伊予灘北東部における中央構造線海底活断層の完新世活動 Holocene activity on a submarine active fault system of the Median Tectonic Line beneath the northeastern part of Iyonada, the Inland Sea, Southwest Japan

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愛媛県伊予郡双海町沖において, 高分解能ソノプローブを用いた詳細な音波探査を実施し, 中央構造線活断層系の正確な分布と形態を記載した。さらに, 断層を挟んだ地点からピストンコア試料を採取し, それらを対比することにより, 断層活動の時期を解読した。この海域に分布する中央構造線は, 左雁行に配列する計4本の断層から構成されており, そのうちの1本には完新世における明瞭な右ずれ運動が認められる。また, この断層は近接する他の断層と同時に活動することにより, 細長い地溝を形成する。その活動時期は, 石灰質化石の^<14>C年代測定から, 約6200年前と約4000年前であると推定された。4000年前以降にも活動があったと思われるが, 残念ながら堆積速度の急減のため, 断層活動が保存されていなかった。本地域での活動性に, 陸上のトレンチ調査から解読された活動性を加味すると, 四国における中央構造線は, 少なくとも3つ以上の領域に分かれ, それぞれが約2000年の間隔で活動を繰り返していると考えられる。

The Median Tectonic Line (MTL), extending about 900 km from central Honshu to Kyushu, is the longest fault in Japan, and divides southwest Japan into Inner and Outer Zones. A partition of MTL from Kii Peninsula to northwest Shikoku, especially that named active fault system of the Median Tectonic Line, has been slipping laterally in the late Quaternary. There, however, has been no historical destructive earthquake along it. It is important to reveal activity and distribution of active fauls, on this part of MTL in particular, for long term earthquake prediction and disaster prevention. An active fault in shallow sea area is an excellent field to study behavior of a fault during Holocene, because the region is continuously in sedimentation, and the seismic event of a fault is recorded in the sediment. On the basis of this advantage, we have investigated by using high-resolution single-channel seismic profiler and piston-corer, active faults of MTL off the coast of Futami town in the northwestern Shikoku, i.e. the northwestern part of Iyonada, the Inland Sea. The main results of seismic profiling are as follows: Four faults having clear activity in Holocene time are recognized. They are named the Koamioki Kita fault, Koamioki-Minami fault, Kaminadaoki-Kita fault, and Kaminadaoki-Minami fault. The former two faults and the latter two form two pairs of faults. They show a left-step echelon pattern. Moreover, Kaminadaoki-Kita fault has a feature of right lateral fault. In short, this area is in right-slip movement in Holocene time. The Kaminadaoki-Kita and Kaminadaoki-Minami faults forma fault sag. For the purpose of revealing development of the fault sag, sediments have been sampled on the north side, the central part, and the southside of the sag by using a piston core sampler, and analyzed for correlation in detail. As a result, the two event of dip slip, on each of the faults are recognized. The development of the fault sag is inferred as follows (1) The Kaminadaoki-Kita and the Kaminadaoki-Minami faults have been activated, at the same time, and the central part bounded by the two faults has been sagged. (2) At least, two seismic events are found, these timings are -6200 y and -4000 y. The recurrence interval of seismic event shows 2000 y approximately. The interval corresponds with the others that were given from results of trench survey on the Okamura and Chichio faults, the central and the northeastern parts of the MTL in Shikoku, respectively. However, these events have not occurred at the same time. At least, three segments on the MTL have been activated periodically with 2000 y interval approximately.

収録刊行物

  • 地質学論集

    地質学論集 (40), 75-97, 1992

    日本地質学会

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各種コード

  • NII論文ID(NAID)
    110003025490
  • NII書誌ID(NCID)
    AN00141779
  • 本文言語コード
    JPN
  • 資料種別
    Journal Article
  • ISSN
    03858545
  • NDL 記事登録ID
    3820893
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-322
  • データ提供元
    CJP引用  NDL  NII-ELS  IR  NDL-Digital 
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