The use of the ELF (extremely low frequency) radio station "Zevs" for the study of the stress-strain state of geological media is considered. Analysis of prior research on the connection between tidal phenomena and apparent resistivity ρa variations at earthquake prediction sites are made. In the ELF range experimental measurements of artificial electromagnetic field impedance and the results of calculation of tidal deformations are considered. A high degree of correlation of ρ_a values and vertical deformations of the Earth surface △H are noted. Diurnal ρ_a changes of 7% due to tidal phenomena, were recorded with measurements accuracy of ±0.6%. To increase the reliability of earthquake prediction it is necessary to correct the electromagnetic monitoring data for tidal effects using correlation equations between ρ_a and △H.