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02-02-2009, 07:09 PM
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Swarm of earthquakes detected off Oregon
Undersea quake swarm puzzles the experts
Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.
According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.
The boundary between the Pacific and Juan de Fuca Plates is marked by a broad submarine mountain chain about 500 kilometers long, known as the Juan de Fuca Ridge. Young volcanoes, lava flows, and hot springs were discovered in a broad valley less than 8 kilometers wide along the crest of the ridge in the 1970's. The ocean floor is spreading apart and forming new ocean crust along this valley or "rift" as hot magma from the Earth's interior is injected into the ridge and erupted at its top.
In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust if forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes above the subduction zone.
Located in the middle of the Pacific Plate, the volcanoes of the Hawaiian Island chain are among the largest on Earth. The volcanoes stretch 2,500 kilometers across the north Pacific Ocean and become progressively older to the northwest. Formed initially above a relatively stationary "hot spot" in the Earth's interior, each volcano was rafted away from the hot spot as the Pacific Plate moves northwestward at about 9 centimeters per year. The island of Hawaii consists of the youngest volcanoes in the chain and is currently located over the hot spot.
Oregon State U. / Hatfield Marine Science Center
The earthquake swarm (with red, yellow, brown, purple dots representing different days) is located in a basin in the middle of the Juan de Fuca tectonic plate. In this undersea chart, the plate's ridge is the angled line to the left of the swarm. The boundary of the North American plate and the Oregon coastline can be seen to the right.
WHAT CAUSES EARTHQUAKES ?
GRANTS PASS, Ore. — 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. the underwater microphones of the Sound Surveillance System (SOSUS) can track singing blue, fin, humpback and
02-02-2009, 07:29 PM
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CLOSE [X] 4.5+ earthquake possible in the San Francisco area. 4.0+ earthquake possible in far NW California. 4.0+ possible in the coastal areas from San Diego north to Los Angeles and then north ot Bakersfield... Please see video for details.
02-02-2009, 09:22 PMOn 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.
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