著者
吉田 英嗣
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.119, no.3, pp.568-578, 2010-06-25 (Released:2010-08-30)
参考文献数
112
被引用文献数
2 2

The Japanese Islands are tectonically active and humid. Thus, erosion and deposition are the primary processes controlling geomorphic development. Catastrophic sector collapses at volcanoes should be considered significant in this context. This study examines the geomorphological role of volcanic sector collapses in Japan, introducing 58 cases with their respective occurrence ages and volumes (≥ 1×108 m3). We find that the frequency of sector collapses becomes exponentially higher as the collapse magnitude decreases. The total volume of the dissected volcanic edifice caused by catastrophic collapses amounts to ca. 6.4 km3 (640×108 m3) during the last 500 years. This value can be translated into an annual denudation rate of 0.53 mm/y per unit area of the Quaternary volcanoes (ca. 24000 km2), which is comparable to the contemporary denudation rate of non-volcanic mountains in Japan. Therefore, although volcanic sector collapses occur intermittently, we have to consider them as sediment sources that are indispensable to an understanding of geomorphology in Japan.
著者
吉田 英嗣 須貝 俊彦 坂口 一
出版者
The Association of Japanese Geographers
雑誌
地理学評論 (ISSN:13479555)
巻号頁・発行日
vol.78, no.10, pp.649-660, 2005-09-01 (Released:2008-12-25)
参考文献数
23
被引用文献数
3 3

岩屑なだれをはじめとする大規模土砂供給イベントが,流域の長期的な地形発達に与える影響を評価する事例研究として,約2.4万年前に浅間火山で発生した大規模山体崩壊に由来する前橋泥流が達した利根川・吾妻川合流点付近を対象とし,河川地形発達史を考察した.本地域では最終氷期初頭以降,泥流流下時までの間,気候変動に対応した段丘発達過程がみられた.本地域に達した泥流は,5~6万年前までに段丘化した段丘面に衝突し,段丘面を覆うローム層を削剥しながら,これをのりこえていった.他方,利根川を遡上し,堆積した泥流堆積物は,速やかに河川に侵食されていった.最終氷期最盛期前後には,泥流堆積物が再堆積するなどして,下流側において小規模な谷埋めが生じ,晩氷期には側刻が卓越し,完新世に入ってから下刻が始まった.最終氷期最盛期以降の利根川本流の河床変動は,泥流イベントの影響を残しつつも,再び広域的な気候変動に対応してきたと考えられる.
著者
吉田 英嗣
出版者
The Association of Japanese Geographers
雑誌
地理学評論 (ISSN:13479555)
巻号頁・発行日
vol.77, no.8, pp.544-562, 2004-07-01 (Released:2008-12-25)
参考文献数
46
被引用文献数
4 4

第四紀の成層火山を持っ流域の地形発達には,大規模山体崩壊など,火山特有の侵食過程に伴う土砂供給が関与し得る.本稿ではその一例として前橋・高崎地域の地形発達を検討した.24ka頃,利根川が形成する扇状地面上に前橋泥流堆積物が堆積した結果,泥流堆積域で地表が緩傾斜化し,河床高度が増した.利根川は泥流堆積面を侵食し,16ka以降,河道を定めて広瀬面を形成した.前橋泥流堆積面である前橋面上には,前橋泥炭層が広範に形成された.11ka頃,前橋泥流堆積面を侵食していた烏川の谷を含む高崎地域に,井野川泥流堆積物が流下し,鳥川は河道を現在の位置に変えた.この烏川は井野川泥流堆積面(高崎面)を下刻し,高崎面が離水した.さらに,高崎面東部で井野川泥流堆積物を刻む水系が,侵食面の井野面を形成した.両泥流の堆積は,本地域における複数回の大規模な河道変遷を招き,ひいては地形発達を長期的に規定してきた.
著者
高場 智博 吉田 英嗣 須貝 俊彦
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.127, no.1, pp.73-87, 2018-02-25 (Released:2018-04-14)
参考文献数
54

This study classifies composite small-scale fans along the eastern foot of Mt. Ikeda into five categories, and estimates their formation ages to discuss factors controlling fan development. Besides, it describes microtopography of fan surfaces and surface geology (sedimentary units), focusing on one of the typical terraced small-scale fans to discuss small-fan forming processes. Methods are interpretation of aerial photographs for terraces division, field observations with microtopography measurements, and radiocarbon dating for overlying sediments of terrace deposits. The results show that the five terrace surfaces are mainly formed by a few debris flow deposits, and the estimated formation ages obtained with radiocarbon dating fall into the general classification of the Last Glacial Maximum (LGM) and the early Holocene. However, the difference between each formation periods is 1,000-3,000 years because estimated formation ages are about 20,000 years (I surface) and about 17,000 years ago (I surface) in the LGM, about 9,000 years (III surface), and about 8,000 years ago (IV surface) in the early Holocene. Geomorphic development of the study area might be difficult to explain using a simple formation model of river terraces with time scales of 104-105 years, expressed by dynamic river fluctuations under the full influence of global climatic changes. This speculation is highly suggestive for further investigations on the timing of debris flow sedimentations forming alluvial fans in this area.
著者
吉田 英嗣 須貝 俊彦 大森 博雄
出版者
日本第四紀学会
雑誌
第四紀研究 = The Quaternary research (ISSN:04182642)
巻号頁・発行日
vol.49, no.2, pp.55-67, 2010-04-01
被引用文献数
1 7

