An Earthquake and Fire Destroy San Francisco

At 5:12 in the morning on Wednesday, April 18, 1906, a major earthquake struck Northern California. The primary rupture along the San Andreas Fault caused violent shaking that lasted some 45 to 60 seconds, according to the US Geological Survey, relieving decades of accumulated crustal strain along a rupture zone hundreds of miles long. Shaking was felt from southern Oregon to Los Angeles and inland across central Nevada. Although the earthquake preceded the invention of the Richter scale, modern seismological assessments generally estimate its moment magnitude at 7.9. Analysis by geologist Andrew Lawson later revealed that the severity of surface damage was closely tied to local ground conditions, with the highest shaking occurring on soft valley soils and reclaimed bay muds prone to liquefaction.

While the initial temblor collapsed structures throughout the region, the fires that ignited immediately afterward generated the vast majority of the damage. Broken gas lines and fractured chimneys sparked dozens of individual blazes across San Francisco. The destruction of underground water mains severely hampered firefighting efforts, and the city fire chief, Dennis T. Sullivan, died from injuries sustained during the opening shock. In desperate attempts to halt the flames, untrained firefighters and soldiers resorted to detonating dynamite to build firebreaks, a tactic that frequently ignited surrounding properties. The conflagration burned for three days across more than 500 city blocks, consuming landmark hotels, scientific laboratories, and municipal records.