One of the greatest earthquakes happened in 20th century is in San Francisco in year 1906. It measured 7.8 degrees. Many building were destructed and several hundred of people got killed. The earthquake also started a fire, which destroyed the central business district. .
The earthquake happened on the San Andreas Fault, which is the major fracture of the Earth's crust. It is trending northwestward through southern and northern California, U.S., for 650 miles (1,050 km) and passing seaward in the vicinity of San Francisco. Movement along this transform fault is of the strike-slip type and is characterized by occasional large earthquakes originating near the surface along the path of the fault. The disastrous San Francisco quake of 1906 and the less serious earthquake of 1989 were both caused by movement along the fault. According to the theory of plate tectonics, the San Andreas results from the abutment of two major plates of the Earth's crust, the Northern Pacific and the North American. Along the fault, the Northern Pacific plate is sliding past the North American plate in a northerly direction, at a relative movement of about 1 cm (0.4 inch) per year over geologic time, though the rate of movement has been 4 to 6 cm (1.6 to 2.4 inches) per year over the past century. Parts of the fault line moved as much as 6.4 m (21 feet) during the 1906 earthquake. The San Francisco Bay Area Rapid Transit System (BART) bored a tunnel right through the fault zone, and various cities, towns, and housing developments lie on or near it.
At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada.