High Mobility of Large-Scale Shallow Landslide Triggered by Heavy Rainfall in Izu Oshima

On Oct. 16, 2013, catastrophic shallow landslides were triggered on the west-side hillslopes in Izu-Oshima Island, Japan, by the heavy rainfall accompanying Typhoon Wipha. The displaced landslide materials were characterized by rapid and long runout movement. These landslides resulted in 36 dead, 3 missing, and 46 buildings being completely destroyed on the downstream area of Motomachi area. To understand the initiation and movement mechanisms of these shallow landslides, we surveyed the field and found that besides heavy rainfall, strong wind might also have play a key role in triggering of slope instability on the deep slopes. We took sample from the source areas and examined the shear behavior of these samples under partially drained or undrained condition. We also triggered landslides within soil layers made up of these samples by rainfall in a flume, and examined the variation of soil moisture, pore-water pressure and landslide movement with the introduction of rainwater. Test results showed that high pore-water pressure can be built up and maintained within the displaced landslide material and then elevate the landslide mobility, even though the thickness of displaced landslide materials is 0.7–1.2 m.