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What Is The San Andreas Fault?

san andreas fault
Oblique aerial view of San Andreas Fault (between white arrows) in southeastern Coachella Valley, near Red Canyon; view to the west.
Credit: USGS.

California's sleeping giant, the San Andreas Fault, marks the slippery yet sticky boundary between two of Earth's tectonic plates. It is responsible for the biggest earthquakes in California, up to at least magnitude 8.1.

Beginnings

The San Andreas Fault was born about 30 million years ago in California, where the North America plate meets the Pacific plate at a continuous, narrow break called a fault. Though there are many fractures and faults that mark the zone where the two plates slide past one each other, the San Andreas Fault is the main boundary between the two plates.

On the west side of the fault sits most of California's population, riding the Pacific Plate northwest while the rest of North America inches south. The Pacific Plate is moving to the northwest at 3 inches (8 centimeters) each year, and the North American Plate is heading south at about 1 inch (2.3 cm) per year.

Researchers have measured identical rocks offset by 150 miles (241 kilometers) across either side of the fault. For example, the volcanic rocks in Pinnacles National Park south of Monterey match volcanic rocks in Los Angeles County (called the Neenach volcanics). Geologists think the total amount of displacement along the fault is at least 350 miles (563 km).

NorCal vs. SoCal

The San Andreas Fault is about 800 miles long (1,287 kilometers), stretching from the Mendocino coast south to the San Bernardino Mountains and the Salton Sea. Geologists divide the fault into northern and southern segments, separated in the middle by a curiously quiet portion that "creeps."

The northern segment runs from Hollister north through the Point Reyes National Seashore, then eventually moves offshore. The southern segment stretches from Parkfield south through the Salton Sea.

The central, creeping section includes everything from Parkfield to Hollister. In historical times, this creeping section has not generated powerful earthquakes similar to those on the "locked" sections.

That's because the creeping section continuously moves, while the locked sections seem to get stuck. These stuck sections of the fault store energy like springs, slowly building up strain until — sproing! — they suddenly unzip and slide past one another in an earthquake.

South of the creeping section, the fault also has a well-known "Big Bend" that helps push up some of southern California's spectacular mountain ranges and the faults that slice up Los Angeles. Near the town of Gorman, the fault suddenly swings east for about 6 miles (10 km), the turns south again. This bend creates geologic squeezing and stretching (compression and tension) between the two tectonic plates that's relieved building mountains (squeezing) and faulting, or breaking, the Earth's crust.

Earthquake prediction

The San Andreas Fault is the site of a massive effort to drill into the Earth's crust and investigate a fault at depth. In 2004, work began near the town of Parkfield on the San Andreas Fault Observatory at Depth (SAFOD) to drill nearly 2 miles (3.2 km) into the fault.

Parkfield, in central California, pops off a moderate earthquake of around magnitude 6 every couple decades, and is a center for earthquake research. It was the site of the first official earthquake prediction by the U.S. Geological Survey. Scientists predicted another earthquake should occur in 1993, but it didn't happen until 2004. Previous quakes hit in 1857, 1881, 1901, 1922, 1934 and 1966.

san andreas fault map
Map of the modern San Andreas Fault in relation to the greater plate-tectonic setting of western North America.
Credit: USGS.

Earthquake history

The largest earthquakes in California since European settlers arrived struck in 1857 and 1906 on the San Andreas Fault. The Jan. 9, 1857, Fort Tejon earthquake in southern California, an estimated magnitude 7.9, offset stream channels by as much as 29 feet (9 meters).

The April 18, 1906, San Francisco earthquake triggered a deadly fire in the growing city and killed some 700 people. The earthquake was an estimated magnitude 8.3 and broke the Earth's surface along a 250-mile length (402 km), from San Juan Bautista to Cape Mendocino.

Researchers worry that the southern section of the San Andreas fault has not suffered a big earthquake for at least 300 years. An event similar in size to the 1857 and 1906 earthquakes would substantially damage Palm Springs and other inland cities.

Experts say the southern portion of the fault is capable of triggering a magnitude 8.1 earthquake, by rupturing all the way from Parkfield to the Salton Sea.

Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+.

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Author Bio
Becky Oskin, OurAmazingPlanet Staff Writer

Becky Oskin

Becky Oskin is a senior writer for Live Science. She covers earth science, climate change and space, as well as general science topics. Becky was a science reporter at The Pasadena Star-News and has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what her latest project is, you can follow Becky on .
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