Three Large Historical Landslide Dams and Outburst Disasters in the North Fossa Magna Area, Central Japan

Large landslides or debris flows caused by heavy rainfall or earthquakes often block rivers in mountainous areas and form landslide dams. The area upstream of the landslide dam is submerged under water and the downstream area is flooded when the landslide dam breaks. In recorded history, as many as 22 landslide dams have formed upstream of the Shinano River and the Hime River, in the northern part of Nagano Prefecture in central Japan, and all except three have subsequently broken. This abundance of landslide dams is probably caused by the geotectonic background of this area, which is located at the western end of the “Fossa Magna” major tectonic line. In this study, we examined three large historical landslide dams and outburst disasters in the north Fossa Magna area.