Thu, Dec 30, 2004 page5

• Quake caused Earth to wobble on axis: scientists

  • AFP
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  The gigantic Asian sea quake, the most dramatic seismic shock in more than 40 years, made the Earth wobble on its axis and permanently changed the geology of the surrounding area, scientists said on Tuesday.

  It was like "flicking a top," said Paul Tapponnier, head of the tectonics laboratory at the Institut de Physique du Globe (IPG), France's leading center for the Earth sciences.

  Tapponnier said the quake deep beneath the Pacific Ocean lasted a "colossal" 200 seconds, building up huge amounts of energy in the sea that drove towering waves onto beaches throughout south Asia.

  "That earthquake has changed the map," US Geological Survey (USGS) expert Ken Hudnut said in Los Angeles.

  The quake, which had an epicenter magnitude of 9.0, struck 250km southeast of Sumatra island.

  One of the four biggest in the last century, it sent gigantic tsunami waves crashing around the Indian Ocean causing 55,000 deaths and leaving thousands of other people missing.

  Hudnut said seismic modelling suggested the quake may have moved small islands by as as much as 20m, and the northwestern tip of the Indonesian territory of Sumatra may also have shifted to the southwest by around 36m.

  "That is a lot of slip," he said.

  The energy released as the two sides of the geological fault line under the sea slipped against each other would have made the Earth wobble on its axis, Hudnut said.

  Tapponnier said the quake caused a 15m to 20m slippage of the Earth's surface along a front extending for 100km.

  He said there may also have been vertical movements that possibly pushed the island of Siberut, 100km west of Sumatra, one or two meters higher, although it would be impossible to check this scientifically because of guerrilla activity in the area.

  Tapponnier said it was also possible that some regions of Sumatra south of the equator have been completely swallowed up.

  He said it was not rare for earthquakes to alter geological features. "Earthquakes are the architects of landscapes," he said. "All the mountains that we see today have been modelled by earthquakes."

  Tapponnier said the massive 1960 earthquake off the coast of Chile shifted the local landscape by 20m. A quake in Alaska in 1964 pushed islands higher and sank oyster beds 12m under the surface.

  And a 6.3 magnitude temblor off the coast of Guadeloupe last month moved the ocean floor several tens of centimeters, he added.

  In Asia, "we are dealing with a quake one thousand times more powerful" than the one off Guadaloupe, a shock powerful enough to make the Earth wobble on its axis, Tapponnier said.

  Hudnut agreed the Earth had slightly wobbled "due to the massive amount of energy exerted and the sudden shift in mass."

  But minor oscillations as the Earth spins like a top are known to astronomers. The principal causes of the slight irregular motion known as nutation are the Sun and Moon as they continually change location relative to one another.

  Another USGS research geophysicist agreed that the Earth would have received a "little jog" from the quake and that the islands off Sumatra would have been shifted.

  However, Stuart Sipkin, of the USGS National Earthquake Information Center in Golden Colorado, said it was more likely that the islands had risen higher out of the sea than they had moved laterally.

  "In this case, the Indian plate dived below the Burma plate, causing uplift, so most of the motion to the islands would have been vertical, not horizontal," he said.
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