# Assessment of Internal and External Factors Causing Slow-Moving Landslides
###### Abstract
Most of these occurred in the past and were not classified as slow-moving landslide phenomena but were determined as general landslides [8; 15]. Recently, it has been found that slow-moving landslides differ in signs, phenomena, frequency, and scale from general landslides [6; 8; 11], and the areas identified as such landslide areas are increasing. In South Korea, forest soil sediment disasters are mostly caused by the loss of balance and collapse of natural slopes due to rapid urban expansion and land development in mountainous or hill areas that have been stable. The causes of the disaster are mainly divided into internal and external factors. Internal factors are potential factors such as stratigraphy, topography, geology, soil quality, and clinical conditions of mountain slopes, which cause progressive destruction, weathering, and erosion to reduce the shear strength of slope components. External factors are changes in topography, increased land surface load, vibration and shock, and fluctuations in groundwater levels, which combine with internal factors to cause forest soil sediment disasters. The most influential factor is known to be rainfall. In particular, it is known that the 7-day antecedent rainfall and continuous rainfall days significantly impact the landslide characteristics [16]. However, slow-moving landslides can occasionally accelerate rapidly, travel long distances, or even collapse catastrophically [17]. Slow-moving landslides occur widely and can cause relatively more damage than caused by landslides or debris flows. Increases in soil temperature and groundwater levels [18; 19] and various types of anthropogenic forest development [20] have been cited as the main causes. In addition, triggering factors for slow-moving landslides including geological properties [17; 21; 22], clay content in the soil [22], rainfall [23; 24; 25], topography [21; 26], slopes in the mountains [27; 28], and reservoir water level [29] have been researched. Until recently, South Korea did not clearly distinguish between slow-moving and general landslides and treated both as general landslides. However, applying existing vulnerability assessment criteria for landslides to slow-moving landslide vulnerability de termination may be problematic regarding accuracy and reliability. Furthermore, incorrect judgments could result in loss of life or property damage. In this study, we conducted an overlap analysis with the national landslide hazard map for 15 slow-moving-landslide-prone areas in South Korea with a clear timing of landslide occurrence. We analyzed the difference in the hazards of occurrences of landslides and slow-moving landslides. In addition, we aimed to present compelling evidence to highlight the urgency of national slow-moving landslide vulnerability assessment and the need to establish an SMLHIS (slow-moving landslide hazard map). Moreover, the geology, topography, and the scale of occurrence of slow-moving landslide areas were analyzed to examine the internal factors of slow-moving landslides. Finally, we analyzed the impact of rainfall, a major influential external factor among the triggering factors for sediment disasters affecting forest soils, and the correlation between rainfall factors and the scale of slow-moving landslides (range, length, and depth). This study aimed to provide information on slow-moving-landslide-triggering factors for developing vulnerability assessment criteria for such landslides. The findings of this study contribute to understanding the geology and geomorphology of slow-moving landslide sites, specifically toward improving our understanding of the impact of rainfall on the magnitude of such landslides and developing effective strategies to mitigate their impact on communities and infrastructure.