A 5.2 earthquake rattled San Diego, California, and the surrounding area on Monday afternoon, according to the US Geological Survey.
The quake struck at 10:08 local time (18:00 GMT) with an epicentre in Julian, California.
The resort town is located in the Cuyamaca Mountains about an hour north-east of San Diego.
Around two hours after the earthquake, Perette Godwin, a spokeswoman for the City of San Diego, told the BBC there had been no reports of structural damage or injuries.
The San Diego sheriff’s office also said it had not yet “received any reports of injuries or major damage caused by the earthquake,” but said it was a “developing situation”.
California Governor Gavin Newsom’s office posted on X shortly after the quake that he had been briefed on the situation.
The USGS continued to report smaller aftershocks in the region in the hour after the quake.
The National Weather Service, meanwhile, said a tsunami was not expected.
In the moments before it struck, emergency alerts issued by the USGS ordered residents to take shelter as far away as Los Angeles.
“Drop, cover, hold on. Protect yourself,” the alert read.
Kevin Manaugh was eating breakfast when he received an alert, and quickly took shelter under a door frame.
“Sure enough, the quaking started to happen,” Mr Manaugh said. “I’ve lived in San Diego most of my life, I was born here, and this is probably the worst quake that I’ve ever felt.”
“It was a bit shocking,” he continued. “Everything shook, it shook a lot. It rattled around and lasted maybe three seconds, and then it was over.”
Though his house suffered no damage, Mr Manaugh said he would likely prepare in case another earthquake struck.
“Between this and the wildfires in Los Angeles, it made me think that it’s probably a good idea to have an evacuation bag,” he said.
Earthquakes are notoriously difficult to predict, but the alert’s lead time – about 15 seconds for Mr Manaugh – was enough to take action.
“The way they were able to predict that, it gave me time to get to a point of safety,” he said.
The USGS uses seismographs and other sensors buried in the ground throughout California to make a “lightning fast” detection when the earth starts to move, said Robert de Groot, a team lead for the agency’s ShakeAlert system.
The sensors convey data to a processing centre, which helps scientists determine the strength and potential scale of the quake.
“We use about a second’s worth of data to determine what is going on,” Mr de Groot said. USGS data is quickly distributed to Google and various earthquake apps to help warn residents.
In the case of larger quakes, like the one that struck Southern California on Monday morning, officials send a notification through the wireless emergency alert system to ensure it reaches anyone who could be in harms way.
On Monday, that included Mr de Groot, who felt the earthquake in Pasadena, 130 miles north of the epicentre.
He saw the alert, and quickly took cover under a table until the rattling stopped.
“Nobody likes the ground shaking,” he said, “even though earthquakes are my day job.”