Three-dimensional Wave Propagation Analysis of Simple Two-dimensional Basin Structures with Special Reference to “the Basin-Edge Effect”

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  • 「エッジ効果」に着目した単純な二次元盆地構造の三次元波動場解析
  • 「エッジ効果」に着目した単純な二次元盆地構造の三次元波動場解析--兵庫県南部地震の際の震災帯の成因
  • エッジ コウカ ニ チャクモクシタ タンジュン ナ 2ジゲン ボンチ コウゾウ
  • Three-dimensional Wave Propagation Analysis of Simple Two-dimensional Basin Structures with Special Reference to “the Basin-Edge Effect”
  • 兵庫県南部地震の際の震災帯の成因
  • The Cause of the Damage Belt during the Hyogo-ken Nanbu Earthquake

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Abstract

During the Hyogo-ken Nanbu earthquake of 1995 we have observed conspicuous concentration of damage in an extended area from Suma in the west end of Kobe to the center of Nishinomiya as a form of the so-called damage belt whose width was very small (≅1km) compared to its length (≅20km). We need to find what caused this damage pattern in Kobe. In our previous study we attributed it to “the basin-edge effect”, the amplification effect caused by the constructive interference of the direct S-wave with the basin-induced diffracted/surface waves. However, several researchers have recently reported that seismic motions at the bedrock have a narrow-band distribution due to the directivity effect and that the enhancement of this distribution should be taking place through the interaction of the directivity with the basin structure.<br>In this paper we analyzed again a simple two-dimensional basin model subject to various types of three-dimensional incident waves to confirm that the basin-edge effect is present irrespective of the input wave field. After proving it then we analyzed a simpler basin to fully understand the wave interference phenomena around the edge. It is found that the interference is actually happening twice, first with the basin-induced diffracted P-wave and second with the basin-induced diffracted S-wave plus Rayleigh wave. Quantitative estimation on the contribution of the edge-induced waves during the Hyogo-ken Nanbu earthquake revealed that it must give at least 50cm/s additional amplitude in terms of the peak ground velocity inside the damage belt.

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