ON CERTAIN PERIODIC VARIATIONS OF THE TELLURIC ELECTRIC FIELD BEFORE AND AFTER THE 1976 TANGSHAN EARTHQUAKE

During the time interval from two months before the Tangshan Earthquake till about half a year after it, at the three stations, Xiji, Baodi, Tanggu, variations of the telluric electric field were observed with definite periods of half a month, one month and half a year respectively. Their amplitudes of from maximum to minimum reach several tens to 100—200 millivolts/km (exceed the average standard error of measurement by 6—20 times). the electric field appears to be linearly polarized, in directions parallel to that- of the nearby fault zones.If such variations should be separated from other disturbing factors and be com-pared with the theoretical gravity values on earth surface, it seems apparent that there exists certain connection with the tidal force of the earth. The extreme values of the electric field of the half-month variation have been found to coincide with the intermittent spouting of two nearby abandoned oil wells. It may indicate that the variation of the telluric electric field, has something to do with the change of movement of underground liquids and that one may suppose there exists a filtration electric field caused by the modulation of the velocity of filtration by tidal forces. Small tidal force can give rise to such change of filtration electric field only during the time period before and after the earthquakes.