What Is a Geologic Fault?

A fault is a crack in the Earth's crust. Many faults are found along the edges of Earth's plates. Over many many years, the pieces of the Earth's crust can move along a fault in different directions. The moving rocks can cause earthquakes. Along some faults the pieces of the Earth's crust no longer move. These are called inactive faults.

There are different types of faults. The pictures below show what the most common types are like.

  • Normal faults
    • Normal faults happen in areas where the rocks are pulling apart.
  • Reverse faults
    • Reverse faults happen when rocks are pushed together.
    • Thrust faults are a special type of reverse fault. They happen when the angle of the fault is close to flat.
  • Transform (strike-slip) faults
    • In these faults, one piece of crust is moving in one direction and another piece of crust is moving in the oposite direction.
    • Strike slip faults do not make cliffs or fault scarps because the blocks of rock are not moving up or down.

Many faults are a combination of types.

What Is an Earthquake?

Where Do Earthquakes Happen?

Why Do Earthquakes Happen?

Seismic Waves: Moving and Shaking During an Earthquake

Movie: Designing Earthquake-resistant Buildings

Picture it: Take a look at a fault in Loma Pietra, California

Making Earthquakes... Indoors - streaming RealVideo (1 min. 6 sec.) from NSF

Last modified January 19, 2010 by Randy Russell.

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