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Earthquakes

Discovery confirms active fault in Portland

A discovery at a middle school is the first direct proof that the Portland Hills Fault could generate an earthquake.

Wednesday, May 30, 2001
By Richard L. Hill and Noelle Crombie of The Oregonian staff (reprinted with permission)


The geologist did a double take as he peeked into a shallow trench at a school construction project.

What Ian P. Madin spotted in the trench's soil sediments is what he had spent the past 14 years looking for: evidence that the Portland Hills Fault is active -- capable of generating an earthquake.

Madin's discovery at Rowe Middle School in Milwaukie is the first direct evidence that the extensive Portland Hills Fault, which runs beneath much of Portland, ruptured in the recent geologic past.

He spotted the deformed soil layers about two weeks ago while looking for signs of the fault. He and other geologists followed up with a close investigation of the folded sediment layers.

"This falls under the heading of dumb, blind luck," said Madin, a geologist with the Oregon Department of Geology and Mineral Industries. "If the trench (dug for a retaining wall) hadn't been there at the time I happened to be there, we would have missed it."

John D. Beaulieu, who heads the geology agency, said the discovery is significant because "the earthquake risk for Portland just got ratcheted up another notch."

He said more research needs to be done, but the finding reinforces his agency's warnings that the Portland area and the rest of Oregon need to be prepared for destructive earthquakes.

The researchers, who reported their findings Tuesday, were unable to say with certainty how large the quake was or when it occurred. The soil deposits in the trench had been deformed about 5 or 6 feet, suggesting the earthquake measured about a magnitude 6.5 -- a moderate quake that could today cause substantial damage because of the urbanized area above it.

The scientists can't put an exact date on when the quake occurred. But based on the evidence, Madin and his colleagues speculate that the fault ruptured perhaps 10,000 years ago -- still recent enough for geologists to label the fault as "active." Madin said only one quake appears to have occurred on the fault.

Rowe Middle School is just one of hundreds of buildings and houses that sit on or near the Portland Hills Fault.

The 30-mile-long fault -- actually a complex "fault zone" containing multiple fractures -- runs in a northwest-southeast direction through Portland. It starts roughly on the northern edge of Forest Park and runs along the foot of Portland's West Hills before turning east on West Burnside Street for a few blocks and then turning southeast again through the heart of downtown. The fault then crosses the Willamette River between the Marquam and Ross Island bridges to Milwaukie and ends about a mile south of the Clackamas River near Oregon City and Gladstone.

The quake that deformed the sediment layers appears to have occurred soon after the soils were deposited by the Missoula Floods that swept down the Columbia River and covered the Portland area with 400 feet of water at the end of the ice age, 12,000 to 15,000 years ago.

Researchers aren't able to pinpoint the quake's epicenter. "It could have been miles away (from Rowe Middle School)," Madin said. "The epicenter could have been under Canby or Mount Scott and anywhere in between."

The scientists emphasized that the fault does not pose any more risk to Rowe Middle School than to any other school or building along the Portland Hills Fault. "You have to put the hazard in perspective," Madin said. "I don't think that anybody should get too excited about the prospect of one event in the past 10 to 12,000 years. Living on or very near a very low-activity fault is more hazardous than not living on it, but how much do you worry about something that happens once every 10,000 years or so? It's pretty long odds that it's going to affect you."

Last fall, Madin and other scientists found evidence for the elusive fault under North Clackamas Central Park, which is about a mile southeast of Rowe Middle School. They had been searching large open areas like parks and schoolyards to look for signs of where the deep fault comes close to the surface.

The same Missoula Flood deposits that the quake folded have made the fault extremely difficult to spot. The thick soil deposits, along with vegetation and urban development, have made the fault tough to study.

Madin and his colleagues plan to conduct more research at the middle-school site beginning in mid-June. The 38-year-old middle school sits on 13.5 acres on Southeast Lake Road in Milwaukie.

Geologists led by Ian Madin of DOGAMI work to understand the process that deformed these sedimentary layers at
Rowe Middle School.

The Portland Hills Fault is one of three large faults that underlie the city, running parallel to each other about one and a half to two miles apart.

The East Bank Fault on the east side of the Willamette River runs under the University of Portland, Mocks Bottom, the Oregon Convention Center, Lloyd Center and Benson and Central Catholic high schools. A third fault, the Oatfield Fault, runs west of Northwest Skyline Road from Sylvan Hill to Germantown Road through Bonny Slope. Scientists hope to study those faults to see whether they might be active.

At the Rowe Middle School Tuesday, small yellow flags mark a jagged line in a wall of earth that measures about 12 feet tall. The flags indicate spots where the earthquake moved the sediment layers.

North Clackamas School District officials sent home letters Tuesday, reassuring parents about the safety of the school. Superintendent Ron Naso said the school passed an earthquake-readiness assessment in 1992, and no retrofitting is planned. He said the school will be evaluated again for its earthquake readiness.

Rowe students and their parents seemed to take the news in stride.

Harrison Fulop, 13, said students were told about the active fault during school Tuesday. "They thought it was pretty cool," he said.

The discovery became the talk of the school, said student Miles McFarland, 14. Students seemed more curious than scared, he said, since teachers said the risk to students is minimal. "They described it to us in a nonthreatening, nonscary way," he said.

Casey Marley, emergency management coordinator for Clackamas County, said the discovery should remind the public about the threat of an earthquake.

"It's going to happen," she said. "Whether it will be in five years or in 500 years, that's the hard part."




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