Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.117.901
T. Takano, Sekita Mountains Collaborative Rese
We studied the morphogenetic movements associated with the formation of the Sekita Mountains and Iiyama Basin situated at the boundary between the Niigata and Nagano prefectures in central Japan. As a result, we obtained following conclusions. (1) The morphogenetic movement of the Sekita Mountains is a semicylindrical upwarping originating from faulting at both sides of the mountains. The origin was an inclined thrust of a basal block. In addition, it is assumed that the upheaval of the mountains due to a change in isostasy caused by erosion resulted in the four-thousand meter thick strata that constitutes the mountains, and erosion originated from a fold in the strata occurring in the middle Pleistocene. (2) The morphogenetic movement constituting the Iiyama Basin is a reverse fault, which originated from tilting and drawing of the basal crust as a result of a semicylindrical upwarping of the Kato Mountains situated at the east side of the Iiyama Basin. The upwarping was caused by magma ascending from a deep part of the crust. Therefore, the Iiyama Basin was formed as a fault-angle basin. The mountain body of Kenasi Volcano tilted toward the west through the process of forming the Iiyama Basin. We think the Iiyama Basin continued to subside at the rate of about one millimeter a year in the Holocene, based on radiocarbon dating of drilling core samples collected from sediments in the alluvial lowlands.
{"title":"A Study on the Morphogenetic Movements Associated with the Formation of the Sekita Mountains and the Iiyama Basin, Central Japan","authors":"T. Takano, Sekita Mountains Collaborative Rese","doi":"10.5026/JGEOGRAPHY.117.901","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.901","url":null,"abstract":"We studied the morphogenetic movements associated with the formation of the Sekita Mountains and Iiyama Basin situated at the boundary between the Niigata and Nagano prefectures in central Japan. As a result, we obtained following conclusions. (1) The morphogenetic movement of the Sekita Mountains is a semicylindrical upwarping originating from faulting at both sides of the mountains. The origin was an inclined thrust of a basal block. In addition, it is assumed that the upheaval of the mountains due to a change in isostasy caused by erosion resulted in the four-thousand meter thick strata that constitutes the mountains, and erosion originated from a fold in the strata occurring in the middle Pleistocene. (2) The morphogenetic movement constituting the Iiyama Basin is a reverse fault, which originated from tilting and drawing of the basal crust as a result of a semicylindrical upwarping of the Kato Mountains situated at the east side of the Iiyama Basin. The upwarping was caused by magma ascending from a deep part of the crust. Therefore, the Iiyama Basin was formed as a fault-angle basin. The mountain body of Kenasi Volcano tilted toward the west through the process of forming the Iiyama Basin. We think the Iiyama Basin continued to subside at the rate of about one millimeter a year in the Holocene, based on radiocarbon dating of drilling core samples collected from sediments in the alluvial lowlands.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"24 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"117142904","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.118.1221
M. Yoshino
The global climate is known to have been relatively warm during the period from the 4th to 10th centuries, although there were slightly different fluctuation patterns locally and regionally. The present article addresses these differences, analyzing the results of previous studies. The warm period is known in Europe as the Medieval Warm Period. Evidence in Japan is also found from the 4th century to the 11th century. Because historical age divisions differ between Europe and Japan, the peak of the Warm Period from the 7th to the 10th century is classified as part of the ancient period in Japan. Therefore, the Warm Period in Japan has been proposed to be called the Nara-Heian Warm Period, Heian Warm Period or Little Climatic Optimum. Based on the water level changes of Lake Shinji in Shimane Prefecture, the present article discusses the warmer climatic conditions in the Heian Period. It also examines old agricultural settlements in the Tohoku District, northern Honshu. People came from Hokkaido or northern Honshu and cultivated rice in the northeastern-most part of Honshu in the 1st century B.C. It is thought that the effect of the warm current branch flowing along the Japan Sea Coast and emerging on the Pacific side through the Tsugaru Straight had an influence on the distribution of rice cultivation at this early stage. Finally, the article shows that the northward shift of the power front of the Central Government (Yamato Chotei) during the 7th to the 9th centuries occurred about 70-80 years earlier in Dewa, an ancient state on the Japan Sea side of Tohoku District, than in Mutsu, also an ancient state on the Pacific side. It is interesting to note, however, that the speed of the northward shift was almost the same on both sides, even though there were different political powers, situations and problems on either side. It is suggested that the northward shift was affected by the warming on the broader space scale.
