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.

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At 8:43 a.m. on June 14, 2008, the Iwate-Miyagi Nairiku Earthquake, which centered on the southern inland of Iwate Prefecture, occurred. In the wake of this earthquake, a landslide caused by the specificity of the geological structure and the vulnerability of the ground occurred in the Ichinonobara region in Ichinoseki, Iwate. In order to determine the slip surface of the landslide, the behavior was observed by using a borehole with a pipe strain gauge. However, no obvious landslide movement was observed. Therefore, various indoor tests were conducted using boring core samples, and data that shows evidence of the slip surface was obtained from colluvial soil at a depth of 47 to 48 m as the result of a quantitative analysis of smectite and an improved swelling pressure test based on an oven drying method. It became apparent that these physical and chemical test methods are effective for determining the slip surface of a landslide where no landslide movement is observed.

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The geomorphological features for the occurrence of landslide dams are examined in a case study of the 2004 Mid-Niigata Prefecture Earthquake based on digital elevation models(DEM)and the actual landslide distribution map. Found quantitatively shown that larger landslides along a narrower valley with a larger amount of river discharge have a tendency to induce landslide dams that cause river blockage. Accordingly, a new map showing the potential risk of landslide dams based on the overground-openness and the catchment area is proposed in this study. The combinational use of this map with the landslide risk map previously proposed by Hayashi et al.(2011) can be useful for the exact prediction of landslide dams that can be induced by a strong earthquake. In addition, the relationships between the accumulation height of the landslide mass and the local overground-openness in the original topography are examined for use in prediction of the height of a possible landslide dam.

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