During morning rush hour on September 19, 1985, a violent earthquake shook Mexico City and its 18 million people. The earthquake measured an 8.1 on the Richter scale. Of the 800,000 buildings in the earthquake zone of the city, 179 had collapsed and 85 more were severely damaged and in danger of collapse. How did masonry buildings perform? In general, they performed well. Part of the reason for this is the type of ground movements that occurred in Mexico City--strong, long-lasting movements with long periods of vibration. Such movements affected buildings having periods of vibration in the same range--buildings 5 to 20 stories high. Because most masonry buildings in Mexico City are less than 5 stories high they behaved well.