Geological approach to the evaluation of landslide hazard on the basis of history, geology, and geomorphology

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We investigated landslide distribution before and after rainstorm hazard events and made detailed geological survey, and clarified that their distribution is strongly controlled by geology. We thus demonstrated that landslide susceptibility in a given area can be evaluated in the context of geology. We have succeeded to clearly find old landslide scars hidden by trees by using the airborne laser scanner. We applied this technique to the areas where numerous landslides occurred in a distributed manner, and found that these areas had also many landslide scars before the events. This indicates that these areas have characteristically high susceptibility to landslide. Our geological investigation also clarified that these high susceptibility is due to characteristic weathering profiles or geological structures. These characteristic weathering profiles were found for unwelded ignimbrite, alternating beds of sandstone and mudstone of the Cretaceous Izumi Group, and granitic rocks. Unwelded ignimbrite is weathered to form a profile with a capillary barrier at its base and weight increase and suction decrease are the main causes of landslide occurrence. Alternating beds of sandstone and mudstone we studied were deeply weathered and intensely weathered materials were loosened near the slope surface to form a soil layer to slide by suction decrease and increase of positive pore pressure. Granitic rocks are weathered in different ways for granite and granodiorite, and decomposed granite is most susceptible to landslide. Granodiorite, however, is also susceptible to landslide when it is micro-sheeted. Specific geological structures of storm deposits on lower shoreface, in which thin clay beds are intercalated in highly permeable sands, provide preferable condition to landslide. Weathered volcanic ash burying small valleys in highly weathered andesitic rocks is also susceptible to landslide induced by rainstorms.