9:00 AM - 9:15 AM
[MIS15-01] Mid-Holocene large-scale landslide discovered in the Amegochi River basin, Mount Tenshu, Yamanashi Prefecture, Japan
Keywords:Landslide, Radiocarbon dating, Southern Fossa Magna, Paleoearthquake, Active fault
The major mountain ranges of the Southern Fossa Magma, which are close to the Suruga Trough and bordered by active faults, contain many large- to medium-scale landslides. In this study, we studied the Amegochi River basin in the Tenshu Mountains and obtained ages of fossil wood fragments in the landslide materials. The Amegochi is a steep river flowing along the west face of Mt. Kenashi. A rockslide type landslide with an estimated original landslide body of 1.3×105 m2 is located on the right bank in the middle of the river. According to the outcrop observation, it is estimated to be about 10m thick of landslide materials. Thus it is understood that volumetric magnitude of this landslide is in the lower class of the large-scale landslide. At Loc. 1 (35°26'8"N, 138°30'16"E), on the margin of landslide body, the landslide material consists of poorly sorted coarse debris with silt- to coarse sand-sized matrix. The debris also contains fossil tree, and the outermost parts of two samples (AME1 and 2) were dated. AME1 and AME2 showed 5694±29 14C = 6560-6400 cal BP and 5662±28 14C = 6530-6320 cal BP respectively (by IntCal20 BP at 2 sigma). Landslide would be occurred in the period from 6530 to 6400 cal BP. The nearby Fujikawa-Kako Fault Zone generated paleoseismic activity in this period, so that the relationship between paleoearthquake and landslide as well as significant paleo heavy rain must be further considered.