Earthquake Devastates Mexico City

In September, 1985, a large earthquake and its aftershocks shook Mexico City, leaving thousands dead and causing more than three billion dollars in damage.

Summary of Event

At 7:19 a.m. on September 19, 1985, Mexico City, the world’s most populous urban area, was shaken by a magnitude 8.1 earthquake, the largest earthquake to strike Mexico City in the twentieth century. Some buildings in the downtown area that were damaged in the initial quake collapsed when seismic waves from an aftershock measuring 7.6 on the Richter scale shook Mexico’s capital on the evening of the next day. Disasters;earthquakes
Earthquakes;Mexico City
Mexico City earthquake
[kw]Earthquake Devastates Mexico City (Sept. 19, 1985)
[kw]Devastates Mexico City, Earthquake (Sept. 19, 1985)
[kw]Mexico City, Earthquake Devastates (Sept. 19, 1985)
Earthquakes;Mexico City
Mexico City earthquake
[g]North America;Sept. 19, 1985: Earthquake Devastates Mexico City[05810]
[g]Mexico;Sept. 19, 1985: Earthquake Devastates Mexico City[05810]
[c]Disasters;Sept. 19, 1985: Earthquake Devastates Mexico City[05810]
Gavin, John
McNally, Karen
Ruiz Durán, Clemente

After the dust and smoke cleared, it was found that at least ninety-five hundred people had died as a result of the quake and its biggest aftershock. Because of the level of devastation wrought by the quakes and the fires that followed, the actual total may never be known. Approximately thirty thousand people were injured, and nearly one hundred thousand people lost their homes. More than four hundred buildings were destroyed, and about thirty-two hundred more were damaged.

The destruction and the losses were selective. Some areas of the city underwent only mild shaking and experienced little damage. In heavily damaged sections of the city, relatively undamaged buildings stood next to collapsed structures. Even in collapsed buildings, some survivors were pulled out relatively unharmed. One man plunged from the fifth floor of the Hotel Regis before it collapsed and survived with only minor cuts and broken ribs. Another who was on the third floor of an eight-story building before it collapsed reached the street safely, dazed, as the building fell backward and away from him, killing many inside.

During the initial quake, the ground in Mexico City shook for about three minutes. As a point of comparison, the strong shaking that took place during the larger 1964 Alaskan earthquake lasted for about thirty seconds. Les Connolly, who was staying on the tenth floor of the Sheraton Hotel in Mexico City on the day of the earthquake, later recalled: “The building swayed five or six feet each way. We were holding on to the walls as it went. It would go all the way one way . . . the next time it would sway all the way the other way and you’d think this time it would crash. Finally it righted itself.”

The damage from the Mexico City earthquake was not restricted to the effects of the shaking of the ground. The quake was responsible for setting off the largest fire in the city’s history. The Hotel Regis was destroyed by the shaking, but six high-rise buildings around the hotel remained standing, only to be destroyed by fire. Approximately four hundred structure fires were reported in Mexico City as a result of the quake and its aftershocks.

The earthquake’s epicenter was not in Mexico City but 220 miles to the west, along the Mexican coast of the Pacific Ocean. A large canyon, or trench, in the seafloor off the Mexican coast marks the point at which two crustal plates of rock collide. The heavier of these two plates, known as the Cocos Plate, Cocos Plate is being forced under the plate on which Mexico is located. As it moves back into the earth, however, the Cocos Plate tends to stick under the plate above it. The rest of the plate continues to move, and eventually it breaks loose, releasing earthquake waves. The plates continually move, stick, and break free, generating a cycle of earthquakes. Recognizing this cyclical nature, seismologists had forecast the 1985 earthquake. The area of the plate boundary where the quake occurred had been quiet for too long and had represented a seismic gap along the Mexican coast. Enough strain from the plate motion had been built up that its release was likely to result in a large earthquake.

When the energy from the 1985 earthquake was released, the resulting patterns of damage were not uniform. Communities along the coastline closer to the epicenter suffered minimal damage and few casualties. Four resort hotels at Zihuantanejo-Ixtapa on the coast had to be evacuated because of damage. At Lázaro Cárdenas on the coast, twenty miles from the epicenter, damage was minor. At least five people died in a hotel collapse, but most structures survived the ground shaking intact. Although all the better hotels were seriously damaged, only the old Playa Azul Hotel, situated on the beach, collapsed.

