Recently available ShakeAlert LA is live. The app is design to give LA County residents a few seconds of warning before the shaking occurs. Once downloaded, the app doesn’t need to be open but must set up phone’s location services to “always on.” The early warning function only works in LA county.

For more info, please read the original article by NBC LA:
https://www.nbclosangeles.com/news/local/Earthquake-Los-Angeles-California-ShakeAlert-LA-Early-Warning-App-503794191.html