1906 San Francisco earthquake

The San Francisco earthquake of 1906 was a major earthquake at San Francisco, California on the early morning of Wednesday, April 18, 1906. Foreshocks and the main quake occurred at about 5:12am along the San Andreas Fault, with an epicenter close to the city. Its violent shocks were felt from Oregon to Los Angeles, and inland as far as central Nevada. The earthquake and fire would go down as one of the worst natural disasters to hit a major city in United States history (see also the Galveston Hurricane and Hurricane Katrina).

Analysis

• One important characteristic of the shaking intensity noted in Lawson's (1908) report was the clear correlation of intensity with underlying geologic conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby bedrock sites, and the strongest shaking occurred in areas where ground reclaimed from San Francisco Bay failed in the earthquake. Modern seismic-zonation practice accounts for the differences in seismic hazard posed by varying geologic conditions.
• An analysis of the displacements and strain in the surrounding crust led Reid (1910) to formulate his elastic-rebound theory of the earthquake source, which remains today the principal model of the earthquake cycle.
• The United States Geological Survey (USGS) estimates that the earthquake measured a powerful 7.9 on the moment magnitude scale. The earthquake caused ruptures visible on the surface for a length of 470 kilometers (290 miles). Modified Mercalli Intensities of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace.