1906 San Francisco Earthquake - Magnitude and Geology

Magnitude and Geology

The most widely accepted estimate for the magnitude of the earthquake is a moment magnitude (M_w) of 7.9; however, other values have been proposed, from 7.7 to as high as 8.25. The main shock epicenter occurred offshore about 2 miles (3.2 km) from the city, near Mussel Rock. It ruptured along the San Andreas Fault both northward and southward for a total of 296 miles (476 km). Shaking was felt from Oregon to Los Angeles, and inland as far as central Nevada.

As mentioned, the earthquake was caused by a rupture on the San Andreas Fault, a continental transform fault that forms part of the boundary between the Pacific Plate and the North American Plate. This fault runs the length of California from the Salton Sea in the south to Cape Mendocino to the north, a distance of about 810 miles (1,300 km). The earthquake ruptured the northern third of the fault for a distance of 296 miles (476 km). The maximum observed surface displacement was about 20 feet (6 m); however, geodetic measurements show displacements of up to 28 feet (8.5 m).

A strong foreshock preceded the mainshock by about 20 to 25 seconds. The strong shaking of the main shock lasted about 42 seconds. The shaking intensity as described on the Modified Mercalli intensity scale reached VIII in San Francisco and up to IX in areas to the north like Santa Rosa where destruction was devastating. There were decades of minor earthquakes – more than at any other time in the historical record for northern California – before the 1906 quake. Widely interpreted previously as precursory activity to the 1906 earthquake, they have been found to have a strong seasonal pattern and have been postulated to be due to large seasonal sediment loads in coastal bays that overlie faults as a result of the erosion caused by "hydraulic mining" in the later years of the California Gold Rush.