San Francisco was jolted by a 3.9-magnitude earthquake Thursday, causing jitters but no injuries on the day California carried out an annual drill for the long-feared Big One.

The temblor's epicenter was only two miles (three kilometers) away from Berkeley and the quake was felt across San Francisco and the East Bay area, reports said.

"My hands are still shaking -- my heart is just slowing down," Krys Freeman, a Web manager for Greenbiz in Oakland, told the San Francisco Chronicle minutes after the mid-afternoon quake.

The relatively minor quake -- temblors of up to magnitude 4.0 are recorded regularly in California, but usually further away from population centers -- came hours after the state carried out its annual Great California ShakeOut.

Some 8.2 million people registered to take part in the statewide drill, with building managers announcing a major quake and asking people to get under their desks.

Earthquakes are regular events in California, mostly triggered by activity along the San Andreas Fault that runs through much of the western state, the most populous in the United States.

Geologists say a quake capable of causing widespread destruction is 99 percent certain of hitting California within the next 30 years. A magnitude 7.8 quake could kill 1,800 people, injure 50,000 more and damage 300,000 buildings.

A 6.7 earthquake in Los Angeles left at least 60 people dead and did an estimated $10 billion damage in 1994, while a 6.9 quake in San Francisco in 1989 claimed the lives of 67 people.