A recent swarm of small, shallow earthquakes in Mexico City has raised new concerns about how moderate earthquakes might impact the city buildings. Researchers found that even relatively weak earthquakes produced surprisingly strong ground shaking prompting them to explore potential damage in future scenarios.
In a study published in the Bulletin of the Seismological Society of America Miguel Jaimes and Gerardo Suárez from Universidad Nacional Autónoma de México modeled the expected damage from moderate earthquakes in three key regions of the Mexico City basin. Their scenarios were based on past earthquakes including a 2023 event in La Magdalena Contreras the 2018 San Juan de Aragón earthquake and the 1995 Milpa Alta and 1985 Juchitepec earthquakes.
Their findings highlight the critical role of underlying geology in determining the extent of damage. In areas built on soft, water-saturated lakebed sediments such as the Juan de Aragón region, a moderate earthquake of up to magnitude 5.5 could damage 15% of one- to two-story buildings. Meanwhile in the La Magdalena transition zone, which lies between the lake zone and the city’s hill area, about 13% of similar structures could suffer damage. In contrast, buildings in the Milpa Alta highlands where the ground is more stable, would face less risk, with only 5% affected.
Suárez noted “We were somewhat surprised that the geographic effect was so dependent on the type of geotechnical region, earthquakes in the lake region have a much greater impact than those in transition or highland zones.”
Microearthquakes Provide Critical Data
Mexico City is no stranger to seismic activity, regularly experiencing both shallow earthquakes from local faults and powerful subduction earthquakes from the Pacific coast. However, recent improvements in seismic monitoring have revealed that even small earthquakes can generate unexpectedly strong ground motions.
Previously, the city’s sparse network of instruments failed to capture the full impact of local microearthquakes. Now, with more advanced monitoring systems, researchers have recorded peak ground accelerations from small quakes that exceed those from the devastating September 19, 1985 (magnitude 8.1) and September 19, 2017 (magnitude 7.1) earthquakes.
“The recent microearthquakes in Mexico City have sparked interest in what a moderate earthquake could do,” Suárez explained. “That’s what we set out to investigate in our study.”
Jaimes and Suárez used ground motion models, local geological data, and records of building stock to estimate how different earthquake scenarios might affect structures across the city. While some taller buildings showed vulnerability, their study found that one- to two-story buildings were most at risk.
“The reason is that the high-frequency seismic waves from local earthquakes match the natural vibration frequency of these smaller buildings,” Suárez said. “It’s like pushing a child on a swing at just the right rhythmit amplifies the effect.”
Urban Planning Must Account for Local Earthquakes
The study underscores a critical lesson for urban planners and engineers: seismic risk in Mexico City doesn’t just come from massive offshore subduction earthquakes but also from local crustal faults. While these smaller earthquakes are less frequent, they have the potential to cause substantial damage.
“This research makes it clear that even moderate, shallow earthquakes in the city itself can be highly destructive,” Suárez warned. “We need to take local earthquake hazards seriously when planning and constructing buildings.”
With Mexico City’s unique geology amplifying seismic shaking, experts say it’s crucial to strengthen building codes, improve infrastructure resilience, and prepare for the unexpected even when earthquakes seem small.