地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
129 巻 , 1 号
選択された号の論文の9件中1~9を表示しています
表紙
  • 2020 年 129 巻 1 号 p. Cover01_01-Cover01_02
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     写真は,南米アルゼンチンのパタゴニア南部の大平原に露出する新生代暁新世(約6500万年から6200万年前)の地層から洗い出された巨大な珪化木である.場所は,アルゼンチン共和国チュブ州サルミエント市郊外にある「サルミエント化石の森」と呼ばれる自然保護区(南緯45度47分,西経68度57分)である.1880 haもの広大な敷地内には,火山灰や灰色,赤色,紫色をした砂岩層が現れていて,遊歩道沿いで地層から洗い出された無数の珪化木を見ることができる.この化石の森は,1927年に発見され1970年代頃から観光地となっていたが,2001年に州の自然保護区に指定された.ヤシ類やシダ類,針葉樹など幹からなる珪化木は,大きなものでは直径1 m大,長さ数mに及ぶものがある.この地域は昼夜の寒暖の差が激しく,地層や化石にひび割れができやすい.厳冬期には,珪化木の割れ目にしみ込んだ水分が凍結・膨張して,珪化木を細かく破壊する.その砕けた無数の小片が,まるでウッドチップのように地面を覆っている場所もある.

    (写真・説明:平田大二)

総説
  • 王 天天
    2020 年 129 巻 1 号 p. 1-19
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     During the past few decades, China has experienced several institutional changes, which have provided a distinctive background for its urban spaces to restructure. In the planned economic era, despite the strong influence of the USSR, the unique “Danwei” system was created to reflect the state of affairs of the country. A Danwei or work unit is not only a place of employment, but also provides welfare benefits such as housing, education, and health care for employees and their families. Workplace, housing, and facilities needed for daily life are usually built inside one gated enclosure, a Danwei compound, which became the basic spatial unit of urban China. Such proximity between workplace and residence indicates that a clear suburban residential area in the Western sense had not existed during the pre-reform era. Thus, urban spaces were formed to a unique cellular structure, which distinguishes itself from not only models considered in Western cities, but also socialist cities in Eastern Europe. Following reforms and opening-up, the land market started to develop from the late 1980s. Spatial differences in land prices led to a massive shift in land use, including the relocation of low-profit factories away from city centers, and the emergence of central business districts in big cities. In the late 1990s, the commercialization of housing was promoted. Welfare housing allocated by Danwei has been converted into private ownership, and suburban areas started to spring up with the new construction of commoditized housing. Under this process, a separation between home and work started to take place in urban China. Thus, suburban residential areas in the Western sense, with residents commuting long distances to a city center, have finally come into existence. However, due to government regulations on the development of low-density detached housing, this emerging suburban growth is dominated by mid- to high-density collective housing developments. In this sense, it is inaccurate to claim that residents of suburban China changed their ways of living to the distinctive suburban lifestyles found in “typical” Western or Japanese suburbs.

論説
  • 工藤 崇, 檀原 徹, 岩野 英樹, 山下 透
    2020 年 129 巻 1 号 p. 21-47
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     The stratigraphy and spatio-temporal distribution of the Early Pleistocene volcanic rocks in the east of Hakkoda Caldera, Northeast Japan were revealed. The Hachimandake Volcanic Group (HVG) was newly distinguished in this area. Volcanic activity of the HVG occurred during 2.5-1.4 Ma. Basaltic andesite magma erupted from several eruptive centers and formed several small volcanic bodies in the east of the present Hakkoda Caldera. Volcanic activity of the HVG is characterized by repeated outflows of basaltic andesite lava flows, each several meters thick. Volcanic activity of the HVG was notably smaller than that of Minami- and Kita-Hakkoda Volcanic Groups from 1.1 Ma to the present. Dacite to rhyolite pyroclastic flows were found to have occurred simultaneously during HVG activity. Those source vents are unknown.

