## 5 Discussion
### New Understanding of Landslide Development
Moreover, the InSAR  
Figure 13: Histograms of landslides in the study area with respect to controlling factors of slope failure: (**a**) altitude, (**b**) slope, (**c**) relief amplitude, (**d**) distance to rivers, (**e**) distance to roads, and (**f**) curvature of slope. The red and blue bars represent the data sources of the InSAR analysis and field investigation, respectively. analysis showed the occurrence of some landslides at a distance of greater than 1500 m from rivers. This indicates the possibility of other controlling factors in addition to river erosion in the study area. The detailed cause analysis of these landslides is beyond the scope of this study. Figure 13e shows that the landslide sites are mainly located within a distance of 100 m from roads, indicating that road construction may be related to landslide development. When road constructions take place in mountainous areas, the slope cut generates free surfaces that essentially lead to the increase in the sliding force and the decrease in the anti-sliding force and cause potential landslides [10; 75]. Furthermore, the InSAR analysis revealed 25 landslide sites in an area located more than 250 m away from the roads. This value is five times more than the counterpart in the existing landslide inventory and also implies that InSAR can detect potential landslides located in inaccessible areas that are difficult to identify through field surveys. Slope curvature, as a controlling factor of landslide development, describes the shape of a slope. Positive and negative curvature values indicate upwardly convex and concave slope surfaces, respectively, while a value of 0 indicates a flat slope surface. Considering the error in measuring the topography, slopes with curvatures in the range of [\(-\)1, 1] were considered as flat slopes in this study. The curvatures of the slopes from both data sets were analyzed, and the histograms are shown in Figure 13f. Both data sets indicated that most of the landslide sites developed on relatively flat slopes. Nevertheless, the InSAR analysis showed more landslides in the curvature range of [\(-\)1, 1]. This can be explained by the high-altitude difference of slopes in alpine regions, providing favorable conditions for material migration. With long-term weathering and erosion, the slope surface tends to be flattened, resulting in a small absolute value of curvature. The abovementioned results suggest that the integration of InSAR with the analysis of landslide development significantly promotes deeper understanding of landslide causation factors such as altitude, distance to roads, and distance to rivers. In addition, the InSAR technique dramatically enhanced the dominant ranges of the factors of slope angle, relief amplitude, and curvature that determine the landslide development. The development of landslides is also controlled by the geographical relationships between the slope aspect and the stratigraphic occurrence, major joints as well as wedges formed by the intersections of joint planes and strata bedding planes [76]. From the geological viewpoint, if the dip directions of the strata bedding planes and wedges are similar to the slope aspect, the area is prone to landslides because the strata bedding planes and joints could act as sliding surfaces due to their low shear strength. In this study, to show the influence of the InSAR analysis on the understanding of landslide development from a geological viewpoint, the stratigraphic occurrence, wedges, and slope aspect of landslide sites were statistically analyzed. As shown in the rose diagram in Figure 14a, the strata in this region mainly dip to the northeast and southwest because of the development of a series of NW-trending folds (Figure 3), and the wedges, which were determined by the kinematic analysis [76], predominantly dip to the west-northwest and east-southeast. The dip direction of strata and wedge indicate that slopes dipping towards these directions favor landslide development.