著者

鈴木 力英

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.556-561, 2010-06-25 (Released:2010-08-30)

参考文献数

10

被引用文献数

1 1

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In mid-November, 2009, a breach of the server of the Climatic Research Unit (CRU) of the University of East Anglia, UK, occurred, and more than 1,000 e-mails were copied and disseminated over the Internet. CRU is one of the leading climatology institutes, and has constructed a long-term world temperature database that was referenced in the assessment report of the Intergovernmental Panel on Climate Change (IPCC) as evidence of global warming. The words “trick” and “hide the decline” in an e-mail, which mentioned the process for preparing the temperature time series, generated the allegation; those words demonstrated that scientists at CRU intended to falsify data to exaggerate warming. However, it can be regarded that the allegation was a manufactured controversy originating from irrelevant interpretations of personal e-mails at CRU. Scientists are now being requested to ensure that scientific knowledge and understanding are convincing not only to scientists but also to the general public.

著者

松本 剛 木村 政昭 仲村 明子 青木 美澄

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.3, pp.286-296, 1996-06-25 (Released:2009-11-12)

参考文献数

27

被引用文献数

6 11

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Precise topographic features at the two major structural boundaries in the Ryukyu Arc, Tokara Gap and Kerama Gap, revealed that there exists a ridge feature of the Ryukyu Arc as a saddle topography. The feature supports the idea recently proposed that a landbridge connecting the Chinese continent, Ryukyu Islands and Japanese Islands, is thought to have been subsided since Late Pleistocene. The saddle is offset by large-scale left-lateral normal faults both of which are related to the opening of the Okinawa Trough. These topographic features and geological structure together with the gravity anomaly and regional tectonics in the Ryukyu area suggest that a north-south tensional stress regime, due to the rolling buck of the Philippine Sea Plate at the Ryukyu Trench, has dominated in this region since last 20, 000 years.

著者

真鍋 淑郎

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.101, no.6, pp.453-457, 1992-12-05 (Released:2009-11-12)

参考文献数

16

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This paper describes the response of a coupled ocean-atmosphere-land surface model developed at GFDL to gradual changes of atmospheric carbon dioxide. It summarizes the results in three recently published papers (Stouffer et al., 1989 Manabe et al., 1991, 1992). They represent the current state of the art in predicting further climate change induced by greenhouse gases in the atmosphere.The warming of the coupled system is reduced by the effective thermal inertia of the earth's surface which is essentially controlled by the vertical mixing of heat in the oceans. This study investigates how such vertical mixing of heat is achieved, resulting in the delay of the greenhouse warming in the joint troposphere-continental surface-ocean system.

著者

村田 昌則 小林 淳 青木 かおり 高橋 尚志 西澤 文勝 鈴木 毅彦

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.130, no.3, pp.379-402, 2021-06-25 (Released:2021-07-20)

参考文献数

41

被引用文献数

2

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Kozushima is a volcanic island, located in the northern part of the Izu Islands, approximately 170 km SSW of central Tokyo. The volcanism of Kozushima Volcano started at 80-60 ka and formed monogenetic volcanoes. The latest eruption occurred in AD 838, and was associated with the formation of the lava dome and pyroclastic cone of Tenjo-san. In order to contribute to the long-term forecasting of volcanic eruptions on the Izu Islands off Tokyo, the tephrostratigraphy and eruption history of Kozushima Volcano during the last 30,000 years is reconstructed. According to the geological surveys, the widespread Kikai-Akahoya (K-Ah; 7.3 cal ka) and Aira-Tanzawa (AT; 30.0 cal ka) tephras from southern Kyushu, and Niijima-Mukaiyama (Nj-My; AD 886), Niijima-Shikinejima (Nj-Sk; 8 cal ka) and Niijima-Miyatsukayama (Nj-Mt; 12.8 cal ka) tephras from the Niijima Volcano 20 km NNE of Kozushima are recognized. Tephras that also erupted from Kozushima Volcano are Kozushima-Tenjosan (Kz-Tj; AD 838), Kozushima-Ananoyama (Kz-An; 7-9 c), Kozushima-Chichibuyama-A′ (Kz-CbA′; 14-12.8 cal ka), Kozushima-Chichibuyama-A (Kz-CbA; 30-22 cal ka), and Kozushima-Chichibuyama-B (Kz-CbB; ca. 30 ka). Kz-An is formed by the activity of the northern volcanic chains (Kobe-yama–Anano-yama–Hanatate) just before the Tenjo-san eruption. Kz-CbA′ is distributed in southern Kozushima. Source vent and distribution of Kz-CbA′ have not yet been identified. The eruption history of Kozushima volcano over the last 30,000 years is as follows. At ca. 30 ka, Kz-CbB erupted in the central Kozushima, and the Nachi-san dome and Takodo-yama dome formed. At 30-22 ka, the Kz-CbA eruption in the southern Kozushima and the formation of the southern volcanic chain occurred. After the eruption of Kz-CbA′, the formation of the northern volcanic chain was followed by the eruption of Tenjo-san volcano. In addition, the eruption rate of Kozushima volcano is estimated during the last 30,000 years to be approximately 0.06 km3/1000 y in DRE.

