On September 19, 1985, at 7:18 in the morning, the residents of Mexico City were jolted awake by an 8.1-magnitude earthquake, one of the strongest to ever hit the area. The effects of the quake were particularly devastating because of the type of ground upon which the city sits. Mexico City is on a plateau surrounded by mountains and volcanoes. The plateau region was covered by lakes in ancient times. As the aquifer under the city has slowly drained, it has been discovered that the city sits atop a combination of dirt and sand that is much less stable than bedrock and can be quite volatile during an earthquake.
The quake on September 19 was centered 250 miles west of the city but, due to the relatively unstable ground underneath the city, serious shaking lasted for nearly 3 minutes. The prolonged ground movement caused several old hotels, including the Regis, Versailles and Romano, to crumble. A building at the National College of Professional Education fell, trapping hundreds of students who were attending early-morning classes. Many factories in the city, built with shoddy materials, also could not stand. Further, the tremors caused gas mains to break, causing fires and explosions throughout the city. When the damage was finally assessed, 3,000 buildings in Mexico City were demolished and another 100,000 suffered serious damage. 10,000 people lost their lives, 30,000 were injured and thousands more were left homeless.
Following the earthquake, a team consisting of four engineers and one seismologist from the National Bureau of Standards (NBS) and the United States Geological Survey (USGS) was dispatched to Mexico City to provide technical advice to the U.S. rescue effort and to assess structural damage. The NBS report, "Engineering Aspects of the September 19, 1985 Mexico Earthquake (NBS BSS 165)," is primarily based on data gathered by the team, but it also contains a compilation of other available information. The report addresses the origin and characteristics of the observed ground motion, the ability of buildings designed in accordance with present and proposed seismic design provisions to resist this type of ground motion, and observed data on structural and foundation failures.