## 2 Study Area  
The study area (102.20\({}^{\circ}\) E-102.51\({}^{\circ}\) E, 30.87\({}^{\circ}\) N-31.12\({}^{\circ}\) N) is located in Xiaojin County, Sichuan province, Southwest China (Figure 1). The terrain is extremely complicated, with a high mountain and canyon landform that formed under the influence of strong Himalayan tectonic movements and surficial erosion. The canyons are mainly distributed along rivers, with an altitude below 3500 m. In contrast, the altitude of the high mountains ranges from 3500 m to about 5000 m. Forests in Xiaojin county are well developed, with a total forest area accounting for 58.8% of the county's total area. Regarding the weather conditions in the study area, the area is located in the plateau monsoon climate zone according to the geomorphological division of China [51; 52; 53]. According to the monthly average rainfall during 1981-2010 (Figure 2), the rainfall in this region mainly occurs during May-October, accounting for 84.2% of the annual precipitation, and the maximum average rainfall occurs in June (126.2 mm), accounting for 20.0% of the annual precipitation. The maximum hourly rainfall (28.4 mm) occurred on 8 June 1984. Under the abovementioned topographical, weathering and rainfall conditions, the concentrated rainfall in summer can saturate the soil on the slope and trigger frequent landslides. Figure 1: Location (**a**) and topography (**b**) of the study area. The background colors in (**a**) show the seismic intensity of the Wenchuan earthquake (Ms = 8.0) in 2008. The topographic data in (**b**) are from the Earth Science Data Systems (ESDS) Program of NASA. Figure 2: Monthly average precipitation in the study area in 1981–2010 ([[http://data.cma.cn/](http://data.cma.cn/)]([http://data.cma.cn/](http://data.cma.cn/)) (accessed on 12 October 2023)). The geological map (Figure 3) shows that the geological setting in the study area is complicated due to the historical tectonic movements. During the late Indonesian Movement, the Proterozoic and Mesozoic strata underwent strong folding and metamorphism, forming a series of northwest-trending folds and metamorphic rocks. The regional faults are barely developed in the study area, while some small-scale faults trending northwest and northeast are sporadically distributed in the study area. There are two major groups of joints in the study area with average dip directions of 143\({}^{\circ}\) and 293\({}^{\circ}\) and dip angles of 76\({}^{\circ}\) and 82\({}^{\circ}\), respectively. Rock outcrops in the study area are mainly Quaternary deposits (Q), the Zhuwo Formation of the upper Triassic system (\(T_{3}zh\)), and the Zagunao Formation of the middle Triassic system (\(T_{3}z\)) (Figure 3). They are characterized as follows:  
* The Quaternary deposits mainly consist of moraine debris, alluvium, proluvium, and eluvium. They are widely distributed in river valleys and high mountainous areas, reaching tens of meters in thickness and forming nearly vertical slopes on roads and residential areas. * \(T_{3}zh\) consists mainly of fine-grained metamorphic sandstone, metamorphic silt, carbonaceous sericite slate, and silty slate. Its thickness ranges from 834 m to 1949 m regionally. * \(T_{3}z\) is foliated metamorphic calcareous sandstone and feldspar quartz sandstone interbedded with dark grey slate. According to the field investigation, the soil and rocks of these strata are usually vulnerable. Geotechnical tests show that the internal friction angle of the soils in some landslides was as low as 20.4\({}^{\circ}\) under unsaturated conditions and 17.8\({}^{\circ}\) under saturated conditions. The historical earthquake catalogue shows that, although the study area had not been struck by strong earthquakes directly, it is located in the impact range of several strong events (Table 1). For example, the study area was affected by the Wenchuan earthquake in 2008 (Ms 8.0 according to the Chinese Earthquake Administration) and the seismic intensity was approximately VII (Figure 1a).