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.
Two types exist: "Warning" (public, issued for predicted maximum intensity 5-lower or above) and "Forecast" (for contracted businesses, covering smaller earthquakes). Forecasts trigger automatic railway braking and elevator stopping systems.
Limitations include insufficient lead time near epicenters and non-zero false alarm possibility. Still, seconds of warning save lives - during the 2011 Great East Japan Earthquake, many took protective action thanks to early warnings. See disaster phone guide for more.