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Series of quakes hit off Oregon coast

SALEM — Five earthquakes ranging in magnitude up to 5.9 have struck off the coast of the Pacific Northwest, but they weren't strong enough to generate tsunamis, scientists said.

The earthquakes were shallow, at 6 miles deep, often a signal of a dangerous quake.

It usually takes an earthquake of magnitude 7 or better to trigger a tsunami, said geophysicist Paul Caruso of the U.S. Geological Survey.

The quakes were clustered in an area about 300 miles west of Coos Bay, Oregon, along what's known as the Blanco Fracture Zone.

"It's a well-known place for earthquakes," said another agency geophysicist, Julie Dutton. "They're frequent throughout the year."

A 2008 agency report said the zone had produced about 70 of magnitude 5 or greater in the previous 28 years, as many as eight some years. In 2008, scientists detected a swarm of hundreds of smaller quakes.

In the Blanco faults, blocks of crust slide horizontally past one other, Dutton said. Faults that feature blocks rising and falling violently in relation to each other are the kind that can generate the energy for tsunamis, she said.

The first of the five earthquakes came a few minutes before midnight Sunday, at magnitude 5.8. The fifth, at magnitude 5.9, came shortly after 1 p.m. Monday. The weakest was magnitude 4.3.

Reports to the geology agency and law enforcement offices suggest that people along the coast barely felt the earthquakes.