Android phones around San Francisco’s Bay Area buzzed with an alert on Tuesday morning: A 4.8 magnitude earthquake was about to hit. “You may have felt shaking,” some of the messages read. More than a million Android users saw the alert. And for some, it arrived seconds before the ground even started moving.

It’s not the first time Android devices have received these alerts, says Marc Stogaitis, the project lead for the Android Earthquake Alerts System. But because the Bay Area is so densely populated, the alert hit enough phones that the larger public took notice. Earthquakes have historically come without warning, catching people off guard and leaving them with no advance notice to drop and take cover. Alerts like this aim to take some of the unpredictability out of earthquakes—even if by just a few seconds.