emergency alert; emergency early warning
Refers to official emergency alerts sent to mobile phones, email, or broadcast systems, such as earthquake or tsunami warnings.
An emergency alert arrived on my smartphone right after the earthquake.
When the emergency alert sounds, please evacuate to a safe place immediately.
Specifically an earthquake early warning, while 緊急速報 is a broader term for any emergency alert.
A general warning or alarm, not necessarily delivered via mobile devices.
Compound of 緊急 (emergency) and 速報 (breaking news / bulletin).