Abstract: Four large earthquake sequences of Ms7.0 occurred in North China during 1966-1976. They are the Xingtai earthquake of 1966,Bohai earthquake of 1969,Haicheng earthquake of 1975 and Tangshan earthquake of 1976. The occurrence processes of these earthquakes are studied from the view point of fracture system propagation in this paper. In terms of the analyses of their activity patterns of foreshock-mainshock-aftershock,it is found that the occurrence of each earthquake sequence is the results from the propagation of the associated fracture system in which the length and geometric distribution of fractures may be different. Based on the theory of fracture propagation,four mechanical models of fracture systems suited to each earthquake sequence respectively are presented. The geometric and mechanical parameters of these models are corrected once more by means of the synthetic inversion for the data of focal mechanism solutions,variations of aftershock activity patterns during different periods and geodetic deformation using the fracture finite element method. Then,the best solutions modelling the fracture processes of the four earthquake sequence are selected. It is also found that the pattern of aftershock distribution of a large strike-slip earthquake sequence always show a Z-shape as a whole due to the propagation and combination of fracture branches when an earthquake sequence is at its final stage. It can be a mark to recognize the termination time of an earthquake sequence.