Some people got an alert about Friday night's magnitude 4.5 earthquake before or shortly after the shaking began. Others got it after it was over. Some got nothing.
The U.S. Geological Survey's ShakeAlert system sent out an alert within about five seconds after the earthquake began.
Bob de Groot, with the USGS, explains it then takes apps - such as MyShake, QuakeAlert USA and ShakeAlertLA - a little more time to put out alerts.
The apps issued thousands of them, as did Google, to Android users who've updated their operating system.
"It was just pretty astounding to see how many alerts were sent out by all of our providers," de Groot said.
De Groot says some people didn't get an alert until after the shaking had ended because they were so close to the epicenter.
"The earthquake sort of outspread the rate that the alerts can get out," he said.
He says some people weren't notified because the estimated intensity of the shaking in their area was too low to trigger an alert.
And sometimes the settings on a person's phone might keep an alert from coming through.