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). 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 main shock 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 on which Mexico City sits. 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.
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.
Preface
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 like San Francisco or Los Angeles, Mexico City is also 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 a massive hydraulic project (known as the Desagüe) in response to major periodic 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 more stable.
Another factor is that the old lakebed resonates with certain seismic waves and low frequency signals. This lakebed has a natural "pitch" of one cycle every 2.5 seconds making everything built on the bed vibrate at the same frequency. This is the same "pitch" as a number of shallow earthquake waves. This resonance amplifies the effects of the shock waves coming from an earthquake far away.
However, only certain types of structures are vulnerable to this resonance effect. Taller buildings have their own frequencies of vibration. Those that are six to fifteen stories tall also vibrate at the 2.5-second cycle, making them act like tuning forks in the event of an earthquake. The low-frequency waves of an earthquake are amplified by the mud of the lakebed, which in turn, is amplified by the building itself. This causes these buildings to shake more violently than the earthquake proper as the earthquake progresses. Many of the older colonial buildings have survived hundreds of years on the lakebed simply because they are not tall enough to be affected by the resonance effect.
Earthquake
The earthquake occurred in the Pacific Ocean, off the coast of the Mexican state of Michoacán, a distance of more than 350 km (220 mi) from the city, in the Cocos plate subduction zone, specifically in a section of the fault line known as the Michoacán seismic gap. The Cocos plate pushes against and slides under the North American plate, primarily along the coasts of the states of Michoacán and Guerrero in Mexico. Volatile trenches along the Cocos plate generally have had seismic events every 30 to 70 years before 1985. This subduction zone outside the Michoacán gap was the source of 42 earthquakes of magnitude 7.0 or stronger in the 20th century prior to the 1985 event. However, this particular section of the subduction zone had not had an event for a much longer time.
Shockwaves from the earthquake hit the mouth of the Río Balsas on the coast at 7:17 am and hit Mexico City, 350 km (220 mi) away, two minutes later at 7:19 am. The 19 September quake was a multiple event with two epicenters and the second movement occurring 26 seconds after the first. Because of multiple breaks in the fault line, the event was of long duration. Ground shaking lasted more than five minutes in places along the coast and parts of Mexico City shook for three minutes, with an average shaking time of 3–4 minutes. It is estimated the movement along the fault was about three metres (9.8 ft). The main tremor was foreshadowed by a quake of magnitude 5.2 on 28 May 1985, and was followed by two significant aftershocks: one on 20 September 1985 of magnitude 7.5 lasting thirteen seconds and the third occurring seven months later on 30 April 1986 with magnitude 7.0 lasting ten seconds. However, at least twelve other minor aftershocks were associated with the seismic event.
The energy released during the main event was equivalent to approximately 1,114 nuclear weapons exploding. The earthquake was felt over 825,000 square kilometers, as far away as Los Angeles and Houston in the United States.
In the port of Lázaro Cárdenas, near the epicenter, the 19 September event registered as IX on the Modified Mercalli intensity scale; in parts of Mexico City, it registered the same, even at a distance of about 400 km (249 mi) away. There was no historic record of such a strong quake in Mexico.
While the fault line was located just off the Pacific coast of Mexico, there was relatively little effect on the sea itself. The earthquake did produce a number of tsunamis but they were small, ranging between one and three metres (3 ft 3 in and 9 ft 10 in) in height. Ecuador reported the highest waves of 60 cm (2.0 ft).
Damage
Most of the earthquake damage was to buildings. Two reasons are the resonance in the lakebed sediments and the long duration of the shaking. The buildings most damaged were from 6 to 15 stories in height. These buildings tended to resonate most with the energetic frequency band of the lakebed motions. One interesting characteristic was that many buildings had their upper floors collapse, leaving the lower floors relatively undamaged. In many damaged buildings, just one floor had collapsed. In some cases the damage was caused by the top of a lower, adjacent building banging against the walls and the supporting columns of its neighbor. Eventually, the columns gave way. In other cases, the first few floors of buildings were designed as parking garages, open lobbies or large shopping areas. These "soft" stories were particularly flexible and tended to collapse after prolonged shaking. Some types of foundations, particularly those involving piles driven into clay and held in place by friction, turned out to be weak. One nine-story building, for example, overturned. Its piles were pulled entirely out of the ground.
