What is a Fault?

Brianna Cowling, Charles Spencer
  • Author
    Brianna Cowling

    Brianna graduated from Henderson State University in 2016 with a B.S. in Psychology and Biology. She has been a secondary science teacher for 5 years and has written curriculum and science lessons for other companies. She is a Certified Google Level 1 Educator and is part of the Edulastic Innovator Team and her campus Leadership Team.

  • Instructor
    Charles Spencer

    Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

What is a fault? Learn the definition of a fault, the types of faults and view diagrams. These include dip slip faults, thrust faults, reverse faults, and more. Updated: 08/05/2021

Table of Contents


What is a Fault?

A fault is a crack in the Earth's lithosphere that occurs between two pieces of rock. Rock pieces are able to move at a fault, meaning earthquakes can occur if the movement pushes those rock pieces into or past one another. Faults can range in length from a few millimeters to thousands of kilometers and are found all over the world. However, many faults are inactive, meaning they do not have earthquakes that occur. Active faults are faults where earthquakes do occur. The term fault came from the Old French term falte, which means opening or gap.

Earthquakes and Creeps

Pieces of rock along a fault are in continuous movement, although most of the time these movements are so slow that people do not notice them. A creep is a