The magnitude-5.4 aftershock, the biggest so far, struck the same desert region as Thursday’s earthquake.

A strong aftershock has hit Southern California as residents were still assessing the damage from the strongest earthquake in the region in 25 years on July 4, which was felt by more than 20 million people.

Friday’s magnitude-5.4 aftershock, the biggest so far, struck the same desert region as Thursday’s earthquake. Its epicentre was about 18km west of Searles Valley, the US Geological Survey said.

The aftershock was also felt in Los Angeles, about 240km to the south, and the surrounding area, with many residents posting on Twitter that they were awakened by it.

There had already been more than 80 smaller aftershocks since Thursday’s magnitude-6.4 earthquake near the city of Ridgecrest, which was felt from Los Angeles to Las Vegas, USGS seismologist Lucy Jones said.

“We should be expecting lots of aftershocks and some of them will be bigger than the 3s we’ve been having so far,” Jones told reporters on Thursday. “I think the chance of having a magnitude 5 … is probably greater than 50-50.”

And yes, for those of you keeping score, @DrLucyJones did reveal the likelihood of an #aftershock with a magnitude of about 5.0. pic.twitter.com/6hcujdESVr — Austin Dave (@AustinDave_) July 5, 2019

Only a few injuries were reported in Thursday’s earthquake, but two houses caught fire due to broken gas pipes, officials said.

Water gushed from zigzagged cracks in the pavement from burst water lines. Deep fissures snaked across the Mojave Desert.

People clean up broken bottles at a store after the powerful earthquake that struck Southern California on July 4 [David McNew/Reuters]

The quake sent 30 residents of the desert community of 28,000 to emergency shelters, knocked some houses off their foundations and left many homeowners wondering if their insurance would cover the damage, Ridgecrest Mayor Peggy Breeden said.

“The most important thing is that we have not had any loss of life or any major personal damage to people,” Breeden told CNN on Friday.

Breeden said officials were still assessing the extent of the damage, but added that emergency state and federal aid would enable the city’s overwhelmed agencies to start the cleanup without worrying about their budgets.

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The Ridgecrest Regional Hospital, from which 15 patients were evacuated earlier, appeared intact – apart from some new cracks in the walls.

Thursday’s quake was the biggest in Southern California since the 1994 magnitude-6.6 Northridge earthquake, USGS geophysicist Paul Caruso said. That earthquake, which was centred in a heavily populated area of Los Angeles, killed 57 people and caused billions of dollars of damage.