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タイトル: 37. 丹那断層(北伊豆・名賀地区)の発掘調査
その他のタイトル: 37. Trenching Study for Tanna Fault, Izu, at Myoga, Shizuoka Prefecture, Japan
著者: 丹那断層発掘調査研究グループ
発行日: 1984年3月31日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第58冊第4号, 1984.3.31, pp. 797-830
抄録: Trenching was carried out in 1980 and 1982 on the alluvial plain in the Tanna basin, Izu, central Japan, to obtain evidence for Holocene activities of the Tanna fault, along which the last seismic displacement accompanied the Kita-Izu earthquake(M7.3) of 1930. A.D. The 1982-trench was seven meters deep. The deposits exposed consisted mainly of silt, peat, volcanic gravel and sand with thin layers of air-fall tephras of known ages, Akahoya ash(ca. 6,300 y.B.P.), Kawagodaira pumice (ca. 2,900 y.B.P.), Zunasawa scoria (2,500 y.B.P.) and Kozushima ash (838 A.D.). The Tanna fault was observed in the unconsolidated deposits as a zone 1-2meter wide, consisting of apparently discontinuous faults, fissure-filling deposits and intrusive silty material from below. The zone stands nearly vertical or steeply dipping to the west. The features useful in recognizing individual faulting events were: 1) unconformity between the lower, more complicated structure and the upper, less complicated one. 2) elastic wedges or pockets filling the open-cracks along the fault zone, protruding down to the underlying beds. 3) alternation of two kinds of beds: beds with uniform thickness and those with different thickness across the fault zone. The latter are marker deposits for an individual vertical displacement, which produced scarplet or flexure on the ground surface along the fault zone. The beds thicken toward the downthrown side across the fault zone. The beds with uniform thickness indicate no scarp-making fault movement during the deposition. We compared the thickness of each time-stratigraphic bed on both sides of the fault zone, taking into consideration the effect of left-lateral strike-slip displacement of the beds which were deposited with thickness increasing toward the south. Nine events are recognized in the deposits after deposition of Akahoya ash, including the last event of 1930 A.D. The third youngest event is probably the historical great earthquake of 841 A.D. reported from this province. Eight of the nine events have the western side of the fault zone downthrown and only one had the eastern side downthrown. The ages of the beds and events are inferred in two ways: (1) based on the four tephras of known age (uncorrected 14C age) and an assumption of a constant rate of deposition between the tephras, and (2) based on corrected "C age (calendar date) for the tephras and selected 14C samples. Recurrence intervals of the events in the past ca. 6,000-7,000 years thus inferred are fairly constant with a period of about 700-1000 years.
URI: http://hdl.handle.net/2261/12902
ISSN: 00408972


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