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Major Oregon earthquake likely in coming years, says study

Coos Bay, OR

An earthquake of the same magnitude that wracked Japan in 2011 could shake the southern Oregon coast in the next few decades, according to a study published by Oregon State University (Corvallis) researchers.

The study, published online by the U.S. Geological Survey, concluded there is a 40 percent chance of a major earthquake in the Coos Bay, OR, region during the next 50 years that could approach the intensity of the temblor that devastated Japan in March 2011.

The study looked at the Cascadia Subduction Zone of the coast of the Pacific northwest and confirmed the region has had numerous earthquakes over the past 10,000 years. The data, said researchers, suggested the southern Oregon coast may be most vulnerable based on recurrence frequency.

 “The southern margin of Cascadia has a much higher recurrence level for major earthquakes than the northern end and, frankly, it is overdue for a rupture,” said Chris Goldfinger, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences and lead author of the study. “That doesn’t mean that an earthquake couldn’t strike first along the northern half, from Newport, Ore., to Vancouver Island.

“But 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,” Goldfinger added. “The probability for an earthquake on the southern part of the fault is more than double that of the northern end.”

 The publication of the peer-reviewed analysis may do more than raise awareness of earthquake hazards and risks, experts say. The actuarial table and history of earthquake strength and frequency may eventually lead to an update in the state’s building codes, said the university in a statement on Aug. 1 about the research.

 “We are considering the work of Goldfinger, et al, in the update of the National Seismic Hazard Maps, which are the basis for seismic design provisions in building codes and other earthquake risk-mitigation measures,” said Art Frankel, who has dual appointments with the U.S. Geological Survey and the University of Washington.

The study took four years to complete and is based on 13 years of research, said the university. At 184 pages, it is the most comprehensive overview ever written of the Cascadia Subduction Zone, a region off the Northwest coast where the Juan de Fuca tectonic plate is being subducted beneath the continent. Once thought to be a continuous fault line, Cascadia is now known to be at least partially segmented.

 This segmentation is reflected in the region’s earthquake history, Goldfinger noted.

 “Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border,” Goldfinger noted. “These would typically be of a magnitude from about 8.7 to 9.2 – really huge earthquakes.

The clock is ticking on when a major earthquake will next strike, said Jay Patton, an OSU doctoral student who is a co-author on the study.

 “By the year 2060, if we have not had an earthquake, we will have exceeded 85 percent of all the known intervals of earthquake recurrence in 10,000 years,” Patton said. “The interval between earthquakes ranges from a few decades to thousands of years. But we already have exceeded about three-fourths of them,” he said.