Scientists Say: Fault

A fault is a crack in Earth’s crust where pieces of rock scrape past each other

The San Andreas Fault in California (pictured) is the boundary between two tectonic plates — the North American Plate and the Pacific Plate.

Kevin Schafer/Getty Images

Fault (noun, “FAWLT”)

A fault is a crack in Earth’s crust where rocks slide past each other. Such fractures in Earth’s surface can span centimeters (inches) or can be hundreds of kilometers (miles) long. The biggest faults are borders between Earth’s tectonic plates. The rocks on either side of a fault can scrape past each other slowly. Or, tension can build up between rocks, causing them to jerk past each other suddenly. Such violent lurches cause earthquakes.

There are a few different types of faults. They are classified based on how the rocks move past each other. At a normal fault, two pieces of rock pull away from each other. This causes one piece to slide down relative to the other. Normal faults can form valleys. At a reverse or thrust fault, two pieces of rock are shoved together. This pushes one slab of rock on top of the other. Such collisions help form mountain ranges. A strike-slip fault is one where two pieces of rock slide horizontally past each other. California’s famous San Andreas Fault is one example. At oblique-slip faults, rocks slide both up and down and side to side.

three illustrations show how rocks move relative to each other in different types of faults
Slabs of Earth’s crust move differently at different types of faults. Reverse or thrust faults shove two pieces of rock together. Normal faults pull two pieces of rock apart. At strike-slip faults, rocks slide past each other. Trista L. Thornberry-Ehrlich (Colorado State University)

In a sentence

The San Andreas Fault in California caused a 1906 earthquake that leveled San Francisco.

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Maria Temming is the assistant editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

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