Several devastating earthquakes have rumbled beneath the Pacific in the past 15 months. In Feb­ruary 2010 a magnitude 8.8 temblor slammed central Chile; last September a 7.0 quake walloped Christchurch, New Zealand, leading to a 6.3 aftershock in February. The magnitude 9.0 mega quake that flattened Japan in March is tied for fourth largest in the past 110 years.

These events have led many people to wonder if they are somehow linked. Most likely, scientists say, their near coincidence is merely a statistical fluke. That doesn’t mean, however, that it is necessarily safe to come out from under the bed. The best gauge of quake risk is the geologic record. And new data on that record tell a disturbing story, especially in the northeastern Pacific.

Although most people may consider southern California to be the most earthquake-prone region in the nation, the Cascadia subduction zone is arguably the biggest seismic hazard in the U.S. It parallels the coast and poses a seismic threat to cities such as Vancouver, B.C., Seattle, and Portland, Ore. At that subduction zone, the tiny Juan de Fuca plate slides eastward underneath North America between 30 and 40 millimeters a year—but this interface has apparently been locked for centuries. “This subduction zone stands out as the big elephant in the corner,” says Chris Goldfinger, a marine geologist at Oregon State University. “It sits quiet for hundreds of years and then goes off all at once.”

New data suggest that the northern portion of the subduction zone, from the middle of Vancouver Island to the Washington-Oregon border, has a 10 to 15 percent chance of suffering a magnitude 8.0 or greater quake in the next 50 years. The southern portion, stretching from the Washington-Oregon border to California’s Cape Mendocino, has a 37 percent chance of the same-size quake over that same interval. Goldfinger and his colleagues expect to publish the data in an upcoming USGS report. The next big one, he says, “is going to happen. It’s just a matter of narrowing down the timeline.”

How do scientists measure the speed of tectonic plates?
The best way to measure how quickly two tectonic plates converge is to use the Global Positioning System. By repeatedly checking the distances between specific points on two different plates, researchers can assess long-term rates of convergence and measure sudden movements, such as Japan’s 2.4-meter (eight-foot) leap eastward during the March 11 quake. Before the advent of GPS, scientists relied on rocks in the ocean floor, which, when they cool, record the direction of the earth’s magnetic field. Knowing when and how often the field has flipped in the past enables researchers to calculate the rate at which new ocean crust forms at mid-ocean ridges. Another technique is to sample and map rock formations on both sides of a tectonic interface—especially formations that have a distinctive composition or an unusual assemblage of fossils.