The 6.4 magnitude quake struck at 10:33 a.m. in the Mojave Desert, about 150 miles (240 kilometers) northeast of Los Angeles, near the town of Ridgecrest, California.
Multiple injuries and two house fires were reported in the town of 28,000. Emergency crews were also dealing with small vegetation fires, gas leaks and reports of cracked roads, said Kern County Fire Chief David Witt.
He said 15 patients were evacuated from the Ridgecrest Regional Hospital as a precaution and out of concern for aftershocks.
Kern County District Supervisor Mick Gleason told CNN there were some structural issues with the hospital and some patients had to be moved from one ward to another and that others were taken to a neighboring building.
Gleason did not say what the structural issues were.
Ridgecrest Mayor Peggy Breeden said that utility workers were assessing broken gas lines and turning off gas where necessary.
The local senior center was holding a July 4th event when the quake hit and everyone made it out shaken up but without injuries, she said.
"Oh, my goodness, there's another one (quake) right now," Breeden said on live television as an aftershock struck.
Gov. Gavin Newsom declared a state of emergency for Kern County. The declaration means that the state will help the county and municipalities in it with emergency aid and recovery efforts.
Ridgecrest Mayor Peggy Breeden praised Newsom for declaring the emergency. She also noted at a news conference that other nearby governments have offered to help the recovery effort.
President Donald Trump said he was fully briefed on the earthquake and that it "all seems to be very much under control!"
Police and fire officials said at a news conference Thursday afternoon that they have enough resources so far to meet needs in the wake of the earthquake. Ridgecrest Police Chief Jed McLaughlin said at a news conference that "we have plenty of resources."
California Highway Patrol Lt. John Williams says officials have found cracks on several roads in the county, but overpasses and underpasses are in good shape.
A series of aftershocks included a 4.5 magnitude temblor, according to the United States Geological Survey.
"It almost gave me a heart attack," said Cora Burke, a waitress at Midway Cafe in Ridgecrest, of the big jolt. "It's just a rolling feeling inside the building, inside the cafe and all of a sudden everything started falling off the shelf, glasses, the refrigerator and everything in the small refrigerator fell over."
Video posted online of a liquor store in Ridgecrest showed the aisles filled with broken wine and liquor bottles, knocked down boxes and other groceries strewn on the floor. Flames were seen shooting out of one home in the community.
Lucy Jones, a seismologist with the California Institute of Technology's seismology lab, said the earthquake was the strongest since a 7.1 quake struck in the area on October 16, 1999.
"This has been an extremely quiet abnormal time," Jones said. "This type of earthquake is much more normal ... The long term average is probably once every five or 10 years somewhere in Southern California."
Jones said that the 6.4 quake centered near the town of Ridgecrest was preceded by a magnitude 4.2 temblor about a half hour earlier.
She said vigorous aftershocks were occurring and that she wouldn't be surprised if a magnitude 5 quake hit but that they were striking in a remote area, sparsely populated area. "This is an isolated enough location that that's going to greatly reduce the damage," she said.
People from Las Vegas to the Pacific Coast reported feeling a rolling motion and took to social media to report it.
Local emergency agencies also took to social media to ask people to only call 911 for emergencies.
"We are very much aware of the significant earthquake that just occurred in Southern California. Please DO NOT call 9-1-1 unless there are injuries or other dangerous conditions. Don't call for questions please," the LAPD said in a statement published on Twitter.
There were no reports of serious damage or injuries in Los Angeles, the department said.
The quake was detected by California's new ShakeAlert system and it provided 48 seconds of warning to the seismology lab well before the shaking arrived at Caltech in the Los Angeles suburb of Pasadena but it did not trigger a public warning through an app recently made available in Los Angeles County.
USGS seismologist Robert Graves said the ShakeAlert system worked properly.
Graves said it calculated an intensity level for the Los Angeles area that was below the threshold for a public alert. The limits are intended to avoid false alarms.
Ashleigh Chandler, a helicopter rescue EMT at Fort Irwin, California, said the quake happened as she was getting ready for a July 4th party.
"I was just in the living room getting everything ready, we start to feel the shaking, so then I look up and then the wine bottles start rattling and I thought, 'They're going to fall.'
"My stepson was in the house and my dog, so we just got everyone outside and then it ended. It was like 15, 20 seconds, maybe. It was pretty good shaking, so I'm out of breath."
"Everyone's OK."
Glenn Pomeroy, the head of California's Earthquake Authority, said the earthquake is "an important reminder that all of California is earthquake country."
Pomeroy urged the estimated 2,000 people in the region hit by the quake who have California Earthquake Authority insurance policies to contact the agency as soon as possible. Information on how to file a CEA policy claim is on the agency's website: EarthquakeAuthority.com, he said.