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Those who experienced the earthquake of l 9 September on the firm ground area in Mexico City knew that the event was exceptionally large. Many of them knew at once that structures on the soft soil area should have suffered significant damage, but very few could have imagined at that moment the magnitude of the destruction that had taken place. Modified Merca11i intensities of VI or more are registered every few years, and it had become customary to survive those earthquakes with moderate degrees of structural and non-structural damage, and only exceptionally with higher damage levels, but the toll of collapsed buildings and lost lives had remained insignificant. Before 1985, the worst experience was that of 1957, when an earthquake of magnitude M_= 7, 7 at a distance of 260 km led to extensive damage, including the cổ11apse of about five buildings, all of which were in the aftermath discovered to …