Acapulco, 150 miles from the epicenter, was virtually undamaged. Telephone service and radar at the city airport were knocked out. Elsewhere, damage between the coast and Mexico City was similarly sporadic and minor by comparison to the devastation in Mexico City. The worst damage was at Ciudad Guzmán, where twenty-five people died in the collapse of a church.

Some of the differences in levels of damage and casualties between Mexico City and the other locales closer to the epicenter may be attributed to the type of rock on which the cities are built. Solid bedrock, which underlies much of Acapulco, usually causes less damage during ground shaking than do soft, unconsolidated sediments deposited by rivers and lake waters. Most of the damage in Mexico City was to structures founded on the soft lake beds of what had once been Lake Texcoco.

Much of Mexico City is built over soft beds of sediment on the site of what was once a large intermontane lake. The Aztecs built their capital city on an island in the middle of this lake for defensive purposes. This lake was drained by the Spanish after their conquest of the Aztecs during the development of early Mexico City. Mexico City is built partly on the bedrock of higher ground around the old lake; near the center of the city, however, many buildings stand on the soft, water-laden sands and clays deposited by the lake. The strength of this material is comparable in many ways to that of the material that underlies Lázaro Cárdenas, yet the levels of damage for these two communities defied expectation, given their relative distances from the earthquake.

The Mexico City earthquake reduced many buildings to rubble.

(National Oceanic and Atmospheric Administration)

The answer to this paradox is in the nature of the earthquake itself. The ground shaking of September 19 did not come from one earthquake but from two earthquakes that occurred close together in time and space, and the waves from these two earthquakes combined in such a way as to produce an unusually long train of earthquake waves with a period of about two seconds between the peaks of the waves in the train. This appears to be a fairly common characteristic of earthquake activity that occurs where two crustal plates collide.

Although the peak amplitudes of these earthquake waves decreased on the 220-mile journey from the epicentral area to Mexico City, they were rapidly amplified when they reached the old lake bed because the sediments of the lake bed had a vibration frequency of about 2.5 seconds, closely matching that of the waves. In turn, the strongly amplified waves caused the most severe damage to high-rise buildings, which acted like upside-down pendulums. Most of the 270 structural collapses attributable to the earthquake involved buildings that were six to fifteen stories high and had weak frame structures.


Recovery from the effects of a large earthquake in an urban area is always an enormous task; elements of such recovery include the rescue of survivors, care of the injured, and restoration of power and water. The list of emergency procedures is extensive, and in the case of Mexico City, it required the efforts of many volunteers and the military. Many countries offered aid. The U.S. ambassador to Mexico, John Gavin, facilitated the arrival of twenty-five demolition experts to destroy thirty unsafe buildings and also oversaw the deployment of U.S. firefighting helicopters, experts on emergency services, and earth-moving equipment.

Most of the recovery efforts were supplied by the Mexican government, soldiers, and citizens of Mexico City. Mexican soldiers patrolled the streets of damaged sectors, controlling looting and warning against open flames in areas where gas lines had ruptured. The army also made available several hundred tanks to transport rescue workers from one location to another.

The restoration of utilities was an immediate concern. Broken water mains and aqueducts had cut off water supplies for one-third of Mexico City’s population and had hampered firefighting efforts. International telephone lines were down, and only one television station was still operating. Ham radio operators stepped in to fill the communications void, relaying emergency messages using battery-operated transmitters.

The rescue of survivors trapped in the wreckage of collapsed buildings was a task shared by military and civilian volunteers. Most of the survivors were rescued in the first two days. Nine days after the earthquake, however, a baby was recovered alive from a flattened hospital. Long after the rescue of survivors concluded, searches for the dead continued. The recovered bodies were marshaled in a sports stadium that was commandeered to serve as a morgue.