  • ─関東・南東北に散在する和泉層群東方延長の白亜系・古第三系砂岩─
    長谷川 遼, 磯﨑 行雄, 堤 之恭
    2020 年 129 巻 1 号 p. 49-70
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー
    電子付録

     To reconstruct the tectono-sedimentary histroy of the fore-arc basin that developed along the Cretaceous–Paleogene arc-trench system in Japan, a provenance analysis is carried out by U–Pb dating detrital zircons, particularly for Cretaceous–Paleogene sandstones/conglomerates sporadically found in five distinct areas of the Kanto and southern Tohoku district, i.e., the Shoya Formation in Saku, the Kanohara Conglomerate in Shimonita, the Yorii Formation in Yorii (Kanto Mountains), the Nakaminato Group and Oarai Formation (northern Kanto), and the Futaba Group (southern Tohoku), for which geotectonic identities have been ambiguous. U–Pb dating of detrital zircons from 14 sandstone samples constrains their depositional ages to the Late Cretaceous and early Paleocene, and also their provenance. These results reveal the following new facts. 1) The Shoya Fm and Nakaminato Gr are of the Maastrichtian age; the Kanohara Conglomerate, Yorii Fm, and Oarai Fm are of the Paleocene (mostly Danian and up to Thanetian); and, the top of the Futaba Gr reaches up to the Campanian. 2) In addition to previously known partial similarities in lithofacies and fossils, newly obtained U–Pb age spectra of detrital zircons confirm that all these units represent the eastern extension of the Izumi Group in Shikoku and Kii Peninsula, and they share the same provenance dominated by late Cretaceous granitoids. 3) These new age data indicate that the spatial dimensions of the Cretaceous–Paleogene Izumi fore-arc basin extended over 1,300 km from western Kyushu to southern Tohoku along the arc, whereas its width may have reached 100 km across the arc. 4) The Median Tectonic Line (MTL) in Kanto is represented by a low-angle fault, which separates Ryoke-derived strata of the Izumi affinity from structurally underlying high-P/T Sanbagawa metamorphic rocks. 5) The MTL runs along the northern margin of the Kanto Mountains and extends further to the east, probably even into NE Japan, off-shore from northern Kanto and southern Tohoku district.

  • ─山梨県北杜市を事例としたスケール論からの考察─
    久井 情在
    2020 年 129 巻 1 号 p. 71-87
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     Japan's municipal merger policy in the 2000s created many merged municipalities without a core area, which seems to have produced more local development problems than in other municipalities. To determine their local development strategies, how they establish and use new local images in their development policies is examined. The focus is on Hokuto City in Yamanashi Prefecture as a case of merged municipalities without a core area. Hokuto City developed from a municipal merger that excluded the regional core, Nirasaki City. The jurisdiction of Hokuto City is divided by cliffs into three areas, which are represented respectively by mountains. It is difficult for the Hokuto City administration to use an image of one of these areas for fear of objections by the other areas. Therefore, it requires new images for the developmental policies of the Hokuto jurisdiction. The city administration is attempting to communicate “sunshine” as the image of Hokuto City. This was once the image of Akeno-mura, a former municipality that later became part of Hokuto, where the longest period of sunshine in Japan was recorded. The image has contributed to attracting mega solar energy systems to a national research project in Hokuto. Following the research, these solar energy systems will be owned by the Hokuto City administration and will support the city's finances by selling the electricity generated. This can be interpreted as a case of the city administration scale jumping the image of “sunshine” from Akeno to Hokuto. This scale-jumped image contributed to attracting the mega solar energy systems; then, the presence of the solar energy systems improved the plausibility of the scale-jumped image. Such scale jumping of images seems to be effective in the local development policies of merged municipalities.

