The Mexico Earthquake of September 19, 1985—An Empirical Model to Predict Fourier Amplitude Spectra of Horizontal Ground Motion

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.