## 3 Data and Methods
### Conditioning Factors  
The occurrence of natural disasters is often the result of the combined effects of multiple factors, including topography, geology, meteorology, and hydrology [53; 54; 55]. Drawing on previous studies and empirical findings [56; 57; 58; 60; 61; 62; 63; 64; 65; 66; 67; 68], this study selected 18 conditional factors from four categories--topography and geomorphology, geological structures, environmental elements, and human activities--to construct a comprehensive susceptibility factor system. 1. Topography: This includes elevation, slope, aspect, topographic wetness index, stream power index, topographic relief, drainage basin shape coefficient, drainage basin area, maximum flow length, and maximum flow gradient. Elevation influences geomorphological and geological evolution, shaping the natural disaster background and indirectly regulating the spatial distribution of human activities. Slope is highly related to the formation and development of debris flows, with suitable slopes facilitating their initiation. Aspect affects microclimatic conditions, which in turn influence vegetation and rock weathering, controlling material availability. The topographic wetness index describes how terrain controls runoff generation and saturation source areas. The stream power index reflects the erosive force of flowing water. Topographic relief indicates the degree of surface erosion and tectonic activity. The drainage basin shape coefficient, drainage basin area, maximum flow length, and maximum flow gradient are the basic parameters for calculating flow paths and velocities, determining debris flow probability and scale. 2. Geology. This includes seismic nucleation density and fault distance. Earthquakes and tectonic faults weaken rock and soil strength, producing loose material that serves as debris flow sources. 3. Environmental factors. These include the normalized difference vegetation index, soil erosion, land use, and annual rainfall. The normalized difference vegetation index reflects vegetation cover, which promotes soil and water conservation and reduces debris flow sources. Soil erosion indicates surface susceptibility to erosion. Land use alters runoff and infiltration processes, potentially triggering debris flows. Annual rainfall is one of the primary triggers for debris flow events. 4. Human activities. These include distance to road and population statistics. Road construction along riverbanks can induce slope instability and accelerate loose material accumulation, raising debris flow risk. Population statistics reflect human activity intensity but are often highest in flatter areas, where geological conditions tend to inhibit debris flow development. To avoid multicollinearity, the Pearson correlation coefficient was used to analyze the factors. A Pearson coefficient close to 1 indicates a strong positive correlation between two controlling factors, while a coefficient near 0 suggests independence [69]. Previous studies have indicated that a Pearson coefficient exceeding 0.7 may lead to multicollinearity issues [70]. The analysis revealed a very strong positive correlation between slope and terrain roughness (PCC = 0.974) (Figure 5). Given the well-established dominance of slope as a driving factor in surface processes [71], slope was retained, and terrain roughness was excluded. This resulted in the establishment of a debris flow susceptibility evaluation factor system comprising 17 independent factors (Figure 6), which were classified into five levels, as detailed in Table 2.  
Figure 5: Debris flow factor Pearson correlation coefficient plot. ELV—elevation; SLP—slope; ASP—aspect; TWI—topographic wetness index; SPI—stream power index; TR—topographic relief; DBSC—drainage basin shape coefficient; DBA—drainage basin area; MFL—maximum flow length; MFG—maximum flow gradient; SND—seismic nucleation density; FD—fault distance; NDVII—normalized difference vegetation index; SE—soil erosion; LU—land use; ARF—annual rainfall; PS—population statistics; DR—distance to road.