北海道日高地方海岸段丘地帯における斜面崩壊の発生史とその古環境

抄録

The temporal frequency of slope failures in the terraces of Hidaka district, Central Hokkaido, and the environment inducing them were studied by tephrochronology, pollen analysis, and radio carbon dating. The morphology of the slope failures caused by torrential rains in 1981 in the area can be classified into two types: (1) deep circular gullies on gentle sloping terraces, and (2) soil slips from the surface layer in the steep slopes of dissected valleys.
Most of the failure material in gullies (1) was formed as a result of the increasing rainfall during the Allerød Interstadial and early Post Glacial period. This indicates that no gully failure (1) has occurred at least for 8, 000 years. Ancient failure deposits accumulated on the floor of dissected valleys show that slope failures (2) may have occurred on an average of every 150 to 200 years during the last 2, 600 years. The occurrence of slope failure has increased especially during the last 300 years. According to pollen evidence, the climate became cool/wet about 2, 600 years ago, and this has continued almost to the present. Climatic changes are probably related to the initiation of slope failures. The recent increase in slope failures is presumably related to the accumulation of loose volcanic ash and to changes in land-use patterns.