P波初動の重ね合わせから推定される東北日本の起震歪力

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書誌事項

タイトル別名
  • Earthquake Generating Stresses in North-Eastern Japan Arc Inferred from Superposition of the Initial Motions of P-Waves
  • Pハ ショドウ ノ カサネアワセ カラ スイテイ サレル トウホク ニホン ノ キシンワイリョク
  • Earthquake generating stress in north-east Japan arc inferred from superposition of the initial motions of P-waves

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説明

An attempt is made to estimate regional earthquake generating stresses in the northeastern part of Japan. It is known from previous studies that mechanisms of earthquakes occurring in a small region are similar to one another, if the division of seismic region is appropriate. In virtue of this fact, we superpose all the data of initial motions of P-waves obtained from earthquakes that occurred in a subdivided region, and project them on the same focal sphere. The distributions of P-wave initial motions thus obtained are analyzed to determine the fault types, that is, normal faulting or reverse faulting.<br>The results obtained are summarized as follows:<br>1) Shallow earthquakes which occurred off the coast of Sanriku (west of 143.5°E and north of 38°N) are characterized by reverse faulting, while earthquakes in the ocean-side (east of 143.5°E and north of 38°N) are characterized by normal faulting.<br>2) Shallow earthquakes which occurred off the coast of Fukushima prefecture are characterized by reverse faulting, and no normal faulting can be found.<br>3) It is difficult to explain the initial motion pattern of P-waves from earthquakes beneath the trench (depth; 60-100km) by a normal-fault-mechanism.<br>4) Pressure axis is nearly parallel to the dip of the seismic plane (down dip compression) deeper than 110km in the north-eastern part of Japan.<br>5) Tension axis is approximately parallel to the dip of the seismic plane (down dip extension) for depths between 100km and about 60km.<br>6) No serious contradiction is found between the mechanism solution derived from the superposed data of many small earthquakes in a divided region and that from the data of a large earthquake in the same region.

収録刊行物

  • 地震　第２輯

    地震　第２輯 26 (3), 241-253, 1973

    公益社団法人 日本地震学会

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