Abstract: The precursory signals of the geoelectric field preceding earthquakes come form the inner of the solid earth, and it is very important to study the characteristics of it. The calculation and analysis in this paper indicate that the strength of geoelectric field would be greatly accelerated when the dimension or scope of geoelectric source region is large. The variation of the length of the electric dipole would influence violently the strength of geoelectric field observed at the ground station. The variation of the geoelectric field would approach a steady value with the increase of the thickness of overburden layers. When the resistivity of rock layer under the overburden becomes high, the strength of geoelectric field observed on the ground would increases relatively. In studying the magnitude of current produced in the focal zone, the action of the polar distance of geoelectric dipole would not be neglected.

Key words: Geoelectric field, Length of the electric dipole, Anomalous precursor