APPROXIMATE EVALUATION OF GROUND-MOTION INTENSITY FROM UPTHROWN BOULDERS

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  • 飛石現象に基づく地震動強さの簡略的評価
  • トビイシ ゲンショウ ニ モトヅク ジシンドウ ツヨサ ノ カンリャクテキ ヒョウカ

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Abstract

The estimation of near-field ground motion intensity is an important subject in earthquake engineering. However, the characteristics of nearfield ground motion are not well known because of the limited amount of recorded data from the epicentral regions. Some post-earthquake investigation reports have pointed out that the upthrow of boulders in the epicentral region might be evidence of extremely strong ground motions. In this paper, the upthrow phenomenon is interpreted through numerical simulations and shaking-table experiments, and a method of estimating ground-motion intensity from boulder upthrow phenomenon is proposed. The conclusions reached on the basis of the results of our studies are as follows: (a) The upthrow phenomenon is mainly the result of the large-velocity amplitude of ground motions at the soil surface. (b) Boulder upthrow is caused by periodic motion rather than impulsive motion. (c) An equation by which the maximum velocity of ground motion is estimated from the horizontal displacement of a boulder has been proposed based on numerical simulations assuming the ground motion to be a sine wave. The proposed formula shows good correspondence with the results of our shaking-table experiments.

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