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12 Apr 2008 11:25
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 recognised 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 240km south-west of Newport. The biggest was magnitude 5,4, and two others were more than magnitude five, 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.
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.—Sapa-AP
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