Historical Large-Scale Landslide on the Northwest Face of Mt. Mitou, Southern Kanto Mountains, Central Japan

Abstract

Rugged peaks occur in the southern Kanto Mountains, 60–90 km west of central Tokyo. The geology of this region mainly comprises NE-inclined sedimentary rocks of the Cretaceous Shimanto Belt. This region is considered to be prone to long-term gravitational rock deformation and resultant catastrophic landslides. However, few researchers have focused on these geomorphic and geologic features. To provide new insights into the geomorphology and erosion-control engineering in this area, we investigated the geomorphology, geology, and age of a large-scale landslide (volume: 2.2×10⁶ m³) that occurred in the area northwest of Mt. Mitou, the central part of the southern Kanto Mountains, and discuss the results. Geological investigation, aerial photo interpretation, ¹⁴C dating, and interview of a local resident were conducted. This landslide occurred immediately below the subsidiary ridge of Mt. Mitou and formed a clear main scarp. The landslide debris ran downslope for about 1.5 km and filled the Tama River valley originating from the northwest face of Mt. Mitou. The main landslide body consists of a thick gravel layer of angular rock clast with jigsaw cracks and appeared to have formed a dammed lake. This landslide probably occurred before or during cal AD 1292–1399 (the Kamakura to Muromachi era in Japanese history), and thereafter secondary landslides took place during cal AD 1469–1794 (the Muromachi to late Edo era). Oral traditions preserved by local residents at the northwestern foot of Mt. Mitou describe the obstruction of the Tama River by the primary landslide and the debris flow runout from the secondary landslide.