Subsoil geology and soil amplification in Mexico valley

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The Mexico Valley earthquake of 1985 was devastating and the extent of the damage is attributed to the very exceptional and extreme geological and topological conditions in the area. Mexico Valley is a 'bowl' of about 22 km in length and 1.5 km in depth. Figure 1 shows the Valley and the distribution of damage and the location of stations where records were obtained. Two of these stations CUMV(1), VIV(2) were on rock and three SCTI(3), CDAO and CDAF(4) were on the lakebed. This 'bowl' is surrounded by solid rock of shear wave velocity of 2800 m/sec at the lower level and the sides; and is filled with fractured rock of shear wave velocity of 1050 m/sec. The fractured rock is topped by a very fine and extremely soft layer varying in depth from 30 to 70 meters with shear wave velocities ranging from 30 m/sec to 100 m/sec t-3. Figure 2(a) shows the soil conditions along a representative cut AG (Fig. 1) in the Valley. Dramatic differences exist between records at stations on the lakebed and stations on rock in both duration and amplitude, Fig. 3. In the study of the seismicity and wave amplification in the Valley, significant attention was focussed on the role played by the top soft layer. While most of the damage at the lakebed was caused by the energy associated with a period of two seconds, the strong motion record outside the Valley at Teocalco, about 70 km WSW of the city, Fig. 4(a), has a broad Fourier spectrum with all frequencies up to 20 rad/sec equally represented with no anomolously high input in the two second period range 34. Records in the Valley, Fig. 3, both on rock and in the soft lakebed, indicate significant variability and amplification with the ones at the lakebed being the most dramatic. Although not as dramatic as on the lakebed, the records on rocky ground particularly CUMV also indicate significant filtering of the broad input record of Teocalco. This can only happen as a consequence of some