Deep quake alerts scientists, seismologists

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February 27, 2009

By Anita Burke

Mail Tribune

An earthquake deep below the mountains of southwestern Oregon rattled a broad swath of the region and could shake up what seismologists know about the types of earthquakes likely here, a state scientist said.

An earthquake with a magnitude of 4.1 was recorded at 1:52 a.m. Thursday in the mountains 30 miles northwest of Grants Pass near the Josephine-Curry county line, the U.S. Geological Survey reported. An earthquake with a magnitude of 4 can do moderate damage in built-up areas, but no damage or injuries were reported.

The quake, which officials described as light, happened about 24 miles below the Earth's surface, the Pacific Northwest Seismograph Network at the University of Washington in Seattle reported. It resulted from the Juan de Fuca plate, a vast slab of basalt on the ocean floor, sliding beneath the North American continental plate, then breaking deep underneath the Earth's surface, explained Ian Madin, chief scientist at Oregon's Department of Geology and Mineral Industries.

"It's the only one of this kind recorded within miles of that location since 1833," he said.

That's as far as human records go back and Madin acknowledges that throughout much of that period, human record-keepers were rare around that location.

More than 130 people reported feeling this quake on Pacific Northwest Seismograph Network's "Did you feel it?" Web page, a USGS system that collects anecdotal observations from the public.

The bulk of the reports — 82 by Thursday evening — came from Grants Pass, the population center closest to the quake. But the reports of strongest shaking came from the coast — Gold Beach, Agness and Coquille.

Although most of the reports came from Josephine and Curry counties, a few trickled in from Rogue River, Medford and even farther afield, including Bend, Corvallis and Eugene.

"When the source is that far down, it affects a fairly large area and the effect can feel much the same over the whole large area," said Bill Steele, a seismologist at the seismograph network's University of Washington office.

In the Puget Sound, deep earthquakes caused as the Juan de Fuca plate breaks up after sliding under the North American plate result in heavy damage, such as the 6.8 magnitude Nisqually quake in 2001, which caused $4 billion worth of damage.

"People had speculated whether that could happen under Oregon," Madin said. "There haven't been well-documented instances, but this could be one."

Scientists might argue about this quake for years, even though it had little effect on people, he said.

"Don't worry about this one," Madin said. "Think of the big Cascadia zone one."

A report prepared last year on the potential for deep earthquakes along the Cascadia subduction zone, a long fault in the ocean floor that separates the Juan de Fuca and North America plates and runs from British Columbia to Northern California, used Beaverton as the most likely location for a massive quake in Oregon.

"There's a higher probability there," said Steele, one of the participants in the Cascadia Region Earthquake Workgroup, which produced the report. "But all of Oregon is earthquake country."

The workgroup focused on 9-magnitude quakes, which would cause extensive damage across the region. Most experts believe such quakes occur every 300 to 500 years and the last to strike Oregon was in January 1700.

Jackson County Emergency Management plans to participate in a drill in April along with 21 other counties to help prepare for that sort of quake, said Mike Curry, the county's emergency management director.

While the county can try to plan for interruptions to transportation and communication systems, and the destruction of buildings, people need to plan to be self-sufficient for up to a week in their homes, he said.

"Since 9/11 and Katrina, we've seen the government can't come in immediately," Curry said.

The county's pamphlet on family preparedness is available at the county offices at 10 S. Oakdale Ave., Medford, or online at www.co.jackson.or.us. Search for "emergency management."

Reach reporter Anita Burke at 776-4485, or e-mail aburke@mailtribune.com.

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