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Previously unknown fault caused earthquake swarm off Oregon's coast
Oregon Coast Notes - News
USGS SubductionA series of seismic faults no one knew about was the source of an unusual swarm of undersea earthquakes off the Oregon coast last spring, geologists say.

The quakes puzzled experts because they appeared to be volcanic outbursts in the middle of the Juan de Fuca plate, the deep slab of the Earth's crust underlying the ocean off Oregon. Earthquakes caused by volcanic activity usually arise along the edges of tectonic plates.
The newly discovered faults may help scientists better understand the complex system of undersea volcanic ridges and colliding tectonic plates that unleash massive earthquakes every few hundreds years in the Pacific Northwest.

From northern California to British Columbia, the Juan de Fuca plate is plunging beneath the North American plate. This so-called Cascadia subduction zone produced the volcanic peaks of the Cascade Range and it poses the greatest known earthquake danger in the Pacific Northwest. The zone produced a magnitude 9 earthquake in the year 1700 and triggered tsunami waves large enough to reach Japan.

Marine geologist Robert Dziak of Oregon State University's Hatfield Marine Science Center said that the new findings should help scientists understand how all parts of the plate system work together to cause earthquakes...