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At 5:12 a.m. on April 18, 1906, the people of San- Francisco were awakened by an earthquake that would devastate the city. The main temblor, having a 7.7–7.9 magnitude, lasted about one minute and was the result of the rupturing of the northernmost 296 miles of the 800-mile San Andreas fault
But when calculating destruction, the earthquake took second place to the great fire that followed. The fire, lasting four days, most likely started with broken gas lines (and, in some cases, was helped along by people hoping to collect insurance for their property—they were covered for fire, but not earthquake, damage). With water mains broken, fighting the fires was almost impossible, and about 500 city blocks were destroyed. The damages were estimated at about $400,000,000 in 1906 dollars, which would translate to about $8.2 billion today.
Uncertain Death Toll. In 1906 San-Francisco was the ninth largest .city with a population of 400,000, and over 225,000 were left homeless by the disaster. In 1906 San Francisco was the ninth largest U. S. city with a population of 400,000, and over 225,000 were left homeless by the disaster. The death toll is uncertain. City officials estimated the casualties at 700 but more modern calculations say about 3,000 lost their lives.
The epicenter of this earthquake has moved around in the past 100 years, as advances in seismology have been made. It was first thought to have been in Marin County, then northwest of the Golden Gate, and most recently, in the Pacific Ocean about two miles west of San Francisco. The plate-tectonics theory that would shed some light on the quake wasn't formulated until the 1920s and not embraced until some three decades later. The San Andreas fault is on the boundary between the Pacific plate and the North American plate. During the earthquake, the ground west of the fault tended to move northward and, in the most extreme instance, a 21-foot shift was measured. Earthquake ruptures are fast movers—seismologists estimated the average speed of this rupture to have been 8,300 mi/hr going north, and 6,300 mi/hr traveling south. The quake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. Shifting Seismologic Knowledge
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