JpGU-AGU Joint Meeting 2017

Presentation information

[JJ] Oral

H (Human Geosciences) » H-DS Disaster geosciences

[H-DS17] [JJ] Geohazards in humid, tectonically active countries and their precursors

Wed. May 24, 2017 3:30 PM - 5:00 PM 102 (International Conference Hall 1F)

convener:Masahiro Chigira(Disaster Prevention Research Institute, Kyoto University), Satoru Kojima(Department of Civil Engineering, Gifu University), Hiroshi YAGI(Faculty of Art, Science and Education, Yamagata University), Taro Uchida(National Institute for Land and Infrastructure Management), Chairperson:Satoshi Ishimaru(Geological Survey of Hokkaido)

3:30 PM - 3:45 PM

[HDS17-07] Development history of ridge-top depressions east of Kamikochi, central Japan: Correlation between Nagakabe Ridge and Tokugo Pass

*Satoru Kojima1, Hidehisa Nagata2 (1.Department of Civil Engineering, Gifu University, 2.Fu-Sui-Do Co. Ltd.)

Keywords:deep-seated gravitational slope deformation, double ridge, landslide

The ridges between Mt. Chogatake and Tokugo Pass in the Japanese Northern Alps, about 2,500 m above sea level (asl), have round-top and steep-slopes. Deep-seated gravitational slope deformations with topographic features such as multiple ridges and ridge-top depressions are well-developed on the NE-SW trending ridges, because the basement rocks in this area are Jurassic accretionary complexes with bedding planes generally striking NE-SW. The sediments accumulated in the ridge-top depressions were cored by using hand-auger boring system at the 2,000 m (asl) point on the Nagakabe Ridge (Point A) and at the 2,150 m (asl) point to the NE of Tokugo Pass (Point B), and the lithology of the sediments were described and the refractive index of volcanic glasses in very-fine sand fractions were measured. The sediments at Point A are mostly composed of silty to sandy mud, although those at Point B are volcani-clastics. Two cores drilled at Point A have peaks of volcanic glass contents of the K-Ah tephra (7,300 cal BP) at 67 and 90 cm depth, respectively, while the core at Point B includes K-Ah glass at all depths. The sediments at Point B are probably originated by the volcanic activities of Mt. Yakedake, which is located about 9.5 km WSW from Point B, about 2,000-5,000 years ago on the basis of their sizes and compositions. The sediment accumulation rates are calculated ca. 0.1 mm/y for the points A, and faster than 0.26 mm/y at Point B. The difference is also related to the distance and direction from Mt. Yakedake of these points.