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On September 19, 1985, at 7:17 A.M., a Richter magnitude 8.1 earthquake occurred on the Pacific coast of Mexico. The damage was concentrated in a 25 km2 area of Mexico City, 350 km from the epicenter. Of a population of 18 million, an estimated 10,000 people were killed, and 50,000 were injured. In addition, 250,000 people lost their homes, and property damage amounted to $5 billion. Over 800 buildings crumbled, including hotels, hospitals, schools, and businesses. Communications between the Mexican capitol and the outside world were interrupted for many days.

Surrounding areas affected by the earthquake included the Mexican States of Jalisco, Guerrero, and Michoacan. Damage in the epicentral area was restricted to a few tourist resorts and industrial estates along the Mexico Pacific coast. A two-meter tsunami also caused some damage in this area.

There are geologic reasons why Mexico and especially Mexico City are vulnerable to earthquake damage. Along the west coast of southern Mexico and Central America the Cocos Plate dips beneath the North American Plate producing a very active seismic zone. Since the beginning of the twentieth century 35 earthquakes of magnitude greater than 7.0 have occurred in this zone. The location of the 1985 earthquake's epicenter near the coast at the border between the states of Michoacan and Guerrero was not a surprise. Prior to the 1985 earthquake this area, located between two areas that had experienced recent earthquakes, was known as the "Michoacan Gap." The "gap" was filled in 1985 by the main shock and a severe aftershock (magnitude 7.5) that occurred the next day.

Mexico City itself lies in a broad basin formed approximately 30 million years ago by faulting of an uplifted plateau. Volcanic activity closed the basin and resulted in the formation of Lake

Texcoco. The Aztecs chose an island in this lake as an easily defended location for their capitol. The expansion of the capitol (Mexico City) and the gradual draining of the lake left the world's largest population center located largely on unconsolidated lake-bed sediments. These soft sedimentary clay deposits amplified the seismic waves, or they subsided carrying buildings down with them. Double resonance coupling between the earthquake waves, the subsoils, and the buildings caused intensity IX shaking in some areas, lasting up to three minutes. Earthquakes in 1957 and in 1979 also damaged Mexico City. However, neither of these earthquakes was quite as devastating as the 1985 earthquake.

In the area of greatest damage in downtown Mexico City, some types of structures failed more frequently than others. In the highest damage category were buildings with six or more floors. Resonance frequencies of these buildings were similar to the resonance frequencies of the subsoil. Because of the "inverted pendulum effect" and unusual flexibility of Mexico City structures, upper floors swayed as much as one meter and frequently collapsed. Differential movements of adjacent buildings also resulted in damage. A flexible building often failed if it was held by adjacent, more rigid lower buildings. Damage or failure often occurred where two swaying buildings came in contact. Corner buildings were also vulnerable to damage. Lessons learned from the patterns of earthquake damage need to be quickly applied to prevent another disaster when an earthquake releases stress that is building in another area along the Mexican coast between Acapulco and Zihuatanejo.