At 5:12 AM local time on April 18, 1906, an earthquake estimated at about 8.0 on the Richter scale hit San Francisco. At the time, the city was mostly made up of brick and wooden buildings, many of which collapsed completely. Fires broke out all across the city. In the end, it has been estimated that about 3,000 people died and 30,000 buildings were destroyed. A majority of the city's population of 400,000 was left homeless.
For more information, see Wikipedia: 1906 San Francisco Earthquake