著者

小林 淳 小山 真人

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.4, pp.431-447, 1996-08-25 (Released:2009-11-12)

参考文献数

35

被引用文献数

1 1

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We made a detailed tephrochronological study in the western and southern foots of Hakone Volcano and revealed the eruptive histories of Hakone and other volcanoes since 240 ka. We recognized four widespread tephras, Aira-Tn Ash (AT), Sanbe-IkedaAsh (SI), Kikai-Tozurahara Ash (K-Tz) and Ontake 1 Pumice (On-Pm1) as well as the tephras from the adjacent volcanoes, such as Hakone-Mishima Pumice (Hk-CC4), the pyroclastic flow associated with Hakone-Tokyo Pumice (Hk-TPfl), and Hakone Da-5 Pumice from Hakone Volcano, Fuji Susono-1 Scoria and Fuji Susono-2 Scoria (F-Sul and F-Su2) and Fuji-Yoshioka Pumice from Fuji Volcano, and the tephras from the Higashi-Izu monogenetic volcano field. Eruption ages of F-Su2, F-Sul, Hk-CC4 and F-YP are estimated to be 36 ka, 37 ka, 43 ka and 85 ka, respectively, by loess-chronometry. The discharge mass and eruptive magnitude M are estimated to be 3.7 × 1011 kg and 4.6 for Hk-CC4, and 4.2 × 1011 kg and 4.6 for Da-5 Pumice on the basis of their isopach maps. The eruption frequency in the study area is 0.11 times/ ky for the last 240 ky in average, showing an abrupt increase to 0.53 times/ky during 120 ka-135 ka. In this period, both Hakone Volcano and the Higashi-Izu monogenetic volcano field seem to have been active simultaneously.The loess thickness, which is interbedded between Hk-CC4 (43 ka) and Hk-TPfl (52 ka), tends to increase toward the eastern foot of Fuji Volcano. The age-depth diagram shows that the accumulation rate of loess is nearly constant in each locality except for the rapid increase at about 50 ka in the Kannami area. The linear accumulation of loess supports the validity of loess-chronometry. The 50 ka increase of loess accumulation rate seems to be associated with enlargement of barren plains, which were located in the eastern of Fuji Volcano and were probably the main source of loess materials.

著者

松本 至巨 尾方 隆幸 内川 啓

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.115, no.2, pp.221-235, 2006-04-25 (Released:2009-11-12)

参考文献数

28

被引用文献数

1 1 3

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The Northern Japanese Alps (Chubusangaku National Park) has experienced a dramatic increase in the number of visitors since the boom referred to as “the Japanese hundred mountains”. The Ushiro-Tateyama Range, located in the northernmost area of the Japanese Alps, provides various alpine landscapes such as cirques, asymmetrical ridges, patterned ground, perennial snow patches and alpine plant communities. Data from 6922 trekkers indicate that the distribution of mountain huts and accessibility to trails control their trekking courses, and that the trekkers concentrate in three mountain areas : Shirouma-mountain area, Goryu-mountain area and Kashima-mountain area. These areas are divided by a landform called kiretto, where a col with steep rockwalls lies along a main ridge, operating as a natural obstacle. Such a concentration suggests that human impacts on mountain geoecosystem occur locally, but intensively.

著者

湊 進

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.115, no.1, pp.87-95, 2006-02-25 (Released:2009-11-12)

参考文献数

55

被引用文献数

21 23

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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.

