In the past 15 months several devastating earthquakes have rumbled beneath the Pacific. In February 2010 a magnitude 8.8 temblor slammed central Chile; this past September a magnitude 7 quake walloped Christchurch, New Zealand, leading to a magnitude 6.3 aftershock this past February. The magnitude 9 megaquake that devastated Japan March 11 was the fifth largest in the last 110 years.

Some may wonder if these quakes are linked. A high-magnitude earthquake in Japan, one notion goes, might redistribute stress in Earth's crust, subsequently triggering another temblor in the following months or years—a quake that could even strike as far away as the western shores of the U.S. But the apparent clustering of major seismic events that take place thousands of kilometers apart is probably coincidental. That the Japanese and Chilean megaquakes occurred only 13 months apart is a statistical fluke, the chance alignment of two independent events. "Over great distances, the chances of stress transfer triggering a major quake are low if not nonexistent," says Chris Goldfinger, a marine geologist at Oregon State University in Corvallis.

Even though the recent Japanese temblor has not boosted seismic risk along the U.S. west coast, that doesn't mean residents there can rest easy. The best gauge of quake risk in the northeastern Pacific is the region's seismic history. Whereas scientific instruments designed to document quakes have been around barely more than a century, the interval between major quakes is much longer than that. Thus, scientists turn to the geologic record to determine the recurrence interval of ancient quakes by carbon-dating organic material in sheets of tsunami debris washed far inland or in the layered remnants of submarine landslides smothering the floors of undersea canyons.

The most hazardous swath of the northeastern Pacific lies along the Cascadia subduction zone (CSZ), a tectonic interface that parallels the coast and poses a seismic threat to cities such as Victoria, British Columbia; Portland, Ore.; and Eureka, Calif. At that subduction zone, the tiny Juan de Fuca plate slides eastward beneath the massive North American plate between 30 and 40 millimeters each year—an interface that, with minor exceptions, has apparently been locked for centuries. "This subduction zone stands out as the big elephant in the corner," Goldfinger says. "It sits quiet for hundreds of years, and then goes off all at once."

The last megaquake to strike this region, one estimated to have been a magnitude 9 or greater, occurred the evening of January 26, 1700. According to new analyses by Goldfinger and his colleagues, soon to be published in a U.S. Geological Survey report, that quake is just one of 19 such quakes to have slammed the region in the past 10,000 years. In the same interval at least 20 other temblors measuring between magnitudes 8 and 9 have occurred.

The big question is, when will the next "Big One" hit? "It's going to happen," Goldfinger says. "It's just a matter of narrowing down the timeline."

The team's new data suggest that the northern portion of the CSZ, which stretches from the middle of Vancouver Island to the Washington State–Oregon border, has a 10 to 15 percent chance of suffering a magnitude 8 or greater quake in the next 50 years. But the southern portion of the zone, stretching from the same border to California's Cape Mendocino, has higher risk, as much as a 37 percent chance of the same magnitude temblor over that same interval.

Although most people may consider southern California to be the most quake-prone region in the nation, Goldfinger says that the CSZ is arguably the biggest seismic hazard in the U.S. "We've got a long way to go to get that into the national consciousness," he notes.