About five million Californians ducked for cover, held onto furniture and rescued false victims covered in fake blood as part of the largest mock earthquake drill in U.S. history, held Thursday to prepare for the feared Big One.

The exercise centred on a hypothetical magnitude-7.8 earthquake that unzips the southern San Andreas Fault in California, an event dubbed the Big One by scientists.

Such a quake would likely cause about 1,800 deaths and $200 billion US in damage.

At exactly 10 a.m. PT, the start of the drill, television news programs announced there was an earthquake disaster drill, then cut to cameras in school classrooms showing children ducking under their desks and holding onto them.

"Right now the simulated earthquake has taken place," said the CBC's Steve Futterman, reporting from one of the drills outside a football field in Los Angeles.

"The shaking, if it was a real earthquake, would just be beginning to calm down right now ... and we're having the make-believe situations taking place throughout Southern California, primarily  for first responders, firemen, policemen, medical personnel."

Some participants staged full-scale exercises complete with search-and-rescue missions and medical triaging of people posing as casualty victims.

"Obviously, roads will not be destroyed in this make-believe earthquake, but in the simulated earthquake they are closing roads," said Futterman.

"So although you can't recreate the real thing exactly, they just want ... these first responders to understand what they would possibly have to go through."

If a 7.5-magnitude quake or greater hits California, scientists say, sections of freeways would collapse, water pipes would burst and certain highrise buildings and older structures would fall.

The state's previous simulated quake catastrophes were smaller in scale with the leading actors mainly first responders and cities testing their emergency preparedness.

Governments, schools, residents involved

Thursday's drill was more of an ensemble cast with governments, schools, hospitals, churches, businesses and residents promising to do their part.

U.S. Geological Survey seismologist Lucy Jones spent the past year organizing the drill, and said there were attempts to make it more of a "communal event."

Scientists estimate it cost $2 million to organize and publicize the undertaking.

Scientists plan to follow up the drill with a Get Ready Rally in downtown Los Angeles on Friday, featuring earthquake preparedness information, presentations by state and city government leaders, vendors and live entertainment.

California is the most seismically active state in the Lower 48.

Earlier this year, the Geological Survey calculated the state faces a 46 per cent chance of being hit by a 7.5 or larger quake in the next 30 years, with the epicentre likely in Southern California.

Interest initially low

Despite the known seismic risks, California has never been as organized as Japan, which holds an annual quake drill to mark the 1923 Great Kanto 8.3-magnitude earthquake in Tokyo that killed more than 140,000 people.

Interest in the California-wide exercise was initially low, Jones said, but peaked after the state was jolted by a moderate quake this summer.

Though a far cry from the Big One, the July magnitude-5.4 temblor centred in the hills east of Los Angeles was the strongest to rattle a populated area of southern California since the 1994 Northridge disaster.

After the shaking stopped, 400 new people signed up for the drill, Jones said.

With files from the Associated Press