Abstract
The Hokkaido-Nansei-Oki earthquake that occurred on July 12, 1993, caused major damage to the slope about 150 km from the epicenter. Nihon University Experimental Forest at Yakumo is located about 100 km apart from the epicenter. The area is covered with hard shale and the pelitic sandstone of the Yakumo Formation. There are about 35 large landslides in this areas. There are only a few reports on the aerial photographic analysis of the process of the recovery of vegetation on landslides. In this study, the process from the occurrence of a landslide to vegetation recovery was discussed with respect to expansion, combination, shrinkage and separation. The landslide land was analyzed based on the aerial photographs taken in 1967, 1983 and 1998. The results are summarized as follows: 1)There was no major change in the distribution of landslides in the past 31 years. 2)Landslide has occurred at 583 sites in 1967, 540 sites in 1983 and 541 sites in 1998. The decrease in the number of landslides after 1967 may have been the result of afforestation. Only a few nets had been produced by the Hokkaido-Nansei-Oki earthquake in 1993. 3)There was confirmed to be a landslide area of 86.9 ha in 1967, 86.1 ha in 1983, and 103.4 ha in 1998, and was largest in 1998. This may have been because even a slight earthquake movement weakened the volcanic ash layer of the soil surface, and subsequent rainfall promoted the expansion of landslides. 4)Most of the landslide sites in the study area were about 0.2 ha. 5)The average expansion coefficient for the sites was 1.53-1.56, and the damaged sites were circular or elliptical in shape. 6)Damaged sites were mostly steep with a slope angle of 30-45°. 7)Many of the slopes with damaged sites faced north, northeast or east. 8)In the study area, the expansion-type of landslide was dominant.
