RESEARCHES ON THE GEO-HAZARDS TRIGGERED BY WENCHUAN EARTHQUAKE,SICHUAN

Wenchuan Earthquake triggered landslides, rock collapses and debris flows at more than 15000 sites, which directly caused about 20,000 fatalities.There are more than 10,000 sites of potential geo-hazards, where an increase in slope collapses is mostly noticeable, which shows the earthquake has substantially abnormal actions on steep slopes in the mountains. In particular, the amplifications at mountain peaks is extremely evident. A comprehensive analysis of quake lake volumes, landslide dam heights and their debris compositions and structures was carried out. An assessment on the 33 landslide dames of dam heights great than 10 m formed by the earthquake was further conducted. Consequently, the landslide dams were classified into four types of the dam burst potential : extremely high, high, intermediate and low. The sliding beds of the landslides induced by the earthquake usually have non-continuous flat sliding beds. Sharp point collision is a common and major characteristic. The sliding beds can be classified into spoon-shaped, raised-shaped and staircase-shaped. Site investigations demonstrate that the vertical vibrations in buildings destroyed by the earthquake adjacent to the landslides were evident. The landslides have three stages: throwing out of ground shocking, collision and collapsing and broken, and high-speed sliding flow. During the high-speed sliding flow, three effects were occurring: (1) high-speed air-bed effect, landslide debris were comprised with larger rock blocks and soils, had enough thickness and the flying distance of 1 to 3 km. (2) Broken debris flow effect, collision broken soils and rocks were in a flowing state, especially they contained abundant water, forming long distance flow sliding. (3) Shovel and scraping, huge collision forces induced collapse of rocks at lower portion, which formed new landslides and collapses, however, their thicknesses are not large and the sliding beds were non-smooth and irregular. The two landslides at western Beichuan county and Dongheko of Qingchuan county were analyzed in detail. They are used as examples to analyze the mechanism of the high speed and long distance sliding and hazard formation of earthquake landslides. The landslide at western Beichuan county buried 1,600 people and destroyed several hundreds houses and buildings, which is the severest landslide disaster triggered by the earthquake and is a rear event in the world. The Donghekou landslide at Qingchuan: Debris flow is a typical high speed and long distance landslide triggered by the earthquake. Its distrace is about 2,400 m. Its high-speed debris flew to the left bank of Qingjiang river and formed landslide dam, resulted 7 villages buried and about 400 fatalities.