地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
115 巻 , 1 号
選択された号の論文の12件中1~12を表示しています
  • 工藤 崇, 植木 岳雪, 宝田 晋治, 佐々木 寿, 佐々木 実
    2006 年 115 巻 1 号 p. 1-25
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    This study shows the stratigraphy, petrography, whole-rock chemistry and paleomagnetic polarity of pyroclastic flow deposits during the latest Pliocene to Middle Pleistocene in the southeastern foot area of the Hakkoda Caldera, Northeast Japan. Six pyroclastic flow deposits are identified in this area : Kumanosawa Pyroclastic Flow Deposits (Ks), Takatoge Pyroclastic Flow Deposits (Tk), Osegawa Pyroclastic Flow Deposits (Os), Hakkoda-Ose Pyroclastic Flow Deposits (Hto), Hakkoda 1st-stage Pyroclastic Flow Deposits (Ht1) and Hakkoda 2nd-stage Pyroclastic Flow Deposits (Ht2), in order of decreasing age. These pyroclastic flow deposits have dacitic to rhyolitic compositions, and show distinct modal compositions and whole-rock major element chemistry in each. Based on stratigraphy, topography and paleomagnetic polarity, eruptive ages of the pyroclastic flow deposits are estimated to be as follows : Ks, 1.95-1.77 Ma; Tk, 1.77-1.07 Ma ; Os and Hto, 0.99-0.78 Ma ; Ht1, 0.76 Ma ; Ht2, 0.40 Ma. The source calderas of the Ks, Tk, Os, and Hto can be estimated from petrological features. The source of the Ks is neither the Hakkoda Caldera nor Okiura Caldera, and is probably an unidentified caldera. The source of the Tk may be a low gravity anomaly area “Nenokuchi Caldera” located northeast of Towada Caldera. The source of the Os is not the Okiura Caldera. The source of the Hto is the Hakkoda Caldera. This study suggests that the two large-scale pyroclastic flow deposits during 1-2 Ma erupted from previously unidentified calderas (one is possibly Nenokuchi Caldera). This proposes a revised volcanic history in which the activities of caldera volcanoes overlap each other in the Hakkoda-Towada Volcanic Region.
  • 濱田 崇臣, 本橋 哲郎, 渡邊 眞紀子, 伊藤 洋
    2006 年 115 巻 1 号 p. 26-40
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    Records of short-term environmental changes caused by urbanization and industrialization are expected to be preserved in lake sediments. Besides, a consistent, high-resolution baseline record is required to interpret extreme influences of human impact such as heavy metal pollution and eutrophication. In this study, the numerical dates of the core samples taken from Lake Teganuma are determined using the 210Pb and 137Cs dating method. Concentrations of heavy metals are obtained for centrifuged water samples with wet core samples, and for HCl and acetic ammonium extracts with oven-dried core samples, to clarify the existing state and the mobility of metals. Total carbon content, total nitrogen content, and C/N ratio are also obtained for ovendried core samples to analyze the influence of human impact on water deterioration of Lake Teganuma. According to the analytical results for total carbon, nitrogen, and C/N ratio, the eutrophication in Lake Teganuma started around 1946 and shifted further to a critical condition in the late 1960's. Zn and Cu pollution began around 1955, while serious Cr pollution occurred around 1963. These responsible factors for the water environmental changes in Lake Teganuma represent stresses of population increase and drainage closure due to reclamation.
