GEOLOGICAL FEATURES AND CAUSES OF THE WENCHUAN EARTHQUAKE TRIGGERED LARGE LANDSLIDES ON RIGHT BANK OF HUANGDONGZI GULLY

Wenchuan earthquake ruptured the Yinxiu-Beichuan fault in Longmenshan thrust zone. More than 100 large-scale landslides were triggered during earthquake along the 300 km fault rupture. Three landslides with volume larger than 1000×10⁴ m³ were developed within 5 km at the right bank of Huangdongzi River in Mianyan City. There are Daguangbao landslide, Hongshigou landslide and Laoyingyan landslide. The Daguangbao landslide was the largest landslide during the earthquake. Geological investigations were conducted in landslide areas and the results showed that the study area was located not only between imbricated thrust belts but also on an anticline. The Hongshigou landslide was close to its axis, while the others were on each wing. Several joint sets and weak interlayers were developed in slopes. The three landslides were triggered with special geology. A bedding fault with breccia rock formed landslide potential boundary. The shear resistance reduced rapidly during earthquake which triggered the landslide initiation. The Hongshigou landslide was located on a cataclastic slope with many breccia shear bands. The landslide was triggered resulting from the shear bands failure which formed the basal shear surface rapidly during the earthquake. A weak interlayer of almost 100 m developed in Laoyingyan slope. The weak layer tensioned rapidly due to strong horizontal seismic shaking which triggered the slope collapse.