Southern California residents awoke on Friday morning to a strong aftershock from the most powerful earthquake to hit the region in 20 years, which was felt by more than 20 million people. There have already been more than 80 smaller aftershocks since Thursday's 6.4 magnitude quake near the city of Ridgecrest, which was felt from Los Angeles to Las Vegas, seismologist Lucy Jones said. The 5.4 magnitude aftershock, the biggest so far, struck the same desert region as Thursday's earthquake. Its epicentre was about 11 miles west of Searles Valley, at just after 4am, the US Geological Survey (USGS) said. The aftershock was felt as far as Los Angeles, about 150 miles to the south. But it is the fear of what comes next that has really rattled people. The southern section of the San Andreas fault that runs near the city has not had a "mega-quake" of more than magnitude-7.5 since 1680 and "the Big One", according to seismologists, is more than a century overdue. Seismologists are collecting data from the quake site to try and establish when the next might hit Credit: Rex Ms Jones, widely known as the "earthquake lady" for her informative TV appearances, has taken a lead role in preparing the state for the "Big One" and warned there could be a bigger quake soon. Ms Jones' predictions immediately after the quake have already proved prescient. "We should be expecting lots of aftershocks and some of them will be bigger than the 3s we've been having so far," Ms Jones told reporters on Thursday. "I think the chance of having a magnitude 5 ... is probably greater than 50-50." Sure enough, a 5.4 magnitude aftershock hit the area on Friday. California earthquake So what does Ms Jones think this means for the Big One? "This does not make [the Big One] less likely. There is about a 1 in 20 chance that this location will be having an even bigger earthquake in the next few days, that we have not yet seen the biggest earthquake of the sequence," she told the LA Times. Ms Jones put it another way for 42,000 Twitter followers prior to Thursday's earthquake: If the daily probability of the Big One was 50/50, then the chance it would happen in the next week would be >99%. Real probability is about 2% per yer, or 1/20,000 each day. Your change of being in a car accident today is ~1/7,000. I still wear my seat belt every day. https://t.co/3ovnAfiPFV— Dr. Lucy Jones (@DrLucyJones) July 3, 2019 Thursday's quake is the largest in Southern California since the magnitude 7.1 Hector Mine quake struck the Mojave Desert in 1999. Water gushed from zigzagged cracks in the pavement from busted water lines. Deep fissures snaked across the Mojave Desert, with passersby stopping to take selfies while standing in the rendered earth. The quake hit the edge of Death Valley National Park about 113 miles northeast of Los Angeles at about 10.30 am on Thursday. It was very shallow, only 6.7 miles deep, amplifying its effect, and was felt in an area inhabited by 20 million people, the European quake agency EMSC said. The aftershocks to the SearlesValley earthquake show that two faults are involved. One strikes northwest and the other northeast. You can see the aftershocks at https://t.co/BwTOi7AGG2pic.twitter.com/gkDzPh4I9G— Dr. Lucy Jones (@DrLucyJones) July 4, 2019 It remains difficult for scientists to predict when the next major quake will hit. But the USGS says its models suggest "there is a small chance (about 2 percent) that [a 7.9 magnitude quake] could happen in the next 30 years". "However, this does not mean that the San Francisco Bay region will not experience damaging earthquakes in the next hundred years," the agency says. The agency also said its models for the Hayward fault, the Rogers Creek fault and the Peninsula section of the San Andreas fault suggest that smaller earthquakes - around a magnitude 7 - are also expected, and are in fact much more likely to cause serious damage than a repeat of the Great 1906 San Francisco earthquake which registered a 7.9 magnitude. "If the 19th century is an example, then we can expect one or more M7 earthquakes on Bay area faults...over the next several decades," it says.