地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
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125 巻 , 5 号
選択された号の論文の7件中1~7を表示しています
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総説
  • 田中 宏幸
    125 巻 (2016) 5 号 p. 647-659
    公開日: 2016/11/18
    ジャーナル フリー
     Atmospheric and anti-electron neutrinos generated inside the solid Earth (geoneutrinos) are potentially powerful tools for imaging the Earth's interior, in order to visualize the spatial distribution of the density of uranium and thorium concentrations. This review is limited to neutrino imaging techniques. Observations of atmospheric neutrinos and geoneutrinos have been reviewed previously and are not discussed here. An elementary introduction to neutrino generation on the Earth and propagation through matter opens the review. After reviewing neutrino tracking methods in the context of today's views of technological developments, the current experimental limits on neutrino imaging are presented. A technique to confront the standard Earth model is discussed in the conclusion. Neutrino imaging of the Earth has been pursued at IceCube. It is fair to mention that it has opened the possibilities of this new elementary particle technique for the first time.
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  • 小松 哲也
    125 巻 (2016) 5 号 p. 661-698
    公開日: 2016/11/18
    ジャーナル フリー
     The Pamir is a mountain region in the westernmost part of the Himalayan–Tibetan orogen. It extends 〜300 km from north to south and 〜300–400 km from west to east. The Pamir lies on a double subduction zone, where two buoyant continental plates have subducted several hundred kilometers deep into the asthenosphere. The southern plate is the northward-dipping Hindu Kush slab, and the northern plate is the southward-dipping Pamir slab. The subduction of the Hindu Kush slab began at 〜8 Ma. This narrow part of the Indian plate subducts to a depth of 〜600 km and has a neck at a depth of between 250 and 300 km, causing lithospheric thinning. The Pamir slab, a part of the Asian plate, probably started subduction at 〜25 Ma accompanied by slab rollback (possibly with the back-arc extension), subduction erosion, subduction accretion, and marginal slab-tear faulting. This slab, subducting to depths of 〜400–450 km, forms a broad arcuate shape split at the center due to a vertical tear extending from a depth of 〜200 to 〜400 km. The current geometry of both slabs implies the possible occurrence of tectonic events, such as slab break-off, in the near geological future. The metamorphic and exhumation history of the crystalline basement domes in the Pamir reveal regional tectonic evolution since the Cenozoic India–Asia collision. Prograde metamorphism of the mid–lower crust, driven by crustal shortening/thickening, continued from 45 to 25 Ma. Subsequently, retrograde metamorphism and exhumation of the mid–lower crust began during the period 25–15 Ma. This change suggests a transition from crustal shortening/thickening to crustal extension. A possible cause of this transition is that the Pamir slab obtained sufficient gravitational potential to enable the gravitational collapse of the crust, mainly due to the isostatic uplift of the orogen, which was triggered by the break-off of the Indian plate during 25–15 Ma. Subsequently, activation of extensional exhumation in the southwest and eastern Pamir was synchronous with magmatism in the eastern Pamir at 〜10 Ma. At present, the Pamir region is characterized by an active E–W extension of the Kongur Shan footwall to the east and a westward lateral extension of the western Pamir into the Tajik–Afghan Basin.
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論説
  • 南雲 直子, 澤野 久弥
    125 巻 (2016) 5 号 p. 699-716
    公開日: 2016/11/18
    ジャーナル フリー
     フィリピン共和国ルソン島で第二の流域面積をもつパンパンガ川流域は雨季の降雨や台風によって深刻な洪水の影響を毎年のように受けており,洪水被害の軽減が重要な課題である。そこで本研究では,地形や土地利用,過去の浸水履歴等に着目して,パンパンガ川流域の洪水特性や地域の洪水脆弱性を明らかにすることを目指した。地形分類図によれば,流域の地形は大きく山地・丘陵,火山,平野に分類され,山地・丘陵は山地,丘陵に,火山は火山斜面,火山山麓緩斜面,火山扇状地に,そして沖積平野は扇状地,段丘,後背湿地,湿地,三角州,谷底低地,自然堤防,メアンダースクロール,旧河道に階層区分された。平野北部のパンパンガ川西側では,フィリピン断層の活動に起因すると考えられる北部山地からの土砂供給によって扇状地と台地が南西方向によく発達する一方,東側では西方向へと台地が発達する。また,平野は山地,丘陵,火山に囲まれており,アラヤット付近で幅20 km程度まで狭くなるため,上流からの洪水が停滞し,サンアントニオ湿地とカンダバ湿地を形成する。そして平野南部では,非常に緩い河川勾配と地盤沈下,潮汐の作用による海水の侵入によって,カンダバ湿地を経て南下してきた洪水が排水されにくく,洪水は深刻化する傾向にある。このような地理的条件から想定される洪水様式をもとに,平野部をzone Iからzone IVに区分した。上流側のパンパンガ川西側及び東側にそれぞれ位置するzone Iとzone IIは比較的排水がはやい。一方,カンダバ湿地付近より南部に位置する洪水脆弱地域(zone III,zone IV)は,農漁業生産において周期的な洪水による恩恵を受けていると同時に,家屋の浸水被害も大きい。しかし,住民は2011年に発生した近年最大の洪水時であっても自宅から避難しない等,洪水を恐れず,共存しながら生活を営んでいる。今後も平野部では人口増が見込まれることから,行政主導の事前の洪水対策を進めていくと同時に,効果的な対策を進めながら洪水に適応した暮らしが維持できるよう,住民それぞれが地域の地理的環境や洪水脆弱性を理解しておく必要がある。
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  • 中畑 浩基, 磯﨑 行雄, 堤 之恭
    125 巻 (2016) 5 号 p. 717-745
    公開日: 2016/11/18
    ジャーナル フリー
