A 6.4 magnitude earthquake rattled Southern California on July 4, and a series of aftershocks followed. Experts said this earthquake was the strongest to hit Southern California in 20 years.

The initial earthquake hit at around 10:33 a.m. local time and hit 7 miles southwest of Searles Valley, 8 miles west-southwest from Trona, and 11 miles east-northeast from Ridgecrest, according to the U.S. Geological Survey (USGS).

After that quake, at least 30 aftershocks hit over the span of about an hour, with the largest registering 4.7 on the Richter scale used to determine the strength of seismic activity, according to the USGS’s map.

President Trump tweeted, “Been fully briefed on earthquake in Southern California. All seems to be very much under control!”

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, Kern County Fire Chief David Witt said.

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.

Several people reported that they felt the shaking. However, there were no immediate reports of injuries or damage from the quakes.

“It almost gave me a heart attack,” said Cora Burke, a waitress at Midway Cafe in Ridgecrest, a town of 28,000 people. “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.”

Veteran seismologist Lucy Jones said the earthquake on Thursday was the strongest to hit Southern California in 20 years.

The previous large quake was a 7.1 and that struck in the area on Oct. 16, 1999, she said.

Jones told reporters at the California Institute of Technology in Pasadena, California, that the 6.4 quake centered in the Mojave Desert near the town of Ridgecrest was preceded by a magnitude 4.3 temblor about a half hour earlier.

She said there was a vigorous aftershock sequence occurring and that she wouldn’t be surprise if a magnitude 5 quake occurred during the aftershocks.

“We should be expecting lots of aftershocks,” 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 911 unless there are injuries or other dangerous conditions. Don’t call for questions please,” the LAPD said in a statement published on Twitter.

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.”

A significant earthquake struck Southern California on July 4, 2019, and a series of aftershocks followed. (USGS)
Merchandise lies on the floor at a Family Dollar store seen through a window after an earthquake Thursday, July 4, 2019, in Trona, Calif. A strong earthquake rattled a large swath of Southern California and parts of Nevada on Thursday, rattling nerves on the July 4th holiday and causing some damage in a town near the epicenter, followed by a swarm of aftershocks. (AP Photo/Matt Hartman)
A road is damaged from an earthquake Thursday, July 4, 2019, in Trona, Calif. A strong earthquake rattled a large swath of Southern California and parts of Nevada on Thursday, rattling nerves on the July 4th holiday and causing some damage in a town near the epicenter, followed by a swarm of aftershocks. (AP Photo/Matt Hartman)
Utility poles are damaged from an earthquake, Thursday, July 4, 2019, in Trona, Calif. A strong earthquake rattled a large swath of Southern California and parts of Nevada on Thursday, rattling nerves on the July 4th holiday and causing some damage in a town near the epicenter, followed by a swarm of aftershocks. (AP Photo/Matt Hartman)
Shattered glass lies on the floor at a vacant business after an earthquake, Thursday, July 4, 2019, in Trona, Calif. A strong earthquake rattled a large swath of Southern California and parts of Nevada on Thursday, rattling nerves on the July 4th holiday and causing some damage in a town near the epicenter, followed by a swarm of aftershocks. (AP Photo/Matt Hartman)
Pipes are damaged from an earthquake, Thursday, July 4, 2019, in Trona, Calif. A strong earthquake rattled a large swath of Southern California and parts of Nevada on Thursday, rattling nerves on the July 4th holiday and causing some damage in a town near the epicenter, followed by a swarm of aftershocks. (AP Photo/Matt Hartman)

Includes reporting from The Associated Press.