Unzen Volcano started its activity about 500 ka (thousand years ago) within the active volcanic graben (>20 km long and -10 km wide). Unzen consists of the Old and Young Unzen volcanoes. The former had grown during the period from 500 to 200 ka, whereas the latter began about 100 ka. Pumiceous-deposits are found only in the initial stage, implying explosive eruptions and being in contrast to effusive eruptions for the other stages. Large-scale collapse events frequently occurred, including the event of the summit caldera formation and the 1792 event that killed 15,000 persons.

The histories of volcanic activity and a volcanic graben can be determined by the scientific drillings. Frequency-and-scale of pyroclastic flow and debris avalanche events during the volcano growth also will be made clear. In-situ measurements of tectonic stress and seismicity in the graben axis are the targets.