The big one is coming. We all know that. We also know the big one might turn out to be the very big one.

The 700-mile-long Cascadia subduction zone that's just off the coast of California, Oregon and Washington will, sooner or later, produce a mammoth earthquake, scientists say.

"If the entire zone gives way at once, an event that seismologists call a full-margin rupture, the magnitude will be somewhere between 8.7 and 9.2," journalist Kathryn Schulz wrote in The New Yorker three years ago. "That's the very big one."

The result, she added, freaking out everyone living in the Pacific Northwest, will be "a seven-hundred-mile liquid wall that will reach the Northwest coast, on average, 15 minutes after the earthquake begins. By the time the shaking has ceased and the tsunami has receded, the region will be unrecognizable."

Of course, the damage won't be equally bad for everyone in the region. After analyzing four years of data from 268 seismometers both on the ocean seabed and on land, University of Oregon researchers have discovered "anomalies in the upper mantle below both ends of the Cascadia subduction zone" that have implications for that next big one -- or the next very big one.

"Our study is worse news for Portland northward to Seattle and for southern Cascadia, but central Cascadia is not off the hook," seismologist Douglas Toomey said in a University of Oregon report published this week at SciTechDaily.com.

What that means: pieces of the upper mantel at the north and south of the zone are rising "because of melting rock and possibly elevated temperatures," said Miles Bodmer, a UO doctoral student who led the study. This could lead to worse quakes in those areas.

Offers a summary of the researchers' peer-reviewed study, which will be published in the journal Geophysical Research Letters:

"Subduction zones typically do not rupture along their full length, instead they release seismic energy over discrete segments. ... We identify two anomalous low-velocity regions beneath the subducting oceanic plate, one below the Olympic peninsula in northern Washington and one beneath northern California. We infer that these are regions where mantle is rising and, due to the presence of partial melt and possibly elevated temperatures, is more buoyant than the mantle beneath central Oregon. These buoyant regions correlate well with areas where the plates are more strongly locked together and where greater seismic tremor occurs."

That is, these regions in northern Washington and northern California are different than the rest of the subduction zone fault, with "increased locking and increased tremor densities." There is less of this tectonic-plate "locking" along the central part of the zone in Oregon.

The researchers say their work highlights the need for an integrated early-warning system for Oregon.

There has recently been a series of small earthquakes off the West Coast, which is not unusual. The last time the Cascadia subduction zone burped up a massive, zone-wide earthquake was way back in 1700. No one knows when it will happen again: it could be this year or more than 100 years from now.

Read more about the UO research at SciTechDaily, and check out a summary of the academic paper at Geophysical Research Letters.

-- Douglas Perry