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For many, it was the second alert of the day after an errant predawn message hit some phones with a voice message announcing the test. The U.S. Geological Survey said it was likely due to a mix-up in time zones set in the test alert system.

The real thing happened a day earlier, however, when a struck southwest of California's capital in the agricultural Sacramento-San Joaquin River Delta region.

The tremor triggered a warning from the ShakeAlert system, which detects the start of a quake and sends warnings to areas expected to experience shaking. The quake proved to be weaker than the near-instantaneous initial estimate and no damage was reported.

The ShakeOut earthquake drills are coordinated by the Southern California Earthquake Center at the University of Southern California. The event focuses on the “drop, cover, and hold on” mantra for basic personal safety but also includes such measures as passenger trains slowing down for several minutes.

At the Los Angeles Regional Foodbank, volunteers and staff got down low or under tables during the drill, then evacuated the building and did an assessment to make sure everyone was safe. They dropped to the ground again, pretending they were protecting themselves from objects that might be falling or flying around in an aftershock.

“We really do recommend that people make themselves a smaller target from those things that cause injuries,” said Mark Benthien, the SCEC communications director and lead ShakeOut organizer.

The ShakeOut drill originated in California in 2008. The first one was based on a scenario of a magnitude 7.8 earthquake on the southern section of California’s mighty San Andreas Fault. It’s the type of that experts warn will happen someday — although they can’t say when.

The drill has since expanded internationally. Schools, government organizations, companies and others sign up to take part. More than 10 million people were registered this year in California, and millions more worldwide, according to organizers.

Compared to 15 years ago, Benthien said, “many, many more people are getting prepared, practicing earthquake safety each year because of ShakeOut.”