A 100 ft 'mega tsunami' could hit the US at any moment. And that’s only the beginning

The Cascadia Subduction Zone, a massive fault line stretching from northern California to British Columbia, has been eerily quiet for 300 years. When it finally ruptures, it’s expected to trigger a colossal earthquake that could rattle the Pacific Northwest for minutes.

Even worse, tsunami waves up to 100 feet (30m) high will likely crash ashore, unleashing devastation along the coast.

But according to a new study published in the Proceedings of the National Academy of Sciences, that will only mark the beginning of Cascadia’s cataclysm.

As if a wave the same size as the 2004 Boxing Day mega tsunami wasn’t bad enough, the new analysis reveals that land along the coast could drop by more than 2.5m (8ft) in a matter of minutes.

“We talk about climate-driven sea level rise, which is occurring at three to four millimetres a year, and that does eventually add up,” Prof Tina Dura, lead author of the study, told BBC Science Focus. “But here we’ll have two metres of sea level rise in minutes. Why aren’t we talking about that more?”

What happens when Cascadia ruptures?

The Cascadia Subduction Zone marks the boundary where the oceanic Juan de Fuca Plate is slipping beneath the North American Plate. But rather than moving smoothly, these plates tend to get stuck – and as they lock together, strain accumulates over centuries.

When it finally releases, the result is a powerful earthquake.

Cascadia is capable of producing quakes of magnitude 9.0 or more, with major events thought to strike every 450 to 500 years. The last one occurred on 26 January 1700, and based on geological records and contemporary reports, its magnitude was likely between 8.7 and 9.2.

According to the US National Seismic Hazard Model, there’s now a 15 per cent chance of a magnitude 8.0 or greater earthquake along the zone’s margin in the next 50 years.

A 2022 planning exercise by the Federal Emergency Management Agency predicted such a quake could directly cause 5,800 deaths, with a further 8,000 resulting from the tsunami.

More than 100,000 people could be injured, and over 618,000 buildings damaged or destroyed – including more than 2,000 schools and 100 critical facilities. The projected economic cost: $134bn.

"This is going to be a very catastrophic event for the US, for sure,” said Dura.

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Aftershocks that last for decades

While the earthquake and tsunami would be devastating in and of themselves, Dura and her colleagues warn the long-term aftermath could be just as damaging – and it is far less understood.

By combining geological records of past Cascadia quakes with numerical models, the researchers assessed how the coastal landscape could be reshaped by subsidence: the sudden sinking of land during an earthquake.

If a Cascadia quake happened today, land at 24 estuaries from southern Washington to northern California could permanently drop by 0.23 to 2.67m (0.76 to 8.76 ft).

This would dramatically expand 100-year floodplains – areas expected to flood at least once every 100 years – by as much as 300km² (115 sq miles). That means more homes, roads, and infrastructure pushed into flood zones. Under high-subsidence scenarios, the team estimated that flood exposure could more than double.

“It’s a dual threat,” said Dura. “The tsunami is going to come in, and it’s going to be devastating. But with tsunamis, we also know that some places are going to be very hard hit, while other places are probably not going to feel as severe effects because of the way the coast is configured. But those areas can still be in the zone of subsidence.”

A map of the Cascadia subduction zone. — The Cascadia Subduction Zone, located in the US Pacific Northwest and southwestern British Columbia, last slipped in 1700, unleashing a massive earthquake. - John Wesley Powell Center for Analysis and Synthesis/USGS

In 1700, there was little infrastructure to damage. Today, the region is packed with critical systems that would be left vulnerable to flooding. These include: 5 airports, 18 emergency facilities (e.g. schools, hospitals, fire stations), 8 wastewater treatment plants, 1 electric substation, and 57 potential contaminant sources such as gas stations and chemical storage facilities.

Recovery would be severely hindered by submerged roads and flooded emergency hubs, while saltwater inundation could salinise farmland, causing lasting economic damage.

At the same time, natural flood defences like intertidal wetlands may drown or erode, weakening protection against future storm surges. Rising tidal ranges could worsen high-tide flooding, and already eroding sandy coastlines would likely retreat even faster.

“After the tsunami comes and eventually recedes, the land is going to persist at lower levels,” Dura said. “That floodplain footprint is going to be altered for decades or even centuries.”

The sooner it hits, the better

While an earthquake hitting at any moment would be bad, the longer Cascadia stays quiet, the worse the eventual disaster may be.

The researchers modelled what might happen if the earthquake struck not today, but in 2100. Thanks to climate change, sea level rise could make everything even more destructive.

Global mean sea level has already risen by 21 to 24cm (8 to 9 inches) since 1880, and National Oceanic and Atmospheric Administration projections suggest it will rise by another 60cm (2ft) or more by 2100.

In Cascadia, however, the story is slightly more complex: as the Juan De Fuca slips under the North American plate, the continental land has been slowly pushed upwards, masking the effects of global sea level rise.

But that slow uplift won’t last. Eventually, climate change will catch up, and sea levels in the Pacific Northwest will begin to rise just like everywhere else. Slowly but surely, two disasters – tectonic and climate – are converging.

“You can imagine, during the next earthquake, when the land drops down, you’re going to suddenly have to contend with multiple centuries of equivalent sea level rise in minutes,” Dura said.

By 2100, even without an earthquake, the researchers estimated that floodplains could expand by 100km² (40 sq miles). Combine that with subsidence from an earthquake that hits in 2100, and flood exposure could triple compared to today, expanding floodplains by up to 370km² (145 sq miles).

What can be done?

Faced with this compounded threat – earthquake, tsunami, subsidence and sea level rise – what can communities do to prepare?

Dura acknowledged there are no easy answers.

“Cascadia poses a unique challenge because it’s so seismically quiet,” she said. “It’s hard to keep residents aware of the hazard without undue panic. How do you get people prepared without scaring them away?”

As a starting point, Dura’s team recommends integrating their findings into flood hazard and tsunami maps and working across agencies to improve preparedness.

Key actions include identifying critical infrastructure in future flood zones and planning for relocation or adaptation, increasing public awareness, promoting nature-based solutions such as wetland restoration, and considering new development in high-risk areas.

Is all hope lost? Not at all, Dura said – but the clock is ticking.

“There are definitely people working on it, and I don’t want to diminish the hard work of those individuals,” she said. “I think we just need more people on the problem. Maybe there’s not enough resources or funding or personnel… But we’re all trying to work together. And there’s a lot of new science coming out now.”