In addition to its immediate physical and social impacts, the Mexico City earthquake had long-term devastating economic impacts. At the time of the quake, the Mexican economy was in trouble; the nation’s foreign debt was $96 billion, and an inflation rate of 59 percent made it nearly impossible for Mexico to finance its debt. In the midst of this miserable economic situation, the earthquake damage required major construction programs in Mexico City and in four coastal states. Overall economic damage inflicted in the earthquake was estimated to be more than three billion dollars. Mexican economist Clemente Ruiz Durán predicted that the long-term economic cost of the earthquake would be immense.

One positive result of the earthquake was the knowledge it allowed scientists to gain, which they later were able to use to moderate the effects of other large earthquakes. Before the 1985 Mexico City quake, few raw data were available about the velocity, acceleration, and directions of ground movement in a major earthquake. Such data are crucial to engineers’ ability to design and construct buildings that are able to survive such ground movements.

The problem in acquiring the data necessary to design earthquake-resistant structures lies in having the necessary instruments operating when the earthquake occurs. In May, 1985, Karen McNally of the University of California at Santa Cruz renewed her earlier warning that a large earthquake was due in the seismic gap along the Mexican coast. Based on her forecast, in the summer of 1985, not long before the September temblor, scientists installed seismographs to measure ground movement along the Mexican coast. These instruments recorded for the first time a complete picture of ground motion near the source of a large earthquake. The records show that the ground moved twice in a 120-mile-long tear on the fault surface for a total of about six feet downward. The movements were twenty-six seconds apart, which explained the disastrously long period of ground shaking in Mexico City. The data gathered were invaluable for the designing of new structures and as a basis for revisions to building codes.

Another result of the Mexico City earthquake was the enactment of emergency regulations that formed the basis for a new building code. Building codes The 1985 earthquake taught building designers several important lessons: that the interaction between a building and the soil in which it is founded can alter the response of the building to ground shaking; that the safety of reinforced concrete buildings subjected to large earthquakes depends, to a large degree, on the quality of the materials used in the concrete; and that the strength of buildings is greatly affected by the duration of motion during an earthquake. Many of the lessons learned from the Mexico City earthquake were subsequently applied in building codes around the world. Disasters;earthquakes
Earthquakes;Mexico City
Mexico City earthquake

Further Reading

  • Bolt, Bruce A. Earthquakes. 5th ed. San Francisco: W. H. Freeman, 2003. Excellent primer on earthquakes for the nonscientist. Includes information on plate tectonics, earthquake magnitude and measurement, and faults. Briefly discusses the Mexico City earthquakes.
  • Boraiko, Allen A. “Earthquake in Mexico City.” National Geographic, May, 1986, 654-675. Excellent overview article for the general reader presents a detailed description of the events and their aftermath. Includes eyewitness accounts, many color photographs, and maps.
  • Cassaro, Michael A., and Enrique Martinez Romero, eds. The Mexico Earthquakes, 1985: Factors Involved and Lessons Learned. New York: American Society of Civil Engineers, 1987. Collection of scientific papers on the Mexico City earthquakes covers a range of topics, including seismicity, building codes, soil behavior, and behavior of buildings.
  • Cluff, Lloyd S. “Firsthand Experience of the Ms8.1 Earthquake that Struck Mexico City on 19 September 1985.” Bulletin of the Seismological Society of America 75 (December, 1985): 1843-1846. Offers a nontechnical description by an eyewitness to the Mexico City tremors.
  • Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) About Earthquakes. Princeton, N.J.: Princeton University Press, 2002. Presents information on earthquake science for lay readers, including discussion of earthquake measurement and the Richter scale.
  • Lomnitz, Cinna, and H. Castanos. “A Same-Day Overflight in the Epicentral Area of the Great Mexico Earthquake of 19 September 1985.” Bulletin of the Seismological Society of America 75 (December, 1985): 1837-1842. Presents an eyewitness account of the damage that occurred in the epicentral area at Lázaro Cárdenas.
  • Magnuson, Ed. “A Noise Like Thunder.” Time, September 30, 1985, 35-43. Overview for the general reader includes numerous quotations from eyewitnesses and sidebars on the cause and the economic impact of the earthquake. Features photographs and maps.

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