Imaging of Inhomogeneous Structure of the Crust beneath Ou Backbone Range, Northeastern Japan, Based on Small Aperture Seismic Array Observations

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  • 短スパンアレイ観測による奥羽脊梁山地周辺の地殻不均質構造のイメージング
  • タンスパンアレイ カンソク ニ ヨル オウウ セキリョウ サンチ シュウヘン ノ チカク フキンシツ コウゾウ ノ イメージング

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Abstract

Ou Backbone range, northeastern Japan, is bounded both at eastern and western edges by active faults with reverse-fault type, and seismic activity is quite high in this range. The known largest event is the Rikuu Earthquake with M7.2 that occurred along the western edge in 1896. Two small aperture seismic arrays were deployed in 1997 and 1998 in order to image spatial distribution of P wave scatterers in the crust beneath the range. Waveforms of 7 and 13 explosions observed by the respective arrays were analyzed in a frequency band of 6-20Hz, Hypocentral distances between explosions and arrays are too large to apply conventional analyses such as CMP stacking or migration to observed data. We developed a method to estimate P-wave scatterer distribution from array observation data. Observed waveforms were slant stacked along various directions from the arrays, and energy density of waveform was further processed by diffraction curve summation. Spatial distribution of P wave scatterers was imaged by the presently developed procedure. The results show that strong scatterers are distributed around the fault plane of the 1896 Rikuu Earthquake and in the mid- to lower-crust of the range. The distribution of scatters in the upper crust correlates to hypocenter distribution of micro-earthquakes.

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