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The Cascadia subduction zone. Image via US Geological Survey.
The Cascadia subduction zone. Image via US Geological Survey.

For the first time ever, scientists have gotten a full picture of the plate that is pushing its way under the Pacific Northwestern coast and putting the entire region in danger of a devastating earthquake. In a paper published in Nature Geoscience, scientists describe some surprises that are lurking in the deep underwater plate.

This summer, the fears over a mega-earthquake hitting the Pacific Northwest were renewed by a stunning New Yorker article detailing how Seattle, Portland and other cities in the region are unprepared for “The Big One.”

The Gorda Plate is at the bottom of the Cascadia subduction zone. Image via NASA
The Gorda Plate is at the bottom of the Cascadia subduction zone. Image via NASA

As some residents were freaking out and disaster officials were reminding people to be prepared, scientists led by the University of Oregon were analyzing data from 70 seismometers that were collecting data on the Juan de Fuca plate for 10 months to get a better understanding of how it’s moving and what that means for the Cascadia fault.

“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, the senior author of the study and chair of earth and planetary science at the University of California, Berkeley in a press release. “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.”

The biggest surprise from the data: the plate isn’t moving uniformly. Instead, a section called the Gorda Plate, located off the coast of Northern California, is moving independently of the mantle underneath, which may mean The Big One might not be quite as big.

“When you look at earthquakes in Cascadia, they sometimes break just along the southern segment, sometimes on the southern two-thirds, and sometimes along the entire length of the plate,” Allen said. “The change in the mantle flow could be linked to that segmentation.”

When a fault like Cascadia breaks along its entire length, it could cause a magnitude 9 quake capable of completely devastating the region, the so-called Big One. But if it breaks in smaller segments, the damage could be much smaller, resulting in a magnitude 7 or 8 quake instead (remember that magnitude is measured logarithmically).

While that might sound like good news, the possibility of a huge earthquake still looms large. The new data helps shed some light on how the plates and the mantle underneath them are moving, but scientists still need to run the models to figure out the future risks of magnitude 9 earthquakes.

In the meantime, it’s still smart to get an emergency kit and follow the advice of local disaster experts.

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