著者

藤森 孝俊

出版者

公益社団法人 日本地理学会

雑誌

地理学評論 Ser. A (ISSN:00167444)

巻号頁・発行日

vol.64, no.10, pp.665-696, 1991-10-01 (Released:2008-12-25)

参考文献数

51

被引用文献数

9 15

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糸静線中央部に位置する諏訪盆地の活断層は,変位様式と活動度,分布に基づいてA~Cの3タイプに分類できる.タイプAの断層は盆地の南東端および北西端にみられるもので,大きな左横ずれ成分 (8~10m/kyr) をもつ.タイプBは盆地底と周辺山地の境界部に位置し,盆地側を低下させるもので,約1~3m/kyrの上下変位速度をもつ.タイプCは周辺山地内に位置し盆地側を低下させるもので,いくつかは並行し盆地側への階段断層となっている.平均変位速度は最大でも0.5m/kyr程度である.これらの活断層の分布・分類は,プルアパートベイズンとしての諏訪盆地の形成過程を示すモデルで説明される.諏訪盆地を開口させる主断層にあたるものがタイプA,開口した地殻の盆地側の面(開口壁)にあたるものがタイプB,開口壁の背後の地殻に発達した重力性の正断層がタイプCの断層である.また,古水系や諏訪盆地の形態から,水平圧縮応力により屈曲した主断層(糸静線)が左横ずれし,屈曲部の地殻が徐々に開口していくモデルが諏訪盆地の形成をよく説明できる.諏訪盆地の長辺方向への弘大速度は約8~10m/kyrであり,形成開始期は約120~150万年前と推定される.

著者

藤森 孝俊 蒔苗 耕司 山口 勝 川口 隆 太田 陽子

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.99, no.2, pp.166-181, 1990-04-25 (Released:2009-11-12)

参考文献数

18

被引用文献数

1

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The 1872 Hamada earthquake (M=7.1±0.2) is one of the major historical earthquakes which accompanied a coseismic uplift and subsidence in the coastal area. This paper intends to examine an implication of the Hamada earthquake for geomorphological development of the Hamada area, based on the investigation of marine terraces, emerged sea level indicators such as sea caves, benches, fossil beds and beach deposits, and fault topography.Two steps of Pleistocene marine terraces are found in the coastal area. Terrace I, c. 40-60 m high, is underlain by weathered beach gravel covered by dune sand which is interbedded by at least four horizons of paleosol. It is considered, that a major interglacial period resulting in strong weathering of Terrace I deposits occurred after the formation of this terrace. Thus, Terrace I can be correlated to the penultimate interglacial, and Terrace II, c. 15-25 m high, underlain by rather fresh gravel bed, to the last interglacial. Accepting this correlation, the uplift rate of the study area must be small, c. 0.1-0.2 m/ka, and is no significant difference in the uplift rate over the study area. Height of emerged sea level indicators associated with the Hamada earthquake is 0.9-1.7 m above the present mean sea level. At least one sea level indicator higher than the emerged sea level at the time of the 1872 Hamada earthquake was found at several locations through the study area including the coast where coseismic subsidence occurred in 1872. Northeast-southwest trending lineaments predominate in the study area and a fault exposure is observed on one of the lineaments suggesting that they are fault origin. Areas of coseismic uplift and subsidence can be seens by turns along these faults.Characteristics of the 1872 Hamada earthquake are summarised as follows, based on the above mentioned geomorphological observation : 1) Coseismic deformation as associated with the Hamada earthquake has not accumulated through the late Quaternary, and this earthquake must be a very rare event with coastal deformation during the Holocene. 2) Distribution pattern of coseismic deformation, that is, the occurrence of small blocks with coseismic uplift and subsidence by turns can be interpreted as an effect of strike-slip movement on the faults.