{"title":"Climatic Change and Human Activities in the 4th to 10th Centuries","authors":"M. Yoshino","doi":"10.5026/JGEOGRAPHY.118.1221","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.1221","url":null,"abstract":"The global climate is known to have been relatively warm during the period from the 4th to 10th centuries, although there were slightly different fluctuation patterns locally and regionally. The present article addresses these differences, analyzing the results of previous studies. The warm period is known in Europe as the Medieval Warm Period. Evidence in Japan is also found from the 4th century to the 11th century. Because historical age divisions differ between Europe and Japan, the peak of the Warm Period from the 7th to the 10th century is classified as part of the ancient period in Japan. Therefore, the Warm Period in Japan has been proposed to be called the Nara-Heian Warm Period, Heian Warm Period or Little Climatic Optimum. Based on the water level changes of Lake Shinji in Shimane Prefecture, the present article discusses the warmer climatic conditions in the Heian Period. It also examines old agricultural settlements in the Tohoku District, northern Honshu. People came from Hokkaido or northern Honshu and cultivated rice in the northeastern-most part of Honshu in the 1st century B.C. It is thought that the effect of the warm current branch flowing along the Japan Sea Coast and emerging on the Pacific side through the Tsugaru Straight had an influence on the distribution of rice cultivation at this early stage. Finally, the article shows that the northward shift of the power front of the Central Government (Yamato Chotei) during the 7th to the 9th centuries occurred about 70-80 years earlier in Dewa, an ancient state on the Japan Sea side of Tohoku District, than in Mutsu, also an ancient state on the Pacific side. It is interesting to note, however, that the speed of the northward shift was almost the same on both sides, even though there were different political powers, situations and problems on either side. It is suggested that the northward shift was affected by the warming on the broader space scale.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"25 1‐2","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"120838417","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.117.45
K. Shiomi, M. Matsubara, K. Obara
By the end of the last century, the rough configuration of the Moho discontinuity beneath the Japan Islands had been revealed based on explosion surveys and natural earthquake observations. Recently, however, some researchers have pointed out that local roughness of the Moho geometry or relative location between continental and oceanic Moho might provide important knowledge about the source regions of large earthquakes. Within the southern portion of the Kinki district, the Philippine Sea plate subducts beneath the continental plate at the Nankai Trough. We detect P-to-S converted wave energy from the Moho velocity discontinuity beneath the Kinki district with receiver function analysis, and compare the results of other recent investigations of the depth of Moho. Both oceanic and continental Moho discontinuities are detected in not only our receiver function analysis but also active-source seismic exploration survey and travel-time tomography analysis. The inferred depths of the subducting oceanic Moho beneath the Kii Peninsula, the southern Kinki district, and the continental Moho beneath the northern Kinki correspond well with each other. However, beneath the central Kinki district, no significant converted phases are observed corresponding to the Moho depth inferred from the travel-time analyses. We interpret that no sharp velocity discontinuity exists around the Moho in the central Kinki district.
