The 1906 San Francisco earthquake
On 18 April 1906, at 5.12am, a major earthquake struck San Francisco. Up to 3500 people were killed and hundreds of buildings destroyed, many by the fires that broke out across the city after gas pipelines were ruptured.
The quake was caused by a modest-sounding movement of about 7m on the San Andreas Fault, probably in the Pacific to the west of the city. It was so large that local seismometers went off-scale, but it is now thought to have measured about 7.8 on the Richter scale. The shock waves from the quake did most of the damage by turning the soil underlying the city to liquid, a process called solifluction which occurs when soil containing water is shaken and turns to a liquid of porridge-like consistency. Today, the city's population is ten times what it was then, but safety standards for buildings have improved massively.