San Francisco (AFP) - A 4.0-magnitude earthquake shook the San Francisco area Monday, the US Geological Survey reported, but no major damage or injuries were immediately reported.

Its epicenter was one mile north of Piedmont, about 13 miles (21 kilometers) east of San Francisco on the Hayward Fault.

"#earthquake tremors felt in #SanFrancisco. Always be prepared!" the city of San Francisco said on its Twitter page.

The quake hit at 6:49 am (1349 GMT), at a depth of 3.3 miles, USGS said

An AFP correspondent in Alameda, across the bay from San Francisco and near the epicenter, said he was awakened by a gentle rumble that ended with a sharp jolt after about five seconds.

He said picture frames rattled on the walls and that dishes bounced around, but he reported no major damage.

Another California resident living near the epicenter also felt the ground shake.

"The center was just down the street. There was no damage, it just woke us up," photographer Frederick Neema told AFP.

An AFP reporter living in San Francisco itself said she felt the quake distinctly but there was no damage in her home.

Oakland police Lieutenant Chris Bolton said on Twitter about an hour after the quake hit there were no reports of damage.

The Bay Area Rapid Transport system meanwhile reported a medical emergency and warned of widespread travel delays.

The tremblor struck almost one year after California's Napa Valley wine country was shaken by a 6.0-magnitude earthquake, the strongest to hit the region in a quarter of a century. There were no deaths reported, but some 130 people sought minor medical care.

Napa's was the strongest earthquake since 1989, when the 6.9-magnitude Loma Prieta quake in the Santa Cruz Mountains killed 63, injured 3,000 and caused widespread damage. One section of the upper span of the double-decker Bay Bridge, which connects San Francisco to Oakland, collapsed and a major viaduct in Oakland also gave way.

The Hayward Fault where Monday's tremblor occurred runs from San Pablo Bay in the north to Fremont in the south, passing through the cities of Berkeley, Oakland, Hayward and Fremont, according to the USGS.

California is the most populated state in the country, with some 38.8 million inhabitants according to US Census data, and scientists have long warned that a major tremblor could hit the earthquake-prone state.