{"title":"Detecting of the Moho Discontinuities Using Seismological Surveys: A Case in the Kinki District","authors":"K. Shiomi, M. Matsubara, K. Obara","doi":"10.5026/JGEOGRAPHY.117.45","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.45","url":null,"abstract":"By the end of the last century, the rough configuration of the Moho discontinuity beneath the Japan Islands had been revealed based on explosion surveys and natural earthquake observations. Recently, however, some researchers have pointed out that local roughness of the Moho geometry or relative location between continental and oceanic Moho might provide important knowledge about the source regions of large earthquakes. Within the southern portion of the Kinki district, the Philippine Sea plate subducts beneath the continental plate at the Nankai Trough. We detect P-to-S converted wave energy from the Moho velocity discontinuity beneath the Kinki district with receiver function analysis, and compare the results of other recent investigations of the depth of Moho. Both oceanic and continental Moho discontinuities are detected in not only our receiver function analysis but also active-source seismic exploration survey and travel-time tomography analysis. The inferred depths of the subducting oceanic Moho beneath the Kii Peninsula, the southern Kinki district, and the continental Moho beneath the northern Kinki correspond well with each other. However, beneath the central Kinki district, no significant converted phases are observed corresponding to the Moho depth inferred from the travel-time analyses. We interpret that no sharp velocity discontinuity exists around the Moho in the central Kinki district.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"13 3","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"120859858","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.118.631
T. Rashid, Md. Hossain Monsur, Shigeyuki Suzuki, Nobuo Ooi
This paper provides a detailed description of sedimentary facies including pollen and diatom analyses, and 14C ages obtained from a hand-excavated outcrop in the central part of the Bengal Lowland. Based on this examination, seven sedimentary facies were recognized in relation to relative sea-level (RSL) changes since the mid Holocene. With the help of a standard reference datum, the required Mean Sea Level (MSL) has been calculated at the surface of the outcrop. The top of the outcrop was about 1.9 m above MSL and the base is 4.1 m below the MSL. The lowermost bioturbated sand flat facies, 3.75 m below the present MSL, contains mangrove pollen and represents an intertidal coastal-plain and estuarine-channel deposits that were dominated by tidal current. A 14C age of 7570-7430 cal BP was obtained for this unit. The evidence supports a transgressive mode in the Bay of Bengal during the middle-Holocene epoch. A subsequent regression interval was found in a salt marsh facies before 6670-6410 cal BP. The salt marsh facies has a sharp contact with an underlying (lower) mud flat facies, shaped by downcutting of the upper bed, indicating a regression. In response to this regression, the environment changed gradually from a mud flat to a salt marsh. The salt-marsh deposits contain abundant mangrove pollens and marine- and brackish-water diatoms, providing support for inflow of shallow-marine water into a supratidal, brackish-water mangal environment. The bioturbated mud flat facies indicates that the succession was dominated by tidal current, and the relative sea-level started to rise again after a small rebound. Hence, the transgressive conditions prevailed during the deposition of this mud flat facies in an intertidal coastal-plain environment. The underlying bluish-black medium-bedded peat layer contains grass pollen. This indicates that after the mid-Holocene the environment around the site changed gradually from mangrove to fresh-water swamp vegetation, in response to regression of the bay between 4080-4030 cal BP. The above evidence suggests that the central part of the Bengal Lowland represented as an intertidal to supratidal coastal-plain estuarine environment that experienced with mid-Holocene relative sea-level changes between 7570-7430 and 4080-4030 cal BP.
本文提供了沉积相的详细描述，包括花粉和硅藻分析，以及从孟加拉低地中部手工发掘的露头中获得的14C年龄。在此基础上，识别出全新世中期以来相对海平面变化的7种沉积相。在标准参考基准的帮助下，计算了露头表面所需的平均海平面(MSL)。露头顶部距地面约1.9 m，底部距地面4.1 m。最下面的生物扰动沙滩相，在目前的MSL下3.75 m，含有红树林花粉，代表潮汐间海岸平原和河口水道沉积，主要由潮流控制。该单元的14C年龄为7570-7430 cal BP。证据支持中全新世时期孟加拉湾的海侵模式。在6670 ~ 6410 cal BP之前的盐沼相中发现了后续的回归区间。盐沼相与下伏(下)泥滩相有明显的接触，由上层的下切而成，显示出退行。随着这种退化，环境逐渐从泥滩变成了盐沼。盐沼沉积物含有丰富的红树林花粉和海水和微咸水硅藻，为浅海水流入潮上微咸水红树林环境提供了支持。生物扰动泥滩相表明，该演替以潮流为主，相对海平面小幅反弹后又开始上升。因此，在潮间带海岸平原环境下，该泥滩相沉积时，海侵条件占主导地位。下面的蓝黑色中层泥炭层含有草花粉。这表明，在中全新世之后，随着海湾在4080-4030 cal BP之间的回归，遗址周围的环境逐渐由红树林植被转变为淡水沼泽植被。上述证据表明，在7570 ~ 7430 ~ 4080 ~ 4030 cal BP期间，孟加拉低地中部为潮间带—潮上海岸平原河口环境，经历了中全新世相对海平面的变化。
