## 2 Materials and Methods
### Landslide Inventory and Landslide Characteristics  
Landslide inventory is the basis for all hazard analyses. The original landslide database was realized by the China Geological Survey, through a series of field investigations and historical landslide data, and later updated by remote sensing, using high-resolution images of Pleiades-1 (22/09/2014) and GF-2(06/12/2015). As landslides within the range of the reservoir area are the main objective of this study, landslides situated far from the riverbank were intentionally excluded from the inventory. The final inventory counted 310 landslides delineated as polygons, including 58 landslides under the control of engineering remedial works and about 30 landslides of interest in ongoing monitoring programs. All the selected landslides can be defined as rotational/translational slides or as complex landslides, with sizes ranging from 1360 m\({}^{2}\) to \(1.28\times 10^{6}\) m\({}^{2}\). The information about the landslide boundaries and engineering projects was collected from the third phase of the Three Gorges engineering project geological disasters prevention and control plan, and the related engineering geological survey and design data was based on the work of the Institute of geological environment monitoring, China Geological Survey.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Stratum** & **Stratigraphic Unit** & **Group** & **Main Lithology** \\ \hline J\({}_{12}\) & Lower Jurassic strata & Zhengzhuchong & Gray quartz sandstone with \\  & & & interbedded shale \\ J\({}_{121}\) & Lower Jurassic strata & Ziliujin & Shale sandwiching quartz sandstone \\  & & & and limestone \\ J\({}_{28}\) & Middle Jurassic strata & Shaximiao & Alternative layers of purple red mudstone \\  & & & and feldspar sandstone or siltstone \\ J\({}_{2x}\) & Middle Jurassic strata & Xintiangou & Gray yellow feldspar sandstone with \\ J\({}_{285}\) & Middle Jurassic strata & Xiahasximiao & Interbedded mudstone \\ J\({}_{3P}\) & Upper Jurassic strata & Penglai & Gray white quartz sandstone with \\  & & & interbedded shale \\ J\({}_{3 sn}\) & Upper Jurassic strata & Suining & Red purple quartz sandstone with \\  & & & interbedded mudstone \\ T\({}_{1j}\) & Lower Triassic strata & Jialingjiang & Limestone and dolomite, karst breccia, and \\ T\({}_{2b}\) & Middle Triassic strata & Badong & Limestone and sandy mudstone \\ T\({}_{3 nj}\) & Upper Triassic strata & Xujiahe & Light gray lithtic sandstone and silty shale \\ \hline \end{tabular}
\end{table}
Table 1: Lithology and stratigraphic unit in the study area.  
Figure 2: Tectonics and geology of the study area. (see Table 1 for explanations of geological units).  
Statistical analysis of the inventory data demonstrated a strong correlation with the geological setting, with landslides basically occurring in strata with dip angles below 15\({}^{\circ}\)[26; 27]. In the Sichuan basin, China, such landslides with near horizontal sliding surfaces, represented by landslides in the Wanzhou district, mainly occurred in the Mesozoic sandstone and mudstone strata. Through the analysis of the weak interlayers and age tests in landslide soil masses, the weak layers rich in hydro-expansible montmorillonite were considered as one of the strongest controlling factors of ancient near horizontal slip surface landslides in the Wanzhou district [28].