By Richard A. Lovett

America's Pacific Northwest has a 37% chance of being hit by a magnitude 8 or larger earthquake in the next 50 years, a new study shows. That's more than double previous estimates of a 10-15% risk, says Chris Goldfinger, a marine geologist at Oregon State University in Corvallis.

In a monograph1 soon to be published by the US Geological Survey, Goldfinger and colleagues examined more than 80 core samples taken from the seabed between Vancouver Island, in south-western Canada, and Cape Mendocino, in northern California, looking for deposits from submarine landslides triggered by massive earthquakes.

All the sample locations lay near the Cascadia subduction zone, a fault zone some 1,000 kilometres long that runs alongside the northwest American coast not far offshore.

Using a combination of carbon-14 dating of trapped plankton and geophysical measures of the sediments' physical and magnetic characteristics, Goldfinger's team looked for landslides that appeared to have occurred simultaneously in numerous locations. "If we can link them together, a great earthquake is the most likely cause," Goldfinger says.

From the team's findings, he concludes that during the past 10,000 years -- the farthest back that the core samples allow him to peer -- the subduction zone has produced two types of earthquake. About half -- 19 by Goldfinger's count -- were megaquakes that rattled the entire region, producing magnitude-9 tremblers comparable to the one in the Indian Ocean that unleashed a devastating tsunami on 26 December 2004. Most of these had been discovered by other studies.

The other type, of which Goldfinger identified 22, affected only the southern portion of the fault zone, producing earthquakes of around magnitude 8. These earthquakes are smaller than the magnitude-9 megaquakes, but are still uncomfortably large. "These are similar to what took place in Chile" on 27 February 2010, he says.

Ticking time bomb

Previous hazard estimates, based on the magnitude-9 earthquakes, had set the recurrence interval for earthquakes in the region at about 500 years, with a 10-15% chance of another in the next 50 years. But Goldfinger's study, by upping the total number of earthquakes to 41, has cut the average recurrence interval to about 240 years.

The last earthquake on the subduction zone was on 27 January 1700. Adding in the new earthquakes, and taking into account the statistical distribution of intervals between them, the next earthquake is overdue, and there's a 37% probability it will occur somewhere along the Cascadia fault in the next 50 years.

"Public officials should maybe look at the new numbers and think about this earthquake as a real possibility in the next 50 years, instead of just a remote possibility," Goldfinger says.

Other researchers have welcomed the work. "Chris has done a huge labor with lots of cores and worked them up very carefully," says Brian Atwater, a US Geological Survey geologist based at the University of Washington, Seattle, who has studied tsunami deposits from earthquakes along the northern half of the subduction zone.

"It's very useful information," says John Vidale, director of the Pacific Northwest Seismic Network at the University of Washington. The US Geological Survey has already convened a workshop this summer for the sole purpose of determining how to incorporate the new findings into earthquake-hazard maps, he says. He notes, though, that scientists must still work though all of Goldfinger's data to make sure that all 41 findings do represent earthquakes.