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的调查和详细的地质调查，阐明了滑坡的分布受地质条件的强烈控制。因此，我们证明，在一个给定的地区，滑坡的敏感性可以在地质的背景下进行评估。利用机载激光扫描仪，我们成功地清晰地找到了被树木掩盖的山体滑坡旧伤疤。我们将此技术应用于多个滑坡分布的地区，发现这些地区在滑坡发生前也有许多滑坡疤痕。这表明，这些地区的特点是对滑坡的敏感性很高。我们的地质调查还阐明，这些高磁化率是由于特征风化剖面或地质结构。这些特征性的风化剖面是在未焊接的熔结凝灰岩、白垩纪泉群的砂岩和泥岩交替层以及花岗岩中发现的。未焊合的熔结凝灰岩风化形成底部有毛细屏障的剖面，自重增加和吸力减小是滑坡发生的主要原因。所研究的砂泥岩互层为强风化层，强风化物在坡面附近松动，形成土层，在吸力减小和正孔压增大的作用下发生滑动。花岗岩和花岗闪长岩的风化方式不同，风化花岗岩最易发生滑坡。然而，花岗岩也容易发生滑坡时，它是微席状。下滨面风暴沉积物的特殊地质结构，即薄粘土层夹在高渗透性砂层中，为滑坡提供了有利条件。风化的火山灰掩埋在高度风化的安山岩中的小山谷也容易受到暴雨引发的滑坡的影响。

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.