ShakeAlert is a USGS app for your phone, the USGS also feeds the data to Federal Emergency Management Agency, which distributes applicable alerts through the U.S. Emergency Alert System on mobile devices.

For more info, please read the original article by The Washington Post:
https://www.washingtonpost.com/weather/2020/02/08/californians-can-now-get-earthquake-early-warnings-their-phone-heres-how-alerts-work/