20091004 Study: Sichuan quake once in 4,000 years

clipped from www.chinadaily.com.cn 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. "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," 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.