FAULT SLIPPING BEFORE THE 1976 f ANGSHAN EARTHQUAKE AND ITS RELATED VARIATIONS OF GROUND SURFACE TILTING AND GROUND WATER LEVEL

In analysing the trend of variations of tilting of the ground surface and the ground water level observed before the 197 6 Tangshan earthquake, their theoretical wave form have been calculated based on a propagating slipping fault model. They indicate that besides the creeping fault segment nearby Tangshan, there also exists another such segment nearby Ninghe to the southwest. Slipping took place below depth of about 1 km. The latter segment started to creep on a portion between Tanggu, futher to the southwest and Ninghe in February and March, 1975. Then along the direction of the Tangshan fault, N30E, it propagated gradually toward the northeast at a rate of 4-5km/month. Meanwhile the fault displacement increased from 0 to about 1 meter. It is believed that the process of development of the Tangshan earthquake consists of the gradual approach of two creeping fault segments, north and south, toward each other, and finally they met with a quick increase of the rate of displacement of the fault.