The 1906 San Francisco earthquake had an approximate magnitude of 7.9. It struck Northern California on 18 April 1906 along the San Andreas Fault, with the main shock beginning at about 5:12 a.m. Pacific time.
Because the Richter scale and modern instrumental measurements did not yet exist, scientists estimate the earthquake’s size using historical accounts, fault displacement and later seismological analysis. The most widely accepted modern estimate is moment magnitude 7.9, although published estimates have ranged from about 7.7 to as high as 8.3. That uncertainty is why “approximately” is important in the question.
The rupture extended for roughly 296 miles, or about 477 kilometers, along the San Andreas Fault. Strong shaking lasted approximately 42 seconds, and fires that began after the earthquake burned for several days in San Francisco. The disaster caused widespread destruction across the Bay Area and helped transform modern earthquake science and building practices.