Mechanisms of rainfall-induced shallow landslides regulated by hydrological subsurface structures: Cases in granite and granodiorite areas in Northern Abukuma Mountains, Japan

Revealing hydrological processes leading to shallow landslides triggered by heavy rainfall contributes to assessing hillslope hazards and understanding the topographic evolution of mountainous landscapes. This study investigates the causal linkages between subsurface structures characterized by distinct physical properties of the regolith and dominant hydrological processes leading to shallow landsliding. Study sites are located on low-relief, steep hillslopes underlain by granitoids in northern Abukuma Mountains, northeastern Japan, where many shallow landslides occurred during a severe typhoon in 2019. We selected representative hillslopes with typical translational landslide scars in adjacent granite and granodiorite areas to investigate their subsurface structures, physical properties of soil and bedrock, and groundwater processes. Slope models were configured based on these field data, to simulate pore water pressure fluctuations under extreme rainfall. In the granite slope, the impervious bedrock inhibits vertical percolation of water through the overlying, highly permeable soil, forming a saturated zone at the soil bottom, which persists due to lateral throughflow. The saturated zone thickens downslope and can eventually rise to the ground surface under extreme rainfall, which may cause shallow landslides and gullies at head hollows. In the granodiorite slope, the thick soil mantle and underlying permeable bedrock exhibit high water retention capacity, which prevents full saturation under usual rainfall condition. The wetting front slows as it crosses the transitional boundary between the loose upper and stiff lower soil layers, reflecting shifts in hydraulic properties. During extreme rainfall, excess water accumulates above the less-permeable lower soil layer, forming a perched groundwater table that may induce shear failure along the translational boundary. These distinct mechanisms might result in different rainfall thresholds for landsliding, which are short intense rainfall for the granite hillslopes and prolonged, large-amount rainfall for the granodiorite terrain.