A survey by the government of the damage done found that few buildings from one to five stories suffered serious damage; the same was true for buildings over fifteen stories. When the buildings were built seemed to have an effect as well. Before the 1957 earthquake, there were no building codes with respect to earthquake resistance. Some regulations were passed in that year and more in 1976 after another, stronger earthquake shook the city. However, none of these regulations had an event like 1985's in mind when passed. Most of the seriously damaged buildings were built between 1957 and 1976, when the city was starting to build upwards, in the six-to-fifteen floor range. In second place were buildings from before 1957, possibly because they were weakened by the earlier earthquakes. Structures built between 1976 and 1985 suffered the least damage.
At the time of the earthquake, Mexico City had one of the most stringent building codes, based on experience gained from earthquakes in 1957 and 1979. However, the codes were not designed for seismic activity of the intensity experienced in 1985. The event was one of the most intense ever recorded, and macroseismic waves arrived in the Valley of Mexico with unusually high energy content. Prior to the event, estimates about ground movement on the lakebed were generally accepted and a number of buildings were built on these estimates.
Several notable buildings were relatively untouched by the quake. One significant example is the Torre Latinoamericana. Despite being 44 stories tall, it survived the 1985 event almost undamaged. It was constructed with two hundred piles extending down over one hundred feet (30 m) into the stable earth stratum.
Localization of the damage
Mexico City is divided into sixteen boroughs. Eighty percent of the earthquake damage was confined to four of them: Venustiano Carranza, Cuauhtémoc, Benito Juárez and Gustavo A. Madero. The damage area corresponds to the western part of the lake zone within two to four kilometres (1.2 to 2.5 mi) of the Alameda Central. Nearly all the buildings that collapsed were located in this lake zone that extended from Tlatelolco in the north to Viaducto Miguel Alemán in the south, Chapultepec Park in the west and to a short distance east of the Zócalo or main plaza.
Building damage in the city
Cuauhtémoc, which includes the historic downtown, suffered the most damage. In this particular area, 258 buildings completely crumbled, 143 partially collapsed and 181 were seriously damaged. The next seriously affected area was Venustiano Carranza where 83 buildings collapsed, 128 partially collapsed and 2,000 structures were seriously damaged. Damage was localized to the center parts of the city, leaving much of the residential outer rim unscathed, but the damage in the affected area was extensive. Over 720,000 tons of debris was removed during the first six weeks after the event. The Metropolitan Commission for Emergencies of the Federal District reported 2,831 buildings damaged for the entire city: 31% or 880 were completely ruined, 13% were reinhabitable with major repairs and the rest, totaling 1581, were recoverable with minor repairs. This translates to more than 30,000 housing units destroyed and another 68,000 units damaged.
Hospital Juárez
One of the most visible government institutions to fall in the event was the tower of Hospital Juárez, one of the oldest hospital institutions in Mexico. It was founded in 1847, converting the old convent of San Pablo to treat wounded soldiers from the Mexican–American War. It originally was called San Pablo Hospital but its name was changed to Juárez Hospital in 1872.
The "Torre de Hospitalización" was built in 1970 with the main building being twelve stories tall. It had two wings, one facing north and the other south, with an inpatient capacity of 536 beds. At the top was a helipad. It was also surrounded by a number of other buildings belonging to the hospital complex such as a blood bank, teaching facilities, offices as well as the original convent. At the time of the earthquake, the hospital was 80% full, and it was shift change time for nurses, doctors and residents. Within minutes, the steel-frame structure collapsed, crushing and trapping many people inside.