本文通过分析区域小震活动的变异,指出:1)唐山大震前不同震级下限的围空图象,实际上是唐山附近的地震平静和外围三支小震条带活动,二者合成的结果;2)空区边缘的应力集中和地震活动增强,主要是断裂带活动的结果.最后用强度和应力分布不均匀的断层模式讨论了围空图象形成的力学过程. By analyzing the variation of small earthquakes in the region, it is pointed out that: 1) The images of the lower earthquakes with different magnitudes prior to the Tangshan earthquake are actually the results of the seismic calm near Tangshan and the three small earthquakes in the periphery ; 2) The concentration of stress and the increase of seismic activity at the edge of the vacant zone are mainly the result of the fault zone activity. Finally, the mechanics process of the formation of the aerial image is discussed by the fault model with uneven distribution of the strength and stress.