Definitions

SanAndre

San Andreas Fault

[san an-drey-uhs]

The San Andreas Fault is a geologic transform fault that runs a length of roughly 800 miles (1,300 km) through California in the United States. 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 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.

Southern, central, and northern segments

The San Andreas Fault can be divided into three segments.

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 where it meets the San Bernardino Mountains. 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. Near Palmdale, a portion of the fault is easily examined as a roadcut for the Antelope Valley Freeway runs directly through it.

After crossing through Frazier Park, the fault begins to bend northwards. 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 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 San Francisco 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. (In this region around the San Francisco Bay Area 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 Arena. After that, it runs underwater along the coast until it nears Cape Mendocino, where it begins to bend to the west, terminating at the Mendocino Triple Junction.

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 Earthquake 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 Earthquake 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 Trench, over a period of perhaps twenty million years. On the other hand, if the plate boundary shifts eastward, then the entire state of California would move in the same direction.

Scientific research

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.

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 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, and Mexicali municipality in Baja California. Such an event would be felt throughout much of Southern California, including densely populated areas of metropolitan Los Angeles, Orange County, San Diego and Tijuana, Baja California.

"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 of 2005.

The 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 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 28 September 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. 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.

The San Andreas Fault in popular culture

  • The television version of the theatrically-released film Earthquake features a prologue about the San Andreas, implying that the fault is responsible for the disaster of the title. There is no mention of the San Andreas fault in the theatrical version of the film.
  • 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 James Bond movie A View to a Kill saw a plot by villain Max Zorin to set off explosives under the San Andreas and Hayward faults in order to destroy Silicon Valley.
  • 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 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. Since the San Andreas Fault is a strike-slip (horizontal motion) fault and not a normal (vertical motion) fault nor a rift, this scenario is not plausible.
  • 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." The 1968 song "Day after Day" by the one-local-hit group Shango warns listeners to "get ready to tie up the boat in Idaho."
  • On Duke Nukem 3D games. Duke explored the San Andreas Fault area on the final stages of Episode 1 LA Meltdown on The Abyss
  • The song "San Andreas Fault" by Natalie Merchant off her 1995 album Tigerlily is about the San Andreas fault.
  • In an Animaniacs song about the 1994 Northridge earthquake, the Warners incorrectly claim that the San Andreas Fault was the cause.
  • In the Three Investigators book The Mystery of the Green Ghost, it is believed that an earthquake caused by the fault line had destroyed the infamous ghost pearls.
  • In the computer game Deus Ex (set in 2052), the player can find mention in in-game literature of a catastrophic earthquake (likely originating at San Andreas Fault) that took place an unknown number of years prior, submerging large parts of the state and, presumably, killing millions. Later, the player explores an abandoned gas station located in Central California that shows severe earthquake damage.
  • The San Andreas Fault is mentioned in The Divine Comedy's song "Here Comes the Flood" (1998) as one of the contestants in "the race to end history".

See also

References

  • Collier, Michael A Land in Motion. UC Press. ISBN 0-520-21897-3.
  • Stoffer, Philip W. 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.
  • 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
  • Lynch, David K. Field Guide to the San Andreas Fault: See and Touch the World's Most Famous Fault on any one of Twelve Easy Day Trips. Thule Scientific. Full color, GPS coordinates, ISBN 0-9779935-0-7.

External links

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