In order to increase the safety factor against seismic damage on new or reinforced structures, it is necessary for civil engineers to have available microzonation maps of the urban areas of the soil ground motion in terms of the vibration dominant period (DP). For the four most important urban areas of Baja California, one of the physical aspects of the seismic site effect were here obtained, the DP of the ground motion. These results are presented by means of contours maps of soil vibration DP. To isolate the site effect, the Nakamura’s spectral ratios method was used. The Nakamura’s method consists in obtaining the spectral ratio between the horizontal and the vertical component of the Fourier spectrum on a given site.