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Partial Anomaly of the Secular Variations of the Geomagnetic Field Before the 1976 Tangshan Earthquake

Sun Romei, Lu Zhenye (The Analysing and Predicting Center, State Seismological Bureau)  
Through aquiring the smoothed curves of the total intensity of the geomagnetic field T and their vertical component Z from 14 geomagnetic observatories both within and around China (25°-55°N,105°-145°E) , the secular annual change rates at each of the observatories and the isoporic distribution over longitude, latitude and time have been obtained.Using the time-space distribution as the reference field of the geomagnetic secular variation, the data from 5 geomagnetic observatories and 10 observation points were corrected. It is found out that the Z component at the Changli observatory systematically deviated from the analog line calculated from the reference field since 1973 to 1976 with the maximum deviation of +10γ in 1975. And from the Tangshan earthquake to the end of 1978, it coincided with the analog line. From 1975 to 1976, the deviation systimatically away from the analog line was-12γ at the Tangshan observation point located in the ma-jestoseismic area. And the T deviation was +8γ at the Ninghe point close to the epicenter of the strong aftershock (Ms=6.9) on the east side of the Ji Canal fault. Meanwhile, the T of the Dongditou site on the west side of the fault deviated by-10γ. The expanding band of T and Z anomalies was basically consistent with the source area. The deviations were less than 2γ at the observatories far from the epicenter.
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