地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
118 巻 , 4 号
選択された号の論文の12件中1~12を表示しています
表紙
論説
  • 山下 亜紀郎
    2009 年 118 巻 4 号 p. 611-630
    発行日: 2009/08/25
    公開日: 2010/03/26
    ジャーナル フリー
     The purpose of this study is to propose a method for analyzing regional factors to understand the quantitative characteristics of water supply-demand system on watershed scale. The Naka River basin and the Kinu-Kokai River basin are the study areas selected for a comparative discussion. In particular, urban residential water supply systems of Mito and Mitsukaido cities, which are located in far downstream areas of the Naka and Kinu-Kokai River basins, respectively, are surveyed. In this study, three regional factors are examined— landform, land use, and water rights. Concretely, basin form ratio and basin relief ratio by main tributary basin are calculated first. Then, land use combination types by tributary basin are clarified using modified Weaver's method. Finally, the spatial and quantitative characteristics of water demand in the Naka and Kinu-Kokai River basins are analyzed with data on water rights. As a result of the case study, the analytical indicators and methodological approach this study proposes demonstrate their utility in a discussion of problems associated with urban water resources on watershed scale.
  • トウヒダ ラシード, Md. ホセイン モンスール, 鈴木 茂之, 大井 信夫
    2009 年 118 巻 4 号 p. 631-645
    発行日: 2009/08/25
    公開日: 2010/03/26
    ジャーナル フリー
     本研究はバングラデシュ,ベンガル低地中央部において,掘削によって現れた完新統露頭の地質調査を行い,古環境の変遷と相対的海水準変動の対応関係の解明に取り組んだものである。露頭の標高は測量によって得られ,-4.1mから+1.9mの範囲である。地層の層相解析,炭素同位体年代測定,花粉分析,珪藻化石分析を行った。
     中期完新世以降の相対的海水準変動による堆積環境変遷を反映した,7つの堆積相が識別された。最下部は生物擾乱の著しい砂質干潟相で-3.75m地点からマングローブの花粉化石が検出され,7570-7430cal BPの年代が得られた。潮流の影響を受ける潮間帯が復元され,中期完新世は全般的に海進傾向にあったことが理解される。この上位の泥炭質な塩水湿地相(6670-6410cal BP)は下位の干潟相を削り込むことから,一時的な小海退が推測される。この堆積物中にマングローブの花粉化石や汽水性珪藻化石を含むことから,マングローブ林をともなう塩水湿地が海側に拡大したと考えられる。この小海退後,生物擾乱を受けた泥質干潟相が堆積していることから海進が再び発生したことが認められ,これにともなって潮間帯が広がる海岸平野の発達が復元される。その後4080-4030cal BPの年代が得られた黒色泥炭層が堆積するが,草本の花粉化石が優勢になり,マングローブ林から泥炭質な後背湿地へと変化していったことが推測される。
     このようにベンガル低地の中央部にまで,中期完新世の相対的海水準上昇の影響を受けていたことが明らかになった。さらにその海進期間中に小海退がみいだされた。
  • 花岡 和聖
    2009 年 118 巻 4 号 p. 646-664
    発行日: 2009/08/25
    公開日: 2010/03/26
    ジャーナル フリー
     The purpose of this article is to evaluate a social simulation using a dynamic spatial microsimulation model for predicting demolitions of Kyomachiya, which are traditional wooden townhouses and core elements of the historical landscape of Kyoto City, Japan. This model is also applied to estimate the number of Kyomachiya surviving when preservation policies are introduced. The results are summarized as follows:
     (a) Spatially disaggregated synthetic microdata of Kyomachiya residents were constructed by combining multiple existing datasets in a manner whereby the sums of synthetic microdata agree with those of census datasets. Using synthetic microdata allows us to analyze detailed household demographics and the process of Kyomachiya demolitions at small area and individual levels.
     (b) Decision-making units such as individuals, households, and Kyomachiya can be modeled in the same way that they exist, behave, and interact with each other in the real world using object-oriented modeling. Another merit is that re-using and extending classes are possible due to object-oriented architecture.
     (c) The results of simulations show that, during the next 15 years, only 67.3% of Kyomachiya will be preserved and the proportion of the population aged 60 and over will increase from 43.9% to 51.6%. On the other hand, when a comprehensive preservation policy is implemented, those numbers are reduced to 82.5% and 49.2%, respectively. In this manner, a dynamic spatial microsimulation model is useful for understanding the process and cause-and-effect of Kyomachiya demolitions under the status quo. Furthermore, what-if simulations on the basis of Kyomachiya preservation policies help to evaluate which policy is most effective for reducing the number of demolitions.
  • 大上 隆史, 須貝 俊彦, 藤原 治, 山口 正秋, 笹尾 英嗣
    2009 年 118 巻 4 号 p. 665-685
    発行日: 2009/08/25
    公開日: 2010/03/26
    ジャーナル フリー
     The depositional process of the Latest Pleistocene to Holocene shallow-marine and fluvial successions is investigated using five 14C-dated borehole cores from the Kiso River delta, central Japan. The delta succession provides high-quality records of the transgression and regression of an enclosed delta system and reveals the development of fluvial-dominated coastal lowlands.
     Based on a facies analysis, the cored sediments are classified into five sedimentary units: A) basal gravel, B) fluvial and intertidal sand and silt, C) transgressive lag deposit, prodelta mud, and sandy silt, D) delta-front-slope sandy silt and sand, and delta-front-platform sand, and, E) delta-plain and fluvial sand and silt. Detailed age-depth curves of these cores are reconstructed based on 108 AMS 14C ages including 83 new 14C ages. These curves show patterns similar to those of temporal variations in accumulation rates, and are divided into four sections (I to IV) from the base to the top: I mainly consists of unit B with rapid accumulation (7.3-21.4mm/yr); II mainly consists of unit C with moderate accumulation (2.4-2.8mm/yr); III mainly consists of unit D with rapid accumulation (6.7-17.8mm/yr); and, IV consists of unit D and E with moderate accumulation (1.3-3.9mm/yr).
     The section boundaries of I/II and II/III, which coincide with the unit boundaries of B/C and C/D, reflect the inundation of a flood plain by seawater, developing a bay environment, and the subsequent progradation of the delta–front slope, respectively. The ages of these boundaries constrain the estimation of the expanding rate of the bay and progradation rates of the delta. The expanding rate of the bay is estimated at 10m/yr during the period 10200-7900 cal yrs BP, and the progradation rates of the delta are 3-4m/yr (6500-4100 cal yrs BP), 5m/yr (4100-1300 cal yrs BP), and 10m/yr (1300 cal yrs BP to the present). These age data indicate that the transition from transgression to regression occurred between 7800 and 7300 cal yrs BP.
     The geological cross-section along the Kiso River along with the isochrones constructed by over a hundred 14C dates indicates the following depositional history of the Kiso River delta: (1) from 10000 to 7280 cal yrs BP (K-Ah horizon) —During the expansion of the bay, onlap of unit C on unit B is visible, and unit C overlapped all the core sites just before the K-Ah fall, and (2) from 7280 cal yrs BP to present—Isochrones younger than 6500 cal yrs BP are similar to each other and cross the unit boundaries of C/D and D/E. This demonstrates that units C–E are contemporaneous heterotopic facies formed in response to delta progradation after the K-Ah fall.
  • Martin RADVANEC
    2009 年 118 巻 4 号 p. 686-699
    発行日: 2009/08/25
    公開日: 2010/03/26
    ジャーナル フリー
     この研究は,1945年8月6日,午前8時15分,広島の原子爆弾爆発による熱線の強力な影響を,2片の被爆瓦片を用いて研究したもので,この瓦片は元安川に架かる元安橋下流約100mの西岸で1977~1982年にかけて発掘されたものである。爆発から1.318秒後,爆心地で破壊された建物から衝撃波によって吹き飛ばされた大変熱くて融解した多数の破片は,元安川西岸に堆積した。この瓦片はおそらく,破壊された清病院の石塀,西向寺および(もしくは)西蓮寺からのもので,川の水で急激に冷却されたものと思われる。
     瓦片は安山岩質の火山砕屑岩からなり,その表面は3.18mmの深さまで融解していた。そのガラス質の層は,安山岩から玄武岩質安山岩の組成をもつ。これらはクリストバル石および/もしくはリンケイ石,ピジョン輝石(XFe = Fe/(Fe + Mg)= 0.37-0.44),角閃石(XFe = 0.33-0.42),曹灰長石(Ab48.2-40.6An51.8-55.5Or0-3.9)およびK-長石(Ab8.2Or91.8)が融解生成したものである。融解が3.18mmの深さにまでおよんでいることと,上記鉱物質のそれぞれ異なる融点および溶融深度から,爆発後の爆心地の物体表面の温度6287℃が算出された。この表面温度は,深さ2.68~3.18mmの区間における残存鉱物と,溶融深度の関係から導き出した。回帰直線T = -1715.1d + 6287(dは深さ)(R2 = 0.989)を外挿して求めたものである。安山岩瓦表層部の温度勾配は1mmあたり1715℃で,深さが2.86mmから3.18mmの間では,融解ガラス質と本来の鉱物質は等量である。3.64mm以下では,火山砕屑安山岩の構造および鉱物組成は,本来の組成を有している。
  • 岩月 輝希, 石井 英一, 新里 忠史
    2009 年 118 巻 4 号 p. 700-716
    発行日: 2009/08/25
    公開日: 2010/03/26
    ジャーナル フリー
     A methodology to estimate long-term hydrochemical evolution of deep underground is indispensable for the safe geological isolation of high-level radioactive waste (HLW). This study demonstrates the methodology by illustrating scenarios of past geological events, processes, and their interrelationships with present-day hydrochemical conditions. Besides, we infer long-term variations of groundwater chemistry at Horonobe, Hokkaido, Japan.
     The region is underlain mainly by Neogene to Quaternary marine sedimentary rocks, (the Wakkanai Formation (Fm) and the overlying Koetoi Fm: siliceous and diatomaceous mudstones). During various events in the geological past, such as deposition, compaction, uplift, and denudation, and the more recent Neotectonic activities in this area, highly permeable hydrogeological structures formed at depths not over than 400m below ground level in the Wakkanai Fm. The hydrogeological system can be subdivided into three hydrogeological sub-systems: (1) overlying, relatively low permeability Koetoi Fm., (2) highly permeable, upper Wakkanai Fm at depths less than 400m and (3) relatively low permeability Wakkanai Fm, at depths greater than 400m. The present-day hydrochemical conditions in each sub-system have been influenced by hydrogeological properties and hydraulic conditions over a long period. In subsystems 1 and 2, recharging with meteoric water flushed connate seawater during uplifting and denudation during the last 1Ma. In contrast, fossil seawater with one-third to one-half the salinity of present-day seawater has been preserved in subsystem 3.
     The relatively low permeability sequence in sub-system 3 was formed by the compaction of diatomaceous mudstone during subsidence prior to 1.0Ma. After that, changes of climatic conditions and geographical features would not have influenced groundwater flow. The groundwater chemistry evolved from seawater during long-term geochemical diagenesis in a relatively closed system. The long-term variations of salinity probably range from that of seawater to that of present-day groundwater. The occurrence of secondary minerals shows that the buffer reactions of carbonate and sulphide minerals have preserved the near neutral pH and reducing condition since initial diagenesis immediately after deposition of the rock formation.
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