Mexican earthquake of 1985
Mexico City – Collapsed Hospital Juárez de México
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  Mexico City
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| Date | 19 September 1985 |
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| Origin time | 13:17:50 UTC |
| Magnitude | 8.0 Mw |
| Depth | 20 km (12 mi) |
| Epicenter | 18°21′N 102°23′W / 18.35°N 102.39°WCoordinates: 18°21′N 102°23′W / 18.35°N 102.39°W |
| Areas affected | Mexico |
| Total damage | Extreme |
| Max. intensity | IX (Violent) |
| Tsunami | 1–3 m |
| Aftershocks | 7.5 MW 20 September 1985 7.0 MW 30 April 1986 |
| Casualties | 5,000–45,000 dead 30,000 injured |
The 1985 Mexico City earthquake struck in the early morning of 19 September at 07:17:50 (CST) with a moment magnitude of 8.0 and a Mercalli intensity of IX (Violent). The event caused serious damage to the Greater Mexico City area and the deaths of at least 5,000 people. The sequence of events included a foreshock of magnitude 5.2 that occurred the prior May, the mainshock on 19 September, and two large aftershocks. The first of these occurred on 20 September with a magnitude of 7.5 and the second occurred seven months later on 30 April 1986 with a magnitude of 7.0. They were located off the coast along the Middle America Trench, more than 350 kilometres (220 mi) away, but the city suffered major damage due to its large magnitude and the ancient lake bed that Mexico City sits on. The event caused between three and four billion USD in damage as 412 buildings collapsed and another 3,124 were seriously damaged in the city.
Much of Mexico's volcanic and seismic activity stems from the movement of the North American plate against the Cocos and Pacific plates and it is one of the most active trenches in the world. Each year more than 90 tremors above magnitude 4.0 are recorded in this zone.
While not on or near any fault line like San Francisco or Los Angeles, Mexico City is vulnerable to earthquakes. The main reason for this is the surface geology of the area, especially the downtown area. The city was originally built on an island in the middle of Lake Texcoco, and Aztec rulers built dikes to prevent flooding while Spanish colonial rulers later drained the lakes in response to major flooding. The near surface geology of this area is classified into three sections: the old lake bed which is soft clay from volcanic ash with a high water content, a piedmont area, much of which is capped by 5 to 30 meters of lava less than 2,500 years old, and an old river delta area.
On the bed of the historic lake, the prevailing silt and volcanic clay sediments amplify seismic shaking. Damage to structures is worsened by soil liquefaction which causes the loss of foundation support and contributes to dramatic settlement of large buildings. Mexico City's downtown area mostly lies on the silt and volcanic clay sediments of the bed of the historic Lake Texcoco, which are between seven and thirty-seven meters deep and have a high water content. Above this is a layer of sand and above this is a layer of sand and rock. The western and northwestern parts of the city are outside the old lakeshores and are located on sands from eroding volcanic cones that surround the Valley of Mexico. The southern part of the city rests on hardened basalt lava flows. The old lakebed, with its high water content, is easily moved or compressed. The old lakeshore area also has a fairly high water content, allowing movement, though not as much as the lakebed. The old lava flows have little water content or movement in comparison and are therefore stable.
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