In recent earthquake events in Japan, many largely-displaced landslides occurred on dip slopes of Neogene sedimentary rocks. In this study, in order to examine the mechanism of earthquake-induced landslides, we took Nigorisawa landslides which occurred in the 2008 Iwate-Miyagi Nairiku earthquake for the research site, and investigated physical and mechanical properties of slip surface soils existing along the bedding plane. Results of ring shear tests performed under shear speeds ranging from 0.02 to 500mm/min revealed that shear strength decreases during high shear rates. To clarify the mechanism of the negative rate effect, additional experiments were performed on medium-plasticity cohesive soils. Test results showed that weakening of shear strength is likely to occur when the shear box bath is filled with water. On the other hand, a long-time fast shearing test under drained condition suggests that shear weakening did not continue and strength recovery gradually occurred due to dissipation of excess pore water pressure. Temporal increase in water content within shear zone is an essential cause for strength weakening during fast shearing. In addition, according to field survey on groundwater conditions, water flow is significant just above the bedding plane, possibly influencing on shear weakening of slip surface soils during the earthquake.
Deposits from a rock avalanche are distributed in the east of Mount Jizo, which is in the northern part of the Akaishi Mountains, central Japan. In order to determine the precise age of the rock avalanche run out, I performed dendro wiggle matching of 14C dates using a fossil log found in the dammed-lake deposits caused by the rock avalanche. It is estimated that the rock avalanche occurred during the period from cal AD 778 to 793（2σ）. Although no records of large historical earthquakes were found in old documents or reports of geological investigations carried out in this period, the old Japanese document Shoku Nihongi describes significant disasters caused by river floods in the late August of AD 779（Julian calendar）around the Akaishi Mountains. Possible heavy rain that caused this flooding could have triggered the rock avalanche.
This study attempted landslides hazard assessment for hydrothermal alteration zone landslides of the Ohekisawa-Shikerebenbetsugawa landslide area in Teshikaga Town, eastern Hokkaido, Japan. The landslide hazard assessment system for landslides used here is based on topography, slope geology, geological structure, and the hydrothermal alteration zone. In addition to those we considered mechanical properties of hydrothermally altered rock based on irregular lump point load strength test results. The landslide hazard was evaluated based on analytic hierarchy process (AHP). A landslide hazard map was created based on the landslide hazard rank of I-V as follows. Rank I : stable hard rock slope, Rank II : stable soft rock slope, Rank III : unstable soft rock slope, Rank IV : potential landslide area, and Rank V : ancient landslide area.