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China quake was very unusual: US scientists
Washington (AFP) July 1, 2008 The devastating earthquake in China was the unexpected result of a seismological oddity and is likely to occur in the area only about once in every 2,000 to 10,000 years, US geoscientists said Monday. A team from the Massachusetts Institute of Technology (MIT) had studied the region around Sichuan province, where the 7.9 magnitude quake hit on May 12, for more than two decades but found nothing to warn them of a major quake. In a study published in the July issue of GSA Today, they blamed the tremor, which left 88,000 people dead or missing, on faults -- splits in the rock in the Earth's crust -- with very little seismic activity. As part of its long-term research, the team had operated 25 broadband seismograph stations in western Sichuan for more than a year before the quake. "Nobody was thinking there would be a major seismological event," said Leigh Royden, professor of geology and geophysics at MIT and a member of the team. "This earthquake was quite unusual" and may have involved a simultaneous rupture of two separate but contiguous faults, she said. However, the scientists warned that because earthquakes can sometimes occur in clusters, there was a possibility of another large-scale tremor in the area. Royden said the region is "extremely unusual geologically" because of the steep slopes at the boundary between the Sichuan Basin and the Tibetan plateau. The elevation rises by about 3,500 meters (more than two miles) over only about 50 kilometers (about 30 miles). The area where the quake occurred is part of the boundary between two of the Earth's tectonic plates, the Indian and Asian plates, whose constant collision has created the Himalayan mountains and Tibetan plateau. In central and eastern Tibet, however, much of the movement of the Earth's crust takes place hidden from view, thickening a weak crustal layer more than 15 kilometers (about nine miles) below the surface. The crust in this layer is flowing eastward away from central Tibet, but when it comes up against the Sichuan Basin it is forced to go around it, Royden said, causing the extreme landscape there. The difference in elevations provides the underlying stress that led to the quake, she said. Community Email This Article Comment On This Article Share This Article With Planet Earth
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