Abstract
Four debris flow deposits and many intraformational folds were found in the late Pleistocene Takigoshi lacustrine sediments near the Ontake volcano, central Japan. The debris flow deposits have been regarded to be supplied from a large slope failure around the Kamikurosawa valley, which is identified in the present study. The size of the failure is estimated to be comparable with the Denjokuzure, which was caused by the Naganokenseibu earthquake on September 14th, 1984. Detailed stratigraphic study of the Takigoshi sediments revealed that debris flows took place three times in 500-2500 years. It is suggested that periodic inland earthquakes generated the debris flows, because a large fraction of historical large-scale debris flows was induced by them and active faults densely occur in central Japan. A brief description of other lacustrine sediments suggests that debris flow deposits and intraformational folds are ubiquitous. It is pointed out that a detailed stratigraphic study of rhythmical sediments provides a significant method for estimating recurrence interval of inland earthquakes.
