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

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 70ms-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.