  • 柳田 誠, 青柳 恭平, 下釜 耕太, 岡崎 和彦, 佐々木 俊法
    2020 年 129 巻 1 号 p. 89-122
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     The 2008 Iwate–Miyagi Nairiku Earthquake (Mj 7.2, MW 6.9) occurred in an area where no active fault had been indicated by maps and previous studies. A movement of the concealed source fault of the earthquake resulted in linear but fragmental surface ruptures over a short range. However, geodetical observations revealed a wide uplift at the mountainous side of the surface ruptures. A geomorphological and geological approach is discussed for estimating the active fault length in the area where the displacement accompanied by the active fault movement hardly appears on the ground surface. In order to estimate the length of the concealed fault, a series of geomorphological and geological observations are carried out focusing on the following points: 1) Wide uplift zone shown by relative heights of fluvial terrace surfaces. 2) Folded zone in Neogene strata with the eastern side steeply inclined. Both are presumed to be the results of surface deformation caused by the concealed fault. Additionally, the folded zone is sometimes accompanied by a flexural-slip fault that causes a displacement in the late Quaternary terrace gravel layer. As a result of integrated surveys on these points, it is suggested that the concealed fault is 30 kilometers long from Shitomae-gawa river to Sanhasama-gawa river with N–S strike and west dip. The fault length is equivalent to Mj = 7.3 from the empirical relationship between earthquake magnitude and fault length (Matsuda's formula), which is comparable to the magnitude of the 2008 Iwate–Miyagi Nairiku Earthquake.

短報
  • 高橋 尚志, 須貝 俊彦
    2020 年 129 巻 1 号 p. 123-140
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     Based on a sedimentological analysis and tephrochronology, development of the Ara River terrace is reconstructed focusing on the Nagatoro Gorge between the Chichibu Basin and the Arakawa Fan. Terrace treads along the Nagatoro Gorge can be classified into Oy1, Oy2 and Hg. Oy1 comprises eroded remnants of the fill terrace probably formed in Marine oxygen Isotope Stage (MIS) 6. Oy2 and Hg are toe-cut terraces formed by tributaries from MIS 3 to MIS 2 and early Holocene, respectively. The possible range of the elevation of the Ara River floodplain in the Nagatoro Gorge during MIS 3 and MIS 2 is reconstructed by extending the cross-sectional profiles of Oy2 terrace treads. The longitudinal profile of Ara River in the Nagatoro Gorge during MIS 3 and MIS 2 continues to that of paired terraces formed in the last glacial period: Kagemori Terrace in the Chichibu Basin and Miizugahara-1 Terrace in the Arakawa Fan. The profile of the Ohnohara Terrace of the Chichibu Basin and the Nagatoro Gorge continues to that of the Hanazono Terrace of the Arakawa Fan. As a result of continued lateral erosion of the Ara River during the two periods, wide floodplains formed throughout the three segments of the Chichibu Basin, the Nagatoro Gorge and the Arakawa Fan in the last glacial period and the early Holocene. During these periods, most sediments passed through the Nagatoro Gorge and were discharged onto the Kanto Plain. The river began to incise in the Holocene throughout all three segments.

  • 松本 良, 青山 千春
    2020 年 129 巻 1 号 p. 141-146
    発行日: 2020/02/25
    公開日: 2020/03/17
    ジャーナル フリー

     An initial estimate of the amount of methane carried by a single methane plume was calculated to be 4 × 109 g (4,000 ton CH4) to 2 × 109 g (2,000 ton CH4) per year (Aoyama and Matsumoto, 2009), based on quantitative echo sounder measurements of the methane plume and bubble capture and release experiments. The estimate generated considerable interest because it suggested the potential importance of plumes as natural gas resources. However, a critical mistake in the calculations was found in converting mole amounts to weight of methane. Revised and corrected estimates of annual methane transported by a single plume are between 2.63 × 106 g (2.63 ton CH4) to 1.60 × 106 g (1.60 ton CH4), which are only 0.07% to 0.08% of the original estimates. For comparison, the revised amount of methane discharged from an individual methane seep is estimated based on direct measurements of gas bubbles from seep sites at Joetsu Knoll and Umitaka Spur, Joetsu basin. A total of 200 ml to 1,150 ml of bubbles were captured within 642 to 481 seconds. Total gas flux depends on the composition of the bubbles. Assuming pure gas, the annual discharge is estimated to be 0.71 ton to 4.84 ton CH4. If the bubbles consist of pure hydrate, the seepage is slightly higher at 1.15 ton to 8.83 ton CH4 per year.

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