A silent colossus lurks off the Pacific coast, threatening hundreds of miles of coastline with tsunamis and devastating earthquakes.

For decades, scientists have been warning about the possibility of a major fault line breaking off from the Cascadia Subduction Zone, a megathrust fault that runs offshore along the coast from northern Vancouver Island to Cape Mendocino in California. The next time this fault, or parts of it, breaks, it could upend life in Oregon, Washington, and Northern California.

Of particular concern are signs of great earthquakes in the region’s geological history. Many researchers have been pursuing clues about the last “big quake,” a magnitude 8.7 earthquake that occurred in 1700. They have pieced together this history using centuries-old tsunami records, Native American oral histories, physical evidence of saltwater-flooded ghost forests, and limited maps of faults.

But no one had ever comprehensively mapped the fault structure until now. The study published Friday A paper published in the journal Science Advances describes the data collected during a 41-day research voyage, in which the ship dragged a mile-long cable along the fault, listening to the ocean floor and piecing together images.

The team completed a detailed map of the subduction zone, stretching more than 550 miles to the Oregon-California border.

Their work will give modelers a clearer picture of the impact of a megaquake in the region — a megaquake that occurs in a subduction zone, where one plate pushes under another — and give planners a more detailed, localized view of the risks to Pacific Northwest communities, which could help redefine earthquake-resistant building codes.

“It’s like wearing Coke-bottle glasses, and when you take them off, they give you the correct prescription,” said lead author Suzanne Calbott, a marine geophysicist and research professor at Columbia University’s Lamont-Doherty Earth Observatory. “Before, we only got very blurry, low-resolution images.”

Scientists have discovered that subduction zones are much more complex than previously thought. They are divided into four segments, and researchers believe each segment could rupture independently or simultaneously. Each segment has different rock types and different seismic properties, which means some segments may be more hazardous than others.

Earthquake and tsunami modelers are beginning to assess how the new data might affect earthquake scenarios in the Pacific Northwest.

Kelin Wang, a research scientist with the Geological Survey of Canada who was not involved in the study, said her team, which focuses on earthquake hazards and tsunami risk, is already using the data to make predictions.

“The accuracy and resolution is truly unprecedented, and this is an incredible dataset,” said Wang, who is also an adjunct professor at the University of Victoria in British Columbia. “This will allow us to better assess risk and inform building codes and zoning.”

Harold Tobin, co-author of the paper and director of the Pacific Northwest Seismic Network, said the data will help fine-tune predictions, but it won’t change the untenable reality of life in the Pacific Northwest.

“It could potentially produce earthquakes and tsunamis that are comparable in magnitude to the largest earthquakes and tsunamis the Earth has ever seen,” said Tobin, who is also a professor at the University of Washington. “It looks like Cascadia could produce an earthquake of magnitude 9 or a little less or a little more.”

A quake of that magnitude could cause shaking for about five minutes and generate a tsunami up to 80 feet high, damaging more than 500,000 buildings. According to emergency planning documents:.

Neither Oregon nor Washington are adequately prepared.

To map the subduction zone, researchers at sea used active seismic imaging, a technique that sends sound waves into the ocean floor and processes the returning echoes, a method often used in oil and gas exploration.

They towed more than nine miles of cables called streamers behind the ship and used 1,200 hydrophones to capture the returning sounds.

“This will give us an idea of ​​what the conditions are like underground,” Calbot said.

Trained marine mammal spotters would alert the crew to any signs of whales or other animals. Sounds produced by this type of technology could be disruptive and potentially harmful to marine life.

Calbot said the new research makes it even clearer that the entire Cascadia Fault won’t rupture all at once.

“The next earthquake in Cascadia could rupture just one of these segments, or it could rupture the entire boundary,” Calbot said, adding that some individual segments are thought to have the potential to produce a quake of at least magnitude 8.

Over the past century, scientists have observed only five earthquakes of magnitude 9.0 or higher, all of which were the kind of giant quakes predicted in the Cascadia subduction zone.

Scientists have compiled the latest insights into the 1700 Cascadia earthquake, based on records of an unusual orphan tsunami that was not preceded by any shaking in Japan.

“It would take a magnitude 8.7 earthquake to send a tsunami all the way to Japan,” Tobin said.

Those in Japan who recorded the event had no idea that the earthquake occurred across the ocean in what is now the United States.

Right now, the Cascadia subduction zone is eerily quiet. At other subduction zones, Calbot says, scientists often observe small, frequent earthquakes that make it easier to map the region. But that’s not the case here.

Scientists have a few hypotheses as to why. Wang said the region could be getting quieter as stress builds on the fault, and that time may be approaching.

“The interval between big earthquakes in this subduction zone is about 500 years,” Wang said. “It’s hard to know exactly when it will happen, but it’s certainly quite late compared to other subduction zones.”