{"title":"Reconstruction of Holocene Paleoenvironment and Evidence of Sea-level Changes in the Bengal Lowland","authors":"T. Rashid, Md. Hossain Monsur, Shigeyuki Suzuki, Nobuo Ooi","doi":"10.5026/JGEOGRAPHY.118.631","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.631","url":null,"abstract":"This paper provides a detailed description of sedimentary facies including pollen and diatom analyses, and 14C ages obtained from a hand-excavated outcrop in the central part of the Bengal Lowland. Based on this examination, seven sedimentary facies were recognized in relation to relative sea-level (RSL) changes since the mid Holocene. With the help of a standard reference datum, the required Mean Sea Level (MSL) has been calculated at the surface of the outcrop. The top of the outcrop was about 1.9 m above MSL and the base is 4.1 m below the MSL. The lowermost bioturbated sand flat facies, 3.75 m below the present MSL, contains mangrove pollen and represents an intertidal coastal-plain and estuarine-channel deposits that were dominated by tidal current. A 14C age of 7570-7430 cal BP was obtained for this unit. The evidence supports a transgressive mode in the Bay of Bengal during the middle-Holocene epoch. A subsequent regression interval was found in a salt marsh facies before 6670-6410 cal BP. The salt marsh facies has a sharp contact with an underlying (lower) mud flat facies, shaped by downcutting of the upper bed, indicating a regression. In response to this regression, the environment changed gradually from a mud flat to a salt marsh. The salt-marsh deposits contain abundant mangrove pollens and marine- and brackish-water diatoms, providing support for inflow of shallow-marine water into a supratidal, brackish-water mangal environment. The bioturbated mud flat facies indicates that the succession was dominated by tidal current, and the relative sea-level started to rise again after a small rebound. Hence, the transgressive conditions prevailed during the deposition of this mud flat facies in an intertidal coastal-plain environment. The underlying bluish-black medium-bedded peat layer contains grass pollen. This indicates that after the mid-Holocene the environment around the site changed gradually from mangrove to fresh-water swamp vegetation, in response to regression of the bay between 4080-4030 cal BP. The above evidence suggests that the central part of the Bengal Lowland represented as an intertidal to supratidal coastal-plain estuarine environment that experienced with mid-Holocene relative sea-level changes between 7570-7430 and 4080-4030 cal BP.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127372259","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.119.COVER03_2
S. Yamakawa, R. Suzuki
図1の写真（撮影：中野 俊）は，表紙とほぼ同位置の13年前の状況を示す．撮影角度がやや異なるが，13年間の氷帽の変化を観察することができる．なお，Thompson et al.（2009）によれば，キリマンジャロ頂上の氷帽の面積は1912～1953年には約1%/yearの割合で減少していたが，1989～2007年には約2.5%/yearと変化し，減少速度を速めた．過去のデータ（Thompson et al., 2002）から外挿して今後を推測すると，2015年には氷河はほぼ消滅するとみられる（岩田・小森, 2010）．
图1的照片(摄影:中野俊)显示了与封面几乎相同位置的13年前的状况。虽然拍摄角度略有不同，但仍可观察到冰帽十三年来的变化。另外，根据Thompson et al.(2009)的统计，乞力马扎罗山峰顶的冰帽面积在1912 ~ 1953年间以约1%/year的比例减少，1989 ~ 2007年变为约2.5%/year，减少速度加快。根据过去的数据(Thompson et al.， 2002)推测今后，预计2015年冰川将基本消失(岩田、小森，2010)。
{"title":"A Comparison of the Ice Cap of Kilimanjaro in January 1996 and January 2009","authors":"S. Yamakawa, R. Suzuki","doi":"10.5026/JGEOGRAPHY.119.COVER03_2","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.COVER03_2","url":null,"abstract":"図1の写真（撮影：中野 俊）は，表紙とほぼ同位置の13年前の状況を示す．撮影角度がやや異なるが，13年間の氷帽の変化を観察することができる．なお，Thompson et al.（2009）によれば，キリマンジャロ頂上の氷帽の面積は1912～1953年には約1%/yearの割合で減少していたが，1989～2007年には約2.5%/yearと変化し，減少速度を速めた．過去のデータ（Thompson et al., 2002）から外挿して今後を推測すると，2015年には氷河はほぼ消滅するとみられる（岩田・小森, 2010）．","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"8 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126035253","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.118.1254
N. Geshi
A large pyroclastic eruption occurred around 7.3 ka from the Kikai caldera about 30 km north of Yakushima Island. Its pyroclastic flow and fall deposits covered the entire area of Yakushima Island and may have influenced the evolution of unique floras and faunas of Yakushima Island. Detailed field survey revealed that the Koya pyroclastic flow deposit spread from NW to SE, covering almost the entire area of Yakushima. A part of the southern coastal area remained from the pyroclastic flow due to local alignment of topographic ridges and valleys, which acted as barriers to the pyroclastic flows. Possible tsunami deposits associated with the Kikai-Akahoya eruption were discovered in the area below ca. 50 m above sea level along the northern coasts of Yakushima and Kuchinoerabujima Islands.
