- San Andreas fault
/san" an dray"euhs/, Geol.an active strike-slip fault in W United States, extending from San Francisco to S California and forming the on-land portion of the western margin of the North American Plate.[after San Andreas Lake, located in the rift, in San Mateo County]
* * *Zone of transform faults at the boundary between two tectonic plates in the western U.S. The fault runs along the coast of northern California for 650 mi (1,050 km) and passes seaward in the vicinity of San Francisco.Movement along the fault is characterized by frequent earthquakes, including the major San Francisco quake of 1906, when parts of the fault line moved as much as 21 ft (6.4 m), and the less serious earthquake of 1989.
* * *major fracture of the Earth's crust in extreme western North America. The fault trends northwestward for more than 800 miles (1,300 km) from the northern end of the Gulf of California (California, Gulf of) through western California, U.S., passing seaward into the Pacific Ocean in the vicinity of San Francisco. Tectonic movement along the fault has been associated with occasional large earthquakes originating near the surface along its path, including a disastrous quake in San Francisco in 1906, a less serious event there in 1989, and a strong and destructive quake centred in the Los Angeles suburb of Northridge in 1994 that occurred along one of the San Andreas's larger secondary faults.According to the theory of plate tectonics, the San Andreas Fault represents the transform (strike-slip) boundary between two major plates of the Earth's crust: the Northern Pacific to the south and west and the North American to the north and east. The Northern Pacific plate is sliding laterally past the North American plate in a northerly direction, and hence the San Andreas is classified as a strike-slip fault. The movement of the plates relative to each other has been about 1 cm (0.4 inch) per year over geologic time, though the annual rate of movement has been 4 to 6 cm (1.6 to 2.4 inches) per year since the early 20th century. Parts of the fault line moved as much as 6.4 metres (21 feet) during the 1906 earthquake.The great majority of California's population lives in the vicinity of the San Andreas Fault. Some cities, towns, housing developments, and roads are actually built on it, and a tunnel of the San Francisco Bay Area Rapid Transit System (BART) is bored right through the fault zone. Measures taken to offset the danger from earthquakes include reinforcing roads and bridges to withstand tremors and constructing buildings to absorb seismic shocks.
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