Earthquake early warning is not a prediction. Once an earthquake has started, sensors near the epicentre detect the first waves and an alert goes out before strong shaking reaches places farther away. The review by Allen and Melgar (2019) makes the distinction clear: by the time a warning is issued, the earthquake is already under way. Why the time gained is limited to seconds is explained with P and S waves in our post on how phones detect earthquakes. Here we look at the systems around the world, what to do when an alert comes, and their limits.

Systems around the world

Japan: The Japan Meteorological Agency (JMA) started its public early warning service on 1 October 2007, through television and radio. The system uses data from seismometers near the focus to compute the location and magnitude, then announces the expected intensity and the arrival time of strong shaking for each area. Alerts are used to slow trains, stop lifts and let people protect themselves.

Mexico: SASMEX, run by CIRES, has been a public service since 1993. It uses 96 sensors along the Pacific subduction zone and broadcasts alerts in several cities, including Mexico City, by radio, television and dedicated receivers. For a coastal earthquake it can give roughly 20 to 120 seconds depending on the city; for an earthquake on the Michoacán coast, more than 100 seconds in Mexico City.

US West Coast: ShakeAlert, run by USGS and its partners, began public alerts in California in 2019, in Oregon in March 2021 and in Washington on 4 May 2021. Alerts reach people through FEMA’s Wireless Emergency Alerts and phone apps.

Android: Google’s system uses phones as small seismometers and sends alerts for earthquakes of magnitude 4.5 and above. Its strongest “Take Action” alert, for expected intensity MMI 5 and above, breaks through Do Not Disturb and plays a loud sound. According to Allen et al. (2025), the system delivered alerts in 98 countries.

Istanbul: Describing the system of Kandilli Observatory, Erdik et al. (2003) write that ten strong-motion stations were placed as close as possible to the Marmara Fault for early warning, and a hundred accelerometers across the city for damage maps. Allen and Melgar (2019) note that in Turkey alerts go to selected user groups, not to the public.

What to do when an alert comes

JMA’s leaflet on responding to an alert is short and clear: you have only seconds, so act fast.

  • At home: Protect your head and get under a table. Don’t rush outside. Don’t worry about turning off the stove.
  • In a crowded building: Follow staff instructions and don’t rush to the exit.
  • When driving: Don’t brake suddenly; turn on your hazard lights, slow down smoothly and stop somewhere safe.
  • On public transport: Hold on tight to a strap or handrail.
  • In a lift: Stop at the nearest floor and get out.
  • Outdoors: Keep away from walls, signs and glass.

USGS likewise advises to drop, cover and hold on when an alert arrives. Do the same the moment you feel shaking, alert or no alert.

The limits

  • Blind zone: JMA notes that near the focus strong shaking may arrive at the same time as the alert. That is often where damage is greatest.
  • Short lead time: According to JMA, the time between alert and strong shaking ranges from a few seconds to a few tens of seconds.
  • False alarms: When data from only one seismograph are used, accidents, lightning or device failure can trigger a false warning.
  • Underestimated magnitude: JMA writes that magnitude estimates are limited, especially for large earthquakes. The Android system also greatly underestimated the magnitude of the 6 February 2023 Pazarcık earthquake.
  • Late alerts: Android’s help page says an alert may arrive before, during or after shaking. In user feedback reported by Allen et al. (2025), 36% received the alert before shaking, 28% during and 23% after.

In short

Early warning is an advantage of a few seconds; it does not replace a strong building and secured furniture. In every case, the most important preparation is knowing what to do, whether an alert comes or not. Use the earthquake guide as a quick reminder for the moment of shaking, and follow the latest earthquakes on the live map.