The Tremors That Keep Mexico City on Its Toes

Mexico City, a sprawling metropolis built atop the remnants of ancient lakes, carries within its ground a geological secret that rumbles every now and then. Often imperceptible to most residents, these tremors—officially recorded as 137 microseisms between 2023 and 2024—are subtle reminders of the Earth’s ever-present tectonic dance.

With 52 such events already noted this year and 85 in 2023, Mexico’s capital quietly witnesses seismic activity regularly. Though many residents may remain oblivious, these microshakes have significant implications for a city already grappling with the challenges of subsidence, self-constructed buildings, and a history of devastating quakes.

Microseisms, as they’re colloquially known, are not a recent phenomenon. The term refers to earthquakes of low intensity, typically measuring less than a magnitude of four. Though unacknowledged by formal seismological terminology, they form part of the fabric of life in this region. But what exactly causes these tremors? According to researchers like Víctor Hugo Espíndola Castro from the National Seismological Service (SSN), which is run by the prestigious National Autonomous University of Mexico (UNAM), these quakes are born of local geological faults beneath the city, stretching anywhere from 500 meters to two kilometers below the surface.

The story of seismic activity in Mexico City is intimately tied to its unique position atop the Trans-Mexican Volcanic Belt, a massive fold where the relentless movement of tectonic plates converges. The Cocos Plate, colliding with the North American Plate, has over time given rise to the region’s majestic volcanic peaks: Popocatépetl, the fire-breathing giant visible from the city on clear days; the stoic Nevado de Toluca; and the towering Pico de Orizaba in Veracruz, among others.

This tectonic friction has far-reaching effects, often rearranging the geological faults that lie dormant for years, even decades, before springing to life again. As Espíndola explains, the capital city’s seismicity stems from these hidden, shifting fractures, particularly in areas like Álvaro Obregón, Magdalena Contreras, Benito Juárez, and Miguel Hidalgo, where tremors have been most frequent. The quakes may be brief—mere jerks lasting fractions of a second—but their accumulation speaks volumes about the undercurrent of tension locked beneath the city’s surface.

An Ever-Watching Eye

Thanks to Mexico’s extensive seismic monitoring network—comprising 100 stations strategically placed throughout the city—the SSN can now pinpoint the exact location of local quakes, offering vital information for urban planning and preparedness. In decades past, a tremor felt in the urban core was easily mistaken as originating from a distant epicenter. But today, with more advanced technologies, experts can trace seismic activity back to neighborhoods within the capital itself, underscoring the city’s precarious geological position.

One of the more fascinating developments in recent years was the discovery of a previously unknown fault line—Plateros-Mixcoac—located in the western part of the city. Alongside well-established faults like Mixhuca, Santa Catarina, and Copilco, this newcomer has added a new layer of complexity to the city's seismic profile, demonstrating that the Earth’s underbelly is never quite finished with its adjustments.

The localized nature of these tremors highlights another critical factor: urban expansion. In previously sparsely populated areas like Tláhuac, tremors went largely unnoticed. But as populations expanded, so did the sensitivity to these events. The same seismic activity that once passed in silence is now documented, analyzed, and felt.

The devastating earthquake of September 19, 2017, which struck the border between Morelos and Puebla, reverberated not only in the visible damage to Mexico City but also in the reawakening of long-dormant faults. With a magnitude of 7.1, the earthquake jolted geological systems that had remained quiet for years, causing a flurry of microseismic activity in the Sierra de las Cruces region. Though this flurry may fade in time, as Espíndola suggests, the pattern speaks to the city’s enduring relationship with seismic instability. For many, that earthquake redefined Mexico City’s landscape of preparedness, and the experience still casts a shadow over the daily routine of drills, alerts, and building inspections.

Why Microseisms Matter in a Macro City

At first glance, a minor tremor may seem insignificant in the grand scheme of things, but in a city like Mexico City, which hosts a patchwork of architectural styles and building heights, even small movements can have major consequences. The problem of self-construction, where homes and buildings are often erected without adherence to strict regulations, compounds the risk. Though microseisms do not have the immediate, catastrophic impact of larger quakes, their cumulative effect on older structures can result in considerable long-term damage.

A small tremor, while unlikely to cause widespread destruction, can still rattle nerves and, in certain conditions, shake up more than just a cup of coffee. Researchers like Delia Iresine Bello Segura stress that while microseisms may seem minor, the logarithmic nature of earthquake magnitudes means that even slight increases can cause disproportionate damage. A magnitude four earthquake, for example, can wreak considerable havoc, especially in older, less structurally sound buildings.

Many residents of Mexico City are familiar with the seismic alert system, a network designed to warn of impending quakes, giving people precious seconds to prepare. But this system is not without its limitations. For local quakes, like the microseisms frequently felt in the capital, the system often fails to provide an advance warning. The waves travel too fast, and by the time the alert sounds, the quake is already in motion. As Bello Segura points out, the system is most effective for larger, more distant quakes, typically those exceeding a magnitude of 5.5 and originating at least 100 kilometers from the city.

This creates an odd paradox: while Mexico City’s residents may be better prepared for distant seismic threats, they are left to face local tremors with little more than instinct and hope.

Living with Earthquakes

In a country where the Earth is always shifting, preparedness is the key to survival. Civil Protection, the governmental body tasked with managing disaster responses, continuously advises on the importance of earthquake drills, both in public spaces and private homes. Knowing evacuation routes, meeting points, and emergency procedures can make the difference between life and death.

While predicting earthquakes remains beyond the reach of science, the hope is that constant vigilance and a culture of preparedness will mitigate their impact. In a world where tectonic plates move according to their own inscrutable rhythms, learning to live with the tremors—both large and small—is perhaps Mexico City’s greatest test of resilience.

Mexico City’s relationship with seismic activity is both intimate and unavoidable. Beneath the hum of urban life, the city is quite literally shifting, creaking along its hidden faults. Microseisms may be the quieter, less dramatic cousins of major quakes, but they are no less significant. They remind us that, beneath the surface, the Earth is constantly in motion, waiting for the right moment to express itself—sometimes with a whisper, sometimes with a roar. In a city of 22 million souls, each tremor carries the weight of geological history, urban vulnerability, and the human spirit’s indomitable will to persevere.