The Mexico Earthquake of September 19, 1985—Earthquake Behavior of Soft Sites in Mexico City

From the september 19, 1985 earthquake there are several acceleration records for the soft (lake bed) sites in Mexico City. One-dimensional propagation models of the incident seismic accelerations incorporate the wave propagation profiles at three of those sites. The undamped computed surface acceleration spectra compare well with the recorded ones for two sites. The other one does not produce adequate results, which is probably due to incomplete information on the shear wave velocities. Further investigation with the site impulse function indicates that the surface accelerations during the september 19, 1985 event had a frequency content determined, to a certain extent, by the natural periods of the site. Direct inspection of the recorded spectra for the september 19, 1985 earthquake at the studied sites shows that the relation between spectral periods corresponds to the closed form solution of an homogeneous layer with a fixed base. This relation may be different for other earthquakes recorded at these sites.