著者

神谷 泉 小荒井 衛 関口 辰夫 佐藤 浩 中埜 貴元 岩橋 純子

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.122, no.5, pp.854-874, 2013-10-25 (Released:2013-10-31)

参考文献数

31

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SAR interferometry is widely used for dense measurements of surface displacements caused by earthquakes, but the method cannot be applied if displacements are too large. The near-epicentral area of the Iwate–Miyagi Nairiku Earthquake in 2008 is an inapplicable case. Therefore, we applied photogrammetry to measure surface displacement caused by the earthquake. The maximum horizontal and vertical displacements were found to be 5.3 m and 2.9 m, respectively. We recognized three lines where displacement changes abruptly. The displacement distribution is like that of a reverse fault along the first line (A–B), an east-rising fault along the second line (F–G–H; west of line A–B), and a left-lateral fault along the third line (B–C; between line A–B and line F–G–H). The earthquake source fault reaches or approaches the ground surface at line A–B, with slippage decreasing toward the ground surface. The fault-like large surface deformation found north of the Aratozawa Dam is on the first line. The fault-like deformation was caused by the motion of the earthquake source fault, and the relative displacement of the fault-like deformation was enlarged by local causes. A gravitational mass movement found north of the fault-like deformation is one cause. Because the width of the rising area is small, only 3.5 km, at the southwestern side of line B–C, the slip of the earthquake source fault is mainly distributed near the ground. Line F–G–H suggests the existence of a geological structure that causes the abrupt changes of vertical displacement without a horizontal displacement, for example a high-angle fault.  We assumed: (1) the slip on the main fault is distributed only in a shallow area at the southern part of the main fault and only in a deep area at the northern part; and, (2) the difference of slip caused two lateral faults between southern and northern parts. The assumption qualitatively explains many observation results, such as why there is an abrupt change of horizontal displacement along line B–C and why line F–G–H has a convex part to the east. We found a correlation between the occurrence of large landslides and abrupt changes of displacement, in other words large surface strain. The following mechanisms are possible causes of the correlation: (1) stress from surface strain increased large landslides; (2) faults (not only the main fault) may exist under the focused areas, rupture of faults caused both large surface strain and large seismic motion, and seismic motion induced large land slides. We also found that landslides and slope failures occurred densely over the slipping area on the main fault, based on the assumptions in the previous paragraph. Because photogrammetric measurements need interactive observations, we could avoid observations on possible embanking areas. Because photogrammetry allows intensive measurements at interesting areas, we revealed a two flexure-like distribution of vertical displacement. Therefore, photogrammetry is an effective method for measuring surface displacement caused by an earthquake.

著者

須貝 俊彦 松島(大上) 紘子 水野 清秀

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.122, no.6, pp.921-948, 2013-12-25 (Released:2014-01-16)

参考文献数

109

被引用文献数

18 19

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The Kanto Plain, the hinterland of the Tokyo metropolitan area, is the largest plain in Japan and is characterized by marked marine and fluvial terrace levels that developed during Marine oxygen Isotope Stage (MIS) 5. Late Quaternary topographical changes to the plain have been controlled by concurrent tectonic activity and glacio-eustatic sea-level changes. The shoreline at the maximum transgression of MIS 11, 9, 7, 5 and 1 is reconstructed based on the distribution of marine sediments revealed by many geologic columnar sections and marine terrace surfaces. A comparison of the magnitudes of the last five full-interglacial transgressions above shows that magnitude decreased over the long term. This is due probably to changes in the tectonic regime in the Kanto basin, from subsidence to uplift along with the northward migration of the depositional center, probably associated with changes in the motion of the Philippine Sea Plate and the collision with the Izu peninsula. The marine transgression has also been controlled by fluvial processes, especially in the north-western part of the plain because of high sediment inputs from the Tone, Ara, and Watarase rivers. Aggradation coupled with regional uplift since MIS 5.4 limited the MIS 1 marine transgression within the incised valley formed during MIS 2. As a result, the Paleo Tokyo bay, which was connected directly with the Pacific Ocean, disappeared. Instead, a large shallow submarine area of about 10,000km2 emerged. The northern part of the present Tokyo bay is still subsiding and large volumes of water and sediments have been concentrated in the bay area during the Holocene. Such natural environmental conditions enable supplies of natural resources, such as fresh water, fertile soil, and flat land for the development of greater Tokyo.

