Abstract: During 1977-1984, four earthquakes of about M 6 occurred respcstively at Liyang, Xingtai, Heze and South Yellow sea, all located in the southern part of North China. This is the most noticeable series of event sin this region since 1937. Analysis shows that: (1) These events all occurred at the nodal points of fracture network on the south boundery of the North China block. (2) The 1976 Tangshan earthquake induced the activity of faults in this area, and resulted in stress concentration towards some weak sections on the network, while the stress in the surrounding areas decreased. This formed the background of stress field which was favourable to the seismogenic process of this group of earthquakes. (3) After a failure happened at a weak point on the network, it would cause a redistribution of the stress and influence the occurrence time of the next earthquake. The fast adjusting process of stress could an important part in strain accumulation and in the occurrence of a series of moderate earthquakes.