Four hundred and seventy-seven kilometres of the San Andreas tore open at once. Studying the offset fences afterward gave us elastic-rebound theory — the foundation of modern seismology.
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DownloadAt 5:12 a.m. on April 18, 1906, the northern San Andreas Fault ruptured along roughly 477 kilometres of its length — from near San Juan Bautista north past Cape Mendocino. San Francisco shook for less than a minute, but the rupture was one of the longest ever documented on a continental fault.
Most of the devastation came not from the shaking itself but from the fires that followed: ruptured gas mains ignited blazes while broken water lines left the city unable to fight them, and they burned for days across much of central San Francisco. The death toll was downplayed at the time and has since been revised upward to around 3,000.
The 1906 earthquake was a turning point for science. Studying the offset fences and roads across the fault, Harry Fielding Reid formulated the theory of elastic rebound — that faults slowly store strain and release it in a sudden slip — which remains the foundation of how we understand earthquakes today.