04/24/2003

By JIM PARKER and DOUG IRVING, kgw.com Staff

A 3.9-magnitude earthquake shook the Portland area during the noon hour on Thursday, but there were no reports of injuries or damage. It was followed later by two separate aftershocks, magnitudes 1.6 and 1.2 respectively.

The quake, centered about 10 miles northwest of downtown Portland and 5 miles west of Vancouver, was classified by seismologists as “minor.” They pinpointed the epicenter near Kelley Point Park.

Residents throughout the metro area called KGW to report feeling the tremor. The quake was felt on Mount Hood, in downtown Portland and as far away as the Washington Coast. More than 400 people had also contacted the seismograph network to report feeling it in southern Washington and up to 25 miles south of Portland.

The Portland Fire Department had one quake-related call, spokesman Neil Heesacker said. A woman reported that her house had begun shaking when she turned off her gas fireplace. Fire crews arrived, checked her house and realized she had happened to turn off her fireplace at the moment the quake began.

"She thought she had caused the shaking by something she had done," Heesacker said.

The UW Seismology Lab at first registered the quake's magnitude at 3.1, but upgraded it to 3.9 after further analyzing it. Lab scientist George Thomas described the quake as "relatively shallow."

"This is something you would expect to see in this area. It's just a run-of-the-mill earthquake. This one was just strong enough to be felt," he told kgw.com.

Scientists studying quake type

Thomas said most Portland-area quakes are "strike-slip" types.

Geologists had not yet determined what kind of earthquake rolled through Portland. The area has felt strike-slip quakes before, similar to those in California, where pieces of the earth's crust grind sideways against each other

'One very large bump'

Whatever you call it, many in Oregon and SW Washington said they felt it.

Jesse Wise lives near Kelley Point, where the quake was centered. Wise told told kgw.com: "It felt like something ran into our house -- twice. I thought something was trying to come through the house."

In LaCenter, Wash., Gordon Sahnow described "two very distinct sharply felt jolts here - enough to rattle glassware and shake bits of mortar from the chimney."

A worker at a cytogenetics lab on SW 3rd St. in Portland said while the quake caused no significant damage, it did make for some scary moments.

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Mike Brown told kgw.com he felt "one very large bump raising the third floor an inch or two and then down again. Several people sitting in soft chairs were moved down and back up several times. Glassware on the shelves rattled briefly but stayed put."

"Just a good stress relieving earth mover," he concluded.

Robert Ewing, on the Rock Creek Campus of Portland Community College, said the quake "felt like a truck slammed into the side of the building."

Small quake no reason to worry

But scientists maintained the tremblor was no cause for alarm.

"It's not anything unusual or surprising," said Amy Wright, a seismic analyst with PNSN. "The magnitude is still small enough that it's not a big earthquake, not something to be feared."

"This is still a very tiny earthquake," added geologist Lou Clark of the Oregon Dept. of Geology. "It's certainly of interest. It's certainly a reminder to all of us that this is earthquake territory."

In this case, "the earth probably moved half an inch" 15 miles below the surface, Clark told kgw.com.

In general, the history of Portland suggests it gets one damaging quake every 30-40 years, Clark said. There was a 5.2 quake centered in Portland in 1963 that knocked down chimneys. Then there was the Spring Break quake in 1993.

Portland probably gets a quake this size (3.9) about every five years or so, she said. "It's not at all unheard of."

Valley soils help magnify quake

Geologists figure damage begins around magnitude 4. This one may have caused tiny sidewalk or foundation cracks, especially along the river, in old flood plains or places with artificial fill, where the quake waves are amplified.

Clark was taught in geology school that, "normally, we would say you almost have to be right on top of (a minor quake) to feel it. We were told if it's a 3, you can't feel it unless you're on top of it."

But, the loose, fine soil of the Willamette Valley -- along with the usual high water content -- tends to magnify quake waves. Clark said she wouldn't be surprised if this was felt from Vancouver to south of the city -- an anomoly compared to other places.

There's really no way to tell if a particular quake is a precursor of something bigger. But Clark maintains "there's no reason to think this would be a precursor to anything larger... There's no reason to think there'll be anything else that people can feel."

The first aftershock was "exactly what we'd expect." She predicted there could be a "couple more tiny ones," but nothing people w ould feel.

Quake dwarfed by Nisqually Quake

Portland's major fault lines. (KGW file graphic)

Clark was on the first floor of her building in northeast Portland and didn't even feel the quake. "I never get to feel these," she said.

The last major earthquake felt in Oregon was on Feb. 28, 2001. The Nisqually quake, centered 17 miles from Olympia, Wash., had a 6.8 magnitude. It injured more than 400 people and caused an estimated $2 billion damage in Washington and Oregon.

The Spring Break Quake of 1993, which was centered near Scotts Mills, measured 5.6 and severely damaged several buildings including Molalla’s High School and the Oregon State Capitol in Salem. A 6-magnitude quake later that same year near Klamath Falls killed one person.

(The Associated Press contributed to this report.)