The earthquake's fame rests in part on the fact that it was the first natural disaster of its size to be captured by photography. Further, it occurred at a time when geology and seismology were just blossoming, and led to the formation of the elastic-rebound theory of earthquake source.
The first foreshock was registered at 5:12 a.m. on April 18, 1906. The quake, which occurred on the San Andreas Fault, lasted up to 60 seconds, and was felt from Oregon to Los Angeles, and inland as far as central Nevada.
The United States Geological Survey estimates that the earthquake had an intensity of up to 7.9 on the movement-magnitude scale, which is very large. Estimates of the size of historical earthquakes, however, are subject to debate. The U.S.G.S. says the earthquake caused ruptures visible on the surface for a length of 470 kilometers (290 miles). By comparison, it says, the 1989 Loma Prieta earthquake had a rupture length of about 40 kilometers (25 miles).
As damaging as the earthquake and its aftershocks were, the fires which burned out of control afterwards destroyed much more property. Fires broke out in many parts of town initially fueled by natural gas mains breaking; as water mains were also broken the city fire department had few resources to fight the fires. One journalist at the time wrote that readers elsewhere should understand that it was not a fire in San Francisco, but rather a fire of San Francisco.
General Frederick Funston declared martial law, and finally got the fire under control by dynamiteing blocks of buildings around the fire to create fire breaks.
More Historic photos of 1906 San Francisco earthquake