Field experiments were carried out to assess the possible changes of the seismic safety of buildings due to seasonal variations in the resonant properties of the near-surface soils. The natural frequencies of a ten-storey building and the surface soil layer were determined from passive seismic measurements. The observations were carried out at the end of January when the top soil layer was frozen because of prolonged exposure to negative temperatures, and at the end of June after the ground was completely thawed. The results of the experiments showed that in areas characterized by alternation of long periods of positive and negative temperatures, the natural frequencies of surface soils can change significantly more than the natural frequencies of the buildings built on them. In certain periods, such changes can lead to the coincidence of some natural frequencies of the soil layer and the buildings standing on it, which ultimately reduces the latter seismic safety.