Abstract
The Mid Niigata prefacture Earthquake in 2004 (M6.8) triggered thousands of landslides in the Miocene to Quaternary sedimentary rocks in Japan. The most common landslides were shallow disrupted landslides on steep slopes without geologic preference, but deep, coherent landslides also occurred in many locations. We studied about 100 deep, coherent landslides by field investigation and by interpretation of aerial photographs and found that many of them had occurred due to the reactivation of previous landslides. These had planar sliding surfaces along bedding planes or slope-parallel oxidation fronts. Planar, bedding-parallel sliding surfaces were exposed or inferred from the geometry of the deformed ground surface, such as “horsts and grabens” and “roll-over antiform”. The bedding-parallel sliding surfaces were made at the boundary between the overlying sandstone and underlying siltstone or along the bedding planes of the alternated beds of sandstone and siltstone. Sliding surfaces along the oxidation front w re made in the area of black mudstone. New landslides (rockslide-avalanches) occurred with the sliding surfaces of several-cm thick tuff interbedded in siltstone. Most of the deep landslides occurred on slopes undercut by erosion or artificial excavation, notwithstanding they are reactivated or new ones. One rockslide-avalanche occurred on a slope where buckling deformation preceded the earthquake. Gentle valley bottom sediments were mobilized in many locations, probably because they were saturated and partial liquefaction occurred by the earthquake.
