Precursors and Motion Characteristics of the 1965 Lannigou Rockslides and the Subsequent Evolution

The Lannigou rockslide is one of the most well-known giant and rapid rockslides with long runout in China. The 1965 Lannigou rockslides caused the most catastrophic consequence in China since 1920’s, and were followed by two more rockslides in 1991 and 2007, respectively. However, it remains unclear that how precursors of the 1965 rockslides were and relationship between the two movement sequences was, so did the following activities in 1991 and 2007. Those unclear questions were explored in this paper based on 23 remote sensing satellite images taken during the period between 1965 and 2020 and the UAV image taken in 2019 as well as field survey then. It is found that: 1) tension cracks could be seen on the northern slope of the source area before the 1965 rockslides, evidencing the precursors there, 2) the 1965 rockslide first occurred on the northern slope, and subsequently induced the rockslide on the southern slope next day resulted from its strong impact, 3) rapid granular flows of the two rockslides jumped and flew over the northern slope of the transition zone, and scrapped, entrained surficial layer there, 4) the granular flow did not reach the Jingsha River, but ended ca. 2.8 km east of the intersection between the Lannigou and the Pufu river; 5) the source area of the 1991 rockslide just neighbored the northwestern boundary of the 1965 rockslide on the northern slope, while the 2007 rockslide neighbored the upper boundary of the 1991 rockslide, and 6) the detached volumes and runouts of the 1965, 1991 and 2007 rockslides showed a decrease order. Recently, tension cracks can be clearly seen on both the northwestern and southwestern slopes. One of tension cracks on the northwestern slope is extending with a rate of 16.7 m/a, while those on the southwestern slope has shown no sign of extension since 2014, indicating that the northwestern slope may fail again. It is thus that an attention should be paid to the northwestern slope via monitoring its deformation and keeping local people away from the area likely affected.