THE VARIATION OF REGIONAL STRESS BEFORE AND AFTER THE 1976 TANGSHAN EARTHQUAKE

In the Dagang oil field about 100 km from the source region of the 1976 Tangshan earthquake, 69 in-situ stress measurements have been made by the hydrofracturing method to show variations of stress at depths of 1000-4000 meters within 8 years from 1973-1980 before and after this main shock.The state of tectonic field prior to the major earthquake was changed as follows:1. The gradient of horizontal stresses decreased, that is, the state of stress relaxed in the directions of maximum and minimum principal horizontal stresses.2. The correlation coefficient between horizontal stresses and depth decreased, that is, the in-situ tectonic stress field was disturbed.3. The fault slip coefficient increased, and aseismic creep appeared on the fault. Besides, in combining geological structure and the result of stress measurements.some anomalous phenomena of the seismogenetic process of the Tangshan earthquake, such as the aseismic creep, variation in ground water level, the stress relaxation in shallower layers and around the source region, the stress transfer along the fault plane to deeper layers and closed regions etc. are also analysed and discussed.