The San Andreas Fault is a geological fault, known as a right-lateral strike-slip fault, that spans a length of roughly 800 miles (1287 kilometers) through California. The San Andreas Fault marks a transform boundary between the Pacific Plate and the North American Plate. This fault is famous for producing large and devastating earthquakes.

Recent Events

On September 28, 2004 at 10:15 am, a magnitude 6.0 earthquake struck at Parkfield, California on the San Andreas Fault. This earthquake was originally expected to strike in 1993 based on then recently discovered theories of earthquake prediction. Eleven extra years elapsed before this prediction was finally fulfilled. Despite the extra time, the earthquake was no larger than originally expected.

Segments

The San Andreas Fault can be divided into 3 segments.

The southern segment begins in 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 northeast. 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 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 before it makes a sharp turn west and goes offshore near Eureka, California.

Scientific Research

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.

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 2.5 miles (4 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.

Other research that monitors slip rates along the fault has shown that Los Angeles and San Francisco (which rest on opposite sides of the San Andreas Fault) move towards one another at a rate of a 1/4 inch (0.6 cm) a year.

Notable Earthquakes

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

• 1857 - 220 miles (350 kilometers) ruptured in central and southern California, from Parkfield to the Cajon Pass (southeast of Wrightwood). 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. Estimated magnitude = 8.0
• 1906 - 270 miles (430 kilometers) ruptured in Northern California, from San Juan Bautista to Eureka. The epicenter was near San Francisco. Approximately 3000 people died in the quake and subsequent fires. Estimated magnitude = 7.8 See: San Francisco earthquake of 1906.
• 1989 - 25 miles (40 kilometers) ruptured near Santa Cruz, California, causing 63 deaths and heavy but localized damage to the portions of the San Francisco Bay Area. Magnitude = 7.1. See: Loma Prieta earthquake.

See also

• Parkfield Geology
• Plate tectonics
• Seismology
• Seismic retrofit

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 Corrizo Plain section of the fault taken from a Kite