A long-feared monster earthquake off California, Oregon and Washington could cause some coastal areas to sink by more than 6 feet, dramatically heightening the risk of flooding and radically reshaping the region with little to no warning.

Those are the findings of a new study that examined the repercussions of a massive earthquake on the Cascadia subduction zone, which stretches from Northern California up to Canada’s Vancouver Island.

The study, published Monday in the journal Proceedings of the National Academy of Sciences, concluded that in an earthquake scenario with the highest level of subsidence, or land sink, the area at risk of flooding would expand by 116 square miles, a swath that’s 2½ times the size of San Francisco.

Such a scenario would more than double “the flooding exposure of residents, structures and roads,” and officials would need to contend with a future of “compromised roadways and bridges,” as well as lifelines and infrastructure that are either more frequently flooded or permanently inundated, the study’s authors wrote.

In other words, a powerful earthquake in this area would risk “drastically altering shorelines and causing profound, lasting impacts to coastal populations, infrastructure, and ecosystems,” the study said. Unlike relative sea-level rise that’s driven more gradually by climate change, a rise resulting from a major earthquake “will happen within minutes, leaving no time for adaptation or mitigation.”

The last megaquake on the Cascadia subduction zone, registering a magnitude 9, occurred in 1700. Based on archaeological evidence, villages sank and had to be abandoned, according to the U.S. Geological Survey.

From California’s North Coast to Washington state, scientists say, the next great earthquake — magnitude 8 or higher — could cause land to sink by 1.6 to 6.6 feet, the same range seen during the 1700 earthquake.

Currently, more than 8,000 people live in flood plain areas along estuaries in the Cascadia coastal region. But in the event of a high level of subsidence after an earthquake, that figure would nearly triple to more than 22,000, the study calculates.

Nearly 36,000 structures would be threatened by the resulting quake-influenced flood plain, a 168% increase from the current figure of about 13,000.

And an additional 777 miles of roadway would be in the new flood plain, nearly tripling the at-risk total to 1,212 miles of road.

The flood plain is defined as areas that have at least a 1% chance of flooding each year, which is considered to be “high risk,” according to the Federal Emergency Management Agency. These are areas that have at least a 1-in-4 chance of flooding during a 30-year mortgage. Flood insurance must be purchased by home and business owners in high-risk areas with mortgages from federally regulated or insured lenders.

Radiocarbon dating suggests there have been more than 11 great earthquakes off the shore of California’s North Coast, Oregon and Washington state over the last 6,000 to 7,000 years — recurring every 200 to 800 years, the study said.

“Gradual climate-driven sea-level rise is not the only inundation threat,” the study said. “Coastal subsidence from the next great [Cascadia subduction zone] earthquake may produce” more than 3 feet of sudden relative sea-level rise sooner than otherwise expected.

When discussing a future megaquake on the Cascadia subduction zone, “We often hear about the tsunami and the shaking. But there’s the subsidence that’s going to persist — for decades to centuries —after the earthquake, and just totally alter the flood plains,” Tina Dura, assistant professor of geosciences at Virginia Tech and the study’s lead author, said in an interview.

One estimate published by FEMA is that a magnitude 9 earthquake along the full length of the 800-mile fault zone would leave 5,800 dead from the earthquake alone. An additional 8,000 would die from the resulting tsunami that could rise as high as 80 feet and offer coastal areas as few as 10 minutes of warning. Total economic losses could hit $134 billion.

In the 1700 Cascadia megaquake, oral histories describe tsunamis more than 50 feet high wiping out coastal villages. In Anacla, a village on what is now called Vancouver Island, only 1 out of more than 600 people survived, according to the U.S. Geological Survey.

The tsunami was so strong that it uprooted trees. When it finally receded, treetops were found strewn with trash and victims’ limbs. “Ghost forests” of rot-resistant trees found in tidal marshes and estuaries in the region are evidence that land sank during the quake and drowned the trees, the USGS said.

The results of the latest study should be a wake-up call to residents and government officials considering a postquake response, scientists say. There already are parts of U.S. Route 101 that routinely flood during exceptionally high king tides, Dura said, “and places like that could be kind of a hot spot for seeing flooding right away after the subsidence” following a megaearthquake.

Another factor officials should consider is whether crucial infrastructure, such as airports, would fall within the quake-expanded flood plain, scientists say.

Authorities may also want to consider avoiding building infrastructure such as schools, fire stations and wastewater treatment plants in areas “that we’ve shown are likely to become in the flood plain,” Dura said.

Notable areas at risk of land sinking after a megaquake along the Cascadia subduction zone, according to the study, include:

• California’s Humboldt Bay region, including the area around Eureka and Arcata.
• In Oregon, Waldport and Bayshore along Alsea Bay; Newport and South Beach along Yaquina Bay; and Gearhart and Seaside along the Necanicum River.
• In Washington, the Willapa Bay area, including the communities of Long Beach, Ocean Park, Tokeland and Raymond; and the Grays Harbor area, including Ocean Shores, Westport and Aberdeen.

Traditionally, scientists and government officials have focused on climate-change-driven sea-level rise to calculate the projected increased risk of coastal flooding. But the study suggests that neglecting the role of major earthquakes would be shortsighted.

“Earthquake-driven coastal subsidence following recent historical earthquakes has had severe consequences for communities, leading to permanent land loss, infrastructure damage and forced relocation,” the study said.

One example was the magnitude 9.1 earthquake off Japan’s east coast in 2011, which caused some land to sink by up to 3 feet. In one area of the town of Ishinomaki, the sinking land forced people to contend with regular flooding, according to news reports.

Another magnitude 9.1 earthquake that struck near Sumatra, Indonesia, in 2004 caused land subsidence of up to 3 feet. Areas used for aquaculture have since suffered chronic tidal flooding, leading to oversalinization, and land has been lost, the study said.

The magnitude 9.2 earthquake in Alaska in1964 caused land to sink by more than 6 feet along the coast, “rendering roads, docks and waterfront areas uninhabitable, in some cases necessitating relocation of communities to higher ground or raising waterfront facilities and airstrips above high tide,” the study said. In some places, the subsidence was even more severe.

Before the magnitude 9.2 earthquake of 1964 in Alaska, the sidewalk in front of stores on the right-hand side of this photo from Anchorage were level with those on the left.

And a magnitude 9.5 earthquake in Chile in 1960 caused up to 8 feet of coastal subsidence, “permanently submerging coastal pine forests and farms and converting them to intertidal marshes, and flooding coastal towns and forcing residents to abandon homes,” the study said.

Besides Dura, there are 19 other co-authors to the study, with affiliations including Singhofen Halff Associates of Orlando, Fla.; the University of Oregon; Rowan University; the University of North Carolina Wilmington; Durham University; the USGS; Cal Poly Humboldt; the Oregon Department of Geology and Mineral Industries; the University of Hong Kong; and Nanyang Technological University in Singapore.