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A major earthquake, similar to what devastated Chile and Haiti, has more than a one-in-three chance of striking the U.S. Pacific Northwest within the next 50 years, scientists say.

Earlier estimates put the chance of such quakes at just once every 500 years. But new analyses by Oregon State University marine geologist Chris Goldfinger and his colleagues have revealed a more complex picture of the Cascadia Subduction zone, where the ocean floor steadily slips below the North American Plate – and where the region's earthquakes originate.

They found that Cascadia represents at least four separate segments, rather than one big subduction zone. Mega-quakes of magnitude-9 or greater occur less frequently in the northern segment and can rupture the entire fault, even as magnitude-8 earthquakes strike more often in the southern segment.

"It is not a question of if a major earthquake will strike, it is a matter of when," Goldfinger said, "And the 'when' is looking like it may not be that far in the future."

A magnitude-9 earthquake that struck the Cascadia region could rip apart highways, collapse bridges and even crumble buildings. A quake epicenter just offshore might give coastal residents as little as 15 minutes warning time before a tsunami swept ashore.

"The biggest offshore quake with a tsunami is certainly the worst case scenario for the coast, and it may be the worst case for inland cities also," Goldfinger told LiveScience. "But a shallower, more local earthquake near Portland or Seattle could be worse for those cities than a subduction earthquake."

That sense of urgency has compelled engineers and scientists to explore different ways of evacuating low-lying areas, including the construction of high-rise, tsunami-resistant facilities.

Rolling the dice

The southern end of the fault running from Newport, Ore., to northern California has a 37-percent chance of triggering a major earthquake in the next half century, with an average rate of roughly one quake every 240 years.

An even more devastating mega-quake has a 10 to 15 percent chance of hitting the northern segment, which stretches from Seaside, Ore., to Vancouver Island in British Columbia. A catastrophic event of that magnitude would come along about every 500 years.

Those revised estimates are based on the historical record, which shows that magnitude-8.2 or higher earthquakes took place 41 times in the Northwest during the past 10,000 years.

"At the southern end of the fault, the earthquakes tend to be a bit smaller, but more frequent," Goldfinger noted. "These are still magnitude-8 or greater events, which is similar to what took place in Chile, so the potential for damage is quite real."

As if following the motto that misery enjoys company, the northern segment may also experience earthquakes in clusters. A thousand years could pass without major incident, before a cluster of earthquakes strikes every 250 years or so, researchers found.

Still, scientists remain undecided about whether the clusters represent random statistical flukes, or if quakes do actually like to come in groups. They also want to discover the chances that the Northwest is currently in an earthquake cluster, if such clusters are significant.

Geological signs

The historical record for earthquakes comes from coarse sediments, called turbidites, which stream down from the continental margins into undersea canyons during ground-shaking from temblors. Those stand out from the finer particles that normally build up between major earthquakes.

Goldfinger and his colleagues used carbon-14 analysis and other methods to date the particles and pin down estimates of when major earthquakes took place. That allowed them to recreate a 10,000-year geological record of earthquakes in the Northwest.

Lower sea levels that existed beyond 10,000 years ago meant West Coast rivers regularly dumped storm debris into the offshore canyons, which made it difficult to distinguish the earthquake turbidites.

The last major earthquake to shake the Cascadia Subduction Zone took place in January of 1700. That event unleashed a 30-foot tsunami that struck Japan, and so scientists knew of the impact from historical records of the damage.

More indirect evidence came from huge physical changes in the coastlines of Oregon and Washington – a sobering reminder of what U.S. residents could face again sooner than later, just as in Chile and Haiti, the researchers say.