Abstract

The Fourier amplitude ratio of the 19 September (Ms = 8.1) to the 21 September 1985 (Ms = 7.6), Michoacan earthquakes obtained from data in and near Mexico City increases from about 3.5 at 0.5 Hz to about 10 at 0.2 Hz. This increase is neither seen in the coastal data nor in the teleseismic broadband GDSN P-wave spectra. It is also not predicted by ω−2 or Gusev's scaling laws. We tentatively attribute the increase at frequencies of less than 0.5 Hz to a special path and/or a depth of energy release effect for the 19 September earthquake. The damage to Mexico City is mostly related to 0.5 Hz energy. Since at f ≧ 0.5 Hz, the Fourier amplitude ratios in and near Mexico City, along the coast, and at teleseismic distances are nearly equal; we attribute this energy to the source characteristics of the 19 September earthquake. The evidence suggests that the 19 September was about twice more energetic at f ≃ 0.5 Hz than expected from scaling laws and from spectral regression of coastal earthquakes recorded in Mexico City.

The analysis of strong motion data recorded in Mexico City shows that the ground motions in the lake bed zone are amplified by 8 to 50 times with respect to a hill zone site in Ciudad Universitaria. The frequency at which the maximum relative amplification occurs varies from site to site and lies between 0.2 and 0.7 Hz. The data suggest that fo = β/4H, where fo = frequency at which maximum relative amplitude occurs, and β and H are the shear wave velocity and the thickness of the clay layer, respectively. Relatively small, if any, nonlinear behavior of the clay is found at strains of about 0.2 per cent during the great 19 September earthquake. For a given lake bed site, maximum relative amplitude and fo are roughly constant, independent of magnitude, depth, and azimuth of sources located more than about 200 km.

Ground motion at lake bed sites in Mexico City with fo ≃ 0.5 Hz appear to be amplified 75 times with respect to hard-rock coastal sites at equal distances from the source. This is also confirmed by ML calculation of the 19 and 21 September earthquakes. For the 19 and 21 September earthquakes, the average values of ML at hard-rock coastal sites (100 ≦ distance ≦ 250 km) are 7.4 and 6.7, whereas they are 8.7 and 8.4 in the lake bed zone (distance ∼300 km), respectively.

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