Landslides are natural phenomena that occur when slope materials move downward or outward along the slope due to gravity or other external conditions such as climate, rainfall or earthquake, which play an important role in landscape evolution by transporting sediment downslope (Cruden and Varnes, 1996; Petley, 2012). Landslides can be divided into recent landslides and ancient landslides according to their formation time (McCalpin, 1984). Generally, ancient landslides refer to the landslides that occurred during river terrace erosion, or before the late Pleistocene. But in the actual field investigation, we frequently called the landslides as "ancient landslides" if they happened before the current stage and are essentially stable at the moment. In fact, the stability of ancient landslides is only temporary. Under specific external conditions, these ancient landslides will reactivate and form a new landslide with larger scale and more serious damage (Li and Ji, 2006; Zhang et al., 2018; Zhong et al., 2021). For example, since the 1980s and 1990s, several ancient landslides have been reactivated after the resettlement and impoundment of the Three Gorges Reservoir Area. The Qianjiangping ancient landslide occurred in July 13 2003 and the landslide volume was nearly 2.0 × 107 m3, which result in 14 deaths, 10 missing and huge economic losses (Wang et al., 2003). The potential threat of an ancient landslide not only causes heavy casualties and property losses, but it can also pose a serious threat to people's property and life, as well as the national economic development (Guo et al., 2020; Zhang et al., 2021a; Zhao et al., 2015).
In recent years, complex geological environment, combined with a large number of human engineering activities, have led to the abundant development of landslides along both sides of Jinsha River, which are characterized by large scale, complex failure mechanism, multi-phase sliding, and huge potential threat (Li et al., 2022; Li et al., 2020; Liu et al., 2020). The Jinsha River has a length of 3300 km, and its potential hydroelectric is as high as 1.12*108 kW, which is one of the twelve planned hydropower bases in China. Wudongde Hydropower Station is situated in the downstream area of Jinsha River (Luo et al., 2022). The reservoir area has strong active tectonic movement, and steep terrain. In addition, the rock fractures are developed, and the rock mass is broken and strongly weathered and the study area has abundant rainfall in rainy season. All above factors has led to abundant development of both new and ancient landslides in the reservoir area (Niu et al., 2015; Shi et al., 2020). Meanwhile, a significant number of ancient landslides have different degrees of sliding as a result of the construction of hydropower station (Li et al., 2022). For example, on 28 June 2012, a massive mudslide occurred in Aizigou area of Ningnan County, Sichuan Province, which is the construction area of Baihetan Hydropower Station. 41 people died and numerous houses and farms are damaged. In 2018, Baige landslide occurred on the Jinsha River's right bank in Baige Village, Jiangda County, which Blocking the Jinsha River and forming a barrier lake, several towns in the upper reaches were flooded. After the barrier lake burst, the downstream infrastructure and many bridges were destroyed, resulting in huge economic losses. (Ouyang et al., 2019; Zhang et al., 2019).The sudden landslide in the reservoir area of large hydropower station will cause damage to the safety of dam site and people's lives and property. Therefore, it is very necessary to study the landslide hazard assessment in this area.
The landslide hazard assessment is used to predict the potential hazard area where the landslide may occur by analyzing and summarizing the relationship between the landslide and the influencing factors (Ma et al., 2020; Shao et al., 2021). The accuracy of the assessment results mainly depends on the completeness of ancient landslide inventory. In previous studies, the focus of catastrophic landslide event in the Wudongde reservoir area mainly includes stability analysis, failure mechanism and deposited characteristics of the single landslide(Xu, 2013; Ya, 2020; Zhao et al., 2018). (Xu, 2013) established the inventory of debris flow in Wudongde near-dam area based on spot5 high-resolution remote sensing image, and these debris flows data provides an important basis for the follow-up spatial distribution, hazard assessment and geomorphic evolution. (Ya, 2020) Identification of 10 Potential landslide resurrection area of ancient landslides in Wudongde hydropower station based on INSAR images. (Wang et al., 2022) established the landslide inventory of Batang-Dege section in the upper reaches of Jinsha river combined with field investigation and remote sensing interpretation and conducted relevant landslide hazard assessment. However, at present, there are few studies on establishment of landslide inventory, distribution pattern and hazard assessment of ancient landslides in the Wudongde hydropower station area.
Thus, this paper aims at establishing a comprehensive ancient landslide inventory associated with this area. Particularly, we analyze the spatial distribution, size and their relationship with geologic, topographic and seismic factors. Based on the landslide influence factors, the potential landslide hazard assessment result in this area are calculated. This study provides basic data and important reference for the distribution characteristics and potential hazard assessment of ancient landslides in the reservoir area of Wudongde hydropower station.