A powerful magnitude 6.4 earthquake — Southern California’s strongest in decades — rattled a large swath of the state and parts of Nevada on Thursday, touching off house fires and damaging buildings.

It struck at 10:33 a.m. PT in the Mojave Desert, near the city of Ridgecrest, 320 kilometres northeast of Los Angeles, the U.S. Geological Survey (USGS) said.

The USGS said the quake, initially reported as a magnitude 6.6, was shallow — only 8.7 kilometres deep — which would have amplified its effect.

Veteran seismologist Lucy Jones said it was the most powerful earthquake Southern California has seen in 20 years. The previous large earthquake was of magnitude 7.1 and struck in the area on Oct. 16, 1999.

Jones told reporters at the California Institute of Technology in Pasadena that the 6.4 quake was preceded by a magnitude 4.3 quake a half hour earlier.

A fire is seen following an earthquake in Ridgecrest, Calif., on Thursday in this still image taken from social media video. Kern Country fire Chief David Witt said some people suffered minor injuries as a result of the quake. (Ben Hood/Reuters)

The quake was felt as far as Los Angeles and was quickly followed by several smaller aftershocks in the area.

People from Las Vegas to the Pacific coast reported feeling a rolling motion that shook shower doors and rocked dining room lights.

Fires, broken gas lines

Officials in Southern California said emergency crews were responding to at least 24 medical and fire incidents in the aftermath of the earthquake.

The Kern County Fire Department sent search and rescue teams to Ridgecrest to deal with two house fires, small brush fires, and gas leaks. The city has a population of just under 29,000, according to 2018 U.S. census data.

“We feel we are going to have the upper hand on this,” said Kern County fire Chief David Witt. “We don’t know the exact number of injuries but so far they have been minor.”

California Gov. Gavin Newsom approved an emergency proclamation, and Ridgecrest Mayor Peggy Breeden declared a state of emergency, a step that enables the town to receive help from outside agencies.

Ridgecrest Regional Hospital was being evacuated, and about 15 patients were moved to other locations, according to local media, for fear of a powerful aftershock.

Breeden asked residents to check on their neighbours, particularly the elderly who make up a significant portion of the town’s population.

“They’re all doing great,” she said. “We’re a close-knit community and everybody is working to take care of each other.”

A 6.4 magnitude earthquake that struck near Ridgecrest, Calif., on Thursday was felt as far as Los Angeles, about 320 kilometres away. (CBC)

Ridgecrest may not get much respite in the days ahead. Jones, the seismologist, said more than 80 aftershocks had hit the area in the hours since the initial quake.

“We should be expecting lots of aftershocks and some of them will be bigger than the [ones] we’ve been having so far,” she told a news conference.

“I think the chance of having a magnitude 5 … is probably greater than 50-50.”

911 only for emergencies: officials

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 Los Angeles Police Department said in a statement published on Twitter.

Ashleigh Chandler, a helicopter rescue emergency medical technician at Fort Irwin, Calif., which is southeast of Ridgecrest, said the quake happened as she was getting ready for a July Fourth 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,” she said.

“My sister 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,” adding she and her family are OK.

Other people shared photos of the damage on Twitter.

Meanwhile, U.S. President Donald Trump tweeted that he’d been briefed on the event.

California is on the so-called Pacific Ring of Fire, a 40,000-kilometre area that hosts more than 400 volcanoes and is prone to earthquakes.