San Andreas Fault: Location, Facts & Earthquakes

San Andreas Fault: Location, Facts & Earthquakes
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  • 0:00 What Is the San…
  • 2:00 Understanding Earthquakes
  • 3:20 San Francisco 1906
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

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

This lesson covers the extremely active San Andreas Fault system in California. You'll learn some basics about the San Andreas and how earthquakes are produced along the 800-mile fault system.

What Is the San Andreas Fault System?

California has a bad reputation for having a lot of earthquakes. The news regularly has stories about quakes shaking the area between Southern and Northern California. Why is this part of the country more prone to earthquakes than other areas? The answer lies in the San Andreas Fault system, which is the focus of this lesson.

The San Andreas Fault system stretches for 800 miles and extends to over 10 miles deep within the rocks of California. It marks the boundary where the Pacific plate is moving to the north and the North American plate is moving south.

To truly master this topic, let's review what a fault is. A fault is a fracture where two blocks of the Earth's crust have moved past one another. Faults are classified based on the direction of the slip between the blocks of earth. The faults in the San Andreas system are called strike-slip faults. At strike-slip faults, one block or both blocks move horizontally past one another. Notice how movement along the fault caused an offset fence and a bend in the river on the surface.

Strike-slip faults occur where two blocks of earth move horizontally past one another.
Illustration of strike-slip fault

Faults can occur anywhere on the Earth's surface. The San Andreas Fault system is not just a simple strike-slip fault. It is actually a transform boundary separating the Pacific and North American tectonic plates. Transform boundaries are where two tectonic plates grind laterally past one another. Movement along the transform boundaries occurs along a complex series of strike-slip faults.

The San Andreas Fault system lies along the boundary between the Pacific and North American plates. The Pacific plate is moving north and the North American plate is moving south.
map showing Pacific and North American plates

The San Andreas Fault is commonly shown on maps as a long single line in the Earth's crust which gives the misperception that the fault is just a single fault. However, it is commonly shown this way just for simplicity, not accuracy. If you focused on any one part of the fault system, you would see that there are numerous smaller faults that make up the whole fault system. This map shows the complex faults included in and adjacent to the San Andreas Fault system. Faults are the dark black lines on this map.

Map of California faults

Understanding Earthquakes

While we think of plates as being completely rigid, in reality, they can bend and behave elastically over short periods of time. Imagine bending a thin twig in your hand. The twig is slightly elastic, so it will bend until you apply enough pressure and it snaps. The Earth's crust behaves much the same way. Faults can become locked due to the friction between the two blocks as they move past each other. More and more energy is built up between the stuck blocks until the elastic limit is overcome and movement immediately occurs. The elastic limit is the strain that must be accumulated before movement can occur along the fault. This theory of faults accumulating energy prior to an earthquake is known as the elastic-rebound theory.

These illustrations show the process of elastic rebound. At time one, no force is applied to the rock. At time two, force is applied and the rock is behaving elastically. At time three, the elastic limit of the rock was exceeded and displacement occurred.

Elastic Rebound illustration

If you've ever been in an earthquake, you know that earthquakes often come in sets. There is a larger initial earthquake, which is followed by smaller quakes called aftershocks. According to the elastic-rebound theory, these aftershocks are caused by the accumulated elastic energy that did not get released during the initial earthquake.

San Francisco 1906

The San Andreas Fault zone is responsible for one of the most destructive earthquakes in U.S. history, which occurred in San Francisco in 1906. While this earthquake lasted only a minute, it had a magnitude of 8.2. Magnitude is a measure of the amount of energy released during an earthquake, which is measured along the Richter scale. Earthquakes with a magnitude above eight are extremely rare and only occur once every few years on Earth.

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