The sound doesn't seem like a notification. It's a continuous, loud alarm that ignores silent mode and fills the entire screen. Those receiving it for the first time take a few seconds just to understand what's happening—and those are precisely the seconds the alert is meant to provide.
It's important to understand beforehand what this alert is and what it isn't. It doesn't predict an earthquake. It warns that a tremor has already started somewhere and that the strongest part of the wave hasn't yet reached you.
Why is it possible to give advance notice?
An earthquake doesn't send just one wave. It sends two that matter. The first, called the P-wave, travels faster through the ground and arrives first, but causes little shaking. The second, the S-wave, comes more slowly and is the one that really shakes things up.
It is in this difference in speed that the warning lies. When the sensors detect the P-wave, the system calculates where it has been and how far it is coming, and sends the message over the internet — which travels much faster than any wave on the ground. The alert wins the race against the S-wave.
This has a practical consequence that many people don't realize: The closer you are to the epicenter, the less warning you receive.. Those directly above the camera can feel the vibration before the phone rings. Those further away gain more time, but feel it less.
The two warning levels
The Android system uses two different alerts, and confusing one with the other will cause unnecessary alarm.
- Slight tremor warning: It arrives as a regular notification, with normal sound. It serves to let you know that there was a tremor and that you might feel something. It doesn't ask for any action.
- Strong tremor alert: This is the one that takes over the screen, plays loudly, and ignores the silent mode. It's the one that demands an immediate reaction.
If your phone rang softly and you only saw one notification, it was the first type. There was nothing you could do.
What to do in the next few seconds
The international protocol can be summed up in three words: drop, cover, hold. It applies to most situations and is simple precisely because it needs to work with the brain in a panic state.
At home or at work
Get down before the shaking knocks you down. Find the nearest protection—a sturdy table will do—and protect your head and neck. If there's no table, lean against an interior wall, away from a window, and cover your head with your arms.
Hold onto whatever is protecting you, because the furniture will move along with the floor. Stay there until it stops.
The most common mistake is the instinct to run outside. During the tremor, glass, plaster, and wiring fall onto the street. Exiting in the middle of the shaking is usually more dangerous than staying. Never take the elevator.
In the street
Move away from facades, posts, walls, and awnings, and go to the most open space you can reach in a few steps. This isn't about running across the block—it's about getting out from under anything that could fall.
Driving
Slow down gently and pull over to the shoulder or the clearest point possible, avoiding overpasses, bridges, and pedestrian walkways. Stay inside the car with your seatbelt fastened and wait for the vibration to pass. The vehicle's suspension masks much of the shaking, so it's common for drivers to only notice it due to the warning lights and the strange behavior of other cars.
After it stops
The first thing is to avoid overloading the network. Mass voice calls bring down the operator precisely when it's most needed. Text messages go through under conditions where calls don't complete, and they use a fraction of the network bandwidth.
If you want to confirm whether there was indeed an earthquake and how strong it was, look for official seismological centers — the National Observatory and the seismological center linked to USP publish the records. This is more valuable than any screenshot circulating in groups.
Aftershocks are normal following a major earthquake. They tend to be weaker, but they affect structures that have already been compromised. If plaster has fallen or walls have cracked, avoid standing underneath them.
What if the alert is wrong?
In February 2025, cell phones in southeastern Brazil simultaneously started ringing with messages announcing a strong tremor off the coast of São Paulo. There was no earthquake at all. The system misinterpreted the sensor data and sent the most serious warning it could possibly send, in the middle of the night, to many people at the same time.
The alert was suspended in the country and returned months later. This doesn't invalidate the tool: a false positive can be alarming, and a false negative can be more so. But it explains why the warning should be treated as what it is — an automatic estimate made in seconds, not a definitive diagnosis.
Where to check if it's turned on on your device.
On Android, the path is located in the system settings, under security and emergency, in an option dedicated to earthquake alerts. The exact name varies slightly depending on the brand and version, but it's always found in that section.
Two things that often cause the alert to fail without anyone noticing: location services being turned off and aggressive battery saving mode, which cuts off background services. If you want the alert to work, both need to be enabled.
There is no equivalent system native to the iPhone. iOS users rely on official civil defense channels, which in Brazil use postal code registration to send alerts via text message.
The honest summary
An earthquake alert on your cell phone isn't a shield. It buys seconds—sometimes just a few—and what you do with those seconds is what makes the difference. Knowing beforehand where to go inside your own house is worth more than any app installed after the scare.
