The fault systems of earthquake are non regular whether in laboratory test scale or in geologic scale, which have self similar structure. After a earthquake, the slip function of seismic fault reached the largest value in the middle segment of fault and reduced to the smallest value in two ends of it. The remainder strain caused by the mainshock was gradually adjusted by the faults of aftershock, which have different scales and different directions. The seismic fracture system formed by the mainshock and aftershocks has shown the fractal character.
Two structure models of foreshock and aftershock system have reported in this paper. Based on this result the non-balanced degree of seismic moment in a seismic fracture system has been defined. It is found that before some strong earthquakes the fractal dimension and the non-balanced degree of seismic moment show temporal and, spatial changes. The applying of fractal dimension and non-balanced degree of seismic moment in earthquake prediction are discussed.