AN ANALYSIS OF THE GEOMAGNETIC SECULAR VARIATION OF NORTH CHINA

In this paper, the geomagnetic observational data of the horizontal intensity (H), the vertical intensity (Z) and the total intensity(F) for the period 1960-1977 at several observatories in North China and its vicinity and also at three observatories in Japan are analysed. The results show that from 1974 to 1976 in North China, the yearly variation of the vertical intensity consists of a stage from decreasing to increasing in values. But there is a local difference in the time length of the stage and rate of change between the northern and southern parts of North China. Also, there appears a displacement of the isomagnetic lines of the rate of annual variation from the south to the north. For H and F of this area, similar regional difference can be seen.The above differences has been interpreted by some authors as the geomagnetic effect of the Tangshan Earthquake. They maintain that there occurred in Beijing and Changli prior to the earthquake a negative anomaly of a few to more than 10 gammas. But according to our analysis, the negative magnetic anomaly is not a local phenomenon but a large scale regional exihibition of a secular magnetic variation.