1906 San Francisco earthquake

The San Francisco earthquake of 1906 was a major earthquake at San Francisco, California, estimated to be approximately 7.8 on the Richter Scale [1], on the early morning of Wednesday, April 18, 1906. Foreshocks and the main quake occurred at about 5:12am along the San Andreas Fault, with an epicenter close to the city. Its violent shocks were felt from Oregon to Los Angeles, and inland as far as central Nevada. The earthquake and fire would go down as one of the worst natural disasters to hit a major city in United States history (see also the Galveston Hurricane, and Hurricane Rita and Hurricane Katrina).

At the time only 478 deaths were reported, a figure concocted by government officials who felt that reporting the true death toll would hurt real estate prices and efforts to rebuild the city. This figure has been revised to today's conservative estimate of 3000+. Some have put it as high as 6000. Most of the deaths occurred in San Francisco but 189 were reported elsewhere across the San Francisco Bay Area. Other places in the Bay Area such as Santa Rosa, San Jose, and Stanford University also received severe damage.

Between 225,000 and 300,000 people were left homeless, out of a population of about 400,000. Half of these refugees fled across the bay to Oakland. Newspapers at the time described Golden Gate Park, the Panhandle, and the beaches between Ingleside and North Beach as covered with makeshift tents.

The earthquake's notoriety rests in part that it was the first natural disaster of its magnitude to be captured by photography. Further, it occurred at a time when the science of seismology was blossoming. The overall cost of the damage from the earthquake was estimated at the time to be around $400,000,000.

Subsequent fires

As damaging as the earthquake and its aftershocks were, the fires that burned out of control afterwards destroyed much more property. Fires broke out in many parts of town, some initially fueled by natural gas mains broken by the quake. Other fires were the result of arson, and campfires set by refugees. Some property owners set fire to their damaged buildings, because most insurance policies covered fire losses while prohibiting payment if the building had only sustained earthquake damage. Captain Leonard D. Wildman of the U.S. Army Signal Corps reported that he "was stopped by a fireman who told me that people in that neighborhood were firing their houses...They were told that they would not get their insurance on buildings damaged by the earthquake unless they were damaged by fire."

As water mains were also broken, the city fire department had few resources to fight the fires with. Several fires in the downtown area merged to become one giant inferno. 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. The fire ultimately destroyed over 500 city blocks of the downtown core from Van Ness Avenue, an arterial thoroughfare that bisects the center of the city, to the docks at the San Francisco Bay.

It was erroneously reported that the mayor Eugene Schmitz and General Frederick Funston declared martial law. Schmitz did, however, issue an edict allowing police and troopers to shoot looters on sight, and some 500 people were shot. They tried to bring the fire under control by detonating blocks of buildings around the fire to create fire breaks, but the black powder they used often set the ruins on fire.

Relocation and housing of displaced

The US Army built 5,610 redwood and fir "relief houses" designed by John McLare to accommodate 20,000 displaced people. They were grouped in eleven camps, packed close to each other and rented to people for 2 dollars per month until rebuilding was completed. They were painted olive drab, partly to blend in with the site, and partly because the military had large quantities of olive drab paint on hand. The camps had a peak population of 16,448 people, but by 1907 most people had moved out of the camps, which were then re-used as garages, storage spaces or shops.

Analysis

• many important characteristic of the shaking intensity noted in Lawson's (1908) report was the clear correlation of intensity with underlying geologic conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby bedrock sites, and the strongest shaking occurred in areas where ground reclaimed from San Francisco Bay failed in the earthquake. Modern seismic-zonation practice accounts for the differences in seismic hazard posed by varying geologic conditions.

• An analysis of the displacements and strain in the surrounding crust led Reid (1910) to formulate his elastic-rebound theory of the earthquake source, which remains today the principal model of the earthquake cycle.

• The United States Geological Survey (USGS) estimates that the earthquake measured a powerful 7.9 on the moment magnitude scale. The earthquake caused ruptures visible on the surface for a length of 470 kilometers (290 miles). Modified Mercalli Intensities of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace.