California earthquake puts early warning system to the test

As sensors picked up the first signs of a strong earthquake jolting the Northern California coast, an alert was blasted to 3 million smartphone users telling them to "drop, cover, hold on." It was hailed as the biggest test yet of the warning system since its public launch.

But the people most rattled by the magnitude 6.4 earthquake early Tuesday said the alert didn't give them enough time to take cover as the temblor shook homes off foundations, knocked out power and water to thousands, and injured more than a dozen people.

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Before alerts get sent to people's phones, multiple seismometers have to detect movement below Earth's surface. That information can then be processed to determine the earthquake's location and magnitude. That process, from seismometer detection to an alert being sent, is all automated, said Robert de Groot, a scientist with the ShakeAlert operations team.

Some people received the alert with 10 seconds' notice. Because of how the system works, those closest to the center of the earthquake may not have received an alert until they felt shaking, de Groot said.

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