Unusual Earthquakes Measured Off Oregon

Apr 12, 5:06 PM (ET)

By JEFF BARNARD

GRANTS PASS, Ore. (AP) - Scientists listening to underwater
microphones have detected an unusual swarm of earthquakes off central
Oregon, something that often happens before a volcanic eruption -
except there are no volcanoes in the area.

Scientists don't know exactly what the earthquakes mean, but they
could be the result of molten rock rumbling away from the recognized
earthquake faults off Oregon, said Robert Dziak, a geophysicist for
the National Oceanic and Atmospheric Administration and Oregon State
University.

There have been more than 600 quakes over the past 10 days in a basin
150 miles southwest of Newport. The biggest was magnitude 5.4, and two
others were more than magnitude 5.0, OSU reported.

On the hydrophones, the quakes sound like low thunder and are unlike
anything scientists have heard in 17 years of listening, Dziak said.
Some of the quakes have also been detected by earthquake instruments
on land.

The hydrophones are left over from a network the Navy used to listen
for submarines during the Cold War. They routinely detect passing
ships, earthquakes on the ocean bottom and whales calling to one
another.

Scientists hope to send out an OSU research ship to take water
samples, looking for evidence that sediment has been stirred up and
chemicals that would indicate magma is moving up through the Juan de
Fuca Plate, Dziak said.

The quakes have not followed the typical pattern of a major shock
followed by a series of diminishing aftershocks, and few have been
strong enough to be felt on shore.

The Earth's crust is made up of plates that rest on molten rock, which
are rubbing together. When the molten rock, or magma, erupts through
the crust, it creates volcanoes.

That can happen in the middle of a plate. When the plates lurch
against each other, they create earthquakes along the edges.

In this case, the Juan de Fuca Plate is a small piece of crust being
crushed between the Pacific Plate and North America, Dziak said.

http://apnews.myway.com/article/20080412/D900I9U80.html
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