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The San Andreas Fault is a continental transform fault that runs a length of roughly through Californiamarker in the United Statesmarker. The fault's motion is right-lateral strike-slip (horizontal motion). It forms the tectonic boundary between the Pacific Plate and the North American Plate.

The fault was first identified in Northern California by UC Berkeleymarker geology professor Andrew Lawson in 1895 and named by him after a small lake which lies in a linear valley formed by the fault just south of San Francisco, the Laguna de San Andreasmarker. Following the 1906 San Francisco Earthquake, it was Lawson who also discovered that the San Andreas Fault stretched well southward into Southern California.

Segments of the Fault

Map of the San Andreas Fault, showing relative motion.
Note that both sides are moving to the northwest, but at different rates.

The San Andreas Fault can be divided into three segments.

Southern segment

The southern segment (known as the Mojave segment) begins near the Salton Seamarker at the northern terminus of the East Pacific Rise and runs northward before it begins a slow bend to the west where it meets the San Bernardino Mountainsmarker. It runs along the southern base of the San Bernardino Mountains, crosses through the Cajon Passmarker and continues to run northwest along the northern base of the San Gabriel Mountainsmarker. These mountains are a result of movement along the San Andreas Fault and are commonly called the Transverse Range. In Palmdalemarker, a portion of the fault is easily examined as a roadcut for the Antelope Valley Freeway runs directly through it.

After crossing through Frazier Parkmarker, the fault begins to bend northward. This area is referred to as the “Big Bend” and is thought to be where the fault locks up in Southern California as the plates try to move past each other. This section of the fault has an earthquake-recurrence interval of roughly 140–160 years. Northwest of Frazier Park, the fault runs through the Carrizo Plainmarker, a long, treeless plain within which much of the fault is plainly visible. The Elkhorn Scarp defines the fault trace along much of its length within the plain.

Central segment

The central segment of the San Andreas fault runs in a northwestern direction from Parkfieldmarker to Hollistermarker. While the southern section of the fault and the parts through Parkfield experience earthquakes, the rest of the central section of the fault exhibits a phenomenon called aseismic creep, where the fault slips slowly without causing earthquakes.

Northern segment

The northern segment of the fault runs from Hollister, through the Santa Cruz Mountainsmarker, epicenter of the 1989 Loma Prieta earthquakemarker, then on up the San Francisco Peninsula, where it was first identified by Professor Lawson in 1895, then offshore at Pacificamarker at Mussel Rockmarker. This is the approximate location of the epicenter of the 1906 San Francisco earthquake. The fault returns onshore at Bolinas Lagoon just north of Stinson Beachmarker in Marin Countymarker. It returns underwater through the linear trough of Tomales Baymarker which separates the Point Reyes Peninsulamarker from the mainland, returning onshore at Fort Rossmarker. (In this region around the San Francisco Bay Areamarker several significant "sister faults" run more-or-less parallel, and each of these can create significantly destructive earthquakes.)From Fort Ross the northern segment continues overland, forming in part a linear valley through which the Gualala River flows. It goes back offshore at Point Arenamarker. After that, it runs underwater along the coast until it nears Cape Mendocinomarker, where it begins to bend to the west, terminating at the Mendocino Triple Junction.


Historical movement of the San Andreas Fault.

The evolution of the San Andreas dates back to the mid Cenozoic, to about 30 Ma. At this time, a spreading center between the Pacific Plate and the Farallon Plate (which is now mostly subducted, with remnants including the Juan de Fuca Plate, Rivera Plate, Cocos Plate, and the Nazca Plate) was beginning to interact with the subduction zone off of the coast of North America. The relative motion between the Pacific and North American Plates was different from the relative motion between the Farallon and North American Plates, so when the spreading ridge was 'subducted', a new relative motion caused a new style of deformation. This style is chiefly the San Andreas Fault, but also includes a possible driver for the deformation of the Basin and Range, separation of Baja California, and rotation of the Transverse Range.

The San Andreas Fault proper, at least the Southern Segment, has only existed for about 5 Ma. The first known incarnation of the southern part of the fault was Clemens Well-Fenner-San Francisquito fault zone around 22–13 Ma. This system added the San Gabriel Fault as a primary focus of movement between 10–5 Ma. Currently, it is believed that the modern San Andreas will eventually transfer its motion toward a fault within the Eastern California Shear Zone. This complicated evolution, especially along the southern segment, is mostly caused by either the "Big Bend" and/or a difference in the motion vector between the plates and the trend of the fault(s).

Plate movement

All land west of the fault on the Pacific Plate is moving slowly to the northwest while all land east of the fault is moving southwest (relatively southeast as measured at the fault) under the influence of plate tectonics. The rate of slippage averages approximately 33–37 mm/year across California.

The westward component of the motion of the North American Plate creates compressional forces which are expressed as uplift in the Coast Ranges. Likewise, the northwest motion of the Pacific Plate creates significant compressional forces where the North American Plate stands in its way, creating the Transverse Ranges in Southern California, and to a lesser, but still significant, extent the Santa Cruz Mountains, site of the Loma Prieta Earthquakemarker of 1989.

Studies of the relative motions of the Pacific and North American plates have shown that only about 75 percent of the motion can be accounted for in the movements of the San Andreas and its various branch faults. The rest of the motion has been found in an area east of the Sierra Nevada mountains called the Walker Lane or Eastern California Shear Zone. The reason for this is not as yet clear, although several hypotheses have been offered and research is ongoing. One hypothesis which gained some currency following the Landers Earthquakemarker in 1992 is that the plate boundary may be shifting eastward, away from the San Andreas to the Walker Lane.

Assuming the plate boundary does not change as hypothesized, projected motion indicates that the landmass west of the San Andreas Fault, including Los Angeles, will eventually slide past San Francisco, then continue northwestward toward the Aleutian Trenchmarker, over a period of perhaps twenty million years.

Scientific research

Research at Parkfield

In central California is the small town of Parkfield, Californiamarker, which lies along the San Andreas Fault. Seismologists discovered that this section of the fault consistently produces magnitude 6.0 earthquakes about every 22 years. Following earthquakes in 1857, 1881, 1901, 1922, 1934, and 1966, scientists predicted an earthquake to hit Parkfieldmarker in 1993. This quake eventually struck in 2004 (see Parkfield earthquake). Because of this frequent activity and prediction, Parkfield has become one of the most popular spots in the world to try to capture and record large earthquakes.

In 2004, work began just north of Parkfield on the San Andreas Fault Observatory at Depth (SAFOD). The goal of SAFOD is to drill a hole nearly 3 kilometers into the Earth's crust and into the San Andreas Fault. An array of sensors will be installed to capture and record earthquakes that happen near this area.

The University of California study on "the next big one"

A study completed by Yuri Fialko has demonstrated that the San Andreas fault has been stressed to a level sufficient for the next "big one," as it is commonly called, that is, an earthquake of magnitude 7.0 or greater. The study also concluded that the risk of a large earthquake may be increasing faster than researchers had previously believed. Fialko also emphasized in his study that, while the San Andreas Fault had experienced massive earthquakes in 1857 at its central section and in 1906 at its northern segment (the 1906 San Francisco earthquake), the southern section of the fault has not seen a similar rupture in at least 300 years.

If such an earthquake were to occur, Fialko's study stated, it would result in substantial damage to Palm Springsmarker and a number of other cities in San Bernardinomarker, Riversidemarker and Imperialmarker counties in California, and Mexicali municipality in Baja Californiamarker. Such an event would be felt throughout much of Southern California, including densely populated areas of metropolitan Los Angelesmarker, Orange Countymarker, San Diegomarker and Tijuanamarker, Baja Californiamarker.

"The information available suggests that the fault is ready for the next big earthquake but exactly when the triggering will happen and when the earthquake will occur we cannot tell," Fialko said. "It could be tomorrow or it could be 10 years or more from now," he concluded in September 2005.

Cascadia connection

Recent studies of past earthquake traces on both the northern San Andreas Fault and the southern Cascadia subduction zone indicate a correlation in time which may be evidence that quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas during at least the past 3,000 years or so. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. The 1906 San Francisco earthquake seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake, and the rupture moved mostly from south to north.

Notable earthquakes

The San Andreas Fault has had some notable earthquakes in historic times:
  • 1857 Fort Tejon earthquakemarker: About 350 kilometers were ruptured in central and southern California. Though it is known as the Fort Tejonmarker earthquake, the epicenter is thought to have been located far to the north, just south of Parkfieldmarker. Only two deaths were reported. The magnitude was about 8.0
  • 1906 San Francisco earthquake: About 430 kilometers were ruptured in Northern California. The epicenter was near San Franciscomarker. About 3000 people died in the earthquake and subsequent fires. This time the magnitude was estimated to be 7.8.
  • 1989 Loma Prieta earthquakemarker: About 40 kilometers were ruptured (although the rupture did not reach the surface) near Santa Cruz, Californiamarker, causing 63 deaths and moderate damage in certain vulnerable locations in the San Francisco Bay Area. Magnitude this time was about 7.1. The earthquake also postponed game 3 of the 1989 World Series at Candlestick Parkmarker.
  • 2004 Parkfield earthquake: On September 28, 2004, at 10:15 AM, a magnitude 6.0 earthquake struck California on the San Andreas Fault. This earthquake was originally expected in 1993 based on the latest earthquake prediction theories of the time, but eleven years passed before the predicted event occurred. Despite the extra time between events, the magnitude of the earthquake was no larger than expected.

See also


  • The California Earthquake of April 18, 1906: Report of the State Earthquake Investigation Commission, Andrew C. Lawson, chairman, Carnegie Institution of Washington Publication 87, 2 vols. (1908) - Available online at this USGS webpage.

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