Abstract: To eliminate the annual cyclic variation in the d_s observational data, we analyzed the correlation between the observed data of earth resistivity at Baodi and Tanggu seismic stations, and the data of the g as pressure, air temperature, relative humidity, water table and seawater level nearby during the corresponding period. We also analyzed the reasons why annual variation of earth resistivity is induced. We consider that the external agencies of the variation of earth resistivity and meteorologic factors are induced by the connate field. In addition, the intrinsic agencies of variation of earth resistivity is based on the change of the thickness of the underground saline water layer and stress of earth crust. Using weather parameter and water table variation, we fit the model value of earth resistivity by least square method; and calculated its residual error. Finally, we eliminated the annual cyclic variation in the observed data of the earth resistivity. The result shows that the short-term precursor anomaly has been existed in the observed data of Baodi and Tanggu seismic stations before earthquake with M≈ 6 occurred in North China.

Key words: Earth resistivity, Annual cyclic aviation, Weather factor, Water table, Correlation analysis, Seismic short-term precursor anomaly