白石一福島活断層系の断層変位地形と最新活動期
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概要
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The Shiroishi-Fukushima Fault System, located on the eastern fringe of the southern Ou Backbone Ranges, has been said to be active mainly during late Quaternary (e. g. Matsuda, 1973). Geomorophic surfaces in this area are classified into tweleve surfaces, namely H1 H2, M1, Older mud flow surface, M2, Younger pyroclastic flow surface, L1, L2, L3, L4, Small fan and Alluvial surfaces, descending order. Most of them except lower terraces (L4 and Alluvial surface) were deformed by many active faults. In order to consider a characteristics of recent faultings, the author investigated geology and topography, especially deformed landforms, of the western margins of the Shiroishi Basin, Kosugo Lowland and Fukushima Basin. The findings can be summarized as follows.1) The Shiroishi-Fukushima Fault System, 50km in length, is consisted of twentyone reverse faults in the NNE-SSW, NE-SW or E-W directions (Fig. 6).2) This fault system can be divided into four fault groups; Kosugo faults, Kori faults, Niwasaka faults and Sahara faults.3) The formative periods of several surfaces were dated by radiocarbon method. Developing above M2 surface and Younger pyroclastic flow surface are older than 35, 000 yr B. P. The formative periods of L1 surface is about 20, 000 yr B. P. That of L3 is about 5, 000 yr B. P. Other surfaces also can be distinguished in chronologic order.4) Judging from the periods of most recent faulting in this fault system, it was not all of the faults had been active at the same time. That is to say, Kosugo faults and Kori faults in the northern part of this fault system (1-14 fault) had been active after the formative period of L3 surface before that of Alluvial surface. On the other hand, Niwasaka faults and Sahara faults in the southern part (15-21 fault) had not been active since the formative period of L3 surface.
- 東北地理学会の論文