People ducked under desks and tables in California and other earthquake-prone areas around the world for an annual drill held Thursday to practice ways to stay safe during quakes.

Up and down the U.S. West Coast, the ShakeOut drill began at 10:19 a.m. PDT with a test alert from the region's ShakeAlert earthquake warning system that popped onto cellphone screens.

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 magnitude 4.2 quake 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.

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 disastrous quake that experts say will happen, 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.

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