An earthquake, measured 8.0 on the Richter scale, struck Wenchuan County, Sichuan Province at 14:28 on May 12, 2008. The epicenter was at Yingxiu, a town in Wenchuan, as shown in Figure 1. As indicated in Figure 1, the middle segment of the Longmen Mountain earthquake zone encloses the epicenter, Yingxiu, with the Longmen Mountain zone being part of the north-south earthquake zone in China. According to records, there have been 9 earthquakes larger than a grade of 8 occurring in the north-south earthquake zone from 1739, and 7 of them were after 1897, when the greatest one was at Haiyun in 1920 and at Chayu in 1950, both of them measured 8.5.