San Andreas Fault
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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 (or sliding) 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.
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[edit] 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 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 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 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
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 averages approximately 33-37 mm/year across California[1]. 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.[2]
[edit] The University of California study on "the next big one"
A study completed by Yuri Fialko[3] 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, and Mexicali municipality in Mexico. 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.
"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 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 villian Max Zorin to set off explosives under the San Andreas and Hayward faults in order to decimate Silicon Valley.
- The video game Grand Theft Auto: San Andreas, is set in a fictional city 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 part - pop apocalypse, part description of California in the 1960s, Last Days of the Late, Great State of California (1968) (and, hints Gentry's book, by Edgar Cayce before then), 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, and the lighter crustal rocks "float" on the denser underlying basalts. Some misconstruction may come from the term continental shelf for shallow offshore regions, which do not at all resemble a common cantilever shelf. (Part of the grounds for the misconception may come from pre-plate tectonics geology, in which some theorists characterized lands as rising or falling in orogenies, without continental drift.)
- 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 misconception also forms part of the plot of The End of the World, a popular Flash animation.
- 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 the San Andreas fault.
- In an Animaniacs song about the 1994 Northridge earthquake, the Warner's incorrectly claim that the San Andreas Fault was the cause.
[edit] See also
- Earthquake
- aftershock
- Richter scale
- Parkfield Geology
- Plate tectonics
- Geology
- Hayward Fault Zone
- Seismology
- Seismic retrofit
- Southern California
- Faults
- Farallon Trench
[edit] References
- ^ Wallace, Robert E.. Present-Day Crustal Movements and the Mechanics of Cyclic Deformation. The San Andreas Fault System, California. Retrieved on 2007-10-26.
- ^ San Andreas Fault Observatory at Depth. USGS Earthquake Hazards Program.
- ^ (2006) "Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault System". Nature 441: 968-971.
- Collier, Michael (Dec 1, 1999). A Land in Motion. UC Press. ISBN 0-520-21897-3.
- 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.
- Lynch, David K. (2006). 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.
[edit] External links
- The Parkfield Experiment
- San Andreas Fault Observatory at Depth
- Southern California Earthquake Data Center: San Andreas Fault
- USGS: The San Andreas Fault
- 360 panorama of the Carrizo Plain section of the fault taken from a Kite
- Scripps Institution of Oceanography (University of California, San Diego) study indicating that an earthquake of magnitude 7.0 or greater is "overdue."
- Source for the Carrizo Plain images (USGS)
- Live Eye on the Earth
- Latest Earthquakes (USGS)
- Andrew Lawson: Discoverer of the San Andreas Fault
- The Day California Cracks at BusinessWeek online (09/19/05)
- Waiting for the big one by Shannon Brownlee (July 1986)
