Tim Fought, Associated Press

SALEM, Ore. (AP) — Scientists say a cluster of earthquakes ranging in magnitude up to 5.9 have struck off the coast of the Pacific Northwest, but haven’t been strong enough to generate tsunamis.

The sixth and seventh quakes in the series struck Monday evening. They were the weakest so far, at magnitude 3.9 and 4.2.

The quakes have been shallow — about 6 miles deep. They’ve been clustered in an area about 300 miles west of Coos Bay, Oregon, along what’s known as the Blanco Fracture Zone. U.S. Geological Survey scientist Julie Dutton says that’s a well-known place for earthquakes.

The first quake came a few minutes before midnight Sunday, Pacific time, at magnitude 5.8.

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

“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. Also in 2008, scientists detected a swarm of hundreds of smaller quakes.

In the Blanco faults, blocks of crust slide horizontally past each 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, Pacific time, 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.

___

Associated Press writer Martha Bellisle in Seattle contributed to this report.

Copyright 2015 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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