San Andreas Fault

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The San Andreas Fault is a geological fault that runs a length of roughly 800 miles (1287 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.

Contents

1 Southern, central, and northern segments
2 Scientific research

2.1 Historical data
2.2 The SAFOD project
2.3 Slip rates
2.4 The University of California study on \"the next big one\"

3 Notable earthquakes
4 Trivia and pop culture references
5 See also
6 External links

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 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.

USGS map of the fault in California

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 results in the fault being able to move without the need of earthquakes.

The northern segment of the fault runs from Hollister, through the San Francisco Peninsula where it briefly goes offshore, then follows the coast of California fairly closely to Cape Mendocino, where there is an unstable triple junction where three tectonic plates come together to form a geologically complex and highly unstable zone. To the north lies the Cascadia subduction zone, where the Juan de Fuca plate and the last bit of its lower section sometimes called the Gorda plate, are being subducted under the margin of the North American plate. And west, along the continental shelf, lies the Mendocino fault zone, the strike-slip boundary between the main Juan de Fuca plate and the Pacific plate. This northern segment in the San Francisco area is paralled to the east by at least three "sister faults", which respond to the same plate motion. See Hayward Fault Zone for an example.

Scientific research

Historical data

The fault was first identified and named by Andrew Lawson in 1895. The small town of Parkfield, California 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.

The SAFOD project

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. [link]

Slip rates

Other research monitoring slip rates along the fault has shown that Los Angeles and San Francisco (which rest on opposite sides of the San Andreas Fault) are moving towards one another at a rate of 1/5 inch (0.6 cm) a year.

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 its 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.

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. 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 near Santa Cruz, California, causing 63 deaths and heavy damage to 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.

Trivia and pop culture references

The plot of the original 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 , is set in a fictional state named after this fault.

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 will move northwest, because the fault is a transform fault (the two land masses are sliding by each other, not moving apart).

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

External links

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