THE CHANGE OF GEOMAGNETIC TOTAL FIELD IN BEIJING AREA BEFORE AND AFTER THE 1976 TANGSHAN EARTHQUAKE

In order to study the seismomagnetic effects of the 1976 Tangshan earthquake, the observed data of the geomagnetic total field during the recent six years (1975-1981) have been used in this paper. We have analysed both the local differences of geomagnetic secular variations and the features of changes in geomagnetic statistical parameters in Beijing area during normal period, and the influences of local differences of secular variations on the statistical parameters.Comparison between changes of the parameters in the period before and after the Tangshan earthquake and that in normal period shows that the influence of the Tangshan earthquake on geomagnetic statistical parameters is obvious.In order to determine the portion of the survey network, where the information induced by the Tangshan earthquake was strongest, the space distribution of the statistical parameters before and after the earthquake has been calculated for different kinds of combinations of the points and profiles of observation.The statistical features of seismomagnetic information in the Beijing area caused by the Tangshan earthquake are as follows:1. The anomalous information is not fluctuations in short period but long time changes lasted for one year at least;2. The anomalies in the S region which is closer to the epicenter, were larger, because of the fact that the changes in the geomagnetic statistical parameters Smmj bsn and Fk2 in this region are more obvious, especially, the difference in space distributions of the local statistical information and secular variations, the former trends chiefly east-west while the latter north-south.3. The changes with time in the mean space difference of the tendency of the anomalies is not so clear in the entire surveyed area but becomes quite clear in the region from March to June of 1976, just before the Tangshan earthquake.