Quakes off U.S. Pacific Northwest found more frequent than previous estimates

A new analysis suggests that massive earthquakes on northern sections of the Cascadia Subduction Zone, affecting the U.S. Pacific Northwest, are somewhat more frequent than has been believed in the past.

The chance of one occurring in the more heavily populated areas within the next 50 years is also slightly higher than previously estimated, said researchers from Oregon State University (OSU), Camosun College in British Columbia, Canada, and Instituto Andaluz de Ciencias de la Tierra in Spain.

The findings, published this week in the journal Marine Geology, are based on measurements from 195 core samples containing submarine landslide deposits caused by subduction zone earthquakes, instead of about a dozen such samples in past research, therefore far more detailed and comprehensive data than anything prior to this latest study.

The Cascadia Subduction Zone runs from northern California to British Columbia, and is believed to be roughly divided into four segments. There have been 43 major earthquakes in the past 10,000 years on this subduction zone, sometimes on the entire zone at once and sometimes only on parts of it. When the entire zone is involved, it is believed to be capable of producing a magnitude 9.1 earthquake.

The last major earthquake on the Cascadia Subduction Zone - pinpointed in time because it caused a tsunami that raced all the way across the Pacific Ocean to Japan - occurred in January, 1700, more than 315 years ago.

It has been known that the southern portions of the subduction zone south of Newport, Oregon, tend to rupture more frequently: an average of about every 300-380 years from Newport to Coos Bay, and 220-240 years from Coos Bay to Eureka, California.

"What this work shows is that, contrary to some previous estimates, the two middle sections of the Cascadia Subduction Zone that affect most of Oregon have a frequency that's more similar than different," said Chris Goldfinger, a professor in the College of Earth, Ocean and Atmospheric Sciences at OSU, and one of the world's leading experts on tectonic activity of this subduction zone.

The new data have changed the stakes for the northern sections of the zone, which could have implications for major population centers such as Portland, Tacoma and Seattle of the United States and Vancouver of Cananda. A section of the zone from Newport to Astoria, Oregon, was previously believed to rupture on average about every 400-500 years, and that average has now been reduced to 350 years. A section further north from Astoria to Vancouver Island, Canada, was previously believed to rupture about every 500-530 years, and that average has now been reduced to 430 years.

"These new results are based on much better data than has been available before, and reinforce our confidence in findings regarding the potential for major earthquakes on the Cascadia Subduction Zone," said Goldfinger, who directs the Active Tectonics and Seafloor Mapping Laboratory at OSU. "With more detailed data we have also changed somewhat our projections for the average recurrence interval of earthquakes on the subduction zone, especially the northern parts. The frequency, although not the intensity, of earthquakes there appears to be somewhat higher than we previously estimated."

Based on these findings, the chances of an earthquake in the next 50 years have been slightly revised upwards: of the part of the zone off central and northern Oregon, the chance has been changed to 15-20 percent instead of 14-17 percent; on the furthest north section of the zone off Washington and British Columbia, the chance has increased to 10-17 percent from 8-14 percent.

The study also increased the frequency of the most massive earthquakes, where the entire subduction zone ruptures at once.

"We believed these earthquakes were possible when the hypothesis was first developed in the late 1980s," Goldfinger said. "Now we have a great deal more certainty that the general concern about earthquakes caused by the Cascadia Subduction Zone is scientifically valid, and we also have more precise information about the earthquake frequency and behavior of the subduction zone." Endit