HGM01-01

日本地球惑星科学連合2016年大会

日本列島の花崗岩山地における化学的風化速度：宇宙線生成核種と地球化学的物質収支法に基づく推定
Chemical weathering rates in granitic mountains in Japan: Estimation from cosmogenic
radionuclides and geochemical mass balance method*八反地 剛1、松四 雄騎2、佐藤 昌人3、小口 千明4、松崎 浩之5*Tsuyoshi Hattanji1, Yuki Matsushi2, Masato Sato3, Chiaki T Oguchi4, Hiroyuki Matsuzaki5
1.筑波大学生命環境系、2.京都大学防災研究所、3.筑波大学大学院生命環境科学研究科、4.埼玉大学理工学研究科、5.東京大学総合研究博物館タンデム加速器分析室
1.Faculty of Life and Environmental Sciences, University of Tsukuba, 2.DPRI, Kyoto University,
3.Graduate School of Life and Environmental Sciences, University of Tsukuba, 4.Graduate School ofScience and Engineering, Saitama University, 5.MALT, the University of Tokyo
Recently denudation rates or exposure ages in mountains underlain by granite were estimated fromcosmogenic radionuclides. Many studies also combined this method with geochemical mass balance
approaches and estimated chemical weathering rates in granitic mountains, assuming steady state
conditions. We applied the modified methods to three mountainous areas (North Alps, Rokko
Mountains, and Abukuma Highland) in Japan with various tectonic and climatic conditions. The threesurveyed areas include small target basins; 9 basins in North Alps, 4 basins in Rokko Mountains, 2basins in Abukuma Highland. All these basins are underlain by biotite granite, and basin-wide
denudation rates were estimated from cosmogenic 10Be in stream sediment. We collected stream
sediment as weathered material in these basins and several fresh bedrock samples in each study
area. Stream sediment is sieved into three fractions; F: 0.063 - 0.25 mm, M: 0.25 - 2.0 mm, C: 2.0- 9.5 mm. We measured chemical compositions of fresh bedrock and the fractions of stream sedimentin each basin with X-ray fluorescence analysis (Phillips PW2400, Saitama Univ.), and calculated
chemical depletion fraction (CDF), which is defined as the ratio of chemical weathering rate tototal denudation rate. Both grain-size distribution of soil and chemical composition in the threefractions of stream sediment were used for calculating mean concentration of immobile elements inweathering materials. Titanium was treated as an immobile element for calculation of CDF. Chemical
weathering rates generally increase with increasing basin-wide denudation rates, although they are
more diverse for basins with high denudation rates (> 1000 mm/kyr). The values of CDF ranged from0.02 to 0.40 for North Alps, 0.12 - 0.33 for Rokko Mountains, and 0.43-0.67 for Abukuma Highland.
Contribution of chemical weathering to total denudation is relatively high in Abukuma Highlandwhere landscape is gentle and denudation rates are low (< 100 mm/kyr). The causes for decrease in
CDF in North Alps would be; (1) physical weathering processes due to cool climate in high elevation
zones, and (2) erosion of saprolite after landsliding in basins with high denudation rates.
キーワード：化学的風化、宇宙線生成核種、削剥速度
Keywords: chemical weathering, cosmogenic radionuclides, denudation rate©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会

