Scientist: Deep Quake in Southwest Oregon Was a Rare One

March 2, 2009

A light earthquake deep under the mountains of southwestern Oregon may have left many slumbering at 2 a.m. on Feb. 26, but a geologist said it was an eye-opener.

The quake was about 24 miles deep and arose from a plate of basalt under the Pacific Ocean sliding beneath the North American continent.

For years, scientists doubted that such deep quakes happened under Oregon, said Ian Madin, chief scientist of the Oregon Department of Geology and Mineral Industries.

“Now it turns out that they do,” he said.

The quake was light, at magnitude 4.1. But Madin said it’s a sign that one of magnitude 6 or 7 could arise from that slab, known as the Juan de Fuca plate.

The plate is the source of many earthquakes of greater magnitude in the Puget Sound area, he said, but it is under greater stress there.

Madin said there are three sources of quakes in the region:

-Faults within the North American continental plate, the kind of faults that geologists can readily detect.

-Deep faults within the underlying Juan de Fuca plate, as happened on Feb. 26.

-The great fault between the two plates — which scientists say generates the dreaded Cascadia zone quakes that recur in cycles of 300 years to 500 years, and promise widespread destruction in Oregon.

On the Pacific Northwest Seismograph Network’s Web site, more than 130 people reported feeling the quake beneath the Klamath Mountains.

The Medford Mail Tribune reported that the bulk of the reports came from Grants Pass, the population center closest to the quake.

But reports also came from along the coast –Gold Beach, Agness and Coquille — and from towns farther afield: Rogue River, Medford, 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.

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