{"title":"Distribution and Flow Mechanisms of the 7.3 ka Koya Pyroclastic Flow Deposits Covering Yakushima Island, Kagoshima Prefecture","authors":"N. Geshi","doi":"10.5026/JGEOGRAPHY.118.1254","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.1254","url":null,"abstract":"A large pyroclastic eruption occurred around 7.3 ka from the Kikai caldera about 30 km north of Yakushima Island. Its pyroclastic flow and fall deposits covered the entire area of Yakushima Island and may have influenced the evolution of unique floras and faunas of Yakushima Island. Detailed field survey revealed that the Koya pyroclastic flow deposit spread from NW to SE, covering almost the entire area of Yakushima. A part of the southern coastal area remained from the pyroclastic flow due to local alignment of topographic ridges and valleys, which acted as barriers to the pyroclastic flows. Possible tsunami deposits associated with the Kikai-Akahoya eruption were discovered in the area below ca. 50 m above sea level along the northern coasts of Yakushima and Kuchinoerabujima Islands.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"288 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114611277","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.119.102
Y. Ota, Mitsuhisa Watanabe, K. Taniguchi, Yasuhiro Suzuki, H. Sawa, Mayumi Tanaka, I. Suzuki, D. Hirouchi, Haeng-Yoong Kim
The Eastern Marginal Fault of the Tokamachi Basin is located on the right bank of the Shinano River, the longest river in Japan, in the Niigata area, within the Neogene fold and thrust belt of central Japan. The activity of this fault is partly responsible for the formation of the Tokamachi tectonic basin. The fault zone is composed of several subparallel fault branches that strike N-S to NNW-SSE and deformed late Pleistocene to Holocene fluvial terraces. These fault branches are characterized by west-facing scarps with some subsidiary east-facing scarps to the east of the basin. Although the destructive 2004 and 2007 earthquakes occurred in the northern part of this fold and thrust belt, no historical rupture has been recorded in the Tokamachi Basin. To obtain paleoseismic records in this tectonically formed basin, we excavated four trenches across different fault branches. The Banba South, Banba North, and Shinmiya trenches are located across the west-facing scarps, and the Miyakuri trench is on an east-facing scarp. Fault exposures and deformed terrace deposits are present in these four trenches. These results confirm that these geomorphic scarps are indeed produced by faulting. At the Banba S trench, three fault traces that have nearly horizontal to very shallow dipping fault planes with upthrown side on the east are present, and show clear evidence of the latest event at ca. 3,500-3,100 yrs BP (BC 1,965-1,630 to BC 1,505-1,145). A penultimate event possibly occurred at after ca.11,000 yrs BP. (BC 11,810-9,800), although its exact timing is not determined. At the Banba N trench, the terrace deposits of ca. 5,300 yrs BP (BC 4,225-3,965) are deformed. This suggests the age of deformation is younger, and probably coincides with the latest event at the Banba S trench. At the Shinmiya trench, the late Pleistocene terrace is divided into two parts by a flexural scarp, probably produced by a blind reverse fault. The scarp is formed prior to 14,000 yrs BP (BC 14,710-13,700). In addition, a younger event is recognized to have occurred between 9,400 and 8,900 yrs BP (BC 8,555-8385 to BC 8,000-7,910). In contrast to those trenches on the west-facing scarps, trench logs at Miyakuri, located on an east-facing scarp, show possibly two events with relatively steep fault planes during the last 32,000 yrs. The latest activity at Miyakuri occurred between 9,500 and 7,400 yrs BP (BC 8,955 to BC 6,365-6,045), but the age of the penultimate event cannot be precisely determined. However, we suggest that the faulting interval at Miyakuri is longer than that of other fault branches. In summary, from this study, we can identify at least three paleoseismic events, namely, I, II, and III, and a possible event IV for the Eastern Marginal Fault of the Tokamachi Basin. Event I is clearly identified at Banba S and N trench. Three faults probably moved simultaneously by Event II. Two other events are only recognized locally. Faulting on the west-facing scarp is more frequen
