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 a massive hydraulic project (known as the Desagüe) in response to major periodic flooding. The main reason for this is the surface geology of the area, especially the downtown area. While not on or near any fault like San Francisco or Los Angeles, Mexico City is also vulnerable to earthquakes. Each year more than 90 tremors above magnitude 4.0 are recorded in this zone. 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. Map showing regional tectonic plates and the Middle America Trench. Then-president Miguel de la Madrid and the ruling Institutional Revolutionary Party (PRI) were widely criticized for what was perceived as an inefficient response to the emergency, including an initial refusal of foreign aid. The event caused between three and five billion USD in damage as 412 buildings collapsed and another 3,124 were seriously damaged in the city. 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 on which Mexico City sits. 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. The sequence of events included a foreshock of magnitude 5.2 that occurred the prior May, the main shock on 19 September, and two large aftershocks.
The event caused serious damage to the Greater Mexico City area and the deaths of at least 5,000 people. 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 maximal Mercalli intensity of IX ( Violent).
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