## 3 Field Geological Investigations
### Evolution Characteristics of Retrogressive Landslides  
The landslides are primarily concentrated in three areas: from Maiodian Village to Zhaitou Village (Zone I), from Xiushidu to Fujia Village (Zone II), and from Yujiabao to Jiangliu to Dabaozi Village (Zone III). As shown in Figure 5a, in Zone I, there were a total of 13 landslides before 2003, one small subsequent slide occurred in 2003; from 2003 to 2011, there were four landslides, including one large-scale flowslide, which was independent of previous landslides; from 2011 to 2013, there were three small-scale landslides; from 2013 to 2017, landslides occurred frequently, totaling 12, with the northwestern part of the area experiencing multiple large-scale landslides, while the central region had several smaller slides, and the southeastern part saw small-scale slides and falls; from 2017 to 2022, there were five landslides, mainly small-scale subsequent slides based on the new flowslides since 2003, predominantly slides. Figure 5b shows that there were thirteen landslides before 2003 in Zone II, with one large-scale flowslide occurring in 2003; from 2003 to 2011, there were four landslides mainly concentrated in the central area; from 2011 to 2013, there were four landslides, all being subsequent slides at the sites of previous landslides; from 2013 to 2022, there were ten landslides, most of which were small-scale slides. In Zone III, there were 11 landslides before 2003, as shown in Figure 5c, with one large-scale  
Figure 4: Statistical parameters of landslide characters: (**a**) number of landslides per year; (**b**) number of landslides per month; (**c**) landslide accumulation area; and (**d**) maximum landslide length.  
flowslide occurring in 2003; from 2003 to 2011, there were three landslides, including two secondary slides of old landslides; from 2011 to 2013, there were five landslides, including three large-scale flowslides in the east, and two landslides distributed in the central area; from 2013 to 2017, there were four landslides, concentrated in the central area, overlapping with the landslide distribution of the previous period; from 2017 to 2022, there were seven landslides, with the central area experiencing another large-scale flowslide followed by smaller subsequent slides, and other areas of the zone also seeing some landslides but with lower frequency.  
In summary, the evolution characteristics of loess landslides in the study area generally show clustering and periodicity. Most of the landslides before 2003 were large in size, densely clustered, and developed continuously, and their deposits remained discernible on remote sensing images for a long time. Most of the new landslides after 2003 occurred on old landslides, mostly secondary or multiple slides, primarily slides and falls, and were small in scale. Some of the landslides were independent of those occurring before 2003 and were large in scale, primarily flowslides. These types of landslides were often experienced as secondary or multiple slides in the years following their occurrence. It can be concluded that loess landslides at the edges of the plateau in the study area generally start with large-scale flowslides, followed by multiple small-scale slides and falls to reach a stable state.  
Figure 5: Evolution of the retrogressive landslides group in: (**a**) Zone-I; (**b**) Zone-II; and (**c**) Zone-III.