The alerts will be issued in two ways: a mobile phone app called MyShake and a more traditional wireless alert system that sends Amber Alerts.

“The California Earthquake Early warning system will marry a new smartphone app with traditional alert and warning delivery methods such as wireless emergency alerts (WEA). The system uses ground-based motion sensors from around the state to detect earthquakes before people can feel them and will notify Californians so they can “drop, cap and hold” in the run-up to an earthquake, ” the California Governor’s office of emergency management said in a press release announcing the system’s launch Thursday.

The California earthquake early warning system is a joint project of the office of emergency management, the U.S. Geological survey, the University of California, Berkeley, CalTech and others.

The system was tested this week when residents of the San Francisco Bay Area and Central California were shaken by a magnitude 4.5 earthquake and a 4.7 earthquake, respectively. The warnings hit phones with an average time of 2.1 seconds in the first case and 1.6 seconds in the last, said Richard Allen, Director of the University of California, Berkeley seismological laboratory.

Authorities say the system is promising but far from perfect.

“The California Earthquake Early warning system is based on innovative technologies that will improve over time. In rare cases, you can get ShakeAlert when there was no earthquake, ” the system said in an announcement.