Source spectra and spectral attenuation of seismic waves from Mexican earthquakes, and evidence of amplification in the hill zone of Mexico City

It is well known that seismic waves are enormously amplified at lake-bed sites with respect to hill-zone sites in Mexico City. It has been suggested that even hill-zone sites suffer amplification. In this paper, we use eight recent well-recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone. Results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside Mexico City at similar hypocentral distances. This implies that lake-bed sites are amplified by a factor of 100 to 500 at some frequencies between 0.2 to 0.7 Hz. The source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone. For two of the largest earthquakes (M_S = 8.1 and 7.6), whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies (0.015 to 20 Hz), we observe clear deviation from an ω² model; the spectra at intermediate frequencies (from about 0.05 to 2 Hz) show a sag. Whether the observed spectra deviate from an ω² model for events with M_S < 6, depends on the reliability of the reported long-period seismic moments. If they are considered reliable, then these events also deviate from the model. If, however, the reported seismic moments of smaller events are ignored, then the observed spectra can be fitted well with the ω² model.