WASHINGTON - An earthquake with a magnitude of 6.0 hit rocked northern California Sunday, damaging historic buildings and injuring dozens of people, including a young child critically injured by a crumbling fireplace, according to the U.S. Geological Survey.

California Governor Jerry Brown declared a state of emergency after the South Napa Earthquake, whose epicentre was located about six miles south-southwest of Napa, California, and 51 miles west-southwest of the state capital, Sacramento. Officials have variously referred to the earthquake’s magnitude as 6.0 and 6.1.

Several buildings in Napa were damaged or on fire in the wake of the earthquake, ABC News station in San Francisco reported.

‘We had multiple structure fires that we’ve been dealing with - a total of about six,’ John Callanan of the Napa Valley Fire Department told reporters. ‘In one of those incidents, it involved approximately six mobile homes together, so that was one single incident. Four of the six are completely damaged. The other two have suffered some major damage.’

Eighty-seven people were being treated at Queen of the Valley Medical Centre in Napa, Napa City Fire’s Darren Drake told ABC News.

Three of those people were considered critical, including the child hurt by the fireplace, who was flown to a neuro-centre for further treatment, Callanan said. The city of Napa’s website also cited dozens of reported gas line and water main leaks, though officials said the water remained safe to drink.

Napa County Supervisor Bill Dodd said he believed the county courthouse had been retrofitted for earthquakes. ‘I’ve been through a few of these and I’ve never seen anything like this, particularly in downtown Napa,’ Dodd said, according to ABC News Radio.

‘The county building is just in total disrepair, and they’ve moved it down to the sheriff’s office - so that’s where the coordinating all the emergency services for the county.’ President Barack Obama was briefed on the earthquake Sunday, a White House spokesman said. Dozens living in the region reported falling dishes and violent shaking inside their homes.