On the morning of Tuesday, September 19, people across Mexico had taken part in the annual national earthquake drill and other remembrance activities, to commemorate 32 years since the 1985 Mw8.0 Michoacán Earthquake. Michoacán was the most devastating earthquake in Mexico’s history, leaving at least 9,500 people dead and more than 100,000 homeless.
No one could have imagined at the time of these drills that a little more than two hours later, at precisely 1:14 pm local time (CDT), they’d be experiencing a real earthquake, the same day as Michoacán, as the Mw7.1 “Puebla” earthquake struck Mexico City and surrounding states. The improbable had become reality.
RMS estimates that the Puebla earthquake caused between US$4 billion and US$8 billion in economic property losses and as much as US$1.2 billion in insured property losses. This estimate accounts for shake-only losses to building, contents and business interruption, including the effects of liquefaction and landslides.
Ground Motion Amplification in the Mexico City Basin
Just as in 1985, this Mw7.1 earthquake caused the heaviest damage and loss of life in the country’s capital, despite the epicenter being nearly 75 miles (120 kilometers) away. Much of Mexico City, including many of its most historic neighborhoods, is situated on a former lakebed and underlain by deep deposits of soft soils that form part of what is known as the Mexico City Basin. The basin significantly amplifies and lengthens the duration of seismic ground motions.
This was evident in the 1985 event, where spectral acceleration — a measure of shaking intensity — was amplified more than five times (depending on the building height) in many locations within the basin, compared with rock sites. We cannot yet say definitively how much ground motions were amplified in Mexico City during the Puebla earthquake — researchers need more time to process the observed ground motion recordings — but we can say with high certainty that basin amplification was triggered and that it contributed to the losses.
More than 50 buildings collapsed or were severely damaged in Mexico City, with the boroughs of Cuauhtémoc, Benito Juarez, Coyoacán and to a lesser extent Tlalpan and Xochimilco bearing the brunt of the loss. Many of these buildings; schools, offices, apartment blocks, were constructed of reinforced concrete frames with unreinforced masonry infill walls. This is a structural system that is widely known to be vulnerable during earthquakes unless special care is taken in its seismic design. These building collapses led to most of the fatalities and some of the most severe cases of property loss, attracting global attention.
Confined masonry buildings, a common structural system for homes and small businesses, were also damaged. But in the coming days we should expect to learn more about building damage that was less life threatening — such as cracks in interior partition walls, flooring, and windows or broken mechanical and electrical systems. Damage to these “non-structural” components will likely contribute significantly to the overall economic and insured losses from this earthquake.
Mexico has a long history of mandatory seismic-resistant design. This started with the 1957 building code for Mexico City that, among other things, set limits on the allowable lateral movement of structures during ground shaking. Over the years the code has improved, often spurred by lessons learned from earthquakes. For example, more stringent seismic requirements were implemented following the 1985 Michoacán Earthquake, with particular attention to reinforced concrete frame buildings.
But as is common around the world, Mexico’s code does not mandate that older structures be retrofitted, leaving a large stock of vulnerable buildings across the country. Based on initial reports, it appears that many of the collapsed buildings are older vintage between three and seven stories tall. Other affected buildings, while perhaps built after 1985, are of a modest size and profile, making it more likely that structural design and code enforcement were lacking.
Additionally, the shorter distance from the epicenter for the Puebla earthquake compared to the 1985 event appears to have contributed to the increased damage to mid-rise buildings compared to taller buildings (typically 8-18 stories) that dominated collapses in the 1985 earthquake.
Mapping Collapsed Buildings in Mexico City
A map of population density overlain with the extent and depth of the Mexico City Basin, and the location and severity of damaged buildings from the Puebla earthquake (see Figure 3 below), highlights the linkages between these variables. The collapsed buildings are nearly all within the basin and are also in areas of high population density. While not evident from the map, these hardest hit areas — such as the neighborhoods of Roma Norte, Condesa, and Centro Histórico — are also some of the oldest developments in the city.
The earthquake has affected more than just Mexico City. Collapsed buildings and damage have also been reported in the cities of Puebla and Cuernavaca and small cities across the states of Puebla, Morelos, and Mexico. Initial reports show many of the impacted buildings to be constructed of unreinforced masonry. In fact, RMS estimates that about 50 percent of the total economic value of buildings that experienced shaking intensities of MMI 5.5 or greater was outside Mexico City.
The 1985 earthquake was a subduction interface event, occurring at the junction between the colliding Cocos and North American plates. This boundary occurs along the west coast of Mexico. In contrast, the Puebla earthquake occurred within the subducting Cocos plate and originated at a depth of about 32 miles (51 kilometers). Because it was generated within the Cocos plate, which subducts steadily eastwards towards the Caribbean, the epicenter was about 155 miles (250 kilometers) from Mexico’s west coast, placing it much closer to Mexico City than the Michoacán event.
But Puebla’s smaller magnitude, coupled with improvements in the country’s building stock, have meant that this earthquake, while destructive, will not surpass the losses, either in economic, insured or human terms, caused by its forebearer, the 1985 Michoacán Earthquake.