活断層調査の例 : 柳ケ瀬断層

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The Yanagase fault is a remarkable active fault in central Honshu, Japan, extending north-northwest-wards from the northeast of Lake Biwa to the northeast of Tsuruga City. Both geomorphological and geological detailed studies have been carried out along the fault zone to clarify the history of its movement especially in Quaternary time. A linear fault line Valley consisting of four different streams in a straight arrangment is developed along the fault zone over a distance of about 25 km, which is recognizable as lineament in a broad sense. Geomorphic features caused by left-lateral faulting such as shutter ridges, offset streams are detected along the southern part of the lineament. And the west-side block of the fault shows subsiding topographic features such as rather wide alluvial plain around Lake Yago, but the east-side uplifting such as elevated river terraces of two to five steps, composite alluvial fans, high triangular terminal facets and so on. The paleozoic system consisting of slate, sandstone, chert, greenstones is sheared along the fault in a zone of a few hundred meters wide, in which rocks are strongly crushed to be transformed into fault gouge in a maximum width 50 meters or so. Porphyrite dykes within the fault zone are also crushed at some localities. Fault displacement in late Quaternary fan deposits has been observed at several places in the southern part of the fault zone. However no evidence of Quaternary fault movement has been found out in the northern part, although the fault zone is overlain by Quaternary fan deposits here and there. Each fault shows different sense of dis placement, that is lateral-slip, east-side uplift dip slip and west-side uplift dip slip. The youngest deposit cut by the fault movement with west-side subsidence and east-side uplift occured at 80,000 to 100,000 years ago along the southern part, seems to have changed its direction to the south as in the present. It is concluded the Yanagase fault has moved repeatedly since late Paleozoic time, and has been active in its southern half in late Quaternary time, on the contrary not or scarcely active in the northern part. Its dominant sense of movement in late Quaternary time, on the contrary not or scarcely active in the northern part. Its dominant sense of movement in late Quaternary is supposed to be left-lateral, west-side subsidence and east-side uplift.
一般社団法人日本応用地質学会の論文
1981-03-30

活断層調査の例 : 柳ケ瀬断層

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