The Great 1906 San Francisco Earthquake


San Francisco City Hall after the 1906 Earthquake

The California earthquake of April 18, 1906 ranks as one of the most significant earthquakes of all time. Today, its importance comes more from the wealth of scientific knowledge derived from it than from its sheer size. Rupturing the northernmost 296 miles (477 kilometers) of the San Andreas fault from northwest of San Juan Bautista to the triple junction at Cape Mendocino, the earthquake confounded contemporary geologists with its large, horizontal displacements and great rupture length. Indeed, the significance of the fault and recognition of its large cumulative offset would not be fully appreciated until the advent of plate tectonics more than half a century later. Analysis of the 1906 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.

At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli Intensities (MMI’s) of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace. 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. Read more…

One response to “The Great 1906 San Francisco Earthquake

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