{"title":"Paleoseismology as Deduced from the Trenching Data in the Eastern Marginal Fault of the Tokamachi Basin, Located in the Thrust and Fold Belt in Central Japan","authors":"Y. Ota, Mitsuhisa Watanabe, K. Taniguchi, Yasuhiro Suzuki, H. Sawa, Mayumi Tanaka, I. Suzuki, D. Hirouchi, Haeng-Yoong Kim","doi":"10.5026/JGEOGRAPHY.119.102","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.102","url":null,"abstract":"The Eastern Marginal Fault of the Tokamachi Basin is located on the right bank of the Shinano River, the longest river in Japan, in the Niigata area, within the Neogene fold and thrust belt of central Japan. The activity of this fault is partly responsible for the formation of the Tokamachi tectonic basin. The fault zone is composed of several subparallel fault branches that strike N-S to NNW-SSE and deformed late Pleistocene to Holocene fluvial terraces. These fault branches are characterized by west-facing scarps with some subsidiary east-facing scarps to the east of the basin. Although the destructive 2004 and 2007 earthquakes occurred in the northern part of this fold and thrust belt, no historical rupture has been recorded in the Tokamachi Basin. To obtain paleoseismic records in this tectonically formed basin, we excavated four trenches across different fault branches. The Banba South, Banba North, and Shinmiya trenches are located across the west-facing scarps, and the Miyakuri trench is on an east-facing scarp. Fault exposures and deformed terrace deposits are present in these four trenches. These results confirm that these geomorphic scarps are indeed produced by faulting. At the Banba S trench, three fault traces that have nearly horizontal to very shallow dipping fault planes with upthrown side on the east are present, and show clear evidence of the latest event at ca. 3,500-3,100 yrs BP (BC 1,965-1,630 to BC 1,505-1,145). A penultimate event possibly occurred at after ca.11,000 yrs BP. (BC 11,810-9,800), although its exact timing is not determined. At the Banba N trench, the terrace deposits of ca. 5,300 yrs BP (BC 4,225-3,965) are deformed. This suggests the age of deformation is younger, and probably coincides with the latest event at the Banba S trench. At the Shinmiya trench, the late Pleistocene terrace is divided into two parts by a flexural scarp, probably produced by a blind reverse fault. The scarp is formed prior to 14,000 yrs BP (BC 14,710-13,700). In addition, a younger event is recognized to have occurred between 9,400 and 8,900 yrs BP (BC 8,555-8385 to BC 8,000-7,910). In contrast to those trenches on the west-facing scarps, trench logs at Miyakuri, located on an east-facing scarp, show possibly two events with relatively steep fault planes during the last 32,000 yrs. The latest activity at Miyakuri occurred between 9,500 and 7,400 yrs BP (BC 8,955 to BC 6,365-6,045), but the age of the penultimate event cannot be precisely determined. However, we suggest that the faulting interval at Miyakuri is longer than that of other fault branches. In summary, from this study, we can identify at least three paleoseismic events, namely, I, II, and III, and a possible event IV for the Eastern Marginal Fault of the Tokamachi Basin. Event I is clearly identified at Banba S and N trench. Three faults probably moved simultaneously by Event II. Two other events are only recognized locally. Faulting on the west-facing scarp is more frequen","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"85 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130572383","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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