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

通过对暴雨灾害前后滑坡分布情况的调查和详细的地质调查，明确了滑坡分布受地质条件的强烈控制。因此，我们证明了在给定地区的滑坡易感性可以在地质背景下进行评估。利用机载激光扫描仪，我们成功地清晰地发现了被树木隐藏的老滑坡痕迹。我们将这一技术应用于滑坡分布较多的地区，发现这些地区在滑坡发生前也有许多滑坡疤痕。这表明这些地区具有滑坡易感性高的特点。我们的地质调查也澄清了这些高敏感性是由于特殊的风化剖面或地质构造。这些特征的风化剖面主要存在于白垩系泉群砂岩与泥岩的交替层和花岗质岩石中。未焊接的褐煤经风化形成底部有毛细屏障的剖面，重量增加和吸力减少是发生滑坡的主要原因。所研究的砂岩和泥岩交替层是深风化和强风化物质在坡面附近松动形成的土层，通过吸力的减小和孔隙正压的增大而发生滑动。花岗岩和花岗闪长岩的风化方式不同，腐烂的花岗岩最容易发生滑坡。然而，花岗闪长岩在形成微片状时也容易发生滑坡。下滨面风暴沉积的特殊地质构造，薄粘土层穿插于高渗透性砂层中，为滑坡提供了有利条件。风化的火山灰在高度风化的安山岩中掩埋小山谷，也容易受到暴雨引起的滑坡的影响。

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.