著者

早川 由紀夫

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.104, no.4, pp.561-571, 1995-08-25 (Released:2009-11-12)

参考文献数

11

被引用文献数

7 6

著者

山本 伸次

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.6, pp.963-998, 2010-12-25 (Released:2011-03-17)

参考文献数

99

被引用文献数

20 20 13

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This is a brief review of tectonic erosion originally proposed by von Huene and Scholl (1991) who have spent most of their academic careers studying marine geophysics along the circum-Pacific subduction zone. Accretionary complex is considered to be formed by trench turbidite resting on the subducting oceanic plate that accretes against a hanging wall at a shallow or deep crustal level due to off-scraping or underplating. This concept was introduced to Japan in the early 1970s and was developed in great detail to propose a new paradigm for accretionary geology that involves ocean-plate stratigraphy. Later, identification of accretionary complex on-land in Japan became the mainstream. A new idea refutes the common occurrence of an on-going accretion process forming accretionary complexes along the circum-Pacific subduction zone. Instead, the concept of tectonic erosion has emerged to explain extensive crustal thinning and subsidence as an on-going process destroying the hanging wall of an older subduction complex or even the basement of the overriding plate at more than half of the active trench. During the past three decades, marine geophysicists and geologists have documented tectonic erosion as a more common process than the formation of an accretional complex in subduction zones, and supeculate that a large volume of the continental crust is subducted into the mantle at both accretionary and erosive convergent margins. A simple calculation of the amount of subducting continental material versus arc producted at the subduction zone suggests a balance, resulting in no growth of continental crust at present (e.g. Clift and Vannuchi, 2004; Scholl and von Huene, 2007). However, considering direct subduction of intra-oceanic arcs and foundering of the continental lower crust into the mantle, we must conclude there is negative growth of the continental crust on the Earth at present.

著者

丸山 茂徳 小宮 剛

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.120, no.5, pp.869-876, 2011-10-25 (Released:2012-01-17)

参考文献数

13

被引用文献数

4 12

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グリーンランド・イスア地域は緑色片岩–角閃岩相程度の広域変成作用を受けてはいるが,見事な枕状溶岩の構造を普遍的に残している。枕の形,急冷縁,マトリックスの構造などが明瞭に残存している。枕状溶岩の存在は38億年前に海洋がすでに存在していたことを示すが,さらに付随する岩石との関係から,典型的な付加体の存在が認められる。海洋プレート層序は付加体が現在の西太平洋地域のような海洋内島弧の一部として形成されたことを示す。付加体の形成は北から南側にプレートが沈み込んだことを示す。さらにCA系列の花崗岩帯が付加体を貫く。これらは太平洋型造山運動が38-37億年前に機能していた証拠である。玄武岩の化学組成から,起源マントルの組成と温度が推定される。海洋の温度を100℃以下とみなすと,38-37億年前の海洋プレートの剛性度とプレートの厚さが求められ,プレートテクトニクスが38億年前にすでに機能していたことがわかる。

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.4, pp.Cover04_01-Cover04_02, 2018-08-25 (Released:2018-10-05)

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長野県・諏訪湖(面積12.8 km2,平均水深4.7 m)では,冬季に全面結氷して数日後に「御神渡」現象が現れることが多い.御神渡は夜間と日中の気温変化で湖氷が収縮・膨張を繰り返すと,亀裂部分の氷がせり上がることで生じる.古来から,その道筋は諏訪大社の男神(上社)が女神(下社)に会いに行った証であるとの伝説がある.また,御神渡が出現すると,その道筋によってその年の農作物の豊凶や景気を占う拝観式が執り行われてきた. こうした諏訪湖の結氷日や御神渡出現日,拝観日の詳細な記録が1444年以降ほぼ連続的に残されており,過去574年間の冬の気候変動を知る貴重な史料として世界的にも注目されている.厳しい寒冬年には12月中に御神渡が出現することがあるが,暖冬年の場合,2月になって出現したり,御神渡ができずに明海(あけのうみ)となる.近年は温暖化の影響で湖が結氷しにくくなっているが,今冬(2017-2018年)は5年ぶりに御神渡が出現し,2月5日に八剣神社の宮坂清・宮司らによって拝観の儀式が執り行われた.写真では,湖の東岸から北岸(赤砂崎)にうねりながら延びる御神渡の様子がよくわかる.(写真:宮坂 清 2018年1月31日撮影;解説:三上岳彦)

著者

藤井 紀之

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.104, no.5, pp.793-796, 1995-10-25 (Released:2009-11-12)

参考文献数

3

著者

伊藤 英之 鈴木 正貴 佐藤 凌太 杉本 伸一 関 博充

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.561-574, 2015-08-25 (Released:2015-09-17)

