Topographic features and landslides in humid subtropical region -Shimajiri Group, Okinawa Island, Japan

The topographic features, transitions of landform and landslides were investigated in the humid subtropical region of Tyunanbu, Okinawa Island. A generation process of landslides then was discussed in order to frame a landslide extraction procedure for a humid subtropical region. Landslides and topographies, which are similar looking to landslides, are found in the Shimajiri mudstone area lying under the humid subtropical climate. A closely resembling topography is sometimes interpreted wrongly as a landslide during the procedure of landslide extraction using aerial photographs and topographic maps. In this sense, comprehension of the topography of semicircular valley head and the investigation of bedrock are important to avoid such misinterpretation of landslide topography. The disintegrative landslides and the colluvial landslides occur in stepped terrain, which originate in terrace topographies. However, scales of these landslides are relatively small. Large, first-time landslides occur in slopes that have a relatively longer length, which were formed by the erosion front reaching up to the high level terrace. In the middle of mesa slopes in the Southern part of the East area, cliffs are formed since the lower layers of the Shinzato formation stand against erosion and landslides. It was considered that the risk of occurrence of first-time landslides increases in cases where the cliffs were eroded by landslides and/or erosion.