A large team of scientists has nearly completed the first map of the mantle under the tectonic plate that is colliding with the Pacific Northwest and putting Seattle, Portland and Vancouver at risk of the largest earthquakes and tsunamis in the world.

A new report from five members of the mapping team describes how the movement of the ocean-bottom Juan de Fuca plate is connected to the flow of the mantle 150 kilometers (100 miles) underground, which could help seismologists understand the forces generating quakes as large as the destructive Tohoku quake that struck Japan in 2011.

“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 and chair of earth and planetary science at the University of California, Berkeley, and the senior author of a paper published online Nov. 2 in the journal Nature Geoscience. “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 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, resulting in segmentation of the subduction zone. Similar segmentation is seen in Pacific Northwest megaquakes, which don’t always break along the entire 1,000-kilometer (600-mile) length, producing magnitude 9 or greater events. Instead, it often breaks along shorter segments, generating quakes of magnitude 7 or 8.

The Juan de Fuca plate offshore of Oregon, Washington and British Columbia is small – about the size of California and 50-70 kilometers thick – but “big enough to generate magnitude 9 earthquakes” as it’s shoved 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, predicting that “an earthquake will destroy a sizable portion of the coastal Northwest.”