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The San Francisco earthquake
A quake and fire destroy much of the city
A major earthquake on the San Andreas Fault struck San Francisco, and fires burned for days. Broken water mains hindered the firefighting.
The earthquake struck just after 5 a.m. on 18 April 1906 when a long stretch of the San Andreas Fault ruptured, and shaking was felt across much of California. Its magnitude is usually given as about 7.9.
Most of the destruction came afterwards. Fires started in damaged buildings, and the quake had broken many of the city's water mains, so firefighters often had little water. The fires burned for about three days and destroyed large parts of the city, leaving a large share of the population homeless. The first official death count was around 375, but later research raised it to more than 3,000.
The quake also changed science. Studying the displaced fences and roads along the fault, geologist Harry Fielding Reid proposed the elastic rebound theory: rock slowly builds up strain until it snaps, releasing energy, which is still the basic explanation of how earthquakes happen.
The disaster shaped modern building codes in California and pushed the founding of earthquake research programs. The Lawson report, a state commission study of the quake, remains a classic of early earthquake science.