Have you received another earthquake warning? The test may have been confused by the time difference

Are the earthquake warnings over? Some Californians apparently received a second alarm in less than 24 hours overnight, waking them up seven hours before a scheduled test of the system.

An alert informing Northern Californians of a magnitude 4.2 earthquake shaking the Sacramento-San Joaquin River Delta region startled many on Wednesday. Residents reported being shaken as the epicenter shook near Isleton, not far from Sacramento. At least 400,000 devices, including many cell phones in the Bay Area, were also notified of the emergency.

Then, just after 3 a.m., some MyShake app users received another message.

The error appears to have occurred, at least in part, because the alert was scheduled for 10:19 a.m. UTC – Coordinated Universal Time, also known as Greenwich Mean Time – rather than Pacific Daylight Time.

It was unclear how many people had received the faulty MyShake test alert.

Thursday is International ShakeOut Day, when people who have registered for earthquake alerts in California are expected to receive another quake notification on their phone. The goal this time as part of the Great California ShakeOut is to be prepared if another earthquake occurs.

It is the largest earthquake drill of all time. Participants will be notified at 10:19 a.m

Earthquake researchers have previously found that when activated, the early warning system sometimes issues warnings of quakes that go unnoticed by recipients.

The earthquake early warning system has worked several times recently with impressive results. About a year ago, many in the San Francisco Bay Area received several seconds of warning before they were rocked by a magnitude 5.1 earthquake that struck beneath the mountains east of San Jose.

“Earthquakes are actually the most devastating disaster that could befall us as a state,” said Brian Ferguson, deputy director of crisis communications for the Governor’s Office of Emergency Services, which is organizing the event.

There are 634,064 people who have signed up to receive alerts in what ShakeOut organizers call the Delta Sierra region — which includes Sacramento, Yolo, Placer, El Dorado and neighboring counties.

According to ShakeOut officials, there are active faults in the Sierra Nevada ridge, Lake Tahoe, the foothills and the western Sacramento Valley.

The Los Angeles Times and The Bee’s Jacqueline Pinedo and Angela Rodriguez contributed to this story.