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San Andreas Fault

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Image:Sanandreasfault srtm.jpg

The San Andreas Fault is a geological fault that runs a length of roughly 800 miles (1300 kilometres) through western and southern California in the United States. The fault, a right-lateral strike-slip fault, marks a transform boundary between the Pacific Plate and the North American Plate.

The fault was first identified in Northern California by UC Berkeley 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 Andreas. Following the 1906 San Francisco Earthquake, it was Lawson who also discovered that the San Andreas Fault stretched well southward into Southern California.

Contents

[edit] Southern, central, and northern segments

The San Andreas Fault can be divided into three segments.

Image:Sanandreas.jpg

the southern segment (known as the Mojave segment) begins near the Salton Sea at the northern terminus of the East Pacific Rise and runs northward before it begins a slow bend to the west when it meets the San Bernardino Mountains. Here, it runs along the southern base of the San Bernardino Mountains, crosses through the Cajon Pass and continues to run northwest along the northern base of the San Gabriel Mountains. These mountains are a result of movement along the San Andreas Fault and are commonly called the Transverse Range. This segment of the fault is the most commonly analyzed of any earthquake fault in the world by geologists. This is due to a cutout of the fault in Palmdale (the second largest city directly sitting on the fault) where the Antelope Valley Freeway passes through it, and the deep layers of "shifted" crust can clearly be seen.

After crossing through Frazier Park, the fault begins to bend north. 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 a recurrence interval of roughly 140 - 160 years. Northwest of Frazier Park, the fault runs through the Carrizo Plain, 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.

The central segment of the San Andreas fault runs in a northwestern direction from Parkfield to Hollister. 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. This term describes the fault being able to move without causing earthquakes.

The northern segment of the fault runs from Hollister, through the Santa Cruz Mountains, epicenter of the 1989 Loma Prieta earthquake, then on up the San Francisco Peninsula, where it was first identified by Professor Lawson in 1895, then offshore at Pacifica at Mussel Rock. This is the approximate location of the epicenter of the 1906 earthquake. The fault returns onshore at Bolinas Lagoon just north of Stinson Beach in Marin County. It returns underwater through the linear trough of Tomales Bay which separates the Point Reyes Peninsula from the mainland, returning onshore at Fort Ross. From there it continues overland, forming in part a linear valley through which the Gualala River flows. It goes back offshore at Point Arena. After that, it runs underwater along the coast until it nears Cape Mendocino, where it begins to bend to the west, terminating at a triple junction with the Mendocino Fracture Zone and the Cascadia subduction zone. To the north lies the Gorda Plate which is being subducted under the margin of the North American plate.

[edit] Plate movement

Image:San Andreas Fault Sequential Diagrams.jpg

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 to the southwest (relatively southeast as measured at the fault) under the influence of plate tectonics. The rate of slippage is approximately of 1/5 inch (0.6 cm) a year. Projected motion indicates that the Gulf of California will expand northward at the same time that the landmass west of the fault, including the Baja California peninsula and the California coast (including Los Angeles) slides past San Francisco, then continuing northwestward as an island mass toward the Aleutian Trench, over a period of perhaps twenty million years.

[edit] Scientific research

[edit] Research at Parkfield

Further south in central California is the small town of Parkfield, California 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 Parkfield 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. [1]

[edit] The University of California study on "the next big one"

A study completed by Yuri Fialko, an associate professor at the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics at the Scripps Institution of Oceanography (an academic unit of the University of California, San Diego), published in the June 22, 2006 edition of the journal Nature, 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 has experienced massive earthquakes in 1857 at its central section and in 1906 at its northern segment (the great 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 Springs and a number of other cities in San Bernardino, Riverside and Imperial counties in California. Such an event would be felt throughout much of Southern California, including densely populated areas of metropolitan Los Angeles and San Diego.

"All these data suggest 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.

[edit] Notable earthquakes

The San Andreas Fault has had some notable earthquakes in historic times:

  • 1857 Fort Tejon earthquake — 350 kilometers were ruptured in central and southern California. Though it is known as the Fort Tejon earthquake, the epicenter is thought to have been located far to the north, just south of Parkfield. Only two deaths were reported. The magnitude was about 8.0
  • 1906 San Francisco Earthquake — 430 kilometers were ruptured in Northern California. The epicenter was near San Francisco. About 3000 people died in the earthquake and subsequent fires. This time the magnitude was estimated to be 7.8.
  • 1989 Loma Prieta earthquake — 40 kilometers were ruptured (although the rupture did not reach the surface) near Santa Cruz, California, 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 Park.
  • 2004 Parkfield earthquake — on 28th September 2004 at quarter past 10 in the morning, 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. Eleven years passed before this prediction finally came to pass. Despite the extra time between events the magnitude of the earthquake was no larger than originally expected.
See also: List of earthquakes

[edit] Trivia and pop culture references

  • The plot of the original Superman movie had Lex Luthor purchasing massive amounts of land east of the fault and then detonating a nuclear missile at the fault line, causing all land west of the fault to plunge into the sea and turning Luthor's previously purchased barren land into beachfront property.
  • The video game Grand Theft Auto: San Andreas, is set in a fictional state named after this fault. Also, what is said to be the fault can be seen running through San Fierro, a fictionalized version of San Francisco.
  • There is a popular misconception, played up in Curt Gentry's pop apocalypse, Last Days of the Late, Great State of California (1968), that, eventually, a portion of California will break away from the continental U.S. at the fault, or drop below sea level. This is not true; it would be more correct to say the land west of the fault will move northwest, because the fault is a transform fault (the two land masses are sliding by each other, not moving apart), at least in the short term.
  • The aforementioned misconception has been referenced by Bill Hicks' album, Arizona Bay, the Tool song, Ænema, the Presidents of the United States of America song, "Naked and Famous" the Warren Zevon song, "Desperadoes Under the Eaves", and the Steely Dan song "My Old School."
  • On Duke Nukem 3D games. Duke explored the San Andreas Fault area on the final stages of Episode 1 LA Meltdown on The Abyssis
  • The song "San Andreas Fault" by Natalie Merchant is about, of course, the San Andreas fault.

[edit] See also

[edit] References

  • Collier, Michael (Dec 1, 1999). A Land in Motion. UC Press. ISBN 0520218973.
  • Stoffer, Philip W. (2006). Where's the San Andreas fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region. USGS. General Interest Publication 16.


[edit] External links

ca:Falla de San Andrés cs:San Andreas de:San-Andreas-Verwerfung es:Falla de San Andrés fr:Faille de San Andreas it:Faglia di Sant'Andrea lt:Šv. Andrejaus lūžis ms:Sesar San Andreas nl:San Andreas-breuk ja:サンアンドレアス断層 pl:San Andreas (uskok) pt:Falha de Santo André simple:San Andreas Fault fi:San Andreasin siirros sv:San Andreasförkastningen

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