1906 San Francisco earthquake and fire

At about 5:12 a.m. local time on 18 April 1906, a major earthquake struck the San Francisco Bay region. The U.S. Geological Survey describes rupture along roughly 296 miles (477 kilometres) of the San Andreas Fault, from near San Juan Bautista to Cape Mendocino. Strong shaking was felt from southern Oregon to south of Los Angeles and inland into Nevada. Geological conditions influenced the severity of shaking: sediments and areas of reclaimed land could experience stronger motion than nearby bedrock.

The earthquake damaged buildings and infrastructure, including water lines, while broken gas mains and other sources ignited fires. The fires burned for about three days and destroyed large areas of San Francisco. The earthquake also caused severe damage elsewhere along the rupture. Because the initial official death toll was incomplete, later estimates vary: the USGS notes more than 3,000 deaths directly or indirectly from the catastrophe, while early official counts and later estimates differed substantially. Around 225,000 people were left homeless, according to USGS historical material.

Residents faced shortages of water, food and shelter. The U.S. Army and local authorities helped organize relief, while people displaced by the disaster sheltered in parks and other open spaces. Congress approved emergency aid for food, water, tents, blankets and medical supplies and provided funds to reconstruct public buildings. Recovery involved rebuilding the city while confronting the risks exposed by the destruction of water infrastructure and the spreading fires.

The 1906 earthquake supplied unusually extensive observations of fault rupture and ground movement. Analysis of displacement and strain contributed to Harry Fielding Reid's elastic-rebound theory, which explains earthquakes as the release of accumulated elastic strain along faults. The earthquake's scale and recorded effects also helped establish the San Andreas Fault as a major structure in California's geology and remain central to modern earthquake science and hazard planning.