赤石山地・地蔵ヶ岳東麓で奈良-平安時代に発生した大規模岩屑なだれ
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概要
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A thick (> 20 m) and poorly sorted gravel bed composed of angular granite clasts forms an elongated fluvial terrace-like surface along the Dondokosawa River in the east of Mount Jizo (2764 m asl), part of the Akaishi Range in central Japan. The gravel bed is ca. 3.1 km long and exists over a range from 1100 to 1900 m asl. The ground surface of the gravel bed produces openwork fields like block streams, with ragged depressions and ridges from several to ten meters high. There is debate on whether this gravel bed is of glacial origin or debris flow origin, and the age of the gravel bed has not yet been established. In this study, the author examined the geology and geomorphology of the gravel bed and carried out radiocarbon dating to re-evaluate its origin and age. Rock clasts in the gravel bed are well brecciated and exhibit jigsaw cracks. Neither cross bedding nor imbricated structure is found. Calendar calibrated ^<14>C ages obtained from the gravel bed and other beds closely related to the gravel bed indicate that the gravel bed was formed in the period from cal AD 780 to 870. Comparison of the morphological and sedimentary characteristics of the gravel bed with that of gravel beds previously studied in other high mountains of Japan, the Himalayas, the Andes, and the European Alps suggests that the gravel bed was formed by a large debris (rock) avalanche that occurred in the head scarp near the summit of Mount Jizo (1900-2300 m asl). Its frictional equivalent coefficient and volumetric magnitude were 0.33 and 1.92 × 10^7 m^3, respectively. The avalanche deposit also created a natural dam across the Odanasawa River parallel to the Dondokosawa River, and the resulting backwater formed a small lake and/or floodplain. It is hypothesized that the lake and/or floodplain survived up to ca.1030 years after the avalanche, following which full emergence by dam breaching occurred during the period from cal AD 1665-1807, or a little later, after cal AD 1807. Torrential rain or paleoearthquakes are considered a possible trigger of the avalanche occurrence.
- 2012-07-25