参考文献数

13

被引用文献数

5

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This study is based on an Internet survey of prospective tourists visiting the Sanriku Geopark, which was conducted to understand their travel habits, impressions of the Sanriku coastal area, general awareness of the Geopark, and motivations for travelling. A principal component analysis (PCA) is also performed using a multivariate analysis technique to examine the characteristics of tourists and images of travel destinations they would want to visit in the future. The results demonstrate low awareness of the Sanriku Geopark, especially in metropolitan areas. The majority of respondents do not have a definitive image of the Geopark. The results also indicate that tourists visiting the Sanriku coast are primarily from neighboring prefectures, such as Miyagi and Aomori, as well as from the Tokyo metropolitan area. This suggests that increasing informational awareness of the Sanriku Geopark in these areas would be effective way for attracting more visitors. Regarding their travel habits, the respondents' answers indicate that they mostly go on two-day family trips in a private vehicle. In the majority of cases, their purpose for travelling is “to feel refreshed” or “to eat delicious food.” Based on these data, a PCA employing variance-covariance matrices reveals that tourists are basically seeking “extraordinary” and “healing” experiences from their travels. The principal component scores from the PCA are used and the average scores of each gender are calculated. A t-test reports no significant difference between genders with regard to seeking an “extraordinary” experience, but it identifies a trend among women of seeking “healing” experiences more actively than men (p < 0.10) at a level of 0.1%. On the other hand, regarding the tourists' impressions of the Sanriku coastal area, the results of the multivariate analysis suggest that prospective visitors tend to perceive the coastal area comprehensively in terms of both “nature and scenery” and “local area and culture” to the same degree as those who have previously visited the place. For the area to be resuscitated as a tourist destination, it is important to construct a regional brand and devise strategies that will lead to the region's rebirth as a top travel destination. This can be achieved by offering higher quality experiences and services without destroying the traditional image of the Sanriku coast.

著者

小泉 尚嗣

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.122, no.1, pp.159-169, 2013-02-25 (Released:2013-03-27)

参考文献数

62

被引用文献数

7 6

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Research on coseismic and postseismic groundwater pressure changes is important for evaluating long-term groundwater stability in seismically active regions such as Japan. It is also important for preventing and reducing earthquake-induced geological disasters such as landslides. This study reviews literature and clarifies abrupt coseismic changes and subsequent postseismic gradual changes in groundwater pressure. Coseismic changes result from ground shaking and coseismic crustal deformation. Postseismic changes are gradual changes, which can be described by a diffusion equation with new initial values and boundary conditions due to ground shaking and coseismic crustal deformation. The effects of crustal deformation on groundwater pressure appear to be limited in the vicinity of the seismic source region, although that of ground shaking can remain in distant areas. Because a variety of factors and conditions affect earthquake-related groundwater pressure changes, each should be investigated to more accurately clarify the effects on coseismic and postseismic groundwater pressure changes. This research may contribute toward clarifying seismicity mechanisms triggered by distant earthquakes.

著者

上村 彩 笠原 順三 日野 亮太 篠原 雅尚 塩原 肇 金沢 敏彦

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.109, no.4, pp.506-516, 2000-08-25 (Released:2010-11-18)

参考文献数

40

被引用文献数

4 4 3

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To predict earthquakes, huge efforts have been devoted to monitoring earthquakes, crustal deformations and water level during past decades, however, has been found to be extremely difficult. A new approach in marine seismic studies on earthquake prediction proposes focusing on the nature of the subducting plate boundary. Some Ocean Drilling Program (ODP) drillings and seismic reflection studies show fluid flows and the existence of hydrous minerals in the decollement zone. Considering that the subducting plate might contain much water, a number of hydrous minerals might be stable down to 150-300 km, in particular, serpentines and lawsonite.In 1998, the authors carried out a seismic experiment at the Izu-Bonin trench using Ocean Bottom Seismometers (OBSs) and controlled sources. The 130 km long E-W line crosses the Torishima forearc seamount, one of the serpentine diapirs at the eastern terminus. The velocity structure obtained along the line shows a relatively high velocity at the top and alow velocity at the bottom of the serpentine diapir. The velocity of the mantle wedge is slower than that of normal mantle. The dip of the subducting slab is initially quite gentle and abruptly becomes steep around 100 km from the trench axis. Hydration of peridotite in the mantle wedge might occur close to the Izu-Bonin trench axis and serpe ntines seem to be raised upward to the ocean bottom.

