不均質地殻内で成長する大地震予知のためのモデル

特集: 大地震の長期予測はどこまで可能か? V. 総合評価と地殻活動予測モデル

抄録

Most of large main shocks have preliminary ruptures, and their duration times are proportion to the earthquake magnitude. This relationship indicates the initial rupture process determines the final earthquake size of main shock. The dynamic crack-crack interaction is considered for the growth process of the earthquake. In the region with high crack density, any crack has a lot of chances of rupture initiation, but almost of ruptures are decelerated and stopped by the strong interaction. In this case, each rupture remains a small event. On the contrary, in the case of low crack density, a crack grows up quickly and becomes a large earthquake, because of weak interaction. We assume the crack density is controlled by the fluid which is distributed widely in the lower crust. The upper extent of fluid body can be detected by the reflected seismic waves. About three months before the 1984 western Nagano earthquake, the percentage of the events with clear reflected waves increased and the top of the fluid came up shallow. In this period, the seismic activity was very low, namely the source region was in the stale of seismic quiescence. In most case, the rupture of large main shock initiates from the sharp change portion of the lower boundary of the seismogenic zone. Besides, the depth variation in lower boundary is proportion to that of the upper extent of reflector. Therefore, we can find the initiation region of large main shock by surveying the sharp depth-change area of reflectors, which lie several km beneath the cutoff depth of seismicity. We can also catch the initiation period of large main shock by monitoring the seismic quiescence and reflection efficiency of seismic waves.