Study on Features of Secondary Disasters in the Worst-hit Areas of Wenchuan Earthquake

The Wenchuan earthquake on May 12, 2008 has caused heavy casualties and property losses. The violent earthquake and numerous intense aftershocks also induced secondary natural disasters in large area, which seriously threaten the regional eco-environment and human settlement environment. Based on the field investigation of some disaster area and analysis on RS data collected after the earthquake, the authors present the following opinions on features of secondary disaster caused by the Wenchuan earthquake. The Wenchuan earthquake occurred at the Longmen Mountain fault zone, which starts from east side of the Jiajin Mountain in Tianquan, Ya'an, and extends northeastward connecting with the Daba Mountain fault zone, approximately 500 km long and 70 km wide. This fault zone is composed of the front mountain the fault, central fracture and the rear mountain fault. Primary disaster area is mostly in the central fault zone. Large geomorphological unit of the disaster area is located at the transitional zone between the upmost ladder and middle ladder of landform in China. Primary disaster area is in the high mountain and middle mountain areas at edge of the Sichuan Basin, where valleys are deeply incised by streams and the mountains are very high with very steep slopes, greater than 25 degrees at most sites. The worst-hit area is of subtropical mountainous humid region under monsoon climate. Tendency of the mountain range is in a northeast-southwest orientation, so it is windward to southwestern monsoon, and has plenteous precipitation. Annual precipitation in Dujiangyan is 1178mm, and 1280mm in Beichuan, which are the center of opulent rainfall region in China. It provides conditions for incompact materials to form debris flow after the earthquake. The strong earthquake altered the landscape in disaster area violently. Under the action of seismic force and gravity, a great quantity of materials loosened in the earthquake slide downwards along steep slopes, thus secondary disasters such as collapse and landslides happen. Secondary disasters are concentrated along Longmen Mountain fault zone and two banks of valleys. For instance, 109 clusters of collapses, 98 clusters of landslides and 12 sites of debris flow develop along 138 km valley of Dujiangyan¨CMaoxian section in the mainstream of Minjiang River, totally 219 sites. Secondary disasters are mainly collapse, landslide and debris flow. The earthquake stimulated tens of thousands of these disasters in the disaster area. Lakes formed by blocked rivers by substance of landslide or collapsed in rivers. More than 30 of the larger ones were investigated.