Volcaniclastic debris avalanche on Motomachi area of Izu-Oshima, Japan, triggered by severe storm: Phenomenon and mechanisms

Abstract

Heavy rainfall and strong wind associated with Typhoon Wipha hit Izu-Oshima Island, Japan, and triggered catastrophic landslides on the western slope of Oshima Volcano on 16 October 2013. These landslides were shallow but large in scale, and the displaced landslide materials were characterized by rapid and long runout movement. A massive landslide resulted in 36 people dead and 3 missing, with 46 buildings being completely destroyed in the downstream area of Motomachi district. To understand the initiation and movement of these shallow landslides, we surveyed the landslide area and found that in addition to heavy rainfall, strong wind may also have played a key role in increasing the instability of the slopes. We took samples from the source areas and examined their static and dynamic shear behaviors under naturally drained or undrained conditions. Test results showed that high pore-water pressure can be built up and maintained within the displaced landslide material and increase landslide mobility, even though the thickness of displaced landslide materials is 0.7–1.2 m. This may then give a rational explanation on the fluidization process of the displaced landslide materials. We also examined the windthrow features of trees subjected to different wind speed through monitoring the ground motion by seismometer and portable anemometer, and then performed cyclic shear tests on the materials laying above the sliding surface. The test results suggest that the windthrow may have promoted the buildup of excess pore-water pressure within the saturated soil layer and thus favoured the occurrence of slope failure. Therefore, it is suggested that in the prediction of shallow landslides on forested slopes subjected to heavy storm, both the effects of rainfall and strong wind should be taken into account.