Geo-history of the Late Quaternary ancient liquefaction in the Karasu River Region, Gunma Prefecture, central Japan

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  • 群馬県烏川中流域の第四紀末期の液状化履歴と変動史
  • グンマケン カラスガワ チュウリュウイキ ノ ダイ4キ マッキ ノ エキジョウカ リレキ ト ヘンドウシ

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The authors identified six stratigraphic horizons of preserved deformation structure by ancient liquefaction (liquefaction structure) that have occurred since 24,000yrs, in loam and black soil layers in the Karasu River region, Gunma Prefecture, central Japan. Loam and black soil layers intercalate several dated pumice layers from the Asama Volcano, which is situated about 30km west of the study area. Ancient liquefaction structures reveal that the liquefied material penetrated upward into the pumice layer and blew out, covering younger pumice layers. The occurrence of ancient liquefactions based on their stratigraphic horizons in the pumice layers from youngest are as follows: 1) between 1108 and 1783AD., apreviously unkown historical earthquake, 2) 818AD., [Kanto-Shokoku earthquake, >M7.5], 3) after 10.000yrs BP., 4) between 16,000 and 14,000yrs. BP., 5) slightly before 20,000yrs. BP., and 6) slightly after 24,000yrs. BP. An earthquake, which occurred after 10,000yrs BP. triggered the large-scale Takasaki mudflow (deposit) and the reactivation of the Yudonosan mega-landslide. These ancient liquefactions and related phenomena in the Karasu River region may have a close relationship with the Karasugawa Tectonic Line or the north segment of the Fukaya Fault System, which bound the southwestern margin of the North Kanto Plain.

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