     To reconstruct the detailed paleogeographic configuration of the Cretaceous arc-trench system in East Asia, shallow marine sandstones from the Ryoke, Sanbagawa, and Chichibu belts in western Shikoku are investigated with age spectra by U–Pb dating detrital zircons with LA-ICPMS. The mid-Cretaceous Shuki and Nigyu formations, unconformably covering the pre-Cretaceous accretionary complex of the Chichibu belt, contain abundant detrital zircons from the Jurassic to Early Cretaceous ages, with small quantities of Permian and Triassic detrital zircons. These age spectra are almost identical to those previously obtained from other coeval formations elsewhere in the Chichibu belt (the Monobegawa Group), which represent the Cretaceous fore-arc setting. The common age spectra suggest that the provenance of the Cretaceous fore-arc domain had ubiquitous compositions of rocks for nearly 1,000 km along the arc, and that Jurassic to Early Cretaceous granitoids were predominant, with associated small quantities of older pre-Jurassic granitoids. From the Ryoke belt, the Campanian (Late Cretaceous) Yamanouchi Formation of the Izumi Group, unconformably covering the mid-Cretaceous Ryoke granitoids, also has a similar age spectrum. This confirms that the provenance of the fore-arc remained more or less the same at least until the Late Cretaceous. The most intriguing age spectrum was obtained from the Maana Formation, which occurs as a klippe sitting on top of the southern Sanbagawa belt. As well as other dated sandstones, Maana sandstone contains Early Cretaceous to Jurassic zircons; nonetheless it accompanies not only Permo-Triassic zircons but also abundant Paleoproterozoic (2400–1600 Ma) grains. This unique age spectrum is correlative solely with those from the Tetori/Jinzu groups in the Hida belt, whose depositional setting has featured provenance with the Precambrian basement. This suggests that the Maana Fm was primarily deposited at the continent side of all the Paleozoic accretionary complexes and their high-pressure metamorphosed equivalents of SW Japan; i.e., the back-arc side of the Cretaceous arc-trench system in East Asia. As to the age spectrum of detrital zircons, in addition to lithofacies, deformation style, and occurrence as klippe, Maana Fm is almost identical to the Atogura Fm located in the northern Kanto Mtn., ca. 800 km to the east. These klippe-forming Cretaceous strata were originally located at the continent-side of the Cretaceous arc, and they were moved toward the ocean side by Cenozoic tectonics, possibly related to the tectonics of the Miocene Japan Sea (back-arc) opening. The present study succeeds first in estimating the displacement of crustal rocks, i.e., over 200 km in the across-arc direction, contemporaneous with the inferred Cenozoic tectonics.
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  • 重野 聖之, 七山 太
    125 巻 (2016) 5 号 p. 747-762
    公開日: 2016/11/18
    ジャーナル フリー
     Features and depositional processes are revealed of sandy event deposits (SED) caused by a storm surge and high waves during the 1959 Miyakojima typhoon around the Hirahama coastal lowland, along the western coast of the Oshima Peninsula, southwestern Hokkaido. Three new trenches were excavated in the lowland to study sedimentary features and grain size. Sedimentary features imply that the 1959 SED was deposited from an unidirectional run-up flow. The deposits can be subdivided into three units: T, S, and F in ascending order. Unit T shows 3D dunes. Unit S shows bedform transition from 2D dunes to ripples. Unit D consists of a mud layer including suspended plants and pieces of wood. Grain-size analysis shows that Units T and S peak at around 2.0 Phi (P-2 population), which is the same as beach sand from the Hirahama Coast, and the wide grain-size distribution is over 0–4 Phi due to mixing with the fluvial bed of Yumiyama River (P-1 and P-3 populations). According to Dmax, Unit T shows coarsening upward from -0.25 to 0.25. On the other hand, Unit S shows finning upward to 0.75 from 0.25. Therefore, Unit T recorded the amplification process of the storm surge and high wave energy due to the typhoon after 09:00 on September 18. Unit S recorded the decay process of high waves and storm surge energy associated with the movement of the typhoon from 13:00 to 14:00 or later. After the period 00:00 to 01:00 on September 19, suspended solids and wood fragments in stagnant water covered Unit S, then deposited Unit D because the typhoon had passed.
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寄書
  • 吉田 英嗣, 笠原 友生
    125 巻 (2016) 5 号 p. 763-773
    公開日: 2016/11/18
    ジャーナル フリー
     There are two geomorphic surfaces (terraces) in the northwestern Kanto Plain: Takasaki Upland (Takasaki Surface) and Inokawa Lowland (Ino Surface). It is considered that about 11,000 years ago the Inokawa Mudflow traveled down from the west, forming the Takasaki Upland as a depositional surface. However, opinions are divided and another theory is that the Inokawa Lowland is also the depositional surface of the Inokawa Mudflow. Therefore, this study examines the subsurface geology around the Inokawa Lowland, based on an expanded dataset of existing borehole data obtained from our portable hand-operated drilling survey. The results clearly show that the Inokawa Lowland is basically a depositional surface of the Inokawa Mudflow. Reflecting upon this conclusion, how the terrace scarp between the two surfaces was formed becomes problematic. There are two possible causes to be clarified by future investigations:
     (1) Mantle bedding of the Inokawa Mudflow on the pre-existing fluvial scarp,
     (2) As-yet-unrecognized vertical faulting after full deposition of Inokawa Mudflow deposits in this region, probably related to the Fukaya fault system.
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