What Is a Geologic Fault?

A fault is a crack in the Earth's crust. Typically, faults are associated with, or form, the boundaries between Earth's tectonic plates. In an active fault, the pieces of the Earth's crust along a fault move over time. The moving rocks can cause earthquakes. Inactive faults had movement along them at one time, but no longer move. The type of motion along a fault depends on the type of fault. The main types of faults are described below.

  • Normal dip-slip fault
    • Normal faults happen in areas where the rocks are pulling apart (tensile forces) so that the rocky crust of an area is able to take up more space.
    • The rock on one side of the fault is moved down relative to the rock on the other side of the fault.
    • Normal faults will not make an overhanging rock ledge.
    • In a normal fault it is likely that you could walk on an exposed area of the fault.
  • Reverse dip-slip fault
    • Reverse faults happen in areas where the rocks are pushed together (compression forces) so that the rocky crust of an area must take up less space.
    • The rock on one side of the fault is pushed up relative to rock on the other side.
    • In a reverse fault the exposed area of the fault is often an overhang. Thus you could not walk on it.
    • Thrust faults are a special type of reverse fault. They happen when the fault angle is very low.
  • Transform (strike-slip) faults
    • The movement along a strike slip fault is horizontal with the block of rock on one side of the fault moving in one direction and the block of rock along the other side of the fault moving in the other direction.
    • Strike slip faults do not make cliffs or fault scarps because the blocks of rock are not moving up or down relative to each other.

However, faults are usually more complex than these diagrams suggest. Often movement along a fault is not entirely of one variety. A fault may be some combination of strike slip and normal or reverse faulting. To further complicate these conditions, faults are often not just one orderly break in the rock, but are instead a number of fractures caused by similar motions of the Earth's crust. These clusters of faults are called fault zones.

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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