RAPID AND LONG RUN-OUT FEATURES OF LANDSLIDES TRIGGERED BY THE WENCHUAN EARTHQUAKE

The extremely rapid and long run-out landslides triggered by the Wenchuan Earthquake were occurred along the Yingxiu-Gaochuan-Beichuan-Nanba-Qingchuan of the main seismic fault.The estimated maximum sliding velocity is greater than 70m·s-1 under the over 1.5g earthquake acceleration,compared with conventional rainfall or liquefaction induced landslides.The run-out distance of landslide-debris is several or several ten times of the rockmass length from crown to exit due to the air layer lubrication in where lots of landslide dams occurred.The maximum run-out distance is 3200m.The three cases are studied from the Niuquangou rockslide-debris flow at the south west segment of the main seismic fault(Yingxiu,Wenchuan),the Chengxi rockslide at the middle segment of the main seismic fault(Beichuan) and the Donghekou rockslide-debris flow at the north east segment of the main seismic fault.The three cases illustrates following common characteristics:(1)parent rockmass is broken under long geologic processing and weathered;(2) Rockmass was separately expanded and thrown under long-period strong ground vibration in which the vertical motion was predominant along the main seismic fault;(3)sliding mass was crashed at the exiting zone and transferred to fragment material;(4) The secondary landslide was triggered underneath the exiting zone due to the powerful crashing and shaving processes that provides more mass to debris body and cushion shape;(5) The a long run-out distance fragment flow is occurred under air layer lubrication.The air layer was pressed and turbulently flowed in the narrow-shaped valley,or laminarly flowed in the extensive flatland.