Geologists Keep Finding More Earthquake Faults In Northwest

Nov 20, 2017
Originally published on November 22, 2017 8:33 am

Shallow, active earthquake faults are being discovered all over Oregon and Washington state. Collectively, these may present a higher risk than the better known offshore Cascadia subduction zone.

There's a line on a map for an active earthquake fault running through the forested hills on the north side of the Olympic Peninsula. But unless you have expert guides, you'd never know when you were standing right on top of it.

A short bushwhack from the side of a logging road will bring you to a low ridge. It’s basically a linear bump on the landscape.

"Probably multiple earthquakes raised it up,” Western Washington University geologist Liz Schermer said. “It's flat up here. It's flat down there. They used to be aligned and now they're offset."

Schermer and two colleagues were scouting on Friday for the best places to bring in a backhoe to dig several short trenches across the bump—called a fault scarp. The Lake Creek-Boundary Creek and Sadie Creek faults stretch between Lake Crescent and the Port Angeles vicinity.

They're hoping trenches and coring of swampy areas will reveal telltale layers. Those could tell us how many times the fault has ruptured and possibly provide opportunities to radiocarbon date the past earthquakes.

This is the kind of basic information scientists and emergency planners need to assess risk. And they don't have it for most of the multiplying number of active, shallow fault zones on land in the Pacific Northwest.

"Several of these faults are actually probably more of a hazard than the Cascadia subduction zone because they are closer to the population centers,” Schermer said. “We think they are capable of producing Magnitude 7 earthquakes, which is a good sized earthquake—especially near a population center."

The offshore Cascadia fault zone is capable of generating Magnitude 8 to 9 earthquakes. The Big One strikes the Northwest once every 240 to 800 years or so according to paleoseismology research published out of Oregon State University.

Schermer said the close by, onshore faults she's studying rupture less frequently.

"More than 1,000 years, less than 5,000 years return time,” she said.


But the rub is there are a lot of these active local faults.

"The next damaging earthquake in Oregon is most likely to be a crustal fault," Oregon Department of Geology Chief Scientist Ian Madin said in an interview Monday.

And researchers keep discovering more of them, thanks in large measure to a technology called LIDAR. When mounted on an airplane, LIDAR uses pulses of laser light to pierce the foliage of trees and bushes and map the ground underneath.

Washington Department of Natural Resources Chief Hazards Geologist Corina Forson said ongoing LIDAR mapping flights are proving revolutionary—really.

"We discover new faults all the time as we collect new data,” she said.

In the above image, a composite of an aerial photo and LIDAR image shows how the Toe Jam Hill fault scarp (a strand of the Seattle fault zone) becomes clearly visible in the landscape of Bainbridge Island, Washington. You can move the slider to see the fault that is obscured to the naked eye by vegetation.

Forson's agency plans to ask the 2018 Washington Legislature for an extra $543,000 to hire specialists to map earthquake and tsunami hazards, and get the info online so counties, cities and citizens can use it.

The federal Bureau of Reclamation and U.S. Geological Survey are also pumping money into earthquake fault investigations.

Western Washington University geologist Colin Amos recently worked on a grant-funded team investigating faults in the Klamath Basin. A pair of earthquakes near Klamath Falls in 1993 caused two deaths and extensive structural damage.

"The scarps that we see in the LIDAR imply a record of larger earthquakes than what they experienced in 1993, Amos said. “That’s part of the motivation for looking at those features."

Other faults that are getting priority scrutiny include ones under Seattle and Tacoma and those that run near dams on the mid-Columbia River and Hagg Lake west of Portland.

"I would love to say we have a fully-funded program... to systematically evaluate all of the faults in Oregon. But I'd be lying.” Madin said. "There is not a lot of funding."

Madin said he is particularly interested in investigating active faults on Mount Hood that were discovered with LIDAR in 2011. He said ongoing LIDAR surveys stretching from the state’s border to border are revealing previously unrecognized faults "in areas we weren't expecting," such as parts of eastern Oregon.

"I just keep adding them to the pile for investigation," Madin said.

Washington state and individual counties are also collaborating to fill in LIDAR mapping gaps.

The geologic mechanism loading these crustal faults with energy is a little different from, but also potentially connected to the grinding of tectonic plates that drives the 600-mile long Cascadia subduction zone.

"A jigsaw puzzle of blocks are moving" within the North American plate is how Schermer described the crustal action. Specifically, northward rotation of the Oregon Coast Range—imperceptible to human senses—presses against Washington state and the Oregon Cascade Range. The landmass of Washington gets crunched by resistance from somewhere in British Columbia. The Olympic Mountains may be making what Schermer and her co-authors in a recent paper called a "westward escape."

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