Mag 5.1 earthquake hits San Francisco Bay

A magnitude 5.1 earthquake has rattled the San Francisco Bay Area, causing delays of some commuter trains. There were no immediate reports of damage or injuries.

The US Geological Survey said the 11.42 am quake was centred 19 kilometres) east of San Jose at a depth of about six kilometres. The area is hill country about 65km southeast of downtown San Francisco.

Lucy Jones, a veteran California seismologist, told KNTV-TV the quake happened on the Calaveras fault, one of eight major faults in the Bay Area.

"The Calaveras fault is one that tends to have smaller earthquakes," Jones said.

It was the largest earthquake in the Bay Area since a magnitude 6.0 jolt in the Napa wine country in 2014, Jones said in a social media post.

The 220km-long Calaveras fault is a major branch of the San Andreas fault and runs from San Juan Bautista in the south to San Ramon in the north, Jones said.

Rich Constantine, the mayor of Morgan Hill, a city next to San Jose, said he was in the kitchen of his home when the "long and steady" quake struck.

"We had a frame in the house fall, everything was shaking but once it stopped, there was no damage," he said.

Constantine said Morgan Hill's City Hall and other city offices were evacuated but that everyone returned to work soon after.

People reported feeling the quake as far south as the scenic Big Sur coast, 120km south of the epicentre in the region of Joseph Grant Ranch County Park, a huge natural area.

A 3.1 aftershock followed about five minutes later, according to the US Geological Survey.

Several commuter train companies, including Cal Train and the Bay Area Rapid Transit system, known as BART, held trains to check for damage. BART returned to normal service by early afternoon.