Oregon doesn’t see the same seismic activity associated with California and cities like San Francisco, but the city of Portland is surrounded by earthquake faults. Research out of Portland State University shows that one of the larger faults, previously considered inactive within recent history, has actually triggered regular earthquakes large enough to rupture the surface.
The Gales Creek fault is about 22 miles west of downtown Portland and has ruptured three times in the past 9,000 years, suggesting major earthquakes happen about every 4,000 years.
“This would have been a pretty large earthquake — at least a magnitude seven,” Portland State University earthquake geologist Ashley Streig said. The estimate is based on a rupture along the entire length of the Gales Creek fault.
Streig and study co-author Alison Horst hand-dug a trench in the forest along the Gales Creek fault.
“It was a really neat spot because we could see the fault trace along the hillslope and then we could see it at higher elevations not that far from there, so we were confident that was the fault,” said Horst, who now works for the Washington Geological Survey.
Then they looked for evidence of large earthquakes in the sediment layers that were revealed. Where they found breaks or deformations in the layers, they collected preserved charcoal samples and used radiocarbon dating to estimate the age of each earthquake.
The three major earthquakes they documented happened approximately 1,000, 4,200 and 8,800 years ago. And because the Gales Creek fault is a crustal fault, the shaking associated with these earthquakes likely would have been intense.
“These faults are not as big, but because they’re shallower, they can cause more damage. The rupture is happening closer to the surface,” Horst said.
Crustal earthquakes are more likely to hit violently, yet be over quickly.
“These crustal faults (in the Portland area) like the Gales Creek fault, the Portland Hills fault, East Bank fault and the Oatfield fault, these are all beneath our feet. They’re going to have very short… periods of seismic shaking, but really high amplitude shaking. So it’s definitely a seismic hazard,” Streig said.
For comparison, the 1993, 5.7 magnitude Scotts Mills earthquake happened about 40 miles south of Portland. It caused approximately $30 million in damage. The earthquakes associated with the Gales Creek fault are estimated to have been larger and closer to the metropolitan area.
“It’s been known for decades that there’s a Gales Creek fault, but the general understanding of it was very limited. These details matter,” said Yumei Wang, a retired geotechnical engineer with Oregon’s geology agency and currently a senior infrastructure and risk advisor in the engineering department at Portland State University.
Wang says there’s a huge difference in terms of human risk if a fault is known to be active in the past 10,000 years versus 2.6 million years, which had previously been the assumption about the Gales Creek fault.
“The threat becomes much, much more real,” she said.
This is not only to the Portland metro area but to the Scoggins Dam, which is built almost right on top of the fault.
All of these northwest-oriented faults in the Portland region are pressure release points that react to the larger movements of the plates below us.
“This fault probably formed in response to this broader clockwise rotation,” Streig said. “There’s a pivot point somewhere in Eastern Oregon, close to the border of Oregon and Washington. And all of the Pacific Northwest is essentially rotating clockwise around that pivot point.”
Although the findings suggest there won’t be an earthquake on the Gales Creek fault for another 3,000 years, the scientists say this past activity is an indication that other similar faults in the region may also be seismologically active.
Wang says these findings are a good indication that more research is needed to understand those other faults.
“We need to understand our local hazards — whether it’s flooding or wildfire or storms or earthquakes — because it helps us understand our current risk and prepare for them,” she said. “And then it also allows us to know how to better design and build to accommodate for future construction.”
The paper was published in the Bulletin of the Seismological Society of America.