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  • Paleoseismicity of the Yugamine fault constituting the Atera Fault System(Summaries of Researches Using AMS)

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The Yugamine fault, 10 km in length is in the north-central part of the Atera fault system. We studied the Late Quaternary activity of the Yugamine fault through the trenching survey. We excavated a trench across the scarplet on the L1 terrace at Obayashi district. Some humic Layers and fluvial gravels are exposed on the trench walls. The Yugamine fault cut through these layers to form a linear depression on its NE side. From the structural evidence along the fault plane and angular unconformity, we recognize at least three faulting events in this trench. Event 1:916±28yr.B.P.〜2871±29yr.B.P. (A.D.1161〜B.C.1125) We can find the angular unconformity between unit 2d and unit 2a on the both walls. This possibly shows that the last event occurred after the deposition of unit 2d (2871±29yr.B.P.) and before the deposition of unit 2a (916±28yr.B.P.). Event 2: 2564±30yr.B.P. 〜4382±32yr.B.P. (B.C.601〜B.C.3019) The F2 fault plane cut a bottom of unit 3a and the uppermost termination of F2 fault plane is covered by a colluvium(unit W) which located middle part of unit 3a. The age of this colluvium(unit W) is constrained to be older than upper part of unit 3a(dated at 2564±30yr.B.P.) , and younger than the upper part of unit 3b (dated at 4382±32yr.B.P.). Event 3: Before 9124±37yr.B.P. (B.C.8269〜) We can reveal at least one faulting event since the deposition of the gravel (Unit 6). The age of the gravel is assumed to be older than the bottom of unit 3b (dated at 9124±37yr.B.P.).



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