A comprehensive analysis of the Cascadia Subduction Zone suggests that, based on recurrence frequency, there is a 40 percent chance of a major earthquake in the Coos Bay, Ore., region during the next 50 years, Oregon State University researchers reported Wednesday.
"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.
That doesn't mean the next earthquake couldn't strike on the zone's northern half, he said, which extends 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 said. "The probability for an earthquake on the southern part of the fault is more than double that of the northern end."
An earthquake in the region could approach the intensity of the Tohoku quake that devastated Japan in March of 2011, the researchers said.
"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 said. "These would typically be of a magnitude from about 8.7 to 9.2 -- really huge earthquakes."
Such a quake could be considered overdue, another member of the research team said.
"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," OSU doctoral student Jay 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."