and 100 critical facilities. The estimated economic impact: $134 billion.

“This will definitely be a catastrophic event for the United States,” Dura stated.

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Aftershocks That Will Last for Decades

While the earthquake and tsunami will be devastating on their own, Dura and her colleagues warn that the long-term consequences could be equally destructive.

By integrating historical records of past Cascadia earthquakes with geological data from numerical models, the researchers assessed how coastal landscapes may be altered due to subsidence from the sudden sinking of land during an earthquake.

If a Cascadia earthquake were to occur today, land at 24 estuaries stretching from southern Washington to northern California could drop anywhere from 0.23 to 2.67 meters (0.76 to 8.76 feet) in minutes.

This could significantly expand the 100-year flood plain by 300 square kilometers (115 square miles), meaning more homes, roads, and infrastructure would fall into at-risk areas. In a severe subsidence scenario, flood exposure could more than double.

“It’s a dual threat,” Dura explained. “The tsunami will be catastrophic, but we also know that some locations will face extreme impacts, while others will experience serious repercussions due to the geographic layout of the coast.”

In 1700, the infrastructure suffered minimal damage. Today, the region is filled with critical systems that remain vulnerable to inundation. These include five airports, 18 emergency facilities (such as schools, hospitals, fire stations), eight wastewater treatment plants, one current, and 57 potential contamination sources like gas stations and chemical storages.

Submerged roads and flooded emergency hubs could severely impede recovery, while saltwater intrusion could damage farmland, resulting in long-term economic repercussions.

Furthermore, natural flood defenses like intertidal wetlands could be inundated or eroded, weakening protection against future storm surges. Rising tidal ranges could compound high-water flooding and may exacerbate the erosion of sandy coastlines.

“Once the tsunami hits and eventually recedes, the land will remain at a lower level,” Dura noted. “The flood plain footprint will evolve over decades, and even centuries.”

The Sooner It Hits, The Better

While the occurrence of an earthquake is inherently negative, the longer Cascadia remains quiet, the worse the ultimate disaster may be.

Researchers simulated what would occur if an earthquake were to strike in 2100 instead of today. With climate change, rising sea levels could exacerbate the consequences.

The global average sea level has already seen an increase of 21-24 cm (8-9 inches) since 1880, as reported by the National Oceanic and Atmospheric Administration. Projections suggest an increase of at least 20 cm (2 feet) by 2100.

However, the situation in Cascadia is somewhat complex. As the Juan de Fuca plate slides beneath the North American plates, the land on the continent is gradually uplifted, temporarily masking the implications of global sea level rise.

This slow upward thrust won’t last indefinitely. Eventually, climate change will catch up, leading to a rise in sea levels in the Pacific Northwest on par with other areas. Consequently, the two challenges—tectonic and climatic—are converging.

“You can imagine that when the land subsides during the next earthquake, you suddenly have to contend with centuries of comparable sea level rise,” Dura remarked.

By 2100, even without earthquakes, researchers estimate that flood plains could expand by 100 square kilometers (40 square miles). This, combined with subsidence from a potential earthquake, could triple flood exposure compared to today, resulting in a flood plain enlargement of up to 370 square kilometers (145 square miles).

How Can You Prepare?

What steps can communities take to prepare for this complex threat that combines earthquakes, tsunamis, subsidence, and sea level rise?

Dura admitted that there is no straightforward solution.

“Cascadia presents a unique challenge due to its prolonged quietude,” she said. “It’s tough to keep residents informed about the risks without inciting undue panic. How do you prepare without instilling fear?”

As an initial measure, Dura’s team suggests incorporating their findings into flood hazard and tsunami maps and collaborating across agencies to enhance preparedness.

Key actions include identifying critical infrastructure in future flood zones, planning for relocation or adaptation, promoting nature-based solutions by raising public awareness, restoring wetlands, and carefully considering new developments in high-risk areas.

Is all hope lost? Dura insists that time is of the essence.

“There are certainly individuals actively working on this. I don’t want to undermine their efforts,” she said. “But we need more people engaged in the issue. Perhaps we lack adequate resources, funding, or personnel, but collective efforts are being made, and significant new research is emerging.”

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About Our Experts

Tina Dura is an assistant professor of natural disasters at Virginia Tech in the United States. She specializes in subduction zone paleogeography, integrating methodologies from coastal stratigraphy, sedimentology, microbiology, paleoconservation, geophysics, and sediment transport modeling, as well as sea level research to reconstruct the extensive history of subsidence along the coastal region.