During the 2004 Mid-Niigata Prefecture earthquake, many landslides occurred in the Imokawa basin. Because the landslide triggered by the earthquake does not occur frequently, geo-technical characteristics which are typical for landslides triggered by earthquakes have not been studied in detail. In this study, we took the landslide of the Imokawa basin Shiotani-kamisawagawa area for an example, and examined the geo-technical characteristics of the slip surface.
First, to investigate the location and the structure of the slip surface, we carried out the acoustic borehole televiewer in borehole. Second, to grasp the soil characteristics of the slip surface, we performed the needle penetration tests, grain size analysis, liquid and plastic limit tests and X-ray powder diffraction analysis. In addition, to grasp the shear strength of the slip surface at present static state and the strength during rapid slide in the earthquake, we performed the direct shear tests and the ring shear tests. Finally, by comparing with the past experiments preformed on Tertiary landslides which are induced by rainfall or snow melting, we point out typical soil properties for landslides during the earthquake.
We have found that angle of shearing resistance of slip surface soil is larger than general values of Tertiary landslide, and cohesive soils of the fractured zone do not contain abundant clay fraction, and show low-plasticity. Also, in the ring shear experiments, shear strength decreased steeply by increasing of shearing speed. It is suggested that such geo-technical property played an important role in the occurrence process of the rapid landslide triggered by the earthquake.

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Based on aerial photographs interpretation and DEM's analysis within the Imo River basin and its surrounding area, the present paper revealed several possible causative factors that are possibly responsible for the landslides induced by the Mid-Niigata Prefecture earthquake in 2004. The study results show that, 73% of the earthquake-induced landslide occurred within pre-existing landslide sites in the study area. As geomorphological factors, earthquake-induced landslides related to the convexity and lower part inclination of pre-earthquake ground surface, and the landslide occurrence ratio tends to increase with the values of these factors. As geological factors, relatively larger landslides tend to occur on slopes composed of sandstone, sandy siltstone, and alternation of sandstone and mudstone, and the landslide occurrence ratio is relatively higher compared to mudstone.

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