Millions of people across southern California have been taking part in what organisers say is the biggest-ever US earthquake drill.
Schools, hospitals and businesses joined in the exercise known as "The Great Southern California Shake-out".
The drill was based on the hypothetical scenario of a magnitude 7.8 earthquake striking the southern portion of the San Andreas Fault.
It was devised by scientists to help people prepare for future earthquakes.
At precisely 1000 local time (1800GMT), when the earthquake is supposed to have struck, millions of participants followed instructions to "drop, cover and hold on" - drop to the ground, take cover under a sturdy desk and hold on until the shaking stopped.
They were guided by a public service message sent to buildings and broadcast by television and radio stations.