三浦半島の活断層-完新世における活動史と問題点 Active Faults on Miura Peninsula, Southwest of Tokyo, and Their Holocene Activities

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三浦半島を横切る衣笠,北武,武山の3断層はほぼ平行して走り,いずれも第四紀後期以降には右横ずれ変位によって特色づけられている.最近のトレンチ調査やボーリング調査によって,これらの断層の完新世における活動史に関する情報が蓄積されてきた.活断層である確証がなかった衣笠断層については,完新世に繰り返し活動した活断層であることが確実となった.北武断層の最新活動期は約1,200~1,400yrs BP,武山断層のそれは2,000~2,200yrs BPで,それぞれ独自に活動した.再来間隔は,北武断層では1,500~2,500年,武山断層ではやや不規則であるが,平均約2,000年となり,いずれも次の活動に近づいている活断層である.変位地形がやや不明瞭な衣笠断層では13,000年間に2回の活動を記録しているにすぎない.どの断層も東部での変位地形が明瞭であり,東方海底に延長する可能性を示す.これらの断層は地下では,おそらく一つに収斂する断層帯を形成しているものと思われ,断層帯としての活動間隔はより短かくなるであろう.

Five subparallel active faults striking NWW-SEE truncate hilly Miura Peninsula, on the west side of Tokyo Bay. Right-lateral offset has predoiminated on these faults during the late Quaternary, associated with some vertical component. Seismic risk is a concern in Miura Peninsula, where the densely populated city of Yokosuka is built near or across these faults.<br>Eleven sites were drilled or excavated across three northern active faults, Kinugasa, Kitatake, and Takeyama faults, in order to obtain faulting history, including the time of faulting and the amount and nature of offset. This research has been carried out as official project of city or prefectural agcucies, supported by funding from the Science and Technology Agency, Japan, especiaally since the 1995 Kobe earthquake, as well as by personal research projects.<br>I summarize here the results of these projects. On Kitatake Fault, the latest activity is estimatetd to have occurred between 1, 200-1, 400yrs BP at seven sites. Repeat time is inferred to be 1, 500-2, 000 years, based on differential thickness of strata on the two sides of the fault. Three events during the last 5, 500 years were identified at Tsukui trench log along the Taleyama Fault. The latest event occurred 2, 000-2, 200yrs BP, certainly older than that of the Kitatake Fault., implying that these two faults activated independently. On the northernmost Kinugasa Fault Neogene rocks are intensively shattered and Holocene deposits are clearly deformed. This confirms that Kinugasa Fault is certainly an active fault which activated two times since ca. 13, 000 years BP.<br>Repeat time is estimated to be 1, 500-2, 000yrs for Kitatake Fault and to be average 2, 000 years along Takeyama Fault. But it may be shorter, because they probably converge in one major fault underground. In any case, these two faults are approaching the next period of activity.

収録刊行物

  • 第四紀研究  

    第四紀研究 38(6), 479-488, 1999-12-01 

    Japan Association for Quaternary Research

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各種コード

  • NII論文ID(NAID)
    10004721354
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    4927030
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  NDL  J-STAGE 
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