## 2 Study Area and Geological Setting
### Geological Conditions  
In the study area, 803.1 mm of annual rainfall is concentrated in the months from May to September, accounting for 86.4% of the total rainfall, and the annual rainfall is characterized by an uneven spatial distribution, with more rainfall in mountainous areas than in river valleys, with heavy rain or showers. The river network consists of a tertiary river--the Dadu River and its tributary--the Nanya River, in which the former runs through the area from west to east and the latter runs from south to north and finally merges into the Dadu River at the county seat.  
The lithology in the district is relatively simple. The lithology can be divided into two main categories: magmatic rocks and loose accumulations (Figure 2a). The magmatic rocks are composed mainly of ordinary granite, rhyolite porphyry, diabase dikes, gabbro, granite porphyry, and ultrabasic rock, and ordinary granite \(\gamma\)22 is the main lithology of the stratum accounting for approximately 50% of the study area; the loose accumulation layer is divided into glacial fluvial deposits and alluvial accumulations and is distributed along both sides of the rivers.  
The geological structure is significantly affected by the geostructure and presents a deep-cut topography, with rivers on three sides at the intersection of the Xianshuihe fault, Longmenshan fault and Anninghe fault. In particular, the Shimian fault is a branch of the Anninghe fault that passes through the study area directly in a nearly north-south direction (Figure 2b), breaking the rock mass and leading to the development of adverse geological engineering phenomena [18; 19].  
The geomorphology is a canyon landform with high terrain in the northwest, east and southwest regions, and low terrain in the central part of the area and the river valley. The study area has an elevation of 850 m, and the highest point is located in Dapinggang, with  
Figure 1: Transportation map of Shimian County.  
an elevation of 2030 m. The lowest point is located most downstream of the Dadu River, with a relative elevation difference of 1180 m (Figure 2c). The amount of topographic relief is 487.6 m/km\({}^{2}\). The gradients are distributed from the river surface to steep slopes, with degrees of 0\({}^{\circ}\) to 82\({}^{\circ}\), due to the steep terrain and strong cutting of the river (Figure 2d). The average slope is 30.8\({}^{\circ}\), which reaches the grading standard of steep slopes and triggers frequent geological disasters in the area.  
In this work, lithology and fault data were from a 1:200,000 geological map. The slope angles of the study area were derived from a DEM with resolution 20 m. The study area is located at the intersection of three fault zones with strong structural movement and frequent earthquakes, and mass geological hazards have occurred because of strong geotectonic movement, the fault passing through the county, and rainstorms. The mountains and rocks are highly susceptible to landslides. Human engineering activities have intensified geological disasters, resulting in frequent seismic activity and high susceptibility to geological disasters [20].