Did you get another earthquake alert? Test may have been shaken up by time difference

Are the quake alerts over? Some Californians appeared to have received a second alarm in less than 24 hours overnight, jolting them awake seven hours before a planned test of the system.

A warning informing Northern Californians about a 4.2-magnitude earthquake jolting the Sacramento-San Joaquin River Delta region startled many Wednesday. Residents reported being shaken as tremors rocked from the epicenter near Isleton, not far from Sacramento. At least 400,000 devices, including many cellphones in the Bay Area, were also notified about the emergency.

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

The error appears to have occurred at least in part because the warning 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 received the errant MyShake test warning.

Thursday is International ShakeOut Day in which people registered for quake alerts in California are expected to receive another notification on their phone about a quake. The goal this time, as part of the Great California ShakeOut, is to be prepared if a temblor were to strike again.

Dubbed as the largest-ever earthquake drill, participants will get notified at 10:19 a.m.

Earthquake scientists have noted previously that the early-warning system, when activated, will sometimes issue warnings for quakes that are not felt by the people who receive them.

There have been a number of times recently when the earthquake early-warning system has worked with impressive results. About a year ago, many in the San Francisco Bay Area received several seconds’ warning before they felt shaking from a magnitude-5.1 earthquake that struck under the mountains east of San Jose.

“Earthquakes are actually the most damaging disaster we could face as a state,” said Brian Ferguson, deputy director of crisis communication at the Governor’s Office of Emergency Services, which is organizing the event.

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

There are active faults found in the crest of the Sierra Nevada, Lake Tahoe, the foothills and the western Sacramento Valley, according to the ShakeOut officials.

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