LOS ANGELES -

California will star in a mock earthquake disaster drill, considered the largest in U.S. history, with some five million people pledging to do everything from ducking for cover to rescuing faux victims.

The exercise centres on a hypothetical magnitude-7.8 earthquake that unzips the southern San Andreas Fault -- an event that scientists call the feared "Big One."

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

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

Thursday's drill is 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.

"We're trying to make it a communal event," she said.

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

Earlier this year, the USGS 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.

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 earthquake.

That was a magnitude-8.3 temblor in Tokyo that killed more than 140,000 people.

Interest in the statewide 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.

If such a quake hit, scientists say, sections of freeways would collapse, water pipes would burst and certain high-rise buildings and older structures would fall.

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