Ancient Liquefactions and Their Significance in the Late Quaternary Tephra Layers in the Karasu River Valley, Gunma Prefecture, Japan.

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  • 群馬県烏川中流域のテフラ層中にみられる液状化現象とその意義
  • グンマケン カラスガワ チュウリュウイキ ノ テフラソウチュウ ニ ミラレル

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Abstract

The authors have identified four liquefaction phenomena in tephra layers in the Karasu river valley, Gunma Prefecture, in central Japan. The last liquefaction must have occurred between 1108 and 1783, because the material spouted by the liquefaction penetrates the tephra layer of 1108 and is covered by the tephra layer of 1783. The other three liquefactions are judged to have occurred after 10, 000yrs BP, at about 17, 000yrs BP and between 21, 000 and 17, 000yrs BP, respectively, based on their stratigraphic horizons in the tephra layers.<br>The Gunma district has been considerd to be at low risk for earthquakes, because it has only a few active faults and has had only one historical earthquake, in 818. However, the last liquefaction reveals that a previously unknown historical earthquake occurred after 818, suggesting a higher frequency of earthquakes in this area than was previously thought.<br>In ancient liquefactions, “draw-in” phenomena can be distinguished from “intrusion-spout” ones . Although no previous papers have reported about the draw-in process, it is important because it follows the intrusion-spout process during the final stage of the liquefaction. Thus, comprehensive observation of all phenomena including the draw-in process is necessary for complete understanding of liquefaction.<br>Tephra layers contain a great quantity of geological information in addition to the ancient liquefactions reported here. We should not only use tephra layers as chronological sources but also study the information recorded in them.

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