著者

磯崎 行雄 丸山 茂徳 青木 一勝 中間 隆晃 宮下 敦 大藤 茂

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.6, pp.999-1053, 2010-12-25 (Released:2011-03-17)

参考文献数

145

被引用文献数

26 74

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The geotectonic subdivision and relevant definitions of geotectonic units in the Japanese Islands are revised on the basis of new data, particularly with detrital zircon dating of U-Pb ages and seismic profiling of the deep arc crust across the islands. In addition to the final confirmation of the subhorizontal structures of the Paleozoic to Cenozoic accretionary complexes and their high-P/T metamorphosed equivalents, several new aspects were recognized; i.e., detection of the eastern extension of the collisional suture between the Sino-Korean and Yangtze cratons in the Higo belt with medium-pressure-type metamorphism in SW Japan, and separation of the traditional Sanbagawa belt into two distinct metamorphic belts characterized by mutually different ages of protolith AC-formation and peak metamorphism. The occurrence and consumption of 4 Paleozoic to Mesozoic granite batholiths, as major provenances for the ancient Japanese Islands, are documented by detrital zircon dating of Paleozoic–Mesozoic sandstones. With respect to these new findings, the definitions of unit boundaries were thoroughly revised in terms of chronological spectrum in “ocean plate stratigraphy–metamorphism”. The geological significance of 5 major tectonic lines (faults) of the Pacific-type (or Miyashiro-type) orogen in Japan, i.e., the Nagato–Hida marginal TL, Osayama–Omi TL, Ishigaki–Kuga TL, Paleo–Median TL, and Butsuzo TL, is discussed. The current revision of the geotectonic subdivision and definitions of component units and their mutual boundaries leads to the following conclusions, which challenge the conventional understanding of the orogenic history of the Japanese Islands. (1) Proto-Japan in the Early Paleozoic was located closer to the South China (Yangtze) craton rather than the North China (Sino–Korean) craton. (2) Ever since 520 Ma, subduction of past Pacific ocean floors formed mature arc-trench systems with a full set of granite batholith, fore-arc basin, accretionary complex, and high-P/T metamorphosed equivalents at least 5 times; however, the former 4 sets were almost completely destroyed, with the exception of smaller tectonic blocks that currently occur within serpentinite mélange. (3) Tectonic erosion played a significant role in consuming ancient fore-arc crusts including 4 granite batholiths of the Paleozoic to mid-Mesozoic. (4) Serpentine mélange represents the former Wadati–Benioff plane along which tectonic erosion took place. (5) The Japanese Islands, which basically developed along the Yangtze continental margin, have experienced multiple episodes of oceanward growth and continentward retreat due to alternating subduction-accretion and tectonic erosion. (6) Net production of juvenile crust occurred on a large scale along the Japan margin during the 500 million year-long oceanic subduction regime since the Cambrian; however, intensive tectonic erosion effectively erased the older crusts from the surface and enriched the underlying sub-arc mantle with heat-generating continental material.

著者

大橋 聖和 大坪 誠 松本 聡 小林 健太 佐藤 活志 西村 卓也

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.129, no.4, pp.565-589, 2020-08-25 (Released:2020-10-02)

参考文献数

108

被引用文献数

4 5

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The 2016 Kumamoto earthquake occurred in the tectonically complex central Kyushu area where several forcing factors such as the subducting Philippine Sea plate, the Median Tectonic Line and the Nankai forearc sliver, the spreading Okinawa trough, and the migrating volcanic front are involved. Neogene–Quaternary tectonics of central Kyushu are revisited by integrating geological, seismological, and geodetical approaches. Deformation histories of the Futagawa and Hinagu fault zones, the source faults of the Kumamoto earthquake, are also established in an attempt to explain the relationship between geologic structures and rupture processes of the earthquake. The results show that present-day tectonics surrounding central Kyushu are considered to have originated in the last 1 Ma or younger, as a transtensional tectonic zone (Central Kyushu Shear Zone) characterized by combined dextral faults and rift zones (or volcanoes). Reflecting spatiotemporal variations of the crustal stress field and rift activity, the Futagawa and Hinagu fault zones show multi-stage deformation throughout the Neogene–Quaternary periods: normal faulting to dextral faulting for the Futagawa fault zone and sinistral to dextral faulting for the Hinagu fault zone. Those diverse histories of stress and strain fields in central Kyushu possibly led to the complexities of fault geometry and rupture process of the Kumamoto earthquake.

著者

関口 武

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.73, no.1, pp.11-22, 1964-02-29 (Released:2009-11-12)

参考文献数

2

被引用文献数

1