Fault movements and stress accumulation before the 1976 Tangshan earthquake in North China

This paper analyses geodetic data including the results of short baseline and short levelling surveys across active faults, and of relevellings over a wide area collected at Tangshan and in its vicinity during the several years before the 1976 Tangshan earthquake of magnitude 7.8. Using a theoretical model for slip on a fault plane with an arbitrary dip in a viscoelastic half-space, the parameters of the aseismic fault slip prior to the shock are obtained, and the stress changes caused in the area of Tangshan by such slip are estimated. The results are comparable with the seismic activity and the changes in time and space of the b-value in the relation N = exp(a - bM) observed in the same period. It is demonstrated that during 1968–1975 the Cangdong fault, the main NNE-trending active fault in the southwest of the seismic region, had gradually started aseismic right-lateral strike-slip and that the occurrence of the Tangshan earthquake was related to the stress field produced by the slip. Finally, two sequences of periodic earthquake migration that took place in North China during 1966–1976 are discussed in connection with the Tangshan earthquake.