The Mexico Earthquake of September 19,1985—A Study of Amplification of Seismic Waves in the Valley of Mexico with Respect to a Hill Zone Site

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.