Seconds before a 6.2 earthquake rattled California, phones got a vital warning

Half a million phones received emergency alerts thanks to system offering a few seconds to take cover

In the moments before a 6.2-magnitude earthquake struck the northern California coast on Monday, roughly half a million phones began to buzz. An early-alert system managed by the US Geological Survey sent warnings out before the ground started to shake, giving residents in the sparsely populated area vital time to take cover.

The earthquake brought significant shaking but minimal damage in Humboldt county, about 210 miles north-west of San Francisco, and officials said it was an excellent test of the alert-system. It was the largest magnitude quake that's occurred since the system, known as ShakeAlert, was officially rolled out across the west coast.