This preliminary report gives the distribution of the active faults, their geomorphological features and the relationship between the parasitic eruption centers and the active faults found on the west flank of the Aso caldera.
1) Many active faults are distributed on the west flank of the Aso caldera. NE trending faults dip north with right-lateral displacement. They are geomorphological manifestations of the Oita-Kumamoto tectonic line.
2) Rate of vertical displacement of the Futagawa-fault, a principal fault in the studied area, is 1.7 to 0.6mm/y. for recent 350, 000 years as indicated by the displacement of the Aso pyroclastic flow deposits.
3) The Hinagu-fault joins the Oita-Kumamoto tectonic line near the southern end of the Futagawa-fault.
4) Lowland along Kiyama to Kashima formed by the active faulting is named Kiyama-Kashima graben. Narrow graben is also developed along the Hinagu-fault.
5) It is noteworthy that the grabens are situated right outside of bending of Southwest Honshu arc from SW to SSW trend, the trend of the Ryukyu arc.
6) Eruption centers of the parasitic volcanoes of the Aso caldera are situated at local tensional stress fields formed by right-lateral faulting of echelon arrangement active faults.