San Andreas Fault Facts

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


Viewed from space, the San Andreas Fault looks like a long, narrow valley that marks where the North America plate meets the Pacific plate. This narrow break between the two plates is called a fault. But viewed up close, there are actually many fractures and faults that mark the zone where the two plates slide past one each other. Sometimes the boundary is a zone of several smaller faults, one or more of which may break during an earthquake. Sometimes it is a single fault. 

On the ground, one can find the San Andreas Fault by looking for landforms it created. For example, sharp cliffs called scarps form when the two sides of the fault slide past each other during earthquakes. "The dominant motion along the fault is primarily horizontal, but some areas also have vertical motion," noted Shimon Wdowinski, a geophysicist at the University of Miami's Rosentiel School of Marine & Atmospheric Sciences who has studied the San Andreas Fault. And stream channels with sharp jogs — the channels are offset across the fault line — can be visited in the central California's Carrizo Plain National Monument.

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.

The San Andreas Fault was born about 30 million years ago in California, when the Pacific Plate and the North America plate first met. Before then, another oceanic plate, the Farallon plate, was disappearing beneath North America at a subduction zone, another type of plate boundary. The new configuration meant the two plates slid past one another instead of crashing into each other, a boundary called a strike-slip fault.

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) since it formed.

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." [Photo Journal: The Gorgeous San Andreas Fault]

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 slowly, 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 visible "Big Bend" that helps push up some of southern California's spectacular mountain ranges. 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 between the two tectonic plates. The stress on Earth's crust is relieved by building mountains (squeezing) and faulting, or breaking, the Earth's crust, such as the faults that slice up Los Angeles, Wdowinski said. 

Earthquake prediction

The San Andreas Fault was the site of a massive effort to drill into 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.

Rocks retrieved from the deep drilling project revealed that slippery clays may be responsible for some of the "creeping" behavior along the San Andreas Fault. "They found some talc-like minerals," Wdowinski said. "This material has less friction than on the northern and southern section of the fault, so the central section of the fault can creep."

Map of the modern San Andreas Fault in relation to the greater plate-tectonic setting of western North America. (Image 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 m). The U.S Geological Survey estimates that a similar-size earthquake today, in the same location, would damage half the buildings in Los Angeles, destroy the city's water supply and injure more than 50,000 people.

After the Fort Tejon quake came the April 18, 1906, San Francisco earthquake, which triggered a deadly fire in the growing city and killed some 700 people. The earthquake was an estimated magnitude 7.9 (or an 8.3 on the Richter scale) and broke the Earth's surface along a 250-mile length (402 km), from San Juan Bautista to Cape Mendocino. [In Photos: The Great San Francisco Earthquake]

The San Andreas Fault has been unusually quiet since these two big earthquakes in 1857 and 1906. Recently, studies looking at the fault's past earthquakes suggest that instead of popping off big earthquakes on a regular schedule, the fault seems to release its pent-up energy through earthquakes that vary in size and timing. Scientists now think the San Andreas Fault needs time to build up a critical stress level before it breaks again.

"What is continuous with time is the accumulation of strain," Wdowinski said. "The plates move at a regular rate. What varies is the method of release, and since the release is not uniform, each earthquake doesn't repeat exactly what happened before."

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Becky Oskin
Contributing Writer
Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she 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.