# Assessment of Internal and External Factors Causing Slow-Moving Landslides
## 3 Results and Discussion
### Analysis of Rainfall Effects  
#### 3.3.1 Effects of Rainfalls  
The area with the highest number of continuous rainfall days before the slow-moving landslide occurred was Site 6, with 21 days of continuous rainfall of 611.7 mm. The areas with more than 100 mm of continuous rainfall comprised sites 1, 3, 6, 9, 10, 11, 12, and 14, which accounted for approximately 53% of the total slow-moving landslide area. It has been shown that when the continuous rainfall is more than 100 mm, the probability of mountain disasters such as slow-moving landslides is high [37]. Furthermore, when slow-moving landslides occurred, Sites No. 1, 2, 3, 4, 5, 6, 8, 13, 14, and 15 accounted for about 67% of the total slow-moving landslide area, with rainfall intensity of 30 mm/h or  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Site** \\ **No.** \\ \end{tabular}  } & \multicolumn{6}{c}{**SLO. **} & \multicolumn{6}{c}{**Based on the Date of Occurrence of Slow-Moving landslides**} \\ \cline{2-13}  & \multicolumn{3}{c}{**Body**} & \multicolumn{3}{c}{**Len. **} & \multicolumn{1}{c}{**Dep. **} & \multicolumn{1}{c}{**Rf. **} & \multicolumn{1}{c}{**C.Rf.D. **} & \multicolumn{1}{c}{**C.Rf. **} & \multicolumn{1}{c}{**Mx.H.Rf. **} & \multicolumn{1}{c}{**Rf.I. **} & \multicolumn{1}{c}{**Mx.H.Rf. **} & \multicolumn{1}{c}{**Mx. more. The remaining sites with rainfall intensity less than 30 mm/h were also close to the rainfall intensity of 30 mm/h, such as Site No. 7 with 29.2 mm/h and Sites No. 9, 10, 11, and 12 with 29.5 mm/h. It has been found that slow-moving landslides mainly occur when the rainfall intensity is 30 mm/h ([PERSON] et al., 2017), and all the sites in this study showed similar results (Table 5). The three-day antecedent rainfall was the highest at Site No. 3 with 162.5 mm, and the five-day antecedent rainfall was the highest at Site No. 8 with 166.0 mm. The 7-day and 15-day antecedent rainfalls were the highest at Site No. 8, with 173.0 and 334.2 mm, respectively. Slow-moving landslides have been reported to occur when the antecedent rainfall is more than 200 mm ([PERSON] et al., 2017), and the areas with 15-day antecedent rainfall of more than 200 mm were Sites 6, 8, 9, 10, and 11. It has been reported that mountain disasters such as slow-moving landslides occur as a direct effect of rainfall ([PERSON] et al., 2017). All sites were found to have experienced slow-moving landslides due to intensive rainfall and internal factors such as strata and terrain vulnerable to slow-moving landslides. 3.3.2 Correlation Analysis of the Scale of Occurrence of Slow-Moving Landslides and Rainfall Factors  
The width of the slow-moving landslide area significantly correlated (0.533) with the number of continuous rainfall days before the slow-moving landslide occurred. The length of the slow-moving landslide area also significantly correlated (0.517) with the number of continuous rainfall days (Table 6). It has been shown that among the factors affecting slow-moving landslides, the number of continuous rainfall days is crucial ([PERSON] et al., 2017). As the number of continuous rainfall days before the occurrence of slow-moving landslides increases, the scale of slow-moving landslides increases. The occurrence of slow-moving landslides is known to be strongly influenced by compressed groundwater in the clay layer or rock layer within the soil layer ([PERSON] et al., 2018). Continuous rainfall was considered a powerful source of moisture in the soil layer. The depth of the slow-moving landslide area was significantly correlated with rainfall on the date of slow-moving landslide and maximum hourly rainfall, with correlation coefficients of 0.765 and 0.659, respectively (Table 6). It was found that higher-than-normal rainfall and maximum hourly rainfall at the time of the occurrence of slow-moving landslides implies relatively more infiltration of rainwater from rainfall, which leads to a deeper-than-usual, slow-moving landslide phenomenon.