Earthquake Early Warning detects initial tremors (P-waves) and issues alerts before main shaking (S-waves) arrives. Since P-waves travel faster, areas distant from the epicenter gain seconds to tens of seconds for protective action: hiding under desks, turning off stoves, opening doors.
Smartphone notification uses Area Mail, broadcasting to all devices in the target area. Alerts force sound even in manner mode or silent mode, with the distinctive "tiron-tiron" alarm designed for instant emergency recognition.
There are two types. The "Warning" is the one that reaches the general public: the JMA issues it once at least two seismometers have detected the waves and the forecast is a maximum intensity of 5-lower or above on the Japanese scale, or a long-period ground motion class of 3 or above. That long-period class was added to the criteria on February 1, 2023. In the 2011 Tohoku earthquake the intensity in Osaka reached only 3, yet high-rise buildings there saw people trapped by halted elevators and interior finishes damaged, so slow swaying of that kind is now covered too. A Warning never states the predicted intensity for your own location or how many seconds are left; it only says the shaking will be strong, because the prediction carries an error margin. Treat it as a cue to move, not a countdown to check. The "Forecast" reaches users through operators licensed by the Director-General of the JMA for earthquake ground motion forecasting. It is issued for smaller earthquakes as well and can carry the predicted intensity and arrival time for a specific site, which is what drives automatic railway braking and elevator control.
Two limits are worth knowing. Analysis and delivery take a few seconds, so for a shallow inland earthquake the alert physically cannot reach areas close to the epicenter before the strong shaking does. Lightning and similar jolts are occasionally mistaken for an earthquake, and a cancellation then follows within seconds to a dozen or so seconds. Conversely, no cancellation is issued when an area forecast at intensity 5-lower turns out to be 3 or below, so some alerts will feel like false alarms. Whether those few seconds are of any use comes down to having decided in advance, at home and at work, what everyone does when the alarm sounds. See the disaster phone guide for more.