1985年10月4日千葉・茨城県境付近に発生した地震の発生メカニズムとその構造的意義について

  • 堀 貞喜
    科学技術庁国立防災科学技術センター

書誌事項

タイトル別名
  • The Earthquake Mechanism of the <i>M</i> 6.1 Event Occurred near the Border of Chiba and Ibaraki Prefectures, Central Japan, on October 4, 1985 and its Tectonic Implication
  • 1985ネン 10ガツ 4ニチ チバ イバラキ ケンザカイ フキン ニ ハッセ

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抄録

The upper mantle earthquake of MJMA=6.1 which occurred beneath the Kanto district on October 4, 1985 was investigated in detail for analyzing its difference from other earthquakes around this event. Most of the upper mantle earthquakes at depths of 60-80km beneath this region has been explained that they are the interplate earthquakes with the thrust type focal mechanisms caused by the subducting motion of the Pacific plate relative to the overriding plate. But the present event was not categorized into such a type of earthquakes; the focal mechanisms of the main- and the aftershocks were the strike slip types rather than the dip slip ones. Redetermination of the hypocenters of the main- and aftershocks by using selected stations and station corrections revealed that the focal are a of the present earthquake was not smoothly connected with other groups of the upper mantle earthquakes. The present event obviously occurred below the upper surface of the Pacific plate. Namely, it is concluded to be an intraplate earthquake. The Pacific plate seems to be Λ-shape structure in the vicinity of the focal area of the M 6.1 event, where the plate is considered to be strongly coupled with the overriding plate. It is likely that the local difference in the subducting rate of the Pacific plate between the northern and southern limbs of the plate caused the intraplate fructure along the hinge plane of the plate. It is suggested that the temporal variation of the pattern of seismic activity around the focal region of the present event is related to an occurrence of the M 6.1 event.

収録刊行物

  • 地震 第2輯

    地震 第2輯 39 (3), 457-468, 1986

    公益社団法人 日本地震学会

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