地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
125 巻 , 3 号
選択された号の論文の10件中1~10を表示しています
表紙
  • 2016 年 125 巻 3 号 p. Cover03_01-Cover03_02
    発行日: 2015/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     阿蘇火山中央火口丘群の北西部に位置する米塚は,約3300年前の噴火で形成されたスコリア丘で,阿蘇ではもっとも新しい火山の一つである.この火砕丘は底面の直径が370~380 m,高さが80 m程度で,山頂部に直径80 m,深さ15 m程度の大きさの火口を有し,お椀を伏せたような形をしている.阿蘇神社の主祭神である健磐龍命(たけいわたつのみこと)がその手で米をすくって貧しい人たちに分け与えたことから,その名がついたとされる.草原に覆われ,均整のとれた美しい山容は阿蘇を代表する風景として知られているが,2016年4月16日未明の熊本地震本震によって山頂の火口縁周辺に多数の亀裂が生じた(地震後写真の矢印).米塚の西方約1.5 kmに位置する蛇ノ尾火山では複数の斜面崩壊が発生した(口絵1-図13)が,米塚においてはスコリア丘を覆う厚さ1 mほどのテフラ層に亀裂が入ったものの,わずかに変位が認められただけで,美しい火砕丘の姿は保存された.米塚の背後には外輪山とよばれる阿蘇カルデラ壁が見える.この北側カルデラ壁の高低差は300~450 m程度で,頂部は阿蘇火砕流によって埋め立てられたために定高性を有している.今回の熊本地震によって,カルデラ壁急斜面でも多数の崩壊が発生した.
    (写真・説明:宮縁育夫 2016年5月1日撮影)
総説
  • 早川 裕弌, 小口 高
    2016 年 125 巻 3 号 p. 299-324
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     High-resolution landform data have become widely available in parallel with the technical revolution in topographic measurements, particularly those related to laser scanning technology. Aerial laser scanning (ALS) has become popular for taking basic topographic measurements and has been frequently applied to geoscientific studies in recent decades. Terrestrial laser scanning (TLS) is applied to geosciences less frequently and is mostly limited to Europe and North America, although in countries such as Japan it may permit detection of rapid topographic changes under a humid, tectonically active environment. The purpose of this review article is to summarize the current situation of TLS applications in geomorphology and related sciences, and to illuminate the future directions of such applications. First, the principles of TLS methodology, including the basics of taking field measurements and post-processing TLS data are briefly explained. Then, some case studies on the use of TLS in geomorphology are reviewed. The examples relate to collapses of sea cliffs, landslides and debris flows, sedimentation of fluvial deposits, soil erosion, and tectonic activity of faults. Then, some issues related to the acquisition, processing, and analysis of TLS-derived point cloud data and digital elevation models (DEMs) with a higher resolution than traditional methods are pointed out. Accuracy and resolution issues are particularly crucial, because selecting appropriate scales for a target material is often directly related to the results of analyses, and appropriate scales should be taken into consideration before acquiring TLS data in the field. This means that a higher resolution is not always better in relation to the scales of target landforms and measurement accuracy. Time-series assessment, which is the most typical and fundamental analysis using such high-resolution data, also strictly depends on measurement accuracy. Robust analyses of TLS-derived high-resolution point clouds and DEMs are also examined. Various analytical methodologies not only for the morphology of Earth surfaces such as roughness, but also other elements including the intensities and waveforms of laser returns need to be developed. To expand the use of TLS in geomorphology and geosciences, systems for sharing well-formatted high-resolution datasets, data processing tools, and instruments should also be established.
