Earthquakes in Mexico City: the alarm, the drill and September

Why Mexico City shakes so much

Mexico City sits on the dried bed of the former Lake Texcoco, a layer of soft clay that amplifies seismic waves in a way that catches newcomers off guard: a tremor barely felt on the coast can shake a high floor in the capital hard, hundreds of kilometres from the epicentre. Most of the quakes felt here originate in the subduction zone off Guerrero, Oaxaca and Michoacán, where the Cocos plate slides beneath the North American one. The 1985 earthquake, with its epicentre along that same stretch of coast, permanently changed the city's building codes.

Shaking is part of daily life in the capital, though most quakes go unnoticed outside the seismological stations. Mexico City registers minor movements almost every week, and residents learn without effort to tell a small tremor that barely stirs a lamp from one worth taking seriously. Collective memory circles around two dates that fell, unnervingly, on the same day of the calendar: the 1985 earthquake and the one in 2017, both on 19 September. That coincidence, more than superstition, explains why that particular date became the official day for the national drill.

Towers in Mexico City's financial district, built on the former bed of Lake Texcoco.

How the alarm sounds and how much it warns

Mexico City's seismic alert sounds through loudspeakers placed across much of the city: a sharp, repeating tone followed by a recorded voice saying 'alerta sísmica' several times over. The system, called SASMEX, uses a network of sensors on the Pacific coast that detect a quake's first waves, which travel faster and cause far less damage, and send the signal ahead of the stronger, slower waves that follow. When the epicentre lies off Guerrero or Oaxaca, that speed difference gives the capital between sixty and ninety seconds of warning, generally enough time to reach a stairwell and walk down calmly before the real shaking begins.

That sixty-to-ninety-second margin is not the rule in every case, and that is the part worth understanding before being caught off guard. Quakes with an epicentre closer to the city, like the 2017 one on the Puebla-Morelos border, give much less time because the waves travel a shorter distance. The alarm may sound only seconds before the shaking starts, or not sound at all if the quake originates beneath the city itself. That is why no guide recommends waiting for the alarm to react: if you feel the movement, you act immediately.

The sound A sharp tone followed by a voice repeating 'alerta sísmica'. Who runs it The SASMEX system, with sensors on the Pacific coast. Typical warning Sixty to ninety seconds if the epicentre is off Guerrero or Oaxaca. Minimum warning Under fifteen seconds, or none, if the quake starts near the city. Loudspeakers Placed on streets, plazas and public buildings across much of the city. At the hotel Not all have their own speaker; a phone may not warn in time.

The uncomfortable part The alarm can give sixty seconds in the best case, but in a quake with a nearby epicentre it may sound with under fifteen seconds to spare, or not sound at all before the shaking is felt. No app or system removes that risk: the real advice is to react to the first tremor, not to the first sound of the alarm.

The 19 September drill

19 September has, for years, been the day of the national civil-protection drill, and the date is no accident: it marks the anniversary of the 1985 earthquake and, through a coincidence nobody planned, of the 2017 quake as well, which struck only hours after that year's scheduled drill had already taken place. That double anniversary turned 19 September into a date loaded with meaning for the city, and ever since, the alarm sounds on a scheduled morning in offices, schools, hospitals and government buildings across the capital, even though no real earthquake is under way.

For anyone staying in the city that day, the drill is a practical demonstration more useful than any leaflet. The alarm sounds at an announced time, hotel staff guide guests down the stairs to a meeting point on the street, and the whole exercise usually takes fifteen to twenty minutes. There is nothing to fear or to solve: it is the one day of the year when you can check, with no real pressure, what the exit route from your own floor actually is and where the meeting point sits. It is worth joining in rather than staying in the room watching the clock.

The Zócalo in the Historic Centre, one of the gathering points during the national drill.

What to do during the shaking

During a quake, what saves you is taking cover, not running. Civil protection guidance is to move away from windows and anything that could fall, find the gap of an interior doorway or the corner between two load-bearing walls, or crouch under sturdy furniture if any is nearby, always protecting your head and neck with your arms. Running down a staircase while the building is moving is more dangerous than staying still in a good spot, and using the elevator during or after a quake is off the table under any circumstance, because it can get stuck between floors if the power cuts out.

Once the shaking stops, leave by walking, not running, down the stairs to the meeting point that every hotel or building has marked in advance. It is not worth going back for a passport, a laptop or any other belonging, because aftershocks can arrive minutes after the main quake and are often stronger than expected, catching people mid-stairwell. Once outside, stay well clear of façades, hanging cables and balconies until staff or civil-protection officials confirm the building has no structural damage. Going back into the room before that confirmation arrives is the single mistake that causes most of the injuries recorded after a quake.

During the shaking Move away from windows, cover your head, and find an interior doorway or sturdy furniture. The elevator Never, during or right after. It can get stuck between floors. The stairs Walk, do not run, and only once the shaking has stopped. Balconies Avoid them during the quake; glass and loose objects are the real risk. Meeting point Locate it on arrival at the hotel, before you ever need it. Aftershocks Can arrive minutes or hours later, and are sometimes stronger than the initial quake.

What to check if you stay on a high floor

The 1985 earthquake forced a complete rewrite of the city's building code, and the revisions carried out after the 2017 quake tightened those standards further still. A hotel built or substantially renovated in recent decades, with a reinforced-concrete structure and documented compliance with current rules, is reasonably safe on almost any floor, including a high one near the top. The floor itself matters far less than the date and quality of the construction: asking at reception when the building went up, or when it was last renovated, gives more useful information than worrying about whether the room sits on the eighth floor or the second.

The practical move, beyond the building itself, is to do your own scouting on arrival: locate the emergency stairwell nearest your room, count how many floors down it is, and keep your shoes and phone within reach of the bed, not at the bottom of the suitcase. An eighth floor takes longer to evacuate than a low one, and that is entirely normal, not a sign of danger. If the September drill falls during your stay, it is worth joining in for exactly that reason: it is the one day of the year the same route gets practised under no real pressure.

Construction Ask at reception the year the building went up or was last structurally renovated. The floor Matters less than build quality; a modern building is safe at almost any height. Exit route Find the nearest emergency stairwell on arrival, not during the quake. Shoes and phone Keep them within reach of the bed at night, not inside the suitcase. Firm-ground areas Lomas, Coyoacán and the west shake less than the Centre and the east. Lakebed areas The Historic Centre, Roma and Iztapalapa amplify shaking through soft subsoil.

The uncomfortable part The subsoil across much of the city, especially the Historic Centre and much of the east, is the former lakebed and amplifies shaking far more than the firm ground of the west and south, like Lomas or Coyoacán. That means a moderate quake, which in another city would barely register, can feel surprisingly strong in a hotel downtown, even with the epicentre hundreds of kilometres away. What the magnitude on your phone says does not always match what you feel.

The mistakes people keep making

The most common mistake is dismissing the alarm after hearing a couple of false alarms or quakes that were barely felt; the system does fail occasionally, but ignoring it as a rule is betting against the odds. The second is rushing to the balcony to film the moment the shaking starts, which is precisely the spot with the highest risk of falling objects and broken glass. The third is assuming the classic doorway offers the same protection in every building: in modern reinforced-concrete construction it gives no real advantage over moving away from windows and covering your head.

The fourth mistake, very typical of visitors, is assuming that an expensive hotel comes with a tacit guarantee of structural safety, when it actually depends on the building rather than the rate. The fifth is comparing the quake to earthquake experience back home and judging, wrongly, how serious it is by that standard, when Mexico City's ground follows rules of its own. The sixth, and the one that causes the most avoidable injuries, is running back for a passport or a camera the moment the shaking stops, instead of waiting at the meeting point until staff confirm it is safe.

Ignoring the alarm Even after past false alarms, always treat it as real. Filming from the balcony It is the spot with the highest risk of broken glass and falling objects. Trusting the doorway In modern reinforced concrete it protects no more than moving away from windows. Assuming price protects you Safety depends on the building, not the room rate. Comparing it to home The way the ground behaves in the capital is not like other countries. Going back for belongings The most common cause of avoidable injuries after a quake.

What people always ask

These are the questions that tend to come up as soon as the subject is raised, arranged from the most immediate to the most existential. They cover the sound of the alarm, how safe a tall building is, what to do if a tremor catches you in the street or on the metro, and whether it is worth worrying before you book a trip. The answers are based on how the system and the city actually behave, not on the dramatic version that sometimes circulates outside Mexico, nor on the relaxed version from someone who has lived here for years and stopped paying attention to the alarm.

It is worth putting the risk in perspective before closing the subject. Mexico City shakes often, but the vast majority of those movements pass without consequences, and millions of people live, work and host visitors in the capital every week without anything worth mentioning happening. The point of knowing the alarm and the drill is not to travel afraid, but to know what to do during the one minute that actually matters, which may never come during an ordinary few days' stay. It is the kind of information you use once in a long while, and worth having before you need it.

How much warning does the alarm give? Sixty to ninety seconds if the epicentre is on the Pacific coast; far less, or none, if it starts near the city. Is a high floor safe? In a modern, well-built structure, yes. The date and quality of construction matter more than the height. What if it hits while I'm in the street? Move away from façades, cables and signage, and head for open space if there is any nearby. What if I'm on the metro? The system usually stops as protocol. Wait for staff instructions before stepping onto the tracks. Should I avoid travelling because of this? No. The vast majority of stays pass without any perceptible earthquake. What is the 19 September drill? A national evacuation exercise that coincides with the anniversary of the 1985 and 2017 earthquakes.

In short The preparation takes a minute and covers the whole stay Learning the sound of the alarm, locating the stairwell on arrival, and knowing the September drill is only a rehearsal are quick habits that change nothing about the itinerary or the budget. What they do change is what happens in the one minute it actually matters, which may never come during an ordinary stay. See Mexico City Back to the blog