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

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

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

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

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

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

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

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

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

Page created June 2, 2006 by Lisa Gardiner. Last modified August 22, 2008 by Lisa Gardiner.
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