Nobody can forecast an earthquake the way a cyclone is forecast. But an earthquake can be detected in its first second, and because a radio signal travels roughly fifty thousand times faster than a seismic wave, a warning can outrun the shaking. Most Android phones already contain the system that does this. Many owners have never switched it on.
How to switch it on
- Open Settings.
- Tap Safety & emergency. On some phones it is under Location → Advanced; the reliable route is to type "earthquake" into the Settings search box.
- Tap Earthquake alerts and turn the switch on.
- Leave Location on, and keep mobile data or Wi-Fi available — the alert is delivered over the network to phones in the affected area.
iPhones have no equivalent built in outside the regions with a national system, so iPhone users need an app.
What is actually happening
Google launched the Android Earthquake Alerts System in 2020. The insight behind it is that the accelerometer in every phone — the sensor that rotates your screen — is sensitive enough to register seismic shaking, and that there are billions of them.
A phone that is still, charging and detects a sharp characteristic jolt reports it, with a coarse location, to Google's servers. One phone proves nothing; people drop phones constantly. But when many phones across a wide area report the same jolt in the same instant, that pattern is an earthquake and nothing else. The server estimates location and magnitude and pushes an alert outward.
Two alert levels are sent. "Be Aware" for weak to light shaking arrives as an ordinary notification. "Take Action" for moderate to extreme shaking takes over the screen with a Drop, Cover, Hold On instruction and overrides Do Not Disturb.
This is not how the alerts work everywhere. In California, Oregon and Washington the same Android notification is driven by ShakeAlert, a USGS system fed by more than 700 professional seismometers installed by the USGS, California's Office of Emergency Services, UC Berkeley, Caltech and the state water department. That network is faster and far more accurate. The crowdsourced phone network is what the rest of the world gets, including Bangladesh — and it is a genuine achievement, because it gives early warning to countries that were never going to fund 700 seismometers. It is also the weaker of the two, and it depends on phone density, so it works best in exactly the dense cities where buildings are the hazard.
The arithmetic nobody puts in the announcement
An earthquake sends out two kinds of wave. The P-wave travels at roughly 6 kilometres per second and is mostly harmless — it is the sharp knock people describe before the real shaking. The S-wave and the surface waves that follow travel at about 3.5 kilometres per second, and they are what damages buildings.
Every early-warning system in the world exploits exactly that gap. Detect the P-wave, send a message at the speed of light, and the message beats the S-wave. The warning time is therefore not a property of the technology. It is a property of your distance from the epicentre.
Work it through. At 40 kilometres from an epicentre, the P-wave arrives after about 6.7 seconds and the S-wave after about 11.4 — a gap of under 5 seconds, from which detection, server processing and network delivery still have to be subtracted. At 150 kilometres the gap is around 18 seconds, which is a usable warning. Directly above the epicentre the gap is zero, and nothing can be sent in time.
That zone is called the blind zone, and it is the honest caveat: the people in the greatest danger are the ones the alert cannot reach first. An early-warning system is not protection for the epicentre. It is protection for everybody else — which is still most people, because a city is large and an epicentre is a point.
Two apps worth adding
MyShake, built by the seismology lab at the University of California, Berkeley, both receives alerts and contributes your phone's sensor readings to a research network, so using it improves detection for others. My Earthquake Alerts is a monitoring app rather than a warning system: a live world map, notifications for events near you, and depth, magnitude and distance published long before a news bulletin carries them. Both are free on Android and iOS.
What no app replaces
A warning only helps if the next move is already decided. Ten seconds is not enough time to think; it is enough time to execute something you have rehearsed. Agree now where the safe spot is in each room, keep the stairwell clear, and never use a lift after a tremor.
And keep the limit in view. In most cities at serious seismic risk, the hazard is not the ground — it is buildings put up without seismic design and inspected without rigour. An alert gives you seconds. What those seconds are worth is decided years earlier, by whoever signed off the structure you are standing in.
