Focal Mechanisms and Spatial Distribution of Intermediatedepth Earthquakes beneath the Kanto District and Vicinity with Relation to the Double Seismic Planes
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概要
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The slab intersection of the Northeast Honshu and Izu-Bonin Arcs beneath Central Japan is modeled from the depth contours of hypocenter distributions of intermediate-depth earthquakes relocated by correcting the Pn station biases. The Northeast Honshu slab bends strongly to the northwest beneath the Kanto District. Intense clusters of subcrustal hypocenters beneath the western part of Ibaraki and Chiba Prefectures are located within the highly stressed region of the Northeast Honshu slab due to intersection with the Izu-Bonin slab. A separation of the inclined seismic zone into double layers is seen for the Northeast Honshu slab from a relatively deeper layer of hypocenters, but the double layers are not seen for the Izu-Bonin slab. A separation of 30km between the upper and lower seismic layers is estimated from the hypocenters beneath the Kanto District by the JHD method. Fault-plane solutions are obtained for 144 out of 286 intermediate-depth earthquakes which occurred at depths of h=80-200km with magnitudes of 4.0 or more during the period from 1963 to 1980. Intermediate-depth earthquakes located on the inland side show the down-dip compression, and the down-dip extension is observed on the Pacific side. The strike-slip, normal fault and hinge fault, which are different from the down-dip types of focal mechanisms, are also observed for intermediate-depth earthquakes in and around the Kanto District. Intermediate-depth earthquakes with reverse faults are located along the Pacific coast, and other types are observed for earthquakes on the Japan Sea side and off Miyagi and Fukushima Prefectures. The strike-slip is predominant for earthquakes beneath the central part of the Kanto District and these are located along a contact plane of the southwestern end of the Northeast Honshu slab with the northern end of the Izu-Bonin slab.関東地方周辺下においてもぐりこむスラブの形状と地震メカニズムについてしらべた.用いた震源は気象庁走時データにPn走時残差の観測点補正を行い決定したものであり,深さ別震央分布図,東西・南北に沿った深さ分布図からスラブ上面のコンターを求めた.東北日本弧は関東地方下で北西方向に彎曲しつつ伊豆小笠原弧と接合し,この彎曲は火山・変成岩帯の配列とも調和する.茨城県南西部・千葉県中部にみられる地殻下地震の高活動域は東北日本・伊豆小笠原弧の接合による高歪力部に位置している.今回用いた比較的大きな地震についても西側に傾むき下がる深発地震帯が2層に分離している.この2層構造は東北日本弧で明瞭であるが,伊豆小笠原弧では不鮮明である.JHD法によって求められた高精度震源の深さ分布から,2層面の間隔は30kmである.深さ80～200km,M≥4.0,1963～1980年の期間に起こった関東地方と周辺の稍深発地震286個のうち144個に対して,初動方向データ10個以上を満足するメカニズム解が数値解法で求められた.逆断層又は地震面に沿った張力のメカニズムは太平洋側に,内陸側には地震面に沿った圧力のメカニズムがみられる.地震面に沿った歪力によるこれら2つのタイプの他,横ずれ・正断層・断裂のタイプもみられ,特に東北日本弧の南西端が伊豆小笠原弧の北端と接する部分では多くみられる.関東地方周辺の稍深発地震の起こり方として,2層面構造によるもの,東北日本伊豆小笠原弧の接合に関連するもの,又各々のスラブ内の過程(断裂など)によるものなど多様な起こり方をしている.
- 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Earthquake Research Institute, University of Tokyoの論文
- 1984-07-10
東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Earthquake Research Institute, University of Tokyo | 論文
- 24. Spatial Variation of Earthquake-Generating Stress in and around the Kanto District, Central Japan
- Focal Mechanisms and Spatial Distribution of Intermediatedepth Earthquakes beneath the Kanto District and Vicinity with Relation to the Double Seismic Planes
- 12. Extended Travel-Time Tables for the JMA Standard Model of the Crust and Upper Mantle Structure beneath the Japanese Islands
- 周期特性(3-15秒)を考慮した日本各地における地震動の増幅度分布〔英文〕
- Hypocenter Distribution in the Japan Trench Region, off Sanriku, Northeast Japan, Determined by an Ocean-Bottom Seismometer Array Observation