This paper deals with the resistivity changes inside a three layer media and corresponding apparent resistivity changes observed in different "epicentral distance" on the surface of the media, which are caused by the additional stress-strain field due to an expanding sphare in the media as a modelling earthquake source.
The results show that for a certain source, the stress field within the detective scale of a given geoelectrical observational system in a given "epicentral distance" varies very much with the layers which have different values of elastic modulus, and that the strain field within the same scale does not vary so much as stress. Thus the true resistivity changes in different layers of the three layer model have been calculated in terms of the "resistivity-strain sensitivity" based on some laboratory experiments, and then the apparent resistivity changes associated with the electrical structure of the same model calculated also, so that we could know something about the relations between the apparent resistivity changes and the strain fields for the earthquake source (expanding sphare). The relations reveal the synthetic influences of both mechanical structure and electrical structure on apparent resistivity changes. For the geoelectrical method as one of the means in earthquake prediction, on one hand, we have to seek the observational site which is of high sensitivity of resistivity-strain for the media, on the other hand, it is necessary that the site has a good response of apparent resistivity change to the true resistivity change inside the media which is of high sensitivity of resistivity-strain.