A 4.0-magnitude earthquake shook the San Francisco area Monday, the US Geological Survey reported.

The tremor's epicenter was one mile north of the city 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.