Witnesses took photos and videos of the massive fireball that filled the sky on Friday afternoon around 2:30 PM PST, Sputniknews reported.
One of the workers appeared to be in critical condition, according to Tony Botti from the Fresno County Sheriff's Office. The Community Regional Medical Center's burn and trauma unit reported three patients in critical condition and one in serious condition from the blast. Seven others were also taken to hospitals to be treated for injuries, while four more were treated at the scene.
Highway 99 was shut down in both directions for several hours and a nearby rail line was also stopped, in fear of a passing train sparking the leaking gas.
No law enforcement officers were injured by the blaze.
Earlier this month, PG&E was hit with the state's largest ever penalty against a utility, fined $1.6 billion for a 2010 blast which killed eight people in the San Francisco suburb of San Bruno. Investigators blamed PG&E's faulty safety practices for the deadly explosion.