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.
We analyze Fourier acceleration amplitude spectra of 14 coastal earthquakes (5.6 ≤ Ms ≤ 8.1) recorded at a hill zone site in Ciudad Universitaria (CU), Mexico City (282 ≤ R ≤ 466 km, R= closest distance from CU to the rupture area). The observed smoothed spectra are fitted to the following model log FS(T) = A(T) Ms + C(T) log(R) + B(T) where FS is the maximum spectral value of either of the two horizontal components at period T. The coefficients A, C, and B are obtained for 18 periods (2.5 ≥ T≥ 0.2 sec). The observed spectra at CU are not reliable for most earthquakes beyond this period range. The predicted FS for the September 19, 1985, Michoacán earthquake (Ms = 8.1) is less than the observed FS for T ≥ 0.6 sec suggesting an anomalously large radiation towards Mexico City during the earthquake at these periods. This anomalous radiation is in agreement with the teleseismic evidence. FS at several sites in Mexico City (many on the lake bed zone) can be obtained from the predicted FS at CU and the known spectral ratios of these sites with respect to CU.
S U M M A R YIn order to better understand the causes of unprecedented damage to Mexico City during the 1985 September 19 Michoacan earthquake (M, = 8.0) spectral ratios of teleseismic P-waves of this earthquake are studied with respect to those from five recent, large Mexican subduction zone earthquakes (7.0 5 M, I 7.7). The data are from vertical-component digital seismographs. It is found that the spectral ratios at stations in the NE quadrant are anomalously more energetic than those predicted by the o-* source model in the critical frequency range for Mexico City (0.3-0.7 Hz). The evidence is especially convincing for the spectral ratios with respect to the earthquakes of 1985 September 21 (M, = 7.6) and 1986 (M, = 7.0) since the data are available from several stations in the NE quadrant. The teleseismic P-wave spectral ratio in this quadrant with respect to the 1985 September 21 earthquake, in the critical frequency range, is close to the acceleration spectral ratio found in and near Mexico City (also in the NE quadrant). Velocity traces in the epicentral region of the Michoacan earthquake, obtained by integrating the accelerograms, also show oscillations with a frequency of about 0.4Hz. Furthermore, a regression study of Fourier acceleration spectra at a hill-zone site in Mexico City demonstrates that the Michoacan earthquake was anomalously energetic in the city at the critical frequencies for an event of that magnitude and at that distance. If the data from 7.0 I M, 5 7.7 events can be extrapolated to estimate the ground motions from M, L 8.0 earthquakes, then the evidence, supports an anomalously large body-wave radiation towards Mexico City between 0.3 and 0.7 Hz during the Michoacan earthquake. This anomalous radiation and the dramatic local amplification of seismic waves in the lake-bed zone of the city (-10-50 times at frequencies between 0.3 and 0.7Hz) appear to be the principal natural causes of the disaster. The anomalous teleseismic P-wave spectral ratios with respect to the earthquakes of 1985 September 21 and 1986 found in the NE quadrant are not observed in the data available from a small number of stations in the other quadrants. If this observation is true then it suggests a directional property to the anomalous radiation.
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