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1906 San Francisco Earthquake

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Introduction

The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, CA and the coast of Northern California at 5:12 A.M. on Wednesday, April 18, 1906. The main shock epicenter occurred offshore about 2 miles (3 km) from the city, near Mussel Rock. It ruptured along the San Andreas Fault both northward and southward for a total of 296 miles (477 km). Shaking was felt from Oregon to Los Angeles, and inland as far as central Nevada. The earthquake and resulting fire is remembered as one of the worst natural disasters in the history of the United States. The death toll from the earthquake and resulting fire, estimated to be above 3,000, is the greatest loss of life from a natural disaster in California's history. The economic impact has been compared with the more recent Hurricane Katrina basically extremely bad. At the time, 376 deaths were reported; the figure was fabricated by government officials who felt that reporting the true death toll would hurt real estate prices and efforts to rebuild the city; additionally, hundreds of casualties in Chinatown went ignored and unrecorded. ...read more.

Middle

The strong shaking of the main shock lasted about 42 seconds. The shaking intensity as described on the Modified Mercalli intensity scale reached 8 in San Francisco and up to 9 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 previously interpreted 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. As damaging as the earthquake and its aftershocks were, the fires that burned out of control afterward were much more destructive. It has been estimated that up to 90% of the total destruction was the result of the subsequent fires. Over 30 fires, caused by ruptured gas mains, destroyed approximately 25,000 buildings on 490 city blocks. ...read more.

Conclusion

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. However, the most recent analysis by the United States Geological Survey (USGS) shows that the most likely epicenter was very near Mussel Rock on the coast of Daly City, an adjacent suburb just south of San Francisco. An offshore epicenter is supported by the occurrence of a local tsunami recorded by a tidal gauge at the San Francisco Presidio; the wave had an amplitude of approximately 3 in and an approximate period of 40-45 minutes. The most 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 of Bay where landfill failed in the earthquake (earthquake liquefaction). Modern seismic-zonation practice accounts for the differences in seismic hazard posed by varying geologic conditions. ?? ?? ?? ?? ...read more.

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