06/29/2002

By DOUG IRVING, kgw.com Staff

A faint earthquake flickered across the south slope of Mount Hood early Saturday, startling residents near the mountain but causing no reported damage.

The brief quake registered 4.5 on the Richter Scale, not quite powerful enough to damage buildings. A series of aftershocks continued through the day, including one that measured 3.8.

The quake was the strongest to hit the mountain in nearly 30 years, said Steve Malone, the director of the University of Washington’s seismology lab. Sudden flurries of earthquakes are nothing new for Mount Hood, an active volcano; but most register only with sophisticated measuring tools.

A seismograph records the beginning of Saturday's light earthquake. (kgw.com Photo/Pacific Northwest Seismograph Network)

“It was just a good shaking of the house,” she said. “Nothing fell, nothing broke and nobody woke up.”

No Damage Reported

The quake rolled through the Mount Hood area at 7:36 a.m. People living closer to the mountain saw furniture twitch and lights swing. The quake was barely perceptible closer to Portland.

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		Data from NW Seismograph
		U.S. Geological Survey

Nobody called Clackamas County’s emergency dispatchers to report the quake, Deputy Angela Blanchard said. The county, which extends up the side of Mount Hood, did not activate its Emergency Operations Center, she said.

At Timberline Lodge, the quake stopped some guests in their steps but caused no damage. Not so much as a postcard slipped from the shelves in the lodge’s gift shop, General Manager Mark Vincent said. “It wasn’t that rattling,” he said of the quake. “Just one big, sharp jolt.”

A magnitude-4.5 quake is not strong enough to damage well-built buildings, said Bill Steele of the University of Washington’s Pacific Northwest Seismographic Network. But Saturday’s quake was shallow enough to make people notice; “luckily, it’s not densely populated there,” Steele said.

Cause Still Unknown

Geologists were not sure Saturday what caused the early-morning quake and the dozens of aftershocks that followed. Seismographs seemed to show the jagged convulsions that characterize “garden-variety” tectonic quakes, Malone said.

That would mean the constant shifting of the Earth’s crust triggered the earthquake. Such quakes are common in the Pacific Northwest, where major underground faults crisscross the geologic map.

seismograph
		See the Quake, Aftershocks

But geologists have not ruled out volcanic activity as the cause, a troubling possibility that could signal renewed commotion deep inside one of the Northwest’s most popular mountains. Such a quake “would be cause for concern,” Malone said.

”That would have implications for volcanic activity,” he said. “It’s certainly too early to tell. Typically, one earthquake does not tell the whole story.”

'Like Something Fell on the House'

Geologists consider Mount Hood an active volcano, and small flurries of earthquakes rattle the mountain almost every year. But most of those barely manage a 1 or 2 on the Richter Scale – dwarfed even by the aftershocks of Saturday’s quake.

For most people living near the mountain, though, the quake wasn’t even strong enough to shake them out of sleep or jostle pictures from the mantle. Kathleen Weigant thought nothing of it when she felt her Welches home shudder as she woke up; her husband didn’t even tell her of the quake when she got out of bed.

”It was very brief,” she said. “It felt like something fell on the house, like something had jarred the back of the house.”

Another Reminder for Oregon

The last major earthquake to shake Portland rolled through in February 2001. The magnitude-6.1 quake was centered near Olympia, Wash., and caused millions of dollars of damage in Washington State.

An earthquake jarred northwest Oregon in March 1993, toppling chimneys and breaking down masonry walls. The so-called Spring Break Quake damaged Molalla’s High School and the State Capitol in Salem. It registered 5.6 on the Richter Scale.

Saturday’s quake was about 15 times smaller than the Spring Break Quake, said Lou Clark, a geologist with the Oregon Department of Geology.

“It was a very small earthquake, by earthquake standards,” she said. “Nobody gets hurt, there’s no damage. But it reminds everybody that we do have earthquakes in Oregon.”