At ten o’clock yesterday morning more than five million Americans dived to the floor and cowered under their desks.

It was not a response to the latest economic news or to a new plunge in the Dow Jones Industrial Average.

It was the largest earthquake drill in American history — a $2 million (£1.3 million) event to help to prepare Southern California for the magnitude 7.8 quake predicted by scientists: “the Big One”.

According to government estimates, the Big One will cause 1,800 deaths and about $200 billion in damage. Towns such as Portola Valley, a community of 4,500 directly on top of one of the most dangerous sections of the San Andreas Fault line, will be ripped in two.

Hence the effort to include as many people as possible, with local councils, schools, hospitals, churches, businesses, and private citizens taking part. “We’re trying to make it a communal event,” Lucy Jones , a seismologist with the US Geological Survey, said.

Earlier this year the USGS calculated that California faced a 46 per cent chance of being hit by a magnitude 7.5 or larger earthquake in the next 30 years, with the epicentre in Southern California. Yesterday’s drill — dubbed “The Great Southern California Shake-Out” — began with a radio, television and internet broadcast.

“This is an earthquake drill,” a calm male voice said, to a rumbling soundtrack of what sounded like the engines of the Starship Enterprise.

“Right now, drop, cover, and hold on,” it went on. “Drop to the ground! Take cover by getting under a sturdy desk or table. Hold on to it until the shaking stops. If you cannot get under sturdy furniture, protect your head and neck with your arms. If you’re indoors, stay indoors — if you’re outdoors, stay outdoors. Injuries are more likely when you try to move around.

“If this were the magnitude 7.8 shake-out earthquake we’re practising for, you would be experiencing sudden and intense back and forth motions of up to 6ft per second. The floor or the ground would jerk sideways, out from under you. Every object around you would likely become airborne, potentially causing serious injury.”

In spite of the potential for a catastrophic earthquake, California has never been as disciplined at practising for the inevitable as Japan — which holds an annual drill to mark the 1923 Great Kanto earthquake, one of magnitude 8.3 in Tokyo that killed 140,000.

Some Californians regard earthquake drills in the way that they used to regard Cold War nuclear attack drills — pointless — because they predict that few would survive such a turn of events.

In the event of a magnitude 7.8 earthquake, scientists say that sections of LA’s freeway system would almost certainly collapse and some skyscrapers would also topple. During the infamous 1906 earthquake in San Francisco most of the 700 to 3,000 fatalities were caused by fires from broken gas mains.

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