房総半島南部保田低地の完新世海岸段丘と地震性地殻変動

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タイトル別名
  • Holocene Marine Terraces and Seismic Crustal Movements in Hota Lowland in the Southern Part of Boso Peninsula, Central Japan.
  • ボウソウ ハントウ ナンブ ホタ テイチ ノ カンシン セイ カイガン ダンキュウ ト ジシンセイ チカク ヘンドウ

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抄録

The Boso Peninsula of South Kanto, central Japan, is a seismotectonically active region which has experienced two historical large earthquakes, the 1703 Genroku earthquake (M=8.2) and the 1923 Kanto earthquake (Taisho earthquake; M=7.9), due to the complex convergence of three plates: the Pacific, North American, and Philippine Sea plates.<br>The Hota lowland in the southern part of the Boso Peninsula developed Holocene marine terraces which are classified into four levels, here named Hota I, Hota II, Hota III, and Hota IV terraces. The Hota IV terrace has been considered to have been created by crustal uplift that accompanied the 1703 Genroku earthquake. However, the 14C age of peat (830yrs BP) and historical remains indicate that the Hota IV terrace emerged before 1703. Further, historical documents and old maps showing the coastline between before and after Genroku earthquake show that the Hota lowland was subsided at the time of the 1703 seismic event. The amount of subsidence was inferred to be about 0-0.9m on the basis of geomorphic data.<br>Hota I and Hota II terraces were formed before 4, 350yrs BP, and Hota III was formed before 2, 200yrs BP, as dated by 14C ages from peat and tree roots. These terraces are not correlated to a series of Numa terraces in the southernmost part of the Peninsula that were generated by Genroku-type earthquakes (Nakata et al., 1980). This fact indicates that the Hota lowland has subsided whenever Genroku-type earthquakes occurred. Therefore, it is considered that a series of Hota terraces have been formed by crustal uplift accompanying Taisho-type earthquakes. The mean recurrence time of Taisho-type earthquakes is deduced to be less than 670 years based on the height of the former shoreline of Hota I (14m asl.).

収録刊行物

  • 第四紀研究

    第四紀研究 38 (1), 17-28, 1999

    日本第四紀学会

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