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Study: Sichuan quake once in 4,000 years

(Agencies)
Updated: 2009-10-04 16:38

BEIJING: China's devastating earthquake last year that left some 90,000 people dead or missing was caused by a geological event that occurs about once every 4,000 years, a study led by Chinese researchers said Sunday.

Researchers said the 7.9-magnitude quake was caused by the breaking of solid rock separating major fault segments, allowing the quake to cascade along multiple faults. Such rock barriers stop the vast majority of quakes, limiting their intensity.

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The cascading rupture in the Longmen Shan fault zone that separates western Sichuan province from the Tibetan plateau could have been as long as 199 miles (320 kilometers), said the paper published in the journal Nature Geoscience, part of Nature magazine.

Such major cascade ruptures happen "about every 4,000 years," the study said.

The quake was China's worst in three decades. It left more than 5 million homeless, created $123 billion in direct economic losses.

"The authors find that the stresses that had built up in the Earth's crust in the Sichuan region were sufficiently large for the fault rupture to break through strong seismic barriers. This resulted in a much larger and more deadly earthquake," said Walter Mooney, the US Geological Survey's top expert on seismic conditions in China, who had no connection with the study.

Leading scientists in China have said there were no signals that could have predicted the May 12 quake on the Beichuan fault line in the Longmen zone, though the fault lines in that area of western China are well known.

The earthquake destroyed the towns of Yingxiu, Beichuan and Nanba, which the study found are near shallow fault intersections that experience particularly strong quake motion when ruptured.

The locations of the intersections and their shallow depth "means they are very much to blame for the very high destruction and human casualties at Yingxiu, Beichuan and Nanba," Zheng-Kang Shen of the China Earthquake Administration, the lead author of the study, told The Associated Press.

"You really have to accumulate enough elastic energy to have them rupture through, but once rupture starts, it would rupture a series of barriers to get a cascade style."

Mooney said the study helps explain why such large earthquakes are relatively rare, and said it's important to identify fault zones that could rupture to prepare communities that lie in their path.

The study involved researchers from Peking University, the University of California and the Sichuan Seismological Bureau.