In Rokko mountain range, by the Hyogo-ken Nanbu Earthquake to occur on January 17th, 1995, many cracks occurred on a mountain slope. In this study, scale and distribution situation of cracks formed on a mountain slope, was grasped. As considering these results, then, numerical experiments were executed and influence of cracks against slope stability was evaluated using the numerical simulation model for predicting hillside landslides. With these results, in the slope downstream cracks, it was confirmed to ascend suddenly in ground-water level and increase 10-55 cm grade in maximum ground-water level, in comparison to the case without cracks. And, in the case that cracks exist on a mountain slope, appearance time of a collapse slope gets earlier and collapse slope total number increases 10% grade, than the case that cracks don't exist. Furthermore, in order to be as an index to plan effective land erosion control master plan after earthquake, influence of topographic shape of a place where cracks exist on hillside landslides, was evaluated quantity-like. The typical results were as follows:
(1) In the case that cracks were formed on parallel and convex shapes than concave shapes, slope stability falls more remarkably.
(2) Because influence of cracks on sediment yield potential is different by a difference of topographic shape of a place where cracks exist, in case that land erosion control master plan will be reconsidered after earthquake, it is necessary enough to consider a topographic shape of a place where cracks exist.

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Once landslides movement initiates, the sliding mass tends to move toward a more stable position. However, landslides that have occurred in Tertiary formations indicate recurrent movement. A possible reason for this behavior is that the mineral composition and properties of slip plane materials become more lubricante as the movement repeats . In order to determine the causes, five Tertiary landslides with different movement histories were studied. Based on a geological investigation, the results below were obtained by studying the development process of slip planes and clays of Tertiary landslides that forms the slip planes.
The clay content of slip plane materials is directly related to the magnitude and history of slide movement . This study indicates that slip planes alter from: (1) striation type (showing only striation on bedrock without clay); to (2) brecciatd type (breccia with clay); then to (3) mylonite type (clay with breccia); and finally to (4) clay type (clay without breccia). If the slip plane area is composed of tuff or tuffaceous materials and subjected to increasing slide movement, the content of smectite in the slip plane clay increases while the content of silicate minerals decreases. Furthermore, the smectite content increases within the same landslide from the head to the toe portions of the landslide (direction of ground water flow). The ground water at the slip plane (above the impervious zone) contains an increased concentration of Ca²⁺ ions. As the concentration of HCO₃^- exceeds over 40 mg/l an environment is created which promotes the formation of smectite. This process indicates that there is an active ion exchange within the ground water near the slip plane that forms smectite. Thus, the formation of smectite in the slip plane requires the presence of tuff and sufficient concentration of HCO₃^- within the ground water.
The study results indicate that frictional resistance decreases as clay content increases. The reason for the repeated movement of Tertiary landslides with tuff or tuffaceous composition at or in the vicinity of the slip plane is attributed to the increase in clay portions along the slip plane as the movement repeats, and the groundwater that promotes the formation of smectite which one of the clay minerals with lower frictional resistance.

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The 1986 Iwato slide occurred during November 1986, at Iwato, situated in the southwestern part of Ikutahara Town, northeastern Hokkaido, Japan. This slide took place over an area at the foot of an ancient slide configuration that was approximately 140 m wide and 100 m long. The dimensions of the 1986 Iwato slide were approximately 25 m wide and 45 m long.
The geology in the vicinity of the Iwato slide area consists of the Lower Cretaceous-Eocene Hidaka Supergroup, Upper Miocene Ikutawara Formation, Neogene rhyolite and basalt dikes, as well as Quaternary river terrace deposit, slide deposit, alluvial fan deposit and Alluvium.
Hydrothermal alteration is mainly characterized by smectitization (smectite ± quartz ± opal-CT ± pyrite), zeolitization (mordenite+heulandite-clinoptilolite series mineral ± smectite ± quartz ± opal-CT ± pyrite), propylitization (quartz + albite + chlorite ± smectite ± calcite ± pyrite), illitization (quartz + illite + pyrite ± chlorite) and Kfeldsparization (quartz+adularia+pyrite ± illite ± chlorite).
The bedrock of the slide area is reconsolidated crushed shale of the Hidaka Supergroup in a crush zone . The shale is believed to have been subjected to hydrothermal smectitization. The slide, therefore, can be characterized as both shear zone slide and hydrothermal alteration zone slide. The slide deposit consists chiefly of debris, such as sand or silt mixed with rubble that consists primarily of shale fragments of the Hidaka Supergroup and tuffaceous conglomerate fragments of the Ikutawara Formation, and is approximately 6 to 7 m in thickness. The displaced material moved along a curved surface. The movement of the 1986 Iwato slide was a slump.In addition, the slip surface is estimated to be inside the debris, or the detritus. Thus the slide can also be characterized as a debris slump, or a detritus slump.

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Many landslides have been occurred in the serpentine belt. It was seemed that the ease of the chemical weathering of serpentine were one of the reason. The chemical weathring characteristics were investigated. The perticles of serpentine were soaked in the carbonated water. The change of pH, clay minerals, cations in the eluate, elements, perticle size and shearing characteristics were obtained. It were clear that much Mg ion flowed out and the angle of shear resistance droped about 10° for three months.

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A quick clay, distributed throughout the Scandinavian peninsula and Canada, has an unstable structure with the high sensitivity ratio of more than 8. In quick clay deposit areas, therefore landslides occur frequently caused by fluvial erosion, freezing and thawing of ground, or vibration due to engineering works . In order to study the relationship between landslide activity, pore pressure and meteorological factors, the authors established a research site at a quick clay landslide area in Norway, and started an automated observation of landslide movement in November of 1997. We observe precipitation, meltwater, air temperature, soil temperature, pore pressure, and displacement of the landslide. A but with power supply was built close to the instrument field, and observation data are collected every 15 minutes by a logger in this hut. We report on this observation system and some results from observations during one year.

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