Since the installation of an extensive digital strong motion array by Fundación Javier Barros Sierra in 1987 three moderate earthquakes have been recorded by the array and by the accelerographs operated by Instituto de Ingeniería, UNAM. Using this new data and results from the analysis of previous accelerograms we present spectral ratios at 40 sites in the valley of Mexico with respect to a hill zone site in Ciudad Universitaria (CU). Clear evidence for nonlinear behaviour of the clay is found at Central de Abastos Oficina (CDAO) site during the great Michoacán earthquake (Ms=8. 1). At four other lake bed sites this behaviour is not seen either because none occurred or because of poorer quality of data. The spectral ratio at a given site appears to be roughly independent of magnitude (except, perhaps, during great earthquakes when lake bed sites may behave nonlinearily), azimuth, and depth of earthquakes with epicenters ≥ 200 km from the city. On the lake bed sites of the valley the relative amplification (RA) varies between 8 and 56 and the natural period lies between 1.4 to 4.8 sec. Relative amplification maps at periods centered at 3, 2.5, 2, 1.5, and 1 seconds are presented. The area where severe damage and collapse of buildings in the city was concentrated during the Michoacán earthquake correlates well with the area with RA≥14 in the period range of 1.75 to 2.75 sec.

Spectral ratios obtained for lakebed sites with respect to the average Fourier spectra of the Southwest hill‐zone sites in the Mexico City Valley are presented. The amount and quality of data used allowed us to measure relevant amplification characteristics. For all earthquakes, the ratios are relatively constant regardless the magnitude, epicentral distance and azimuth of subduction and normal faulting earthquakes. With data recorded at Station Central de Abastos, it is shown that these ratios are a much better approach than horizontal to vertical ratios, which yield only a rough estimation of site amplification. Also for Central de Abastos, where little nonlinear behavior has been observed, an empirical approximation is used to account for these effects. We also built maps of equal amplification that exhibit those zones that have experienced damage during earthquakes and those at the highest risk. The ratios presented here are useful and reliable to accurately predict the amplification of motion.