論説
  • 井口 隆
    2016 年 125 巻 3 号 p. 325-351
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     The litho-biostratigraphy of Paleogene coalfields in northwestern Kyushu, Japan is reviewed on the basis of data obtained from the Miike Coalfield below the Ariake Sea. The Miike Coalfield extends over a wide area below the Ariake Sea as far as the west coast. The Paleogene stratigraphy of the coalfield is currently defined as the Ginsui Formation and the Ohmuta and Manda Groups in ascending order. The Manda Group consists of the Kattachi and Yotsuyama Formations in ascending order. The Kattachi Formation contains large quantities of glauconite in the basal part, the base of which has been defined as the base of the marine Manda Group. However, the Kattachi Formation contains coal seams and varies laterally, with glauconite content decreasing westerly. No significant facies variation with the underlying Ohmuta Group can be seen. The Kattachi Formation is, therefore, re-interpreted as the uppermost zone of the underlying coal-bearing Ohmuta Group. Glauconite sandstone of the basal part of the Yotsuyama Formation overlies underlying strata with a clear boundary throughout the coalfield. No significant facies variation exists in the Ohmuta Group. As a result, the Paleogene stratigraphy of the Miike Coalfield is revised into the Ginsui, Miike and Yotsuyama Formations in ascending order. The marine Yotsuyama Formation reaching ca. 800 m in thickness below the Ariake Sea is divided into four lithostratigraphic members: Ia (glauconite sandstone), Ib (shale), IIa (glauconite sandstone), and IIb (shale) Members in ascending order. The Yotsuyama Formation spans the late Middle Eocene (Subzone CP14a of Okada and Bukry, 1980) to the late Late Eocene (Subzone CP15b) age. Marine sequence, biostratigraphically correlative with Ia to IIb Members, is distributed in the Takashima and Amakusa Coalfields, while marine strata correlative with IIb Member are distributed in the Sakito-Mastushima and Isahaya Coalfields. These facts indicate that marine transgression occurred from the south and that a marine environment prevailed in northwestern Kyushu in the late Middle to late Late Eocene and peaked during the Yotsuyama Formation deposition.
  • 中畑 浩基, 磯﨑 行雄, 堤 之恭, 岩本 直哉
    2016 年 125 巻 3 号 p. 353-380
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     To clarify the supply/deposition pattern of terrigenous clastics of Cretaceous Japan, U–Pb and Pb–Pb ages of detrital zircons are analyzed by laser-ablation induced coupled plasma mass spectroscopy (LA-ICPMS) of Lower Cretaceous sandstones in the Chichibu belt, SW Japan. Target rocks include Lower Cretaceous fore-arc sandstones in the Sanchu graben and the Choshi area of the southern Kanto district; e.g. Hauterivian Shiroi Formation, Barremian Ishido Fm, and Aptian–Albian Sanyama Fm in the Sanchu graben, and Barremian Ashikajima Fm, Early Aptian Inubozaki Fm, and Aptian–Albian Nagasakihana Fm in the Choshi area. All these fore-arc sandstones are dominated by Mesozoic (ca. 250–100 Ma) detrital zircons with minimal amount of (300–250 Ma) Permian grains, and Precambrian grains are extremely rare. The similar age spectra of these sandstones suggest a common provenance, despite the current along-arc separation of the Sanchu graben from the Choshi area for ca. 130 km. Hauterivian–Barremian sandstones from both areas are characterized by the abundance of Permian, Triassic, and Jurassic zircons, where as Aptian–Albian sandstone by monotonous dominance of Early Cretaceous grains. This stratigraphic change in zircon age spectra reflects a secular change in the exposure/erosion conditions of older granitoids in the provenance, in remarkable accordance with that of coeval sandstones from other areas; e.g., Ryoseki Formation in Shikoku (fore-arc basin) and Kanmon Group in northern Kyushu/western Honshu (intra-arc basin). This stratigraphic change commonly detected in the Cretaceous fore-arc and intra-arc records the growth/erosion history of a new crust of the volcanic arc developed along the East Asian margin. The disappearance of older Permian, Triassic, and Jurassic zircons in sandstones during the Barremian–Aptian interval suggests large-scale tectonic erosion along the East Asian active margin. The abundance of Proterozoic zircons in coeval sandstones deposited at the back-arc side highlights the remarkable contrast in sedimentary flux between fore-arc/intra-arc settings and back-arc domain. The uplift of arc crust to expose new Cretaceous granitoids probably formed a great barrier to sedimentary flux from the continental interior to the fore-arc domain in East Asia.
