On April 18, 1906, a devastating earthquake struck San Francisco. Its epicenter was a few miles from the city, but at an estimated magnitude of 7.9, it was a severe blow to what was the largest west coast city of its time. The quake was felt as far north as Oregon and as far east as Nevada. A massive fire broke out as a result of the earthquake, and the fire further devastated San Francisco, ultimately making as many as 300,000 people homeless (in a city whose population was 410,000 at the time).
Many photographs of the earthquake damage and fire exist, but movies are relatively rare. Edison shot some, and another important motion picture is simply called San Francisco earthquake and fire, April 18, 1906. It shows some of the cleanup efforts in the still-smoldering ruins; this silent film has been preserved by the Library of Congress. In 2009, an assemblage of motion pictures (apparently including new footage) was unveiled showing the aftermath. The first part is the most interesting: a drive down main city streets, which are covered with ash and dust. There are some intact buildings, and even streetcars running. People wander among the debris, dressed in fine suits and dresses -- it seems that many are there just to witness the destruction, some are headed somewhere (to get food?), and a very few are already rebuilding. It's mesmerizing watching these people in the ruins of their city. Later footage shows the tent camps that appeared in Oakland and other nearby regions. Take a look:
Also of note: the excellent Wikipedia page on the earthquake, and a comprehensive program showing archivist Rick Prelinger's footage of San Francisco, including a very similar scene before the destruction (1905), and a bit of similar post-quake footage, though it looks a bit later than the film above. Prelinger's program also includes 1917 footage -- it's amazing to see how much has been reconstructed by that time.