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New quake threat off the Pacific Northwest coast

The most comprehensive study of earthquake history off the Pacific Northwest coast has found a new threat of a major, destructive earthquake on the scale of the 2011 Japan quake and tsunami.

Scientists at Oregon State University have been studying the Cascadia Subduction Zone, analyzing clues beneath the surface of the Pacific Ocean such as the the movement of mud, sand, sediment and fine particles, trying to go back 10,000 years. That history shows 19 huge earthquakes along the Cascadia Subduction Zone, typically 8.7 to 9.2 on the Richter scale.

Chris Goldfinger, a professor in OSU’s College of Earth, Ocean and Atospheric Sciencies, believes there is a 40 percent chance of a major, Japan-sized earthquake in the next 50 years, but more likely in the southern end of the subduction zone, meaning Oregon.

“Major earthquakes tend to strike more frequently along the southern end – every 240 years or so – and it has been longer than that since it last happened,” writes Goldfinger.

Art Frankel, with the U.S. Geological Survey and the University of Washington has reviewed the new findings, based on 13 years of research. He calls it important but not wholly accepted and still being “digested by the scientific community.”

The research concludes, in part, that the southern part of that Cascadia Subduction Zone has a higher recurrence of major quakes and thus a higher risk.

“The clock is ticking on when a major earthquake will strike the zone,” wrote a co-author of the study. If there is no big quake by 2060, we’ll have exceeded 85 percent of all known intervals of earthquake recurrence in the last 10,000 years, he concluded.

“It’s not clear whether our record is complete in the north. There may be additional earthquakes in the northern part of the zone that we just haven’t really detected,” suggested Frankel.

Frankel said there might be more than one explanation for what researchers found beneath the sea.

“We’re looking back in time with a very cloudy lens and each type of observation, either offshore or onshore has its limitations and sometimes has multiple interpretations,” he argued.

Goldfinger, the lead author of report, says the southern end of the earthquake zone “is overdue for a rupture.” And, he says, the risk of a major quake in the southern end is double that of the northern end. That doesn’t mean that the Seattle area can relax.

“That would be a serious mistake. [The report] basically says that maybe the hazard in the southern part is higher than we thought it was but it’s still high in the north. The hazard is still high along the whole Cascadia Subduction Zone,” said Frankel.

Frankel and others at the USGS are in the process of updating our earthquake hazard maps, which is done every six years. He says the new findings will change the calculations.

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