  • 山口 正秋, 須貝 俊彦, 大上 隆史, 藤原 治, 大森 博雄
    2006 年 115 巻 1 号 p. 41-50
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    Three-dimensional structures of the Latest Pleistocene to Holocene shallow marine and fluvial sequence at the Nobi Plain, Central Japan, are reconstructed from 510 boring logs using a 1 km × 1 km grid system on GIS. The sequence unconformably covers the upper Pleistocene Atsuta formation, and consists of 5 depositional units - basal gravel bed, Lower Sand (LS), Middle Mud (MM), Upper Sand (US), and Top Mud and Sand (TM/TS), in ascending order. The basal gravel bed is the fluvial gravel bed, part of which fills a valley cutting the depositional surface of the Atsuta formation. The depositional surface of the gravel bed is slightly undulating, and is dissected in the north-south direction by incised valleys, which were formed during the Last Glacial Maximum. Unit LS, mainly the fluvial deposit, fills the incised valleys on the basal gravel bed. The upper boundary of LS is fiat and tilts toward the Yoro active fault, which is the western boundary of the Nobi Plain. Unit MM is mainly composed of prodelta mud. There is westward tilting and thickening of the unit MM. The Unit US is submarine deltafront sand, and is thickly distributed along grooves in the Unit MM. The thick part of the Unit US shows the main axis of the delta progradation or the main river channels. Unit TM/TS is a fluvial deposit covering the upper surface of the unit US. In this study, we confirm the effectiveness of the method using many boring logs and GIS to reconstruct the three-dimensional pattern of valley fill.
  • 鶴我 佳代子, 笠原 順三, 三ケ田 均, 山岡 耕春, 藤井 直之
    2006 年 115 巻 1 号 p. 51-71
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    Large earthquakes along the subducting plate boundary occur repeatedly in the area of an asperity that consists of a strongly coupled zone between two plates. Other areas along the subduction zone are considered to be stable-quasi-stable slip region, which is called a non-asperity, might release strain energy caused by oceanic plate subduction. The physical states of large asperities under the ocean are not well known at present because of the lack of offshore stationary observation networks (e.g., geodetic, seismic and electromagnetic networks).
    Strong PP reflections from the subducting plate boundary were found in aseismic zones along the Japan Trench and in the slow slip region in the Tokai region. These features suggest the presence of low-Vp/soft materials and/or fluid along the subducting plate boundary. Such regions might cause continuous or intermittent aseismic slow-slips. If we can map areas of strong PP reflections from observations such as refraction-reflection studies using Ocean Bottom Seismometer (OBS) -airgun surveys, we will be able to obtain the distribution of asperities along the plate boundary.
    Assuming that slip acceleration at non-asperity regions might trigger a large earthquake at adjacent asperities, a sudden change of physical states in a non-asperity region might suggest a high probability of plate-boundary earthquakes. Changes due to slip acceleration might be detected by continuously monitoring seismic reflection intensity at non-asperity regions. To perform continuous monitoring, we propose the Accurately Controlled Routinely Operated Signal System (ACROSS), with an integrated active monitoring method using continuously transmitting seismic and electromagnetic sinusoidal waves, which are accurately controlledby a GPS clock with a sophisticated signal analysis method.
    The ACROSS seismic source at Toki city in central Japan has been operated continuously for more than 2 years. A field experiment in the Tokai region, central Japan, using this transmission method provided sufficient S/N ratios for the Pg phase traveling 60 km through stacking the data for one month.
    Submarine cable OBS systems near the trenches enable us to continuously monitor seismic reflection signatures provided by ACROSS systems located on land. The planned submarine cable OBS in the Tonankaki region might be a good real-time receiver system.
    The Exploration of Asperities-Reflectors System (EARS) is proposed for integrating the necessary research components-mapping, monitoring, and real-time continuous monitoring of the Earth's crust. In this paper, we describe the analytical method and important points in such a study.
  • 春山 成子, 志田 健
    2006 年 115 巻 1 号 p. 72-86
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    The Mekong delta is experiencing flood damage at a time when the population is growing and farmland is expanding. Because the Mekong River Delta is one of the important grain belts in Vietnam, planning disaster mitigation is government policy. The purposes of this study are to present a method of visualizing flooded areas, classify geomorphology, and make flood risk map using JERS-1 SAR of the Mekong River Delta. JERS 1 SAR data, including geo-reference, noise reduction, re-sampling, and calculation of Normalized Radar Cross Section (NRCS), were processed by TNTmips. The flooded area in the Mekong River Delta is visualized and flood dynamics are discussed. The relation between NRCS and micro-geomorphology was clarified by a field investigation and a geomorphologic land classification map is presented. After evaluating flood risk in four levels, a flood risk map was created by visualizing the evaluation. Differences between NRCS of JERS 1 SAR before and after a flood are calculated, and the flooded area is visualized for differences lower than - 1.8 (dB). This value (-1.8 dB) was defined by a field investigation. Flooded area maps for 1995, 1997, and 1998 were created. The relation between NRCS and geomorphology was clarified by field investigations, and a detailed geomorphologic map in the Mekong delta was created with reference to the geomorphologic land classification map.
