A recent swarm of small shallow earthquakes in Mexico City in 2019 and 2023 caused surprisingly strong ground shaking, prompting researchers to wonder how shaking from a moderate sized earthquake might impact buildings across the city.
In the Bulletin of the Seismological Society of America, Miguel Jaimes and Gerardo Suárez at Universidad Nacional Autónoma de México describe potential damage forecasts from ground shaking scenarios for moderate earthquakes in three regions of the Mexico City basin.
The three locations of the modeled earthquakes were based on recent seismic events: a 2023 earthquake in La Magdalena Contreras, the 2018 San Juan de Aragón earthquake, and the 1995 Milpa Alta and 1985 Juchitepec earthquakes.
Their model earthquakes, up to magnitude 5.5, produced ground shaking that led to different amounts of damaged buildings, with the extent of the damage depending on the region's underlying geology, Jaimes and Suárez concluded.
For instance, their study found that ground shaking from a moderate earthquake in the Juan de Aragón lake zone would damage 15% of one- to two-story buildings, due to the area's soft and supersaturated lakebed sediments.
By contrast, shaking from an earthquake in the La Magdalena transition zone between the lake zone and the city's hill area would damage 13% of these buildings, while shaking from an earthquake in the Milpa Alta highlands to the southeast of the city would damage 5% of one- to two-story buildings.
"We were somewhat surprised that the geographic effect was very dependent on the type of geotechnical region the earthquake occurred," said Suárez. "The potential damage is very different for earthquakes in the transition zone between the old lakebed and the highlands, than in the case of an earthquake in the lake region."
Mexico City is no stranger to seismic events, feeling the effects of both shallow crustal earthquakes and deeper subduction earthquakes.
"The large ground accelerations shown by the recent small earthquakes in the city had not been recorded before because the instrumentation was very sparse. Improved seismic instruments, in both numbers and quality, installed in recent years have increased our capacity to record these local microearthquakes and their resulting accelerations," said Suárez.
Peak ground accelerations from the microearthquakes were larger than those recorded during the devastating 19 September 1985 magnitude 8.1 and the 19 September 2017 magnitude 7.1 earthquakes.
"The recent microearthquakes recorded in Mexico City have awakened the interest of what effect an earthquake of moderate magnitude would have," Suárez noted. "This is what we set out to investigate in the paper."
Jaimes and Suárez estimated ground motion intensity measurements for the three earthquake scenarios, based on ground motion models from similar tectonic regions, along with data to capture the effects of local geology and a record of each region's building stock.
While some taller buildings were also impacted, one- to two-story buildings received the most damage from local earthquake shaking from local earthquakes in the three scenarios.
"The reason is that the high frequency content of the seismic waves of local earthquakes coincides with the frequency of vibration of these structures," Suárez explained. "It is similar to swinging a child on a swing following the period – or rhythm – of the swing."
Suárez said "the lesson is clear" that urban planners, earthquake engineers and others "must consider that seismic risk of Mexico City stems not only from the large and frequent subduction earthquakes, but also from local crustal sources that, although rare, may cause substantial damage."
