Major Quake Could Triple Flood Threat Across Pacific Northwest and California, Scientists Say

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The Pacific Northwest is being taken for a ride.

At the Cascadia Subduction Zone—a 700-mile fault that runs all the way from California to British Columbia—two giant tectonic plates are locked in a high-stakes standoff. The North American plate is slowly crushing the Juan de Fuca plate beneath it, according to experts, lifting the region millimeter by millimeter over centuries. But that pressure won’t build forever.

When the plates finally slip, they’ll unleash what scientists call a megathrust earthquake, or “the great quake” in layman’s terms. This magnitude 8 or 9 earthquake will be capable of violently shaking the region for minutes and trigger a massive tsunami.

But there’s another, less visible threat that could have longer-lasting consequences: subsidence, or the sinking of land. In a matter of minutes, entire coastal communities could drop by up to 6 feet, permanently redrawing the floodplain and placing thousands of homes and lives at risk.

New research shows subsidence following the great quake could double, or even triple, the number of people and properties exposed to catastrophic flooding—an impact that most flood maps and insurance models still fail to capture.

What is subsidence (and why it matters more than you think)?

Make your hands into fists and press them hard against each other. Now, tilt one fist upward, like it’s riding over the other. When the pressure becomes too much, one fist suddenly slips past the other and, in an instant, drops.

Now, imagine your fists are the Earth’s crust. That drop is called subsidence.

“During one of these over magnitude 8, probably up to magnitude 9 earthquakes, the land actually deforms and drops down up to 6 feet or 2 meters,” explains Tina Dura, a coastal geologist and lead researcher on a new study examining flood exposure in the Pacific Northwest recently published in Proceedings of the National Academy of Sciences.

“Basically causing sea level rise of 6 feet within minutes.”

This instant ground-level drop doesn’t just mean higher tides. It redefines the baseline for future flooding. Areas that were once safely above water could suddenly become permanent flood zones, or even become permanently under water—changing the region’s coastline, increasing insurance costs, and putting thousands of homes at risk.

What happens to coastal communities when the ground drops

The last time a megathrust earthquake rocked the Pacific Northwest was centuries ago, long before cities, suburbs, and infrastructure blanketed the region’s coast. But scientists like Dura are looking to the geologic record to understand what the quake left behind, and what could happen when the next one strikes.

“Cascadia has repeatedly dropped down up to 6 feet in these earthquakes,” she explains. “We find these marsh soils that are overlain by really clean gray tidal mud, showing that these wetlands dropped down and were converted to tidal flats repeatedly.”

In other words, when the ground sank, the ocean rushed in, flooding areas that were once comfortably above water.

“On the coastal records, we can look back about 7,000 years or so, and [we’re] seeing these catastrophic changes in these estuaries,” she adds. “I’m constantly thinking about the land level change that’s going to occur.”

That land level change is what makes subsidence so dangerous. What was once safe, dry ground could become a flood-prone tidal flat in a matter of minutes. And while the region once held quiet wetlands, it’s now home to entire communities.

The risks go far beyond flooded basements. Entire systems critical to public safety and daily life could be thrown into crisis. Wastewater treatment plants, electric substations, and roads built at elevations sensitive to sea level rise could be compromised.

When the coastline drops, so does everything built on it, putting lives, homes, and infrastructure directly in harm’s way.

A doubling, and eventually tripling, of exposure

Until recently, most models focused on immediate impacts: shaking, waves, and storm surges. But Dura’s new research reveals what happens when you factor in subsidence—and the results are startling.

If a megathrust earthquake were to happen today, the 100-year floodplain—the key metric used to determine which homes are at risk of flood, and what homes need to purchase flood insurance policies—expands by 115 square miles.

“That doubles the exposure of resident structures and roads,” Dura explains.

In other words, half of the residents who would need flood insurance to cover damage caused by flooding following a megathrust earthquake are unlikely to know that they are at risk right now. And that’s just the start.

By 2100, due to the combined effects of sea-level rise from global warming, the floodplain expands by 145 square miles, tripling the exposure.

That one-two punch is a combo that few current models account for.

“The studies that have been conducted so far … haven’t really taken into account the long-term impacts of the subsidence,” Dura notes.

The result: a massive blind spot in our understanding of flood risk in the Pacific Northwest. And without that knowledge, thousands of homeowners might be dramatically underestimating their need for flood insurance.

What homeowners can do now to understand and reduce risk

Just one inch of floodwater can cause up to $25,000 in damage, according to FEMA. Yet, as Janet Ruiz of the Insurance Information Institute warns, flood damage and “damage from a tsunami is not covered under most standard homeowners, renters, or business insurance.”

For homeowners in the Pacific Northwest, Dura’s study offers a critical opportunity to get ahead of that risk.

“Residents can use our maps to locate themselves … to see whether their house might lie within one of the projected post-earthquake floodplains,” she says.

Even homes outside FEMA-designated flood zones could be pulled into future floodplains in an instant, making flood insurance a smart move now, before disaster strikes.

“The major insurer of flood is the National Flood Insurance Program … [which] provides coverage up to $250,000 for a structure and $100,000 for personal possessions,” Ruiz notes.

That might not be enough. Flooding following a tsunami triggered by the megathrust earthquake could destroy entire homes, not just damage the first floor, so owners of higher-value homes might need additional protection.

And the risk isn’t only long-term. Areas newly vulnerable to subsidence might also start experiencing nuisance flooding, king tides, and coastal erosion well before the next major quake hits.

“Twenty-five percent of flooding happens outside the designated flood zones,” Ruiz adds, which makes proactive planning even more essential.

Homeowners should start by taking the following steps:

• Locating their property on Dura’s projected floodplain maps (PDF or interactive map, if available).
• Comparing National Flood Insurance Program policies with options from private insurers.
• Reassessing their property’s flood risk based on elevation, proximity to water, and changing coastal conditions.

How likely is this ‘next great quake’?

Scientists refer to recurrence intervals to estimate how often major earthquakes occur on a given fault. For the Cascadia Subduction Zone, the geological record shows a pattern of megathrust quakes occurring roughly every 250 to 800 years.

“We’re over 300 years out from the last earthquake,” says Dura. “Geologically speaking, we’re well within the window of possibility.”

In human terms, that can seem abstract, but the odds are very real. Unlike storms or seasonal weather events, earthquakes strike without warning, and the longer the buildup, the greater the risk.

“Fifty years in geologic time is well within our plus or minus uncertainty,” Dura adds. “It really could happen any day.”