  • 湊 進
    2006 年 115 巻 1 号 p. 87-95
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    Nationwide in-situ measurements of terrestrial gamma ray dose rates have been carried out using a scintillation counter. A database of over 4300 entries has been compiled by adding data taken from literature to the data collected by the mentioned survey. A contour map of dose rates in Japan calculated from the database is presented along with a table of dose rates for each bedrock type.
  • 日本地学史編纂委員会 , 東京地学協会
    2006 年 115 巻 1 号 p. 96-109
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 下川 浩一, 佐藤 正, 江口 孝雄
    2006 年 115 巻 1 号 p. 110-115
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 高橋 雅紀
    2006 年 115 巻 1 号 p. 116-123
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
    The Euler pole position of the Philippine Sea Plate (PHP) relative to the stable Eurasian Plate (EUP) between 15 and 3 Ma can be estimated at around 150°E, 36°N, on the basis of the geological constraint that the intersection of the Izu-Ogasawara Arc with Southwest (SW) Japan has not moved from South Fossa Magna since 15 Ma. The timing of the migration of the Euler Pole to its present location (154°E, 47°N) should have occurred at 3 Ma because the fore-arc basin in SW Japan was once interrupted by the Kurotaki Unconformity at 3 Ma.
    PHP moves northwestward and subducts beneath SW Japan at a convergent rate of 4 cm/yr. The Izu-Ogasawara Trench (IT) also moves at the same rate as the westward component (ca. 3 cm/yr.) of the PHP motion. Both the trench-trench-trench (TTT) triple junction and the Japan Trench (JT) should migrate westward, because the thick, cold, and sturdy Pacific Plate (PAP) has never been cut by the transform fault at the TTT junction. Northeast (NE) Japan would also move westward because tectonic erosion along JT would not be sufficient for westward migration of the JT. Thus, the present PHP movement causes the westward migration of IT, TTT junction, JT and then NE Japan. This westward motion of NE Japan against the sturdy oceanic lithosphere of the Japan Sea has caused an E-W contraction of NE Japan since northward motion of PHP changed to NW at 3 Ma.
    It is expected that rifting of the thin, heated lithosphere of the Izu-Ogasawara Arc would reach break-up before the thick, cold lithosphere of the PAP would be torn by the right-lateral transform fault at the TTT junction. Once rifting reaches break-up, the northwestward movement of the PHP would be compensated by back-arc spreading, and this motion would not propagate to the IT, the JT nor NE Japan. Therefore, the present E-W contraction in Japan would cease in the geologically near future when back-arc rifting along the Izu-Ogasawara arc reaches break-up.
    McKenzie and Morgan (1969) discussed how the TTT triple junction was unstable except under a few uncommon geometrical and kinematic conditions. However, the PHP actually selected this particular Euler pole position at 15 Ma, and the TTT triple junction had been stable for more than 10 m.y. Although the present TTT junction is in an unstable condition, it would become stable again through back-arc basin spreading of the PHP in the geologically near future. Thus, the TTT triple junction offshore central Japan, which controls tectonics of Japan, would be in a stable state in nature.
  • 笠原 順三
    2006 年 115 巻 1 号 p. Plate1-Plate2
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 山口 正秋, 須貝 俊彦, 大上 隆史, 藤原 治, 大森 博雄
    2006 年 115 巻 1 号 p. Plate3-Plate4
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
  • 笠原 順三
    2006 年 115 巻 1 号 p. Plate5
    発行日: 2006/02/25
    公開日: 2009/11/12
    ジャーナル フリー
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