Abstract: There may be foreshock activities occurred in hypocenter and the surrounding region before the strong earthquakes. There are two types of foreshock activities: first, the continuity one—the frequency and magnitude of foreshocks increase with time varying till the main shock; second, the discontinuity one—the frequency and magnitude of foreshocks increase earlier, then go down till the main shock. The phenomena are not only observed rupture test, but also before many strong earthquakes occurred in China, for example, the Haicheng earthquake with M_S=7.3 (March 4, 1975). In this paper, our research based on simulation using cellular automation model, in which we introduced the time-dependent friction mechanism, roughly indicates the friction characteristics of real faults. This model is close to the natural seismogenic system owing to the combination of the cellular automation model and the time-dependent friction mechanism. We use the model to research the temporal evolution of the foreshock activities. The model used in the paper is a simple model of seismogenic system on single fault. In our research, the model is varied with varying the parameters: the number of cells, the increment coefficient of stress loading, the increment coefficient of the internal static friction and the redistribution coefficient. Both two types of the foreshock activities can be found in long time rupture series obtained from the model. The results indicate that the distribution of static friction on the fault is important for the temporal evolution of seismicity before strong earthquakes.

Key words: Cellular automation model, Time-dependent friction mechanism, Single fault, Foreshock activities