火山麓に分布する流れ山は,大規模山体崩壊が過去に発生した証拠として,また,岩屑なだれのメカニズムを推察するうえで,重要な研究対象とされてきた.本研究では,流れ山地形がなお崩壊や岩屑なだれに関して地形学的に重要な情報を提供してくれるものと捉え,岩屑なだれの流下方向における流れ山の分布様式を検討し,流れ山地形に新たな地形学的意義を与えることを試みた.研究対象は,日本における4つの岩屑なだれが形成した流れ山であり,これら岩屑なだれは山麓に拡散した典型例とみなされる.空中写真判読により抽出した流れ山の数は,尻別火山の172,有珠火山の262,岩木火山の200,那須火山の643であり,GISを用いて流れ山の形態データを取得した.<BR>いずれの事例も,流れ山地形は山麓の下部斜面から平地にかけて緩やかな斜面として存在する.そして,流れ山のサイズは下流方向に減少する傾向が認められる.この減少傾向は,流れ山のサイズと給源からの距離との回帰分析によれば,指数関数で近似しうる.まず,回帰関数は,距離ゼロ(給源)における流れ山のサイズが崩壊の体積に規定されていることを示している.すなわち,崩壊の規模に応じて,崩壊部に発生する初期段階での割れ目の大きさが決まるらしい.他方,流れ山のサイズの減少割合は,等価摩擦係数の逆数で示されるような岩屑なだれの流動性に規定されていると考えられる.換言すれば,流動性の小さい岩屑なだれでは流れ山が急速に縮小するのに対し,大きい岩屑なだれでは緩やかである.以上の検討により,流れ山のサイズと給源からの距離との関係は,火山体ならびに岩屑なだれの流動特性を反映していることが明らかとなった.
著者
吉田 英嗣
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.124, no.4, pp.575-586, 2015-08-25 (Released:2015-09-17)
参考文献数
36
被引用文献数
1 1

Hummocks are conically shaped mounds, which occur following catastrophic sector collapses, and are mainly observed around volcanoes. Empirical relationships of hummocks proposed by Yoshida et al. (2012) are basically applicable if hummocks now distributed on a debris avalanche depositional area do not have remarkable disturbances due to topographical barriers or modifications after deposition. Using these empirical relationships, the Old Yotei volcano, whose mega-collapse in the Late Pleistocene has not been clarified yet, is examined. The results show that the size-distance relationship of hummocks (N=297), distributed west of the volcano can be expressed by an exponential function as,   A=α exp (−β D),where A is the average area of a hummock within a certain span and D is the average distance from the source. For the Old Yotei volcano, the values of coefficients α and β of the fitted curve are 170286.4 and 0.000429, respectively. Based on a geomorphological interpretation of both α and β values, it can be concluded that the collapse volume of the Old Yotei volcano is ca.1.3 km3, and the equivalent coefficient of friction of the debris avalanche is 0.129. The latter also reveals a high probability that the pre-collapse summit of the Old Yotei volcano reached 1700 m a.s.l. Thus, the image of the flank collapse of the Old Yotei volcano has been restored more decisively by the distribution of hummocks.
著者
須貝 俊彦 水野 清秀 八戸 昭一 中里 裕臣 石山 達也 杉山 雄一 細矢 卓志 松島 紘子 吉田 英嗣 山口 正秋 大上 隆史
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.116, no.3-4, pp.394-409, 2007-08-25 (Released:2009-11-12)
参考文献数
26
被引用文献数
5 9

The northern Ayasegawa fault is a part of the Fukaya fault system, which is the longest active fault in the Kanto district. The paleoseismology of the northern Ayasegawa fault was revealed by a combination of arrayed boring and ground penetrating radar (GPR) survey. The northern Ayasegawa fault produced a fold scarp with the NW-SE direction running along the boundary between the Oomiya 2 (O2) surface and fluvial lowland. The O2 was formed in Marine Isotope Stage 5a, and was slightly deformed with a wide warping zone. Sixteen sediment cores arrayed across the warping zone contain a series of tephra layers such as Hk-TP (ca. 60-65 ka), KMP, AT (26-29 ka), As-BP group (20-25 ka), and As-YP (15-16.5 ka). These key beds except Hk-TP were deposited and deformed parallel to each other, suggesting that no faulting events occurred between KMP and As-VP fall. The timing of the last faulting event is after the As-YP fall, and is probably younger than 10 ka based on an interpretation of GPR profiles and 14C ages. KMP should be deposited horizontally because it intervened in the peaty silt layer, which accumulated conformably on lacustrine deposits overlapping the fold scarp. Thus, the KMP horizon roughly indicates the vertical offset produced by the events occurred after the As YP fall. The events were probably singular, and the last one formed a vertical offset of more than 4 m. The older event occurred at around 70 ka between Hk-TP fall and O2 formation. Vertical deformation of the O2 was at least 7 m, indicating the possibility that the vertical offset caused by the penultimate event is at least 3 m. The vertical slip per event might reach 5 m, and the average vertical slip rate is nearly 0.1 mm/yr because the warping zone detected by the arrayed boring above is within the flexure zone shown by the P-wave seismic profile. The northern Ayasegawa fault is considered to be a single behavioral segment because of its longer recurrence interval and lower slip rate of 0.1 mm/yr in comparison with those of the other part of the Fukaya fault system.