GEOMAGNETIC ANOMALOUS CHANGES AND EARTHQUAKES IN NORTH AND NORTHEASTERN CHINA

In this paper the method of distance-dependent weights and least squares surface fitting has been applied to the calculation and analysis of the changes from the year 1970.0 through year 1979.0 of the geomagnetic declination D, horizontal intensity H, vertical component Z and total intensity F at 50 geomagnetic observatories in East Asia and its vicinity.The results show that there exist geomagnetic anomalies in the four magnetic quantities mentioned above in the years from 1971 through 1976 in a region about 13104km2 covering latitudes(37-40) N, longitudes (123-126) E. A good correspondence has been found between the appearance of the negative anomalies and the two earthquakes in Northeastern China which occurred in 1975 at Haicheng with M = 7.3 and in 1976 at Tangshan with M = 7.8. This anomaly did not last long after the latter earthquake. The maximum negative geomagnetic anomalies of declination and intensities at the central part of the anomalous region before the Haicheng and the Tangshan earthquakes were respectively D=1.5' and 2.0' per year all pointed toward the epicenters; H=10 and 25nT per year; Z =10 and 30nT per year; F = 10 and 45nT per year. It seems that geomagnetic anomalies were caused by the su-bduction of the Pacific Plate beneath the China Plate along the Japan Trench and by the flow of mass in the upper mantle.