Scientists say they don't known when the next major earthquake will strike the Northwest U.S., but they're getting a better idea of where it will hit following extensive mapping of the region's tectonic plate.

A team of researchers says it's almost finished the first complete mapping of the Earth's mantle beneath the undersea Juan de Fuca plate, which is diving under the Pacific Northwest and presenting Portland, Seattle and Vancouver with a risk of a large earthquake and possible tsunamis.

A quake along the Cascadia fault line, where the 700-mile-long plate slides under Canada and the U.S., is at least 70 years overdue, the researchers say.

The flowing of the mantle 100 miles beneath the Juan de Fuca plate influences the plate's movement, they explain.

In a $20 million effort known as the Cascadia Initiative, 70 seismometers were place along the plate and gathered data for 10 months, measuring the plate's interaction with the mantle beneath.

"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," says Richard Allen of the University of California, Berkeley's department of earth and planetary science.

"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," says Allen, lead author of a study appearing in the journal Nature Geoscience.

A major unexpected finding of the research was that the mantle under one small region of the Juan de Fuca plate is moving in a different direction from that under the remainder of the plate, the researchers say.

This is causing segmentation along the subduction zone, further complicating efforts to understand the processes that could be behind a major quake, they say.

"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 says. "The change in the mantle flow could be linked to that segmentation."

While the Juan de Fuca plate is small — around the size of California and 30 to 45 miles thick — it is "big enough to generate magnitude 9 earthquakes," he says.