Abstract
The authors studied the face of excavation slopes along forestry roads in the Yachi landslide area, investigating the distribution range of pyroclastic flow deposits and the internal structure of the landslide bodies. A tephra analysis was also conducted. The results revealed two types of pyroclastic flow deposits in the area, identifiable as either the Yuhama pyroclastic flow deposits emitted in the Early Pleistocene or the Fudoyama pyroclastic flow deposits emitted approximately two hundred thousand years ago. The pyroclastic flow deposits were found in three states:1) diffused in mudstone fragments, 2) adjoined with fractured mudstone or other type of pyroclastic flow deposit at a steep angle, 3) covering fractured mudstone. These field observation findings and aerial photo interpretations of the landslide microtopography show that the slopes have been formed by recurrent landslides for more than two hundred thousand years including the compound landmass movement since the time before the accumulation of pyroclastic flow deposits. They also include the formation of present microtopography.