短報
  • 植村 円香
    2016 年 125 巻 3 号 p. 381-394
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     This study examines agricultural practices and roles of elderly farmers in Ugo Town, Akita Prefecture. Previous studies suggest that elderly farmers do not become important leaders in the area's agriculture because they farm as a hobby. Elderly farmers do not seem to engage in agriculture for their livelihoods because they receive pensions and retirement benefits. However, some elderly people do not receive adequate pensions. For these individuals, agriculture is the main source of their livelihoods, and they might be involved in strategic farm management. Through interviews with elderly farmers in the study area, it was found that there are differences in agricultural management between elderly farmers who had stable employment and those who engaged in migrant work during their middle age. The former group could enjoy farming as a hobby by entrusting part of its farm work to the latter group. On the other hand, the latter group could earn part of its agricultural income through consignment farming. In other words, it becomes clear that the relationships of consignment and entrustment of agricultural work in both groups of elderly farmers play an important role in preserving the area's agriculture and farmland.
  • 丹羽 雄一, 須貝 俊彦, 松島 義章
    2016 年 125 巻 3 号 p. 395-407
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     In the Sanriku coastal area, discrepancies have been suggested in crustal movements between long (104–105 years) and short (101–102 years) time scales. To clarify the cause of these discrepancies and reconstruct the tectonic history of this area, knowledge of incised valley fills with many radiocarbon ages provides basic and important data. Although the southern Sanriku coast has some small alluvial plains in the environments of ria coasts, the formation process of valley fills has not been discussed on the basis of a number of radiocarbon ages. In this study, a sediment core, TY1, is acquired from the lower reaches of the Tsuya Plain, southern Sanriku coast. Core sediments show a shallow marine succession influenced by Holocene sea-level change. Based on twelve radiocarbon ages, the accumulation rate is high (>5 mm/yr) at 9,000 to 7,100 cal BP, low (ca. 1 mm/yr) at 4,080 to 2,800 cal BP, and high (3–5 mm/yr) after 2,800 cal BP. High accumulation rates in bay mouth deposits during the middle Holocene, when relative sea-level rise decelerated, indicates sedimentation from the seaward area during the period of marine transgression. This marine transgression can be explained by the deposition of terrestrial sediments in small basins upstream from the Tsuya Plain and setback in the deceleration of relative sea-level rises by the Holocene subsidence trend. In the regressive phase since the middle Holocene, a low accumulation rate in deltafront deposits and a high accumulation rate in delta plain deposits coincide with changes of accumulation rate in the coastal area where relative sea-level has risen in a millennium scale. This also implies that the Tsuya plain subsided during the Holocene.
寄書
  • 小元 久仁夫, 伊藤 佑始
    2016 年 125 巻 3 号 p. 409-419
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     Along Wakimisaki beach, in the southwestern part of Nagasaki peninsula, beachrock developed in the intertidal zone, and is designated a natural monument by Nagasaki prefecture. The beachrock consists of sand and gravels cemented by carbonates in a muddy matrix, and is a conglomerate. The beachrock can be divided into five formations from its strike, dip, and location. Eight sample materials collected from the beachrock were radiocarbon dated. Calibrated radiocarbon ages suggest the beachrock formed between ca. 5,700 cal BP, and ca. 600 cal BP. There is no geomorphic and geologic evidence to suggest tectonic movements on the Wakimisaki coast. This means sea-level at the Wakimisaki coast has been stable since ca. 5,700 cal BP.
  • 宮縁 育夫
    2016 年 125 巻 3 号 p. 421-429
    発行日: 2016/06/25
    公開日: 2016/07/14
    ジャーナル フリー
     The Kumamoto earthquake (Mj 7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types: landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25° at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300 m high, 130–200 m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (<600 m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important information for preventing or mitigating future landslide disasters in the Aso caldera region.
口絵
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