STRESS VARIATIONS IN THE REGION AROUND BEIJING AND TIANJIN BEFORE AND AFTER THE 1976 TANGSHAN EARTHQUAKE

This paper deals with the variations of tectonic stress in the region around Beijing and Tianjin before the great Tangshan Earthquake of 1976, based on the analysis of the continuously recorded data of over 400 small earthquakes occurred during the last 20 years. Such variations are chiefly concerned with the horizontal stress, indicating compression in the east-west direction and extension in the north-south. It is essentially consistent with the results from the earthquake source mechanism solution and the results of field macroscopic observations of the main Tangshan earthquake. The variations lasted for four and half years and recovered to the original state after the great earthquake.Through an analysis of the stress variations, the spatial distribution of the seismic activity of small earthquakes of M≥3.0 occurred in this region and their source mechanism solutions, the relation between the stress concentration and the fault activity before the Tangshan Earthquake has been discussed. It can be shown that before the earthquake the strain energy was concentrated at the northern end of the cangdong fault belt and it was released for the first time during the last several decades which reflects the background for the occurrence of the Tangshan Earthquake.