<P> The 1906 earthquake preceded the development of the Richter magnitude scale by three decades . The most widely accepted estimate for the magnitude of the quake on the modern moment magnitude scale is 7.9; values from 7.7 to as high as 8.3 have been proposed . According to findings published in the Journal of Geophysical Research, severe deformations in the earth's crust took place both before and after the earthquake's impact . Accumulated strain on the faults in the system was relieved during the earthquake, which is the supposed cause of the damage along the 450 - kilometer - long segment of the San Andreas plate boundary . The 1906 rupture propagated 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 . </P> <P> A strong foreshock preceded the main shock by about 20 to 25 seconds . The strong shaking of the main shock lasted about 42 seconds . 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 . </P> <P> For years, the epicenter of the quake was assumed to be near the town of Olema, in the Point Reyes area of Marin County, because of evidence of the degree of local earth displacement . In the 1960s, a seismologist at UC Berkeley proposed that the epicenter was more likely offshore of San Francisco, to the northwest of the Golden Gate . The most recent analyses support an offshore location for the epicenter, although significant uncertainty remains . An offshore epicenter is supported by the occurrence of a local tsunami recorded by a tide gauge at the San Francisco Presidio; the wave had an amplitude of approximately 3 in (8 cm) and an approximate period of 40--45 minutes . </P> <P> Analysis of triangulation data before and after the earthquake strongly suggest that the rupture along the San Andreas Fault was about 500 km in length, in agreement with observed intensity data . The available seismological data support a significantly shorter rupture length, but these observations can be reconciled by allowing propagation at speeds above the S - wave velocity (supershear). Supershear propagation has now been recognized for many earthquakes associated with strike - slip faulting . </P>

Epicenter of the 1906 earthquake in san francisco
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