東北地方北部の三次元P波速度構造

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タイトル別名
  • Three-Dimensional P Wave Velocity Structure beneath Northern Part of the Tohoku District
  • 東北地方北部の3次元P波速度構造
  • トウホク チホウ ホクブ ノ 3ジゲン Pハ ソクド コウゾウ

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P wave arrival times from 119 local events recorded by a dense network of seismograph stations located on northern part of the Tohoku district, Honshu, Japan, are inverted to determine lateral variations in crust and upper mantle structure to a depth of 230km. The maximum velocity difference in each layer decreases with increasing depth, from 18% in the first layer (0-12.5km) to 5% in the sixth layer (131-164km). In the uppermost layer (0-12.5km), we find low-velocity zones beneath the Tsugaru Plain, Noshiro Plain, and Akita Plain where quaternary sediments are thick. The second layer (12.5-32km) is marked by a high-velocity zone along a part of the Pacific Coast. This zone coincides with the zones of both positive geomagnetic anomalies and positive Bouguer gravity anomalies reported by other workers. In the uppermost mantle layer (32-65km), we find low-velocity zones beneath the active volcanoes near the volcanic front. Since this relationship has already been found for the central and southern parts of the Tohoku district, the present study shows that the same relationship holds over a wide area of the Northeastern Japan arc. In contrast, no conspicuous low-velocity zone is found beneath Mount Iwaki, an active volcano located 80km west of the volcanic front. High-velocity zones in the uppermost mantle layer extend over areas along the coastal lines of both the Pacific Ocean and the Japan Sea. This is in good agreement with the regional variation of Pn velocity estimated from explosion seismic data previously.<br>Cross-sectional projection of the velocity perturbation along east-west profiles shows a high velocity zone extending parallel to the inclined seismic zone of intermediate focus earthquakes. This high velocity zone is interpreted as the subducting Pacific plate. The fine structure in and around the inclined high velocity zone appears to differ regionally along the strike of the island arc, though its reliability must be confirmed in further elaborate inversion. We compare this regional variation of velocity structure with the seismicity of intermediate focus earthquakes, and find no clear correlation between them.

収録刊行物

  • 地震　第２輯

    地震　第２輯 42 (4), 419-437, 1989

    公益社団法人 日本地震学会

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