A major fault in the Earth's crust running for about 950 km/600 mi through NW California to the Colorado Desert. It marks the boundary between the Pacific and American crustal plates, which are slipping past each other at an average rate of 1 cm/? in a year. Sudden movements can cause earthquakes, the most notable of which devastated San Francisco in 1906. Serious movement also occurred in 1989, 1993, and 1994.
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.
Southern, central, and northern segments
The San Andreas Fault can be divided into three segments. 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. 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. 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 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.
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. 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.
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.
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. 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. 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.
"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.
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. 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. See also: List of earthquakes
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