"We will continue to have a lot of aftershocks," seismologist Dr. Lucy Jones warned at a news conference following the quake.
A magnitude 5 aftershock is likely, Jones said.
The Fourth of July quake, which struck at about 10:30 a.m. local time, was centered in a remote area near Searles Valley in the Mojave Desert, about 150 miles northeast of Los Angeles.
In Kern County, the quake has sparked fires as well as evacuations at Ridgecrest Regional Hospital and some apartment buildings, officials said.
The mayor of Ridgecrest confirmed to ABC News that they have declared a state of emergency in the city.
So far, 86 earthquakes have occurred just northeast of Ridgecrest, according to the National Weather Service office in Las Vegas.
Stacy Bindschatel, a nurse who takes x-rays at Ridgecrest Regional Hospital, told ABC News the quake sent computer there flying to the ground.
"All the cupboards and doors were opening. So, immediately I thought, 'this is really happening, it's not stopping. And so we came outside because we didn’t know if there was going to be structural damage and anything falling down," she told ABC News. "We stayed outside, there was a couple more after effects, we kept feeling the trembles. We waited until there was no trembles happening for quite a while before we went inside and were stilling the after effects."
Bindschatel said they've seen multiple patients with injuries including those who had objects fall on them.
“Our kitchen is completely destroyed. Everything fell of the walls, out of the cabinets," said Sara Beren, park mangaer at Trousdale Estates in Ridgecrest.
Residents in Ridgecrest also saw severe damage to their homes.
Homeowner Charlotte Sturgeon said the foundation of her house is completely off and is leaning in one direction. The floor has also dropped in her front room.
"This can’t be saved, I’m pretty sure," Sturgeon said.
In San Bernardino County, the quake broke water mains, downed power lines, cracked buildings and caused rock slides on roads, officials said. No injuries or fires were reported in San Bernardino County, officials added.
Los Angeles saw no injuries or significant damage, the fire department said.
But Angelenos noted how rare it was to feel such a long quake.
Director Ava DuVernay tweeted, "Been living in Los Angeles all my life. That was the longest earthquake I’ve ever experienced. Not jerky. Smooth and rolling. But it was loooong. It was so long I thought for the first time ever 'Is this the big one?'"
"This was a strong one," the LAPD tweeted, "and a good reminder to be prepared."
A tweet from the Los Angeles International Airport said no runways were damaged and the airport was operating as normal.
The quake was even felt around Las Vegas, though police there said no damage was reported.
A tsunami is not expected, according to the National Tsunami Warning Center.
"This is an isolated enough location that that's going to greatly reduce the damage," Jones said at a news conference.
She added, "This is a large enough earthquake that it probably did break through to the surface of the earth."