Temporal Change in the Magnitude-Frequency Relation for the January 1983 Izu Earthquake Swarm, Japan

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  • 1983年1月伊豆群発地震活動における規模別頻度分布の時間変化について
  • 1983ネン 1ガツ イズ グンパツ ジシン カツドウ ニ オケル キボベツ

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Abstract

Hypocenters, focal mechanisms and magnitude-frequency relations were studied in detail for a swarm activity which took place off east coast of the Izu Peninsula, central Japan, in January 1983. Hypocenters were restricted in a small source volume of a few kilometers in diameter. Most of the focal plane solutions present strike-slip type with the maximum pressure axis in the NW-SE direction, that is consistent with the tectonic stress field. At the beginning of the swarm activity, b value was larger than the value before the swarm (normal value). It decreased with time corresponding to activation of seismicity. After the largest shock (MJMA=4.5), b value increased exceeding the value at the beginning, and finally it resumed the normal value. This characteristic pattern of the variation in magnitudefrequency relation is interpreted by temporal change of average stress in the focal region on the assumption that the parameter p of the ‘Go game model’ (probability that a fault rupture propagates to the adjacent zone) depends on the stress in the source area.

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