姶良Tnテフラの古地磁気方位 Paleomagnetic Directions of the Aira Tn Tephra Deposits

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姶良Tnテフラの広域性と同時性に注目して,テフラ堆積時の日本列島における地磁気分布を明らかにすることを目的に,その残留磁化測定を行った.姶良Tnテフラは2.2~2.5万年前の一時期に,南九州姶良カルデラから噴出したものである.17府県の44地点から約800個の定方位試料を採取した.すべての試料について段階交流消磁を実施し,安定な一次磁化を抽出した.<br>測定結果を10地域に分け比較したところ,残留磁化方位は地域内だけではなく地域間でもよく一致し,偏角が現在の地磁気偏角より約15°東偏する特徴がみられた.一方,岡山・広島地域では偏角がその周辺地域より約5°西偏し,富山地域では伏角がその周辺地域より約7°も深くなる異常値がみられた.

Paleomagnetic measurements have been carried out on the Aira Tn tephra, which erupted about 22, 000-25, 000 yrsBP from Aira caldera in Southern Kyushu. The tephra is distributed widely in Japan, and is the important marker bed in the Quaternary research. We collected about 800 oriented samples at 44 sites of outcrops of the tephra. The stable component of remanent magnetization was isolated from all samples through stepwise alternating field demagnetization up to 70mT. The direction of stable component was obtained using principal component analysis, anchored to the origin of Zijderveld diagram.<br>The VGPs obtained from different sites agree well with one another, which, means that their remanent magnetizations were acquired under the same geomagnetic dipole field (DF) at the time of eruption of the tephra. The pole of the DF can be assumed to be located on the all-mean VGP (Long=104.6°W, Lat=81.1°N, A<sub>95</sub>=1.3, N=44). Declinations and inclinations over the Japanese islands calculated from the pole range between 8°E and 10°E, and between 45° and 55°, respectively. The surveyed area was divided into 10 regions, and the local geomagnetic field (LF) direction of individual regions was calculated from the regional mean VGP. The difference between the DF and the LF direction at each region is less than 3°, except in Okayama Prefecture and Toyama Prefecture. The declination of the LF in Okayama Prefecture is deflected westerly by about 5° from those in the nearby regions, and the inclination of the LF in Toyama Prefecture is steeper by about 7° than those in the nearby regions. These local anomalous directions might be caused by the non-dipole field in the regions.

収録刊行物

  • 第四紀研究  

    第四紀研究 34(4), 297-307, 1995-10-31 

    Japan Association for Quaternary Research

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各種コード

  • NII論文ID(NAID)
    10002633263
  • 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
    3924632
  • 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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