Tuesday, April 18, 2017

The Great San Francisco Earthquake

The Great San Francisco Earthquake 1906


1906 San Francisco earthquake
1906 San Francisco earthquake is located in California
Eureka
Eureka
Chico
Chico
Truckee
Truckee
Santa Rosa
Santa Rosa
Salinas
Salinas
Fresno
Fresno
Paso Robles
Paso Robles
1906 San Francisco earthquake
DateApril 18, 1906
Origin time05:12 local time
Magnitude7.8 Mw[1]
Depth5 mi (8.0 km)[2]
Epicenter37.75°N 122.55°W
[2]
TypeStrike-slip[3]
Areas affectedNorth Coast
San Francisco Bay Area
Central Coast
United States
Max. intensityXI (Extreme)[4]
TsunamiYes[5]
Casualties3,000+[6]
The 1906 San Francisco earthquake struck the coast of Northern California at 5:12 a.m. on April 18 with an estimated moment magnitude of 7.8 and a maximum Mercalli intensity of XI (Extreme). Severe shaking was felt from Eureka on the North Coast to the Salinas Valley, an agricultural region to the south of the San Francisco Bay Area. Devastating fires soon broke out in the city and lasted for several days. As a result, about 3,000 people died and over 80% of the city of San Francisco was destroyed. The events are remembered as one of the worst and deadliest natural disasters in the history of the United States. The death toll remains the greatest loss of life from a natural disaster in California's history and high in the lists of American urban disasters.

The San Andreas Fault is a continental transform fault that forms part of the tectonic boundary between the Pacific Plate and the North American Plate. The strike-slip fault is characterized by mainly lateral motion in a dextral sense, where the western (Pacific) plate moves northward relative to the eastern (North American) plate. The 1906 rupture propagated both northward and southward for a total of 296 miles (476 km). This fault runs the length of California from the Salton Sea in the south to Cape Mendocino in the north, a distance of about 810 miles (1,300 km). The maximum observed surface displacement was about 20 feet (6 m); geodetic measurements show displacements of up to 28 feet (8.5 m). 
USGS ShakeMap showing the earthquake's intensity.
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 7.8; 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 main shock epicenter occurred offshore about 2 miles (3.2 km) from the city, near Mussel Rock. Shaking was felt from Oregon to Los Angeles, and inland as far as central Nevada.
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
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 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 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.

Damaged houses on Howard Street (left), and ruins in the vicinity of Post and Grant Avenue
At the time, 375 deaths were reported, partly because hundreds of fatalities in Chinatown went ignored and unrecorded. The total number of deaths is still uncertain today, and is estimated to be roughly 3,000 at minimum. Most of the deaths occurred in San Francisco itself, but 189 were reported elsewhere in the Bay Area; nearby cities, such as Santa Rosa and San Jose, also suffered severe damage. In Monterey County, the earthquake permanently shifted the course of the Salinas River near its mouth. Where previously the river emptied into Monterey Bay between Moss Landing and Watsonville, it was diverted 6 miles south to a new channel just north of Marina.
Between 227,000 and 300,000 people were left homeless out of a population of about 410,000; half of those who evacuated fled across the bay to Oakland and Berkeley. Newspapers described Golden Gate Park, the Presidiothe Panhandle and the beaches between Ingleside and North Beach as covered with makeshift tents. More than two years later, many of these refugee camps were still in operation.
Selected Mercalli intensities
MMILocations
XI (Extreme)San FranciscoSanta Rosa
X (Extreme)SebastopolSan Bruno
IX (Violent)San JosePoint Arena
VIII (Severe)EurekaSalinas
VII (Very strong)TruckeeParkfield
VI (Strong)WillowsFresno
V (Moderate)ChicoPaso Robles
U.S. Earthquake Intensity Database, NGDC
The earthquake and fire left long-standing and significant pressures on the development of California. At the time of the disaster, San Francisco had been the ninth-largest city in the United States and the largest on the West Coast, with a population of about 410,000. Over a period of 60 years, the city had become the financial, trade and cultural center of the West; operated the busiest port on the West Coast; and was the "gateway to the Pacific", through which growing U.S. economic and military power was projected into the Pacific and Asia. Over 80% of the city was destroyed by the earthquake and fire. Though San Francisco rebuilt quickly, the disaster diverted trade, industry and population growth south to Los Angeles, which during the 20th century became the largest and most important urban area in the West. Many of the city's leading poets and writers retreated to Carmel-by-the-Sea where, as "The Barness", they established the arts colony reputation that continues today.

The 1908 Lawson Report, a study of the 1906 quake led and edited by Professor Andrew Lawson of the University of California, showed that the same San Andreas Fault which had caused the disaster in San Francisco ran close to Los Angeles as well. The earthquake was the first natural disaster of its magnitude to be documented by photography and motion picture footage and occurred at a time when the science of seismology was blossoming.

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