Geological and Geomorphological Causes of Two Historical Deep-Seated Catastrophic Landslides Induced by the 1892 Heavy Rainfall Event in the Shimanto Accretionary Complex, Tokushima, Japan

The catastrophic Hose and Ogawatuetui landslides occurred coincidentally in 1892 following heavy rainfall in the upper reaches of the Kaifu River, Tokushima, Japan. The area is underlain by a steeply dipping coherent formation with many faults, and lies in the Shimanto Belt where deep-seated catastrophic landslides are known to occur following heavy rainfall and large subduction zone earthquakes. The main geological and geomorphological causes of the landslides were found to include local relief ≥ 350 m (1200-m-diameter analysis windows); knickpoints and slope breaks at 90–150 m above sea level, competent massive and thick-bedded sandstone with open fractures on the upper slope, and alternating thin beds of incompetent fine sandstone and shale beds of high deformability and low permeability on the lower slope.