DEFINITION OF SAN ANDREAS FAULT

The transcurrent plate boundary

between the North American and Pacific plates in southern

California is defined by a number of related faults known as

the San Andreas Fault system. The system consists of a number

of predominantly right-lateral (dextral) slip faults that

accommodate most of the relative motion between North

America and the Pacific plate and form a belt about 50–100

miles (80–160 km) wide. Movement along these faults in

southern California has caused a number of destructive earthquakes,

including the Fort Tejon earthquake of 1857, the

1906 San Francisco earthquake, the 1989 Loma Prieta earthquake,

and many others. The fault system has been active for

hundreds of millions of years, and as relative plate motions

continue between the Pacific and North American plates, it is

certain that more large earthquakes will be generated along

the system. Since southern California is so densely populated,

the San Andreas is one of the most intensely studied and

monitored faults in the world.

The San Andreas Fault system extends from an incipient

oceanic spreading center in the Gulf of California, past San

Diego, Los Angeles, and San Francisco, then joins with the

Mendocino transform at Cape Mendocino, where the North

American, Pacific, and Gorda plates meet. Individual fault

zones along this system range from 0.3 to 0.6 mile (0.5–1 km)

wide and extend along strike for hundreds of kilometers. These

fault zones are all associated with a number of smaller fault

elements, including faults, fault branches, and fault strands.

A number of major elements of the San Andreas Fault

system include the main San Andreas Fault, the Hayward and

Calaveras Faults (and their extensions) in central California,

and the San Jacinto and Elsinore Faults in southern California.

North of Los Angeles the San Andreas is met at a high

angle by the Garlock Fault, which is linked to the active

Owens Valley Fault. Most of the faults of the San Andreas

system trend about N35°–40°W, but there are several major

bends in the system. The faults bend in a left-stepping manner

in a compressional bend north of Los Angeles near Santa

Barbara, forming the Transverse Ranges, as relative motion

between either side of the fault is compressional, uplifting the

mountains in the area. There are many thrust faults and folds

associated with the fault in this area. East of San Diego the

fault system steps in the opposite direction to the right, forming

an extensional bend. The Mojave segment of the San

Andreas terminates along the east side of the Salton Sea, and

motion is picked up on the Imperial Fault on the west side of

the sea. Relative motion across this step is extensional, causing

stretching and motion on normal faults, and subsidence

in a basin now occupied by the Salton Sea.

The San Andreas Fault system is segmented into a number

of different sections with different patterns of behavior,

and likelihood of major earthquakes. Some sections are characterized

by a slow and steady creep without major earthquake

events, whereas other segments move by stick-slip

behavior where long periods of quiescence are interrupted by

great earthquake events when major slip occurs suddenly.

The main segments of the fault, rated from most likely to

least likely to slip include the Parkfield segment, the

Coachilla Valley, Mojave, San Bernadino Mountains, San

Francisco Peninsula, Carizzo Plain, and Southern Santa Cruz

Mountains that slipped in 1989.

See also TRANSFORM PLATE MARGIN PROCESSES.

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