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Seattle, Portland and Vancouver are at risk of being hit by a megaquake, scientists have warned. Pictured is the Juan de Fuca and Gorda plates offshore of the Pacific Northwest. The plate moves eastward from the midocean ridge and spreading center on the plate's western edge to the trench on the eastern margin
Seattle, Portland and Vancouver are at risk of being hit by a megaquake, scientists have warned.

Researchers believe a killer quake along Cascadi - a fault line which runs from Cape Mendocino, California, to Vancouver Island - is 72 years overdue.

The 'Big One' will hit when Juan de Fuca, a 700-mile chunk of the Pacific Ocean, slides under Canada and America.

Now, scientists have a better idea of how this will happen after creating the first ever detailed map of the mantle under the tectonic plate.


The map shows the movement of the famous Juan de Fuca plate is connected to the flow of the mantle 100 miles (150 km) underground.

'This is the first time we've been able to map out the flow of mantle across an entire plate, so as to understand plate tectonics on a grand scale,' said Richard Allen, a professor at the University of California, Berkeley.

The major surprise, Allen said, is that the mantle beneath a small piece of the Juan de Fuca plate is moving differently from the rest of the plate, creating a divide in the subduction zone.

A similar pattern is seen in Pacific Northwest megaquakes, which don't always break along the entire 600 miles (1,000-kilometer) length, producing magnitude 9 or greater events.

The Juan de Fuca plate offshore of Oregon, Washington and British Columbia is small - about the size of California.

It is around 40 miles (70km) thick but 'big enough to generate magnitude 9 earthquakes' as it's placed under the continental North American plate, Allen said.

Because of the hazard from this so-called Cascadia Subduction Zone, a recent New Yorker article portrayed the area as a disaster waiting to happen.

It predicted that 'an earthquake will destroy a sizable portion of the coastal Northwest.'

But little is known about the tectonic plates submerged under the oceans, how they are linked to processes inside the Earth.

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An ocean bottom seismometer being retrieved after spending 10 months on the floor of the Pacific Ocean to map the mantle 100 miles underneath the Juan de Fuca plate
The Juan de Fuca plate is one of seven major and dozens of minor plates that cover the Earth like a jigsaw puzzle, pushed around by molten rock rising at mid-ocean ridges.

They dive under other plates or ram into them to generate mountain ranges like the Himalayas.

The largest of Earth's tectonic plates, the Pacific Plate, is moving eastward and plunging under the entire western edge of the Americas, creating a 'ring of fire' dotted with volcanoes and mountain ranges and imperiled by earthquakes.

Until now, however, scientists have used only a handful of seismometers on the seabed worldwide to explore the mantle underlying these plates, said Allen.

His team are working with new underwater and on-shore seismic instruments to measure the plate's interaction with the mantle, and monitor quake and volcanic activity at the trench off the coast where the Juan de Fuca plate subducts under the North American plate.

Based on the first three years of data, Allen and his team confirmed what geophysicists suspected.

At the mid-ocean Juan de Fuca ridge about 300 miles (500km) offshore of Seattle - the western edge of the Juan de Fuca plate - the flow of the mantle below the plate is perpendicular to the ridge.

As the plate moves away from the ridge, the mantle flow rotates slightly northward toward the trench.

At its eastern margin, the plate and underlying mantle move in alignment, perpendicular to the subduction zone, as expected.

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Devastation: Experts say citizens have 20 minutes to run when a tsunami of around 9.0 on the Richter scale will hit the Pacific Northwest when the Juan de Fuca plate, a 700-mile chunk of ocean, slips under the region
Researchers believe the subducted part of the plate deep under the trench is pulling the massive plate downward at the same time that the emerging lava at the mid-ocean spreading ridge is elevating the plate and pushing it eastward.

Allen found, however, that a part of the Juan de Fuca plate called the Gorda Plate, located off the northern California coast, is not coupled to the mantle, leaving the mantle beneath Gorda to move independently of the plate above.

Instead, the Gorda mantle seems to be aligned with the mantle moving under the Pacific plate.

'The Juan de Fuca plate is clearly influencing the flow of the mantle beneath it, but the Gorda Plate is apparently too small to affect the underlying mantle,' he said.

This change in mantle flow produces a break in the forces on the plate, possibly explaining segmentation along the subduction zone.

'Our goal is to understand large-scale plate tectonic processes and start to link them all the way down to the smallest scale, to specific earthquakes in the Pacific Northwest,' he said.

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Because of the hazard from this so-called Cascadia Subduction Zone, a recent New Yorker article portrayed the area as a disaster waiting to happen. Due to the porous nature of the land, many roads will turn to liquid when the quake hits. This diagram show the areas where land will be most affected by the shocks