Residents of southern California took part in what organisers say is the biggest-ever earthquake drill in the United States.
Five million people were expected to become involved in Thursday's exercise. Schools, hospitals and businesses took part, as well as rescue workers.
Participants across the region followed instructions to "drop, cover and hold on" - drop to the ground, take cover under a sturdy desk and hold on until the shaking stops.
The drill is based on the hypothetical scenario of a magnitude 7.8 earthquake striking the southern portion of the San Andreas Fault.
This part of the fault usually moves every 150 years, but it has not shifted in about 300 years, the BBC reports.
California is overdue for a major earthquake and experts believe there is a 99% chance that a magnitude 6.7 or stronger quake will shake California in the next three decades.
Organisers say 300 scientists have worked together on the scenario for Thursday's drill.
The imagined earthquake would, they say, cause 2,000 deaths, 50,000 injuries and $US200 billion in damage.
California quake-prone
California is one of America's most earthquake-prone states.
The last major earthquake to strike California was in 1994, when a quake measuring 6.7 on the Richter scale left 57 people dead.
The great San Francisco earthquake of 1906 was about 7.8 and caused an estimated 3,000 deaths in collapsed buildings and fires.
