San Andreas Fault

San Andreas Fault
The fault (right) and the Carrizo Plain (left)
Arrows show relative motion of the North American plate (southeastward) and the Pacific plate (northwestward)
Named by	Andrew Lawson
Year defined	1895
Coordinates	35°07′N 119°39′W / 35.117°N 119.650°W / 35.117; -119.650
Country	United States, Mexico
State	California, Baja California, Sonora
Cities	San Francisco, San Bernardino, San Juan Bautista
Characteristics
Segments	Calaveras, Hayward, Elsinore, Imperial, Laguna Salada, San Jacinto
Length	1,200 km (750 mi)
Displacement	20–35 mm (0.79–1.38 in)/yr
Tectonics
Plate	North American & Pacific
Status	Active
Earthquakes	1857, 1906 (Mw ≈7.8), 1957 (Mw 5.7), 1989 (Mw ≈6.9), 2004
Type	Transform fault
Movement	Dextral
Age	Neogene-Holocene
Orogeny	Gorda-California-Nevada

The San Andreas Fault is a continental right-lateral strike-slip transform fault that extends roughly 1,200 kilometers (750 mi) through the U.S. state of California.^[1] It forms part of the tectonic boundary between the Pacific plate and the North American plate. Traditionally, for scientific purposes, the fault has been classified into three main segments (northern, central, and southern), each with different characteristics and a different degree of earthquake risk. The average slip rate along the entire fault ranges from 20 to 35 mm (0.79 to 1.38 in) per year.^[1]

In the north, the fault terminates offshore near Eureka, California, at the Mendocino triple junction, where three tectonic plates meet. The Cascadia subduction zone intersects the San Andreas fault at the Mendocino triple junction. It has been hypothesized that a major earthquake along the Cascadia subduction zone could trigger a rupture along the San Andreas Fault.^[2][3][4]

In the south, the fault terminates near Bombay Beach, California, in the Salton Sea. Here, the plate motion is being reorganized from right-lateral to divergent. In this region (known as the Salton Trough), the plate boundary has been rifting and pulling apart, creating a new mid-ocean ridge that is an extension of the Gulf of California. Sediment deposited by the Colorado River is preventing the trough from being filled in with sea water from the gulf.

The fault was first identified in 1895 by Professor Andrew Lawson of UC Berkeley. In the wake of the 1906 San Francisco earthquake, Lawson was tasked with deciphering the origin of the earthquake. He began by surveying and mapping offsets (such as fences or roads that had been sliced in half) along surface ruptures. When the location of these offsets were plotted on a map, he noted that they made a near perfect line on top of the fault he previously discovered. He concluded that the fault must have been the origin of the earthquake.

This line ran through San Andreas Lake, a sag pond. The lake was created from an extensional step over in the fault, which created a natural depression where water could settle. A common misconception is that Lawson named the fault after this lake. However, according to some of his reports from 1895 and 1908, he actually named it after the surrounding San Andreas Valley.^[5] Following the 1906 San Francisco earthquake, Lawson also concluded that the fault extended all the way into Southern California. In 1953, geologist Thomas Dibblee concluded that hundreds of miles of lateral movement could occur along the fault.

A National Science Foundation funded project called the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, California, involved drilling through the fault from 2004 to 2007. The aim was to collect core samples and make direct geophysical and geochemical observations to better understand fault behavior at depth.^[6]