Unusual Earthquake Swarm Off Oregon Coast Puzzles Scientists [View All]

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http://www.sciencedaily.com/releases/2008/04/080413184801.htm

Unusual Earthquake Swarm Off Oregon Coast Puzzles Scientists

ScienceDaily (Apr. 14, 2008) — Scientists at Oregon State University’s Hatfield Marine Science Center have recorded more than 600 earthquakes in the last 10 days off the central Oregon coast in an area not typically known for a high degree of seismic activity.

This earthquake “swarm” is unique, according to OSU marine geologist Robert Dziak, because it is occurring within the middle of the Juan de Fuca plate – away from the major, regional tectonic boundaries.

“In the 17 years we’ve been monitoring the ocean through hydrophone recordings, we’ve never seen a swarm of earthquakes in an area such as this,” Dziak said. “We’re not certain what it means. But we hope to have a ship divert to the site and take some water samples that may help us learn more.” The water samples may indicate whether the process causing the earthquakes is tectonic or hydrothermal, he added.

At least three of the earthquakes have been of a magnitude of 5.0 or higher, Dziak said, which also is unusual. On Monday (April 7), the largest event took place, which was a 5.4 quake. Seismic activity has continued through the week and a 5.0 tremor hit on Thursday. Numerous small quakes have continued in between the periodic larger events.

Few, if any, of these earthquakes would be felt on shore, Dziak said, because they originate offshore and deep within the ocean.

The earthquakes are located about 150 nautical miles southwest of Newport, Ore., in a basin between two subsurface “faulted” geologic features rising out of the deep abyssal sediments. The hill closest to the swarm location appears to be on a curved structure edging out in a northwestern direction from the Blanco Transform Fault toward the Juan de Fuca ridge, Dziak said.
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