On September 19, the Mw7.1 “Puebla” earthquake struck Mexico City and surrounding states, causing the most fatalities (369) and damage of any temblor since the Michoacán event in 1985. The Mexican government has not yet published overall economic or insured loss tallies, but six days after the earthquake RMS publicly released an estimate of economic property losses between US$4 billion to US$8 billion and no more than US$1.2 billion in insured property losses.
To better understand the impact of this event, RMS sent a reconnaissance team to the affected region from October 1 to October 6. The RMS team spent most of the trip surveying sites in Mexico City but also visited Cuernavaca, Puebla de Zaragoza, and other cities in the states of Morelos and Puebla that were closer to the epicenter than the capital (Figure 1). This blog shares some of the key observations made by the RMS team.
The earthquake caused the heaviest impacts in Mexico City, where 228 people died and more than 10,000 buildings were damaged, despite the epicenter being nearly 75 miles (120 kilometers) away. Besides the capital having a concentration of people and property, much of the city, including many of its historic neighborhoods, also sits on former lake beds and is underlain by deep deposits of soft soils that form what is known as the Mexico City Basin. The basin significantly amplifies and lengthens the duration of seismic ground motions. Preliminary reports following the earthquake indicate that the largest recorded ground motion amplifications occurred along the western edge of the Mexico City Basin for building periods roughly between one and two seconds. This finding is consistent with government data showing the locations of the most severely damaged buildings in Mexico City.
At the same time, the RMS team observed that buildings 5-10 stories high had the highest rate of significant damage, which is consistent with the lower half of the period range that experienced the largest recorded amplifications. For example, only 1-in-25 of the surveyed three-story buildings sustained building damage ratios greater than 30 percent, while nearly 10-in-25 of the surveyed eight-story buildings sustained building damage ratios greater than 30 percent. In other words, among the buildings surveyed, eight-story buildings were nearly ten times as likely as three-story buildings to have experienced significant damage. From a loss perspective, this trend is particularly important when you consider that eight-story buildings house many more people, provide office space for many more workers, and have much higher replacement costs than three-story buildings.
The RMS team also observed other important recurring themes linked to the damage within Mexico City:
- Pre-1987 buildings: Most of the damaged buildings surveyed (75 percent) appeared to be constructed before 1987, when a new building code was introduced for the city. This new code, which included the requirement to design buildings to higher shaking intensities, emerged in response to the widespread damage and lessons learned after the 1985 Michoacán earthquake.
- Construction class: Most damaged buildings surveyed (about 70 percent) had one of these structural systems:
(i) reinforced concrete (RC) frames with unreinforced masonry (URM) infill
(ii) RC columns with waffle or flat slabs with URM infill, or
(iii) URM, typically using either concrete blocks or clay-fired bricks
Each of these structural systems has inherent seismic vulnerabilities unless special care is taken in their seismic designs. Nevertheless, the first two structural systems are still widely used in new construction in Mexico City, especially for mid-rise buildings.
- Paseo de la Reforma: The team did not observe any exterior damage to the tallest skyscrapers along Paseo de la Reforma, an important avenue with some of the most prominent buildings in the city. This observation is unsurprising as most of these very tall buildings are newer and well designed and constructed, and their fundamental periods of vibration are outside the range of significant amplification for this earthquake.
- Corner buildings: A large percentage of surveyed buildings that sustained heavy damage were located on street corners (Figure 4). This observation has been corroborated by preliminary reports published by other reconnaissance teams. For example, an Earthquake Engineering Research Institute team found that 55 percent of surveyed “heavily damaged or collapsed” buildings in Mexico City were located on corner lots. To put this in perspective, on a typical block with 16 buildings (considering both sides of the street), four buildings (or 25 percent) would be on corner lots, meaning that they disproportionately sustained heavy damage or collapsed during the earthquake. One reason corner buildings can be especially vulnerable is that they often have large openings, such as for windows or easier street access, on both of their street-facing sides.
Outside of Mexico City, the RMS team observed a wide range of damage levels within the visited cities. Overall damage levels were low in the city of Puebla de Zaragoza, the state capital of Puebla. In Puebla de Zaragoza, the team observed damage at historical churches and other buildings, and a regional hospital which sustained heavy damage was closed for repairs. In contrast, Jojutla de Juárez, a small city in the state of Morelos, experienced widespread damage in a six block by four block area of downtown. The zone was cordoned off at the time of the team’s visit, and all buildings within the zone, whether they had sustained damage or not, were closed.
A full report summarizing the reconnaissance team’s findings is available for RMS clients on RMS Owl.