Many Californians dived under desks and tables early yesterday in a major drill of the "drop, cover and hold on" earthquake survival skill.
Organisers said 9.5 million Californians signed up to take part in the drill at 10:17am local time and another 15 million in earthquake-prone regions elsewhere in the world also participated.
The Great ShakeOut was first held in California in 2008 and participation has since spread around the globe.
More than 900 firefighters on duty at 106 fire stations around Los Angeles conducted the drop drill, then drove around their districts to practise surveying for damage and assessing need for rescues, said department spokesman Brian Humphrey.
California has not experienced a devastating quake since the 1994 Northridge disaster that killed 60 people and injured more than 7,000 in metropolitan Los Angeles.
Thursday was also the 24th anniversary of the 1989 Loma Prieta disaster, a magnitude-6.9 earthquake in San Francisco that killed 63 people, injured almost 3,800 and caused up to $10bn (€7.3bn) in damage.
Last month, California Governor Jerry Brown approved a law directing state emergency officials to fund a state-wide quake early warning system by 2016.