琉球列島久米島東部における堡礁地形の高解像度マルチビームマッピング
High-resolution multibeam bathymetric mapping of barrier reef geomorphology of eastern
Kume Island, the Ryukyus*菅 浩伸1、長尾 正之2、藤田 和彦3、長谷川 均4、中島 洋典5、堀 信行6*Hironobu Kan1, Masayuki Nagao2, Kazuhiko Fujita3, Hitoshi Hasegawa4, Yosuke Nakashima5, Nobuyuki
Hori6
1.九州大学大学院比較社会文化研究院、2.産業技術総合研究所地質情報研究部門、3.琉球大学理学部物質地球科学科、4.国士舘大学文学部地理学教室、5.有明工業高専、6.奈良大学
1.Graduate School of Integrated Sciences for Global Society, Kyushu University, 2.ResearchInstitute of Geology and Geoinformation, National Institute of Advanced Industrial Science andTechnology, 3.Department of Physics and Earth Sciences, Faculty of Science, University of theRyukyus, 4.Department of Geography and Environmental Studies, Kokushikan University, 5.Ariake
National College of Technology, 6.Nara University
In contrast to the vast knowledge that has been accumulated regarding terrestrial landforms, few
previous studies have dealt with shallow seafloor landforms. Since a broadband multibeam
echosounder (Sonic 2022, R2 Sonic, LLC) and its accessory system were introduced to H. Kan’slaboratory in 2010 using JSPS Grant-in-Aid for Scientific Research A, we have been conducting
high-resolution multibeam bathymetric surveys around the Ryukyu Islands, southern Japan, andmapping the coral reef geomorphology with a horizontal grid size of 1–2 m.
Accurate descriptions of coral reef geomorphology improve our understanding of reef environments.
Here, we introduce our mapping project, which covers of a total of 39 surveyed areas across fiveislands, and present an example illustrating the barrier reef and island shelf geomorphology ofeastern Kume Island (1.8 ×6.5 km bathymetric area, with a depth range of 0.4–161.3 m) combined withSCUBA and VTR observations.
The bathymetric area comprises two contrasting reef edge formations: a double reef in the westernarea, and a deep breakwater reef in the eastern area. The latter shows the high-energy window
condition which is accompanied by spur and groove formation in the lagoon, formation of circularspits or bowl-shaped depressions behind the lagoon patch reefs, and traces of movement of coral
boulders in the lagoon following typhoon storm surges. These observations enable us to understandand reconstruct the paleo-geomorphology and sedimentology in reefs associated with the Holocene
high-energy window, and can also contribute to predictions of coastal environmental change
associated with future sea level rise.
On the island shelf, we observed several reef terraces at depths between 80–95 m, and isolated
patch reefs at 135 m depth. The discovery of these features may contribute to reconstructions ofpast sea level and reef growth in the northwestern Pacific.
キーワード：サンゴ礁、マルチビーム測深、礁湖、海底段丘
Keywords: coral reef, multibeam ecosounder, lagoon, submarine terrace©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会

The significance of landslide dams on large Chinese rivers*David Laurence Higgitt1
1.University of Nottingham Ningbo China
Landslides forming natural dams across rivers pose considerable hazards, particularly the risk ofcatastrophic flooding after dam failure. In China, documentary records of large earthquakes andriver blocking landslides have enabled inventories to be compiled. However, the record is far fromcomplete as the remote and rugged terrain shelters evidence both of recent landslide dams and ofancient landslides which occurred before documentary records. Geospatial technologies enable thedetection of anomalies in fluvial geomorphological systems which may indicate the past influence ofriver blocking landslides. Field evidence of sediment deposited behind landslide dams, enables thelocation of palaeo-landslides to be inferred. Recent discovery of an ancient landslide which dammed
the main stem of the Yangtze River, China (crest height 200 m; lake volume 11.4 ±1.3 km3) indicatesthat very large rivers have –and can be - impacted by persistent river-blocking landslides. As themagnitude of this landslide dam was much greater than examples in the historical record, it isargued that the potential for large earthquakes to generate enormous river-blocking landslides may
be underappreciated as a mega-hazard.
Keywords: landslide dam, flood hazard, Yangtze River©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-03 -HGM01-04

日本地球惑星科学連合2016年大会

Solitary Step in Bedrock Rivers*Yan Ma1, Norihiro Izumi1
1.Hokkaido University
A solitary step is often observed to be formed in the mixed alluvium and bedrock rivers, which will
be described later as “bedrock river” for simplification. Once a step is formed on the river bed,
due to bedrock erosion, it is wondered whether the step is maintained and propagates in the
upstream direction, or change its profile continuously. Compared with the bed of alluvial rivers
which is easily eroded by flow, the mechanisms of bedrock erosion are more complicated, mainly
including abrasion by bedload, plucking and macroabrasion. Based on this complexity, in this paper,
we employ the Macro-Roughness Saltation Abrasion Alluviation model to analyze the evolution of aconvex solitary step, featuring a subcritical region upstream and supercritical region downstream,in abrasion-dominated bedrock rivers. From the results of the analysis, we obtain that the convex
solitary step cannot migrate in the direction of upstream with maintaining its profile. In
addition, the erosion rate tends to be constant in the downstream reach, while the erosion rateincreases drastically in the upstream reach. From a numerical simulation, it is found that theslope of the upstream region becomes larger and the curvature of the step downstream reach ismaintained in relatively long time.
Keywords: Bedrock river, Solitary step

©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-04 -HGM01-05

日本地球惑星科学連合2016年大会

Velocity and Sediment Concentratin Distributions of Turbidity Currents*Sytharith Pen1, Norihiro Izumi1
1.Hokkaido UniversityAbstract. In this work, we present a mathematical model to describe thedistribution of the velocity and the particle concentration of the turbiditycurrents. The turbidity current is described by the Navier-Stokes equations
with the use of Boussinesq approximation to approximate the e ect of sediment concentration on theow, the di usion/dispersion equation for thesediment transport, and k- turbulence model for the problem closure. The
model is simpli ed by a roof-like con guration which allows a steady equilibrium state of theow. The numerical solution of the distribution of theowvelocity, the sediment concentration, the turbulence energy, and the dissipation rate are obtained by the nite-di erent numerical scheme. The study
shows that the entrainment capacity of sediment due to turbulence and thedampening characteristic of turbulence is aliated with Richarson number
and non-dimensional settling velocity. The model is capable of capturing thefundamental characteristics of the turbidity current which provides a vital
base state for the analysis of morphodynamics generated by the turbidity
current.
Keywords: turbidity current©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-05 -HGM01-06

日本地球惑星科学連合2016年大会

異種災害の比較と類似性の抽出
Comparison of different kind disasters and find of the similarity*瀬戸 真之1、高木 亨2*Masayuki Seto1, Akira Takagi2
1.福島大学うつくしま福島未来支援センター、2.熊本学園大学
1.Fukushima Future Center for Regional Revitalization, Fukushima University, 2.Kumamotogakuen
University
This study compared the damages and revival processes of Kawauchi village with Miyake Island.
Kawauchi village was damaged by the 2011 Off the Pacific Coast of Tohoku earthquake. On the otherhand, Miyake Island was damaged by volcanic eruption.
Kawauchi village is located in Fukushima prefecture, northeastern Japan. The population of 2010 was2997 people. In this village, depopulation and aging advanced. The main industry was tertiary.
Kawauchi village damaged by the 2011 Off the Pacific of Tohoku earthquake. The most serious damage
was nuclear hazard. This village evacuated outside in 16, March, 2011. The refuge continued untilJanuary, 2012.
Miyake village is the Island of active volcano belonging to Tokyo city. The population of 1999 was3872 people. The main industry before the volcanic eruption was sightseeing business. Miyake
volcano erupted in June, 2000. A large quantity of volcanic gas and volcanic ash were spouted out.
Landslide disasters were occurred frequently. All islanders of Miyake village evacuated outside of
Miyake Island in September, 2000. The refuge continued until February, 2005. The service industry,
the retail trade, the wholesale trade were decreased greatly. A village office moved under theinfluence of volcanic gas.
This study divided the damages of the two villages into two types. The first type affected theindustry. Another one affected the community. We found same kind of damages and revival processes
in different disaster.
キーワード：東日本大震災、三宅火山、災害
Keywords: The Great East Japan Earthquake, Miyake Volcano, Disaster©2016. Japan Geoscience Union. All Right Reserved.

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