[in Japanese]

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The 1995 Hyogoken Nanbu Earthquake caused landslides in the Rokko Mountains. We studied geological and topographical features of those landslides in reference to an existing database which had been constructed for investigation of sediment related disasters around the Rokko Mountains. We compared those features of the landslides by the earthquake with features of past landslides caused by rainfall. The landslides by the earthquake distributed chiefly in north-eastern part of the Rokko mountains. Many of them are located near reverse faults. Most landslides by the quake occurred at north-east or south-west faces with steep convex slopes of granite, while the old landslides by rainfall had occurred at concave and less steep slopes without any preferable direction. Nevertheless, a certain number of landslides by the earthquake occurred at slopes of similar condition with the old landslides. Relationships between the occurrence of the landslides and movement of the faults during the earthquake are not clear, for the landslides distributed around steep fault scarps of those reverse faults.

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The mechanism inducing the infiltration rate lowering of the Unzen volcanic ash was studied through permeability tests applying varying electrolyte concentration, ESP (Exchangeable Sodium Percentage) measurement of the fallout deposits, and SEM (Scanning Electron Microscope) observation. The results of the permeability tests showed that the permeability decreased with the number of experiments using destilled water, whereas the permeability kept constant using electrolyte. The ESP in the field condition is sufficiently higher than the forest soil. These results indicate that the mechanism of infiltration lowering of the Unzen volcanic ash is due to formation of chemical crust by physico-chemical processes of the ash. The observation of the SEM also support the results. The experimental results suggest that infiltration rate of Unzen volcanic ash could be increased by applying electrolyte for reducing surface runoff and debris flow initiation.

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Debris flows from streams across roads in mountainous areas, sometimes damage the roads during heavy rainfall and cause severe damage to the local community and economy. To prevent or reduce the damage by such debris flows, it is desirable to identify streams which are likely to cause debris flows and to implement rational and effective countermeasures.
The method of assessing the danger of debris flows occurring is studied by such means as aerial photographs and topographic maps in order to pick out those streams which are likely to cause debris flows in a large area with many mountain streams. The report also compares the method with some of the methods proposed in the past to assess the danger of debris flows in stricken areas by the precision of predicting and the amount of work required for surveys. The validity of the method was confirmed.

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The Unzen Volcano resumed volcanic activities on November 17, 1990 after a dormancy of 198 years. A series of repeated volcanic activities including magmatic eruptions, lava dome building, and pyroclastic flows occured due to collapses of lava domes and caused substantial changes in the topography of both on the mountain and the land around the foot of the mountain. Collapsing lava domes occasionally produced largescale pyroclastic flows, and their deposited debris was washed down by rainfall, resulting in large- scale debris flows. These events caused severe disasters in the region, as human lives were lost and houses were either destroyed, buried, or burned down, mainly in the Mizunashi River basin and the Nakao River basin.
To obtain basic information on the pyroclastic flow and debris flow, we used aerial photographs and topographic maps to analyze the growth of lava domes and the topographic changes on the mountain over a period of four years beginning in late May 1991 (the lava domes began to collapse) until May 1995 (it was announced that this volcanic activity had ceased) and estimated the time sequential changes in the total amount of lava (sum of volume of lava dome and amount of deposited debris), the volume of the lava dome and the deposited debris in a watershed area.
The results of the aerial photographs analysis showed that on May 12, 1995, the volume of lava dome, the amount of deposited debris, and the total amount of lava in the entire Unzen Volcano watershed area were about 94 million cubic meters, 119 million cubic meters, and 213 million cubic meters respectively. The maximum amount of lava produced per day was about 310, 000 cubic meters for a period of between May 18 and October 3, 1991, but its daily average had fallen into 3, 000 cubic meters, or about 1/100 of the maximum volume by March 1995.

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