The risk of more earthquakes striking Southern California is "approaching certainty" according to seismologists, following a 7.1-magnitude tremor which rattled LA and caused fire and building damage near its rural epicentre.
The most recent quake hit at 8.19pm local time on Friday, 11 miles from Ridgecrest in the Mojave Desert, which was the site of Thursday's 6.4-magnitude quake.
Dr Lucy Jones, a seismologist with the California Institute of Technology, warned that Friday's tremor was part of the sequence that produced the earlier quake, saying Thursday's was a "foreshock".
Dr Jones warned there was about a 10% chance another 7.0-magnitude quake could hit in the next week, and that the chance of a 5.0-magnitude tremor was "approaching certainty".
Offices in downtown Los Angeles shook for around 30 seconds on Friday, and the quake was also felt in the Hollywood Hills, Las Vegas and parts of Mexico.
The earthquake is the strongest to hit the region in 20 years, with experts giving it an early rating of 6.9 to 7.1 on the Richter scale.
There are "significant reports of structure fires, mostly as a result of gas leaks or gas line breaks" in Ridgecrest, said Mark Ghillarducci, head of the California Office of Emergency Service (OES).
Nearly 200 people have been housed in shelters, although Mr Ghillarducci said that the damage from the back-to-back earthquakes was not as extensive as expected.
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The governor of California, Gavin Newsom, has declared a state of emergency and placed the state's OES on its highest level of alert.
There have been minor injuries but no reports so far of anyone seriously hurt or killed.
Some California residents posted videos on social media of their swimming pools sloshing around as the tremors struck.
Disneyland in Orange County and Six Flags Magic Mountain in Santa Clarita closed their rides, and a NBA Summer League game in Las Vegas was stopped.
Celebrities in the area also reacted to the quake, with actor Stephen Fry tweeting: "Whoa! Was that aftershock just now or a whole new quake? 8:20pm #Earthquake."
"Certainly felt it here in the Hollywood Hills," added Fry.
Rapper Nicki Minaj posted "That earthquake was intense", while Big Bang theory star Kaley Cuoco posted a video of her chandelier shaking and called the quake "freaky".
Reality TV star and make-up entrepreneur Kylie Jenner tweeted "These earthquakes" next to two anguished-looking emojis.
Amy Lee from Middlesbrough, who is on holiday in the area, told Sky News: "We're in Torrance at an Airbnb cottage and the entire room felt like it started rocking! It was like the room was swaying, the ceiling light started shaking and flickering."
President Donald Trump has issued an emergency declaration so California can get federal aid.
Hundreds of aftershocks have jolted the area since Thursday's initial earthquake and experts say they could continue for three years.