Evidence Discovered that the Manaitayama Rockslide in Northern Nagano Prefecture Occurred Prior to the AD 1502 Echigo-Nanseibu Earthquake: Short Discussion Based on Newly Obtained Radiocarbon (¹⁴C) Dates

A large-scale rockslide occurred at the western side of the summit of Mount Manaitayama in the Otari Mountains. Several estimates have been proposed regarding the timing of the rockslide. Two small plant macrofossils were discovered directly below dammed-lake deposits formed by the rockslide, and their radiocarbon (14C) ages were measured. Using the latest data set and a calibration program, it is found that the two samples died between cal AD 1427 and 1442. Considering a tree-ring wiggle-matched 14C single date of the tree trunk sample obtained from the same stratigraphic position by a previous study, it can be concluded that this massive slope failure occurred before the first half of the 15th century. On the other hand, it is unlikely that the 1502 Echigo-Nanseibu Earthquake, which is thought to have occurred near Manaitayama based on another study, caused the Manaitayama rockslide. At present, there are no historical records indicating that strong earthquakes occurred in this area in the first half of the 15th century. Thus, it is necessary to continue to investigate potential causes of the Manaitayama rockslide such as torrential rain or rapid snow ablation.