Periodicity of large-scale landslide revealed by the landslide-lake sediments and ancient documents

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Prediction of large-scale landslides with more than one million cubic meters of landslide debris is important in central Japan with steep-slope topography, because they cause heavy damages on infrastrucutes. Three largest landslides (Nan'notani, Neoshiratani and Tokuyamashiratani landslides) are know to occur during last one hundred years in the Mino terrane. Two of them occurred at the sites with specific geologic structures : the resistant limestones against weathering and erosion are thrust over the weak melanges. These geologic structures formed at the time of accretion, and are useful to predict the future slides in the Mino terrane. Similar geologic structures are present in the Funafuseyama, Ibukiyama, Ryozensan and Fujiwaradake areas. Very large-scale landslides in the western slope of Ibukiyama dammed-up the Anegawa River flowing on the foot of the mountain, and formed landslide lakes. We measured the 14C ages of the lake sediments and revealed the ages of the lakes to be 34,000 and 5,200 years before present. The Sekigahara pass south of Ibukiyama has been the most important traffic route connecting eastern and western Japan, and many ancient documents described landslide events at Ibukiyama. We analyzed these documents and found that the landslide events occured at about 890, 1500, 1700 and 1909 (Anegawa earthquake) AD. These events made no landslide lakes, and the events 34,000 and 5,200 years before present making the landslide lakes were quite large in scale