A little after 1 p.m. on September 19, 2017, Alejandra Vera Izquierdo suddenly felt her building in the southern Mexico City neighborhood of Xochimilco begin to shake violently for about 20 seconds. She was physically unscathed in the 7.1-magnitude earthquake, but she was angry: She claimed that the city’s earthquake alarm, which would have given her enough time to exit the building, hadn’t sounded on any of the loudspeakers nearby.

Terrified by the deadly aftermath of that earthquake and fearing the city’s alarm system might fail again, she downloaded SkyAlert, an earthquake warning app. Almost six years later, Vera Izquierdo still relies on it. She told Rest of World how, in April 2019, when a less intense earthquake hit, “SkyAlert warned me several seconds before the city alarm, which helped me get out of the building sooner.” 

Some residents consider Mexico City’s earthquake warning system, which has been fully operational since the early 1990s, to be increasingly flawed. Since 2017, the Seismic Alert System, called SAS, has triggered false alarms, failed to work in certain neighborhoods, or ended up not sounding the citywide alarm at all during an earthquake. For private app developers, though, these issues have presented an opportunity. Several have created their own mobile earthquake warning apps, of which SkyAlert is the most popular. With just under 7 million users, according to the company’s own figures, SkyAlert is also the only earthquake alert app legally permitted to operate in Mexico. 

Despite weaknesses of their own, earthquake alert apps have gained people’s trust in Mexico City. Users told Rest of World they consider the apps better than the government’s official alarm system for a variety of reasons, such as portability and the ability to use them outside Mexico City. “You never know when the loudspeakers are going to fail,” Vera Izquierdo told Rest of World. “I know SkyAlert is going to ring very, very loudly on the phone next to me, so I can’t miss it.”

Most of Mexico sits upon a seismically active zone, and the densely populated capital is more vulnerable to earthquakes than many other cities. SAS is run by a nonprofit named CIRES, created in 1986 in the wake of an earthquake that had devastated the city the year before. 

For years, SAS has been hailed as a life-saving innovation by experts and locals alike; it is the first of its kind worldwide. Its 97 sensors are distributed in open-air areas and linked to a radio wave transmitter. If at least two sensors detect a tremor that measures over 5.5 on the Richter scale at no more than 350 kilometers from the city, a radio wave alert — traveling faster than the seismic waves, according to SAS — will set off an alarm in Mexico City at least 50 seconds before the shaking starts, depending on how close the epicenter is to the city. In 2015, its distinctive siren was introduced, blasting the sound of the earthquake alarm out of almost 14,000 loudspeakers linked to the city’s ubiquitous surveillance system. 

Rest of World reached out to CIRES but received no response. 

In 2011, Álvaro Velasco and Alejandro Cantú founded SkyAlert to warn users about incoming earthquakes through their phones. Currently, the app’s warning system works with its own network of over 120 internet-connected sensors. They cover about 80% of the country’s seismic areas, including the Pacific coastline, “which is important because it’s not only Mexico City that is highly vulnerable to earthquakes,” Francisco Catalá, SkyAlert’s chief technology officer, told Rest of World. 

In contrast to SAS sensors that sit atop radio towers, each SkyAlert sensor is encased in a plastic box and secured onto the ground, inside buildings like schools or hotels. When sensors identify the early tremors of an earthquake of any magnitude, they immediately trigger the app’s warning system.

The app sends a loud push notification and a voice announces, “Earthquake detected,” followed by its estimated intensity — weak, mild, moderate, strong, violent, or severe. All earthquakes over the “strong” category automatically trigger the alarm. Users can configure the types of earthquakes for which they want to receive notifications. They can also choose if they want their phone to vibrate or light up when the alert goes off. Catalá said that, depending on how far the quake’s epicenter is from a certain city, the warning might reach a user up to 120 seconds before it hits. 

The app has its limitations. Since its sensors are connected through the internet, any phone not connected to mobile data or Wi-Fi will not be alerted. This is on top of the company’s panic-inducing early struggles. When SkyAlert first launched, it relied on the SAS system. According to Velasco, the company’s CEO, the app sent out an alarm that turned out to be false — not only confusing users, but making them angry and anxious. The company has now launched its own independent sensor network — larger than that of SAS.

“If there’s one notification on my phone that I don’t ignore, it’s definitely the one that sends out earthquake information.”

SkyAlert rose to prominence after the devastation of the 2017 quake. That day, the app’s warning went off about 12 seconds earlier than the SAS alarm. For the next three days, the company said, SkyAlert was the most-downloaded app on Apple’s App Store in Mexico. Catalá told Rest of World it had over 1 million downloads on Android devices after five days. Before the earthquake, it had just over 100,000 active users; it now has over 6 million, most of whom are in Mexico City. The app’s users told Rest of World they consider it to be a convenient alternative or even a complement to the city’s alarm. 

“It’s not like I don’t trust the city’s alarm, but I think SkyAlert is much handier,” Emmanuel García Gama, a Mexico City resident, told Rest of World. He is one of about 1 million users who have paid for a SkyAlert subscription, which costs up to 249 pesos (just over $14) a year. The app can be used for free, but a subscription allows users to get more data on an earthquake, including its estimated time of arrival — rather than just a warning.

SkyAlert also makes money by operating earthquake alarm systems for buildings. City authorities mandate that all public buildings should have a CIRES-operated alarm, but many buy an additional alarm system, provided by companies like SkyAlert. For instance, all WeWork buildings in Mexico City have a SkyAlert-managed alarm, a WeWork employee told Rest of World, requesting anonymity since they were not authorized to speak for the company. Catalá said about 60% of SkyAlert’s income comes from selling their product to businesses rather than individual app users. 

In the past six years, more earthquake alert apps have emerged, despite SkyAlert being the only one legally approved to operate in Mexico. Some users told Rest of World they had switched from SkyAlert to other apps, like SASSLA, because they claimed SkyAlert hadn’t been working properly — that it had warned them about very mild quakes away from their location, but not sent an alert when a quake was actually deadly. 

Velasco told Rest of World the company is working to improve these flaws, sometimes making up for a missed alert by sending users a subsequent message with additional information. 

In recent weeks, Mexico City has experienced a series of small but strong “micro-earthquakes.” Neither SAS nor SkyAlert have been able to send an early warning for these, since they’ve originated inside the city, far from where each of their sensor networks are installed. Nadim Matuk, a SkyAlert user in Mexico City, told Rest of World he has nevertheless been especially attentive to the app since then. Even when it doesn’t send a warning, it provides information on what happened. 

“If there’s one notification on my phone that I don’t ignore, it’s definitely the one that sends out earthquake information,” he said.