## 2 Study Area  
Lushui is a subordinate Autonomous County of Yunnan Province in Southwest China, located between the longitudes of 98 deg34' E and 99 deg09' E and the latitudes of 25 deg33' N and 26 deg32' N, with a total area of approximately 3100 km2 (Figure 1). Topographically, the Nujiang River Grand Canyon runs through the whole area from north to south. The elevation ranges from 740 m to 4160 m, and increases from the middle of the canyon to both sides. Areas with the slope angle between 15 deg and 35 deg account for 59.7% of the study area, and areas with slopes over 40 deg cover 13.2%. The region has a tropical monsoon climate, with an annual average temperature of approximately 15.1 degC. The rainy season extends from May to October each year, and the rainfall during the rainy season accounts for 70% of the total annual rainfall. The largest daily rainfall can reach to 105.3 mm during the rainy season. The annual average rainfall is approximately 1200 mm. The surface runoff of the study area mainly consists of the Nujiang River system and its tributaries.  
Geologic units in the study area range from Precambrian to Quaternary and include metamorphic, igneous, and sedimentary rocks as well as unconsolidated sediments (Figure 2). The oldest geological units are represented by the Proterozoic Gaoligongshan (Ptgl) and Chongshan (Ptch) groups, which widely appear in the eastern part of the study area and along the area west of Nujiang Fault. Strata from the Paleozoic (Pz) to Mesozoic (Mz) outcrop in the area east of the Nujiang Fault. The youngest Quaternary sediments consisting of alluvial and pluvial deposits are widely distributed in the terrace of the Nujiang River, and are the major slope-forming materials. Knowledge of geology is critical because different rock types and sediments have different physical hardness,  
Figure 1: Geographical position of the study area and landslide inventory map.  
interlayer structures, and weathering and erosion resistance, and those properties are related to slope susceptibility. In particular, soft clay or other phyllosilicate minerals are highly plausible causes of landslide occurrence, while the rocks or sediments with strong mechanical strength and good weathering resistance usually have a better inhibiting effect on slope deformation. In this study, the geological unit was reclassified into seven engineering rock groups (ERGs) according to the lithology, structure, hardness, and weathering degree of the rocks. Table 1 gives a detailed description for the ERGs.  
Lushui County is one of the key monitoring zones for geo-hazards in Yunnan Province. A complex geographic and geological setting and seasonal heavy rain make the area prone to geo-hazards. For example, on 5 July 2018, falling rocks induced by a landslide nearby a road of Chenggan hit running cars and directly caused three deaths, with seven injured. On 3 October 1997, unwise mining activities at the Shiganghe disturbed the unstable strata located on a fracture zone, triggering slope sliding and causing 23 deaths and economic losses of US$2.5 million. Landslides have caused huge damage in this area, prompting the urgent need for effective regional landslide susceptibility mapping.  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**ERG** & **Code** & **Lithology** \\ \hline
1. Layered, massive hard & Ptgl\({}^{1}\), Ptgl\({}^{2}\), Ptch\({}^{1}\), Ptch\({}^{2}\), metamorphic rock & Mtch\({}^{3}\) & Migmatite, granulite, gneiss, and mylonite \\
2. Layered, flaky soft mudstone, shale, and sandstone & \(\text{O}_{3}\text{p},\text{S}_{1}\text{r},\text{P}_{1}\text{b}\) & Mudstone, shale, and sandstone \\ \hline
3. Layered karst medium hard & \(\epsilon_{3}\text{s},\text{S}_{2}\text{r},\text{D}_{2}\text{hy},\text{D}_{3} \text{d},\text{C}_{1}\text{x}\), carbonate rock & C\({}_{1}\text{p},\text{P}_{1}\text{d},\text{T}_{2}\text{h}^{1},\text{T}_{2} \text{h}^{2}\), \(\text{T}_{3}\text{nn}\) & Migmatite, granulite, gneiss, and mylonite \\  & \(\text{T}_{3}\text{nn}\) & intercalated with mica schist, quartztite \\  & & \\