## 2 Materials and Methods
### Materials
#### 2.1.2 Landslide-Related Predisposing Factors  
In this study, we selected 14 landslide-related predisposing factors as input variables of the PSO-MLP, MLP-only, BPNN, and IV models. These predisposing factors were topography factors (digital elevation modeling (DEM), slope, aspect, relief amplitude, plan curvature, and profile curvature); hydrological factors (distance to river, topographic wetness index (TWI), and modified normalization difference water index (MNDWI)); lithological factors (rock types); land cover factors (normalized difference building index (NDBI), normalized difference vegetation index (NDVI), total surface radiation, and population density index). The DEM was obtained from the Global Digital Elevation Model of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER GDEM), and the MNDWI, NDBI, and NDVI factors were calculated from Landsat 8 TM images (obtained on 5 October 2013, path/row 121/42, 30 m resolution). These 14 predisposing factors were  
Figure 1: Geographic location and landslide distribution of Shicheng County. selected according to the related literature and studies in areas with similar physical geography and geological environments [11; 43; 44]. In this study, the data format of DEM and original remote sensing images was grid cells with a spatial resolution of \(30\times 30\) m. Moreover, the raster format has the advantages of fast subdivision, simple expression, and high computational efficiency. Hence, the raster format with grid cells was applied to express all 14 predisposing factors in Shicheng County. The recorded landslides were subdivided into 2709 landslide grid cells. (1) Topography factors in Shicheng County  
DEM (Figure 2a), which indicates the altitude above sea level in the study area, was the data source of the five other topographic factors. These six topographic factors have important effects on the probability of landslide occurrence [45]. Slope (Figure 2b) has a direct influence on the landslide stability coefficient, and it is one of the basic elements of landslide evaluation [46]. Aspect (Figure 2c) influences landslide occurrence by affecting moisture movement, soil properties, and so forth [47]. Relief amplitude (Figure 2d) directly reflects terrain complexity through the variations of DEM in a certain area [48]. Plan curvature (Figure 2e) and profile curvature (Figure 2f), respectively defined as the slope of the aspect and the slope of the slope, can also effectively reflect the terrain complexity of the study area [49]. Terrain complexity affects landslide evolution by influencing soil erosion, slope structure, sediment transportation, and so forth. Figure 2: Topography factors in Shicheng County: (**a**) DEM, (**b**) Slope, (**c**) Aspect, (**d**) Relief amplitude, (**e**) Plan curvature, (**f**) Profile curvature. (2)  
Hydroological, lithological, and land cover factors  
Hydroological factors are very important factors of landslide evolution which influence the surface water distribution, water and soil saturation, water sources, runoff generation, and so forth [50]. The distance to river and TWI calculated from DEM and the MNDWI were used in this study as hydrological factors (Figure 3a-c). Distance to river reflects the water scour and soil erosion effects of surface runoff on landslide evolution [49], and TWI reflects the influence of terrain on soil saturation, rainfall infiltration, and runoff [50; 51]. MNDWI suggests the surface humidity features, which indirectly present the moisture content in the slope mass [13]. The lithological factor in Shicheng County was expressed using rock types in this study, which remarkably affect the slope soil types, slope structure, and soil shear strength [11; 43]. The rock types in Shicheng County are mainly defined as metamorphic, carbonate, and clastic rocks (Figure 3d). Land cover is also a key factor for LSP. The NDBI, NDVI, total surface radiation, and population density index were adopted in this study to express the land cover factors (Figure 3e-h).