It might be relatively easy to discern non-tectonic faults from tectonic ones, if they die out toward the depth, or their dipping angle is low like a sliding surface of landslide and the texture and/or rock facies of the hanging wall is completely different from those of the foot wall. Faults observed at natural outcrops, however, are not usually so discernible between non-tectonic and tectonic, and it tends to become an issue on the occurrence. In this paper, we present two cases of non-tectonic faults, which look like tectonic but formed by landslide movement in fact. Those examples are observed in Toyama Prefecture and their mechanism is discussed. Both cases build up peculiar“landslide structure”with some faults. One example is“step fault”occurred at a graben due to landslide movement, and another one is“piercement structure”which might be caused by the undulation of sliding surface of a rock slide.

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Range front thrusting of hard basement rocks over the Osaka Group of soft sediments is prone to turn its nature into landslide successively in the basin. The soft sediment of the Osaka Group of the foot wall undergoes shortening as a result of drag fold by thrusting. The range front thrust becomes low angles due to the shortening of the foot wall, and subsequent to gravitational creep of the hanging wall along the mountain slope. The shortening of the foot wall produces bedding parallel shear zones in marine clays of Ma0-Ma2 of the Osaka Group. Because the direction of tectonic movement of the hanging wall along the bedding parallel shear zone coincides with that of its gravitational valleyward sliding, it is easy to cause the conversion of tectonic movement to gravitational movement.

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There are discernible hydrothermal alteration zones along the Median Tectonic Line in central and western parts of Shikoku. The hydrothermal alteration zones mainly consist of smectite bearing clay around intrusive rhyolite body of the middle Miocene. The rhyolite body is not available in north east Shikoku, but in this part also, the hydrothermal alteration along the Median Tectonic Line consequently ensued lot of smectite bearing clay minerals in rockmass. The bedrock deterioration by the hydrothermal alteration was overlooked in the previous geological investigations, because it had been interpreted as the fault gouge and crushed materials. In this study, various problematic sites of hydrothermal alteration along the Median Tectonic Line were investigated in terms of clay mineralogy, shear strength and slope stability. The soils consist of smectite and smectite bearing mixed-layer clay minerals collected from fault gouge as well as clay veins possesses low shear strength. Therefore, due to continue reduction of soil strength, the landslides having low angle slip surface were observed even in the cut slope that were established several years ago.

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