The extensive damage to high-rise buildings in Mexico City during the September 19, 1985 earthquake is primarily due to the intensity of the ground shaking exceeding what was previously considered credible for the city by Mexican engineers. There were two major factors contributing to the catastrophe, resonance in the sediments of an ancient lake that once existed in the Valley of Mexico, and the long duration of shaking compared with other coastal earthquakes in the last 50 years. Both of these factors would be operative again if the Guerrero seismic gap ruptured in a single earthquake.