San Andreas Fault

Tanya Hausman, Amy Lange
  • Author
    Tanya Hausman

    Tanya has taught for 21 years, anywhere from 1st through 9th grades, as well as STEM. She has a bachelor's in elementary education with a middle school math endorsement from Oklahoma Wesleyan University. She has a current professional teaching license and years of experience creating interesting, engaging lessons for her students.

  • Instructor
    Amy Lange

    Amy has taught university-level earth science courses and has a PhD in Geology.

Explore the San Andreas fault. Learn the definition of the San Andreas fault, its location on a map, activity in California, and key facts about the fault system. Updated: 03/03/2022

Table of Contents


What Is the San Andreas Fault?

The San Andreas Fault (or SAF) is the most well-known fault in the world. Stretching along California, USA, the San Andreas Fault system covers highly populated areas and receives a lot of attention whenever something happens. From the famous 1906 San Francisco earthquake to less devastating earthquakes that have happened more recently, the SAF gets significant news coverage whenever the ground shakes. Many people have therefore at least heard about this fault system.

To understand what the SAF is, it's important to first know the definition of a fault. A fault is a crack in the Earth's crust where the two sides move against each other. There are different types of faults, including divergent faults, convergent faults, and dip-slip faults. The San Andreas Fault definition is that of a strike-slip, or transform, fault. In a strike-slip fault, the two sides of the fracture move horizontal and parallel to the fault. The two sides move horizontally past one another. To be even more technical, the SAF is a right lateral transform fault, meaning if a person was standing on each side of the fault, each would see the other person move to the right.

How Deep Is the San Andreas Fault?

Faults do not go forever until the center of the Earth. The depth of a fault can vary depending on multiple factors. How deep the San Andreas Fault is, approximately, is 10 miles (16 km). In length it stretches about 800 miles (1,287 km), curving somewhat north and south through much of California.

The blue line is the San Andreas Fault line.

color map of California showing the San Andreas Fault

San Andreas Fault Location

The SAF is where two tectonic plates touch. The Pacific Plate is moving northwest while the North American Plate is moving southeast. The San Andreas Fault location begins in Northern California, south of Cape Mendocino. It moves southeast going through major cities such as Santa Rosa, San Francisco, Desert Hot Springs, San Jose, and winds down to San Bernardino outside of Los Angeles and the Salton Sea. There are several branching faults that come out from the SAF, making up the fault system. It also runs along less populated areas, such as the northern edge of the San Gabriel mountains.

Areal view of the San Andreas Fault through one of the unpopulated stretches.

color areal view of the San Andreas Fault

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San Andreas Fault Map

The SAF is often shown on maps as a long, single line. This is not accurate, but is done for simplicity. As stated in the previous section, several branching faults come out from the SAF, creating an entire system. In the San Andreas Fault map image in this lesson, there are a few thinner blue lines that show some of those branching faults that come off of it. In the tectonic plates map, notice where the Pacific Plate and North American Plate share a border. That border creates the San Andreas Fault.

Tectonic plate boundaries in North America. Where the Pacific and North American Plates meet is the San Andreas Fault.

color image of tectonic plates covering North America

San Andreas Fault Boundary Type

The San Andreas Fault Boundary type is a transform boundary; that is, where two plates slide past each other. The Pacific Plate and North American Plate move relative to each other about 2.5 inches each year (about the same speed as your fingernails grow). The average slip rate (how much the plates slip past each other) for the SAF is only about 1.5 inches each year, due to rocks from the two plates getting stuck on each other. At any of the places these two plates touch, they can get stuck and cause an earthquake. The difference between these amounts of movement are constantly being compensated for by the many slip faults there are within the SAF system.

San Andreas Fault Facts

Here are some more San Andreas Fault facts:

  • The San Andreas Fault can be seen from space. It looks like a valley where the two plates meet.
  • SAF is unique because it is a transform boundary on land. Most of these plate boundaries exist in the ocean.
  • The fault divides into three sections with different characteristics and earthquake risks. The area with the highest risk is the southern section, which passes close to Los Angeles.

Impact of Faults

The largest impact faults have is earthquakes. When the plates get stuck on each other (also called "locked"), the rocks deform and build up stress. Eventually, the rocks break or slip, allowing the plates to suddenly move which creates an earthquake. This entire process is called the elastic rebound theory. As the plates move and scrape against each other they create seismic waves that shake the ground above. According to this elastic rebound theory, usually not all of the accumulated energy is released in the initial earthquake. What then happens is what is called "aftershocks", as smaller earthquakes take place after the main one to expel the rest of the energy.

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Frequently Asked Questions

What happens if the San Andreas fault moves?

The San Andreas fault moves all the time, about 2.5 inches every year. If the rocks in the fault lock, build up stress and then suddenly release, it can cause an earthquake.

What cities does San Andreas fault run through?

The SAF runs through and by several major cities. Cities such as Desert Hot Springs, San Francisco, San Jose, and Los Angeles are all on or near the fault line.

Where is the San Andreas fault located?

The San Andreas Fault is located in California, USA. It starts in the northern part of the state and moves southeast.

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