05/22/2007

Associated Press

If you feel the earth move beneath your feet it may — or may not — be true love. It could be something much deeper.

Geologists say Oregon and Washington state get about 1,000 earthquakes a year of magnitude 1.0 or greater, most from several miles underground. But only a handful are powerful enough to be felt by humans, and even fewer do damage.

The Pacific Northwest has had about 25 damaging quakes since 1872, In the 20th century quakes in the region claimed about 17 lives.

The largest ones in modern times were the so-called Spring Break Quake at Scotts Mills and two quakes near Klamath Falls, all in 1993.

The Spring Break Quake, with a magnitude of 5.6, badly damaged Molalla Union High School, a church in Mount Angel and the dome of the State Capitol. It did damage estimated at $30 million.

The strongest quakes in Oregon in modern times occurred about 15 miles west of Klamath Falls on Sept. 20, 1993. They registered 5.9 and 6. Two people died.

The Oregon Department of Geology and Mineral Industries estimated 1,000 buildings in the Klamath Basin sustained damage totaling about $10 million.

They were centered on the West Klamath Lake fault zone, about 15 miles west of Klamath Falls. Smaller aftershocks continued for months.

The U.S. Geological Survey says that zone has faults up to 10 miles long that could cause a magnitude 7 quake.

There are occasional quakes from volcanic activity, mostly from Mount Hood and Mount St. Helens, and while humans can detect them they tend to do little more than rattle dishes.

Not all earthquakes that affect Oregon take place here. Quakes in Washington state are often felt in Oregon, as was the San Francisco quake of 1906, computed later at 7.6 and felt at least as far north as the Coos Bay area.

Oregon suffered tsunami damage from the monster 9.2 quake in Alaska in March 1964. An earthquake in Del Norte County, Calif., in 1873 caused chimney damage as far north as Port Orford, Ore., and was felt in Portland.

Chimneys cracked in the Portland area from an Oct. 12, 1877, earthquake, probably a magnitude 7, centered in the Cascades, and there have been others.

Most people can't detect quakes with magnitudes less than 2.5, although some animals can — and can even feel them coming.

An increase on the scale of one whole number, say from 4.1 to 5.1, means a quake's magnitude is 10 times as great and in theory there is no upper limit.