City and County of San Francisco (S.F.) is the only administrative zone with the integration of county and city in California, and is also the economic and cultural center in Northern California and Bay Area. With the population more than 800,000, San Francisco is the second densely populated city in USA, only after New York. The Gold Rush in 1849 made this sparsely populated city quickly become one of the major cities in the west of USA. In 1906, this city was hit by a heavy earthquake, and after the reconstruction, the Bay Area of this city was about 1,126km2, and became the important naval base and trade harbor in USA.