Major fault line off West Coast could trigger catastrophic magnitude 9 earthquake, study finds

An underwater fault line along the U.S. West Coast could trigger a megaquake more destructive than California’s “Big One,” a new study suggests.

Using underwater mapping techniques, scientists have mapped the Cascadia Subduction Zone – a 600-mile-long fault line stretching from southern Canada to northern California – in unprecedented detail.

The fault appeared to divide into four segments, rather than being one continuous band like most fault lines. The discovery could prove more catastrophic, as tectonic plates can slide under each other, creating more pressure and stronger earthquakes.

Researchers have concluded that the Cascadia Subduction Zone has the potential to trigger earthquakes of magnitude nine and above.

California’s San Andreas, by comparison, is prepared for earthquakes of up to magnitude 8.3.

If a magnitude 9 earthquake were to strike the US West Coast, it could generate a tsunami of 100 feet or more, kill more than 10,000 people and cause over $80 billion in damage in Oregon and Washington alone.

Emergency plans in Oregon and Washington warn that in the wake of such a large earthquake, they could face a wave of long-term deaths from diseases caused by exposure to dead bodies, animal carcasses, contaminated water and releases of hazardous substances from commercial, industrial and domestic sources.

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A similar fault zone off the coast of Japan erupted in 2011, creating a magnitude 9 earthquake that triggered a devastating tsunami that hit the country and killed nearly 20,000 people.

Now, scientists fear that a similar calamity could hit the US in the coming years, saying that Cascadia tremors occur roughly every 500 years, with the last occurring in the 1700s.

“The recurrence interval for this subduction zone for major events is on the order of 500 years,” Wang said.

“It’s hard to know exactly when that will happen, but certainly when you compare it to other subduction zones, it’s pretty late.”

Cascadia’s four segments make it more dangerous than other major fault lines because they have different rocks and sediments, with the most worrisome stretch running along northern Oregon, into Washington and southern British Columbia.

“It requires a lot more study, but for places like Tacoma and Seattle, it could mean the difference between alarming and catastrophic,” said study co-author Harold Tobin, a geophysicist at the University of Washington.

This part of Cascadia is flatter and smoother than the other three parts, which means it could cause the largest earthquake to spread further into the US and affect all coastal communities in Washington.

“We have the potential for earthquakes and tsunamis as large as the largest we’ve experienced on the planet,” Tobin told NBC News.

“Cascadia appears to be able to generate a magnitude nine or a bit smaller or a bit larger.

Suzanne Carbotte, the study’s lead author and a marine geophysicist at the Lamont-Doherty Earth Observatory, said this is the first clear picture of the Cascadia zone, adding that all emergency response models are based on “old, poor-quality data from the 1980s.”

The researchers hope their findings will help states in the impact zone prepare for a worst-case emergency response and evacuation scenario if the Cascadia Subduction Zone ruptures.

Neither Oregon nor Washington state are adequately prepared for this type of disaster, according to researchers, due to limited information in the 1980s Cascadia model.

However, they said new readiness assessments could be published as early as next year.

The subduction zone map was created using active source seismic imaging – which sends sounds down to the ocean floor and processes the echoes – giving a sharper look at how it can affect the surrounding area.

Columbia Climate School researchers attached a streamer — a nine-mile cable — to the back of a ship that used 1,200 hydrophones that picked up the echoes to update their ocean acoustic tomography models that provide images of the disturbance.

Hydrophones measure the time it takes for sound to bounce off structures on the ocean floor and reach the surface, allowing them to detect differences in rock height that mark recently active fault lines.

“The accuracy and this resolution is truly unprecedented. And it’s an amazing data set,” Kelin Wang, a scientist with the Geological Survey of Canada who was not involved in the study, told NBC News.

“It allows us to do a better job of risk assessment and have information for building codes and zoning.”