Researchers: New, active fault lines found at Mt. Hood


PORTLAND, Ore. (KOIN) — For the last few years, the Pacific Northwest has braced for the possibility of “The Big One,” an earthquake that could happen on the Cascadia Subduction Zone, a massive fault line that stretches from Northern California to Canada. 

A new study, however, says there’s another possible earthquake Oregonians have to consider.

Researchers from Portland State and the Oregon Department of Geology and Mineral Industries found an “extensive network of active geological faults” near Mt. Hood that have the possibility of triggering an earthquake with a magnitude of up to 7.2. For comparison’s sake, a spokesman for Portland State, who announced the release, said it could be larger than the one that hit the San Francisco Bay Area in 1989. 

“I think it’s an important message because we think about big earthquakes, we think about Cascadia, we think about the subduction zone. We don’t think about the faults that are literally beneath our feet,” Dr. Ashley Streig, an assistant professor of geology at PSU, told KOIN 6 News.

“Subduction zone quakes are deeper below the surface, they last longer – as long as seven minutes – but they are lower in amplitude. The kind of quake we would get from Mt. Hood would be shorter – 20 seconds to less than a minute – and would be strong enough to knock you off your feet,” she continued.

An earthquake on the active fault line would have an epicenter closer to Portland that the Loma Prieta earthquake had to San Francisco in 1989, “which means it could be even more damaging.”

The faults are north, south and southwest of Mt. Hood. If the fault ruptures, Dr. Streig said the Columbia River Gorge communities would see the worst impact — including Hood River, White Salmon, Cascade Locks, Government Camp and Stevenson. The Portland metro would experience “strong ground motions” and “could suffer liquefaction damage along waterfront areas.”

Dr. Streig and her colleagues hope to further their investigation to figure out what could trigger an event like this and when.

“Get information on fault behavior so we can better inform people of seismic hazards in the Portland area, in Eugene, in Corvallis,” she said.

What would trigger an earthquake along these faults? Would it be an eruption of seismic event?

While researchers don’t have exact answers just yet, those are the questions Dr. Streig and he colleagues are determined to figure out.

“What we think we’re seeing is similar chain of faults and map out,” Dr. Streig said. “Are they related to large volcanic events? Or are they related to the regional tectonic stresses? So that isn’t so clear right now, so that’s ongoing work. But either way, if this is rupturing intendant of volcanic events, we want to understand that. To do that we need to get better age dates, trench the faults and look for offset stratigraphy and understand the timing of prehistoric earthquakes.”

The next steps include gathering more detailed mapping to find all the faults, conducting field investigations on the faults themselves, getting age dates along the offset surfaces and trenches to find out the slip rate.

According to Dr. Streig, if we know the slip rate, then we’ll know if these faults are tectonic related vs. volcano related — which will help us determine the hazards to our area and get a more accurate estimate of the magnitude.

Differences between Cascadia and crustal faults  

1.  Cascadia Subduction Zone: Large and deep, but far from us. We’d experience really long durations of shaking 3-7 minutes, like a rolling feeling.

2. Crustal fault: Movement happening in the crust, very near to us — creating a high amplitude, but short duration. So tens of seconds to maybe a minute of shaking, however, Dr. Streig said “That high amplitude meas it could knock you to your feet.”

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