# Assessment of Internal and External Factors Causing Slow-Moving Landslides
###### Abstract  
There have been limited studies on slow-moving landslides in South Korea despite their frequent occurrence. Moreover, a national slow-moving landslide hazard information system (SML-HIS) is needed. Herein, we conducted an overlap analysis of 15 slow-moving landslide areas with clear occurrence timings with national landslide hazard maps (LHMs) using the geographic information system data. Additionally, internal and external factors causing slow-moving landslides were analyzed. The results of the overlap analysis showed that slow-moving landslide areas occurred in low-hazard and excluded non-hazard areas on the LHM. The study of internal factors revealed that slow-moving landslides occurred mainly in the Gyeongsang supergroup, which has sedimentary rock type and sandy loam. The analysis of external factors, e.g., rainfall, showed that slow-moving landslides occurred during intensive rainfall, with continuous and 15-day antecedent rainfall exceeding 100 and 200 mm, respectively. The longer the continuous rainfall duration before a slow-moving landslide; the greater the rainfall on the day of the landslide; the greater the maximum hourly rainfall; the greater the 3-, 5-, and 7-day antecedent rainfalls; and the greater the rainfall intensity during the landslide, the greater the size of the slow-moving landslide. This study provides information for developing a national SMLHIS, presenting novel perspectives for slow-moving landslide research. land creep; landslide hazard map; rainfall; slow-moving landslide; triggering factor +
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the point at which it is not noticeable. In other words, the movement is caused by internal shear stress sufficient to cause deformation of the terrain but insufficient to cause sudden breakage. Creep is also widely distributed worldwide and is the most common type of landslide. It often occurs before faster and more destructive types of landslides [6]. Meanwhile, South Korean slow-moving landslides can be explained as \"earth flow\" according to [PERSON] landslide type classification [7]. It was only in 1996 that slow-moving landslides were recognized as a natural disaster with a different underlying mechanism than that of landslides. In South Korea, a landslide that occurred in 1995 was first reported as a slow-moving landslide rather than a general landslide [8], and this sparked interest in slow-moving landslides in the academic community. However, until now, South Korea has yet to develop SMLHIS through a selection of slow-moving-landslide-vulnerable areas at the national level (e.g., a slow-moving landslide hazard map). Consequently, slow-moving landslides are still being managed by the public authorities as general landslides. In addition, damage to communities and infrastructure is not classified as forest soil sediment disasters. However, slow-moving landslides differ from typical landslides in their mechanism of occurrence [9; 10]. Slow-moving landslides typically occur during heavy rainfall in areas with clayey soils and abundant groundwater. The clayey soils in these slow-moving landslide areas are saturated by rainwater, and as the clay is pushed in the direction of gravity under the weight of rainwater, the soil moves downward with the water, forming tension cracks and steps along the slope plane [8; 11]. In contrast, a landslide is a phenomenon wherein rainwater fills the bedrock and the soil on the bedrock slides downward. Thus, a landslide is a phenomenon wherein water acts as a lubricant at the interface of soil and bedrock, causing the mountain to collapse instantaneously [11]. In addition, the occurrence mechanism of landslides can be better understood through centrifuge modeling research based on typical landslide triggering factors such as centrifugal acceleration, rainfall, earthquakes [12; 13], or machine-learning-based reservoir slope stability research [14]. The number of slow-moving landslide areas is gradually increasing in South Korea.