Many San Francisco Bay Area residents were given several seconds' warning before shaking arrived from a magnitude 5.1 earthquake that struck under the mountains east of San Jose, California, this week.
The activation of the U.S. Geological Survey's ShakeAlert system is the second time in six weeks that Bay Area residents got a warning before they felt shaking. Tuesday's magnitude 5.1 earthquake was the most powerful to hit the Bay Area since the destructive Napa earthquake of 2014, which was magnitude 6.0.
More than 2.1 million earthquake alerts were issued to users Tuesday, said Robert de Groot, part of the ShakeAlert operations team.
