**1. Introduction**

Landslides are among the most destructive geological disasters with features of rapid speed, long runout distance, and entrainment effect [1–3]. Catastrophic landslide events are often triggered by heavy rainfall, earthquake, and engineering activities [4–6]. According to the spatial characteristics and trajectory of the sliding body, the entire process of landslide movement is mainly divided into three stages: The starting stage at the slide source area, the propagating stage, and the deposition stage [7–10]. In addition, entrainment, base liquefaction, and air cushioning occur during the landslide movement [11–13]. To reduce the landslide hazard loss, risk assessment is often requested [14–16]. The landslide runout analysis is a very effective method to assess a landslide hazard [17]. Landslide runout analysis involves two aspects: The simulation of previous landslides and the prediction of potential landslides [18]. Runout analysis could be used to design remedial engineering measures, such as barricades and berms [17]. The maximum runout distance, propagation velocity, and the deposit thickness, and provision of the basis for the design of remedial engineering measures, are obtained by landslide runout analysis.

Landslide runout analysis methods mainly include empirical-statistical methods and numerical models [19,20]. Empirical methods establish the geometric relationship between the landslide volume, height difference, and angle of reach (i.e., the angle of the line between the highest point of the rear edge and the farthest point of the sliding distance) to predict the sliding distance. The empirical methods could not precisely predict the runout distance in the different complex geological environments, including entrainment, friction resistance, and impaction. Compared with the empirical method, the numerical models could give more information about the dynamic features of the sliding mass under different geological environments, such as the scraping depth, the thickness of the accumulation area, velocity, and the scope of dangerous area. The numerical simulation methods include the discrete element methods and the continuum methods. The discrete element method is based on Newton's second law and is used to analyze the interaction of particles constituting a landslide. It is suitable for landslides with debris flow patterns, such as the MatDEM (i.e., Fast GPU Matrix computing of Discrete Element Method, Nanjing University) and PFC (i.e., Particle Flow Code, Itasca) [21,22]. The continuum method, based on the momentum and mass equations incorporating the earth pressure theory, simulates the motion characteristics of the slip mass to obtain the velocity, position, and thickness of the slip mass [23,24]. The continuum methods have been successfully used to simulate previous hazards and predict potential hazards, such as debris flow, landslides, landslide bam, and avalanches [25–27]. The methods evolved into models such as the GeoFlow-SPH [28], LS-RAPID [29], Flow-2D [30], Kinematic model [31] and DAN model [23]. Based on the fluid continuity equation and motion equation, Hungr proposed the landslide dynamics model software DAN-W, which regards the sliding body as an equivalent fluid and can accurately calculate sliding motion characteristics [23]. These models provide a good method for the risk assessment of the geologic hazard.

The loess geological hazards frequently occurred in Tajik and Kazakhstan Tian shan area, which have also become a focus [32]. The loess has quite a widespread distribution in the world and it occupies approximately 10% of the total global land area. China is the country with most widely distributed loess area in the world. Loess is mainly distributed in the northwestern part of China, on the Loess Plateau, which covers an area of nearly 630,000 km2, accounting for 4.4% of China's land area [33,34] (Figure 1a). Due to its special geological structure, loess has high water sensitivity (i.e., loess undergoes a structural collapse when wetted) and is prone to geological hazards. The main types of hazards are loess landslides, such as the Heifangtai Landslide Group and Jingyang Landslide Group [35–39]. Among them, high-speed and long runout loess landslides have caused considerable losses in terms of human lives and property and have become an important research topic. Due to the porosity, weak cementation and water sensitivity of loess as the water content increases, the shear strength of loess declines sharply, and the loess structure is destroyed [40–43]. In addition, the strength loss in loess might also be a chemo-mechanical problem that involves volume and stress changes of the finest component due to changes in pore water salinity. Consequently, a loess slope loses stability and slides. Furthermore, the pore water pressure rises during the sliding process, and the phenomenon of motion liquefaction occurs, which readily forms a high-speed and long runout landslide [44–47].

**Figure 1.** Location map of the Panjinbulake loess landslide (**a** is modified from [33]; **b** is modified from [48]). (**a**) The distribution of loess in China. (**b**) The distribution of loess in the study area.

In the study, we studied the characteristics of the Panjinbulake loess landslide through a field geological survey and aerial image analysis using drones. We used the landslide dynamics model DAN-W and multiple sets of rheological models to calculate the dynamic characteristics of this landslide. By using the different rheological models to simulate the different stages of the loess landslide (i.e., triggering in the sliding source area, propagation in the debris flow channel area, and deposition in the accumulation area), the best rheological model groups and parameters were obtained to improve the accuracy to analyze the loess dynamic characteristic. The potential secondary failure of the landslide was evaluated. This study could offer a basis to predict the potential landslide runout distance and define the hazard area, make necessary measures to prevent landslide induced damages (e.g., engineering measures, landslide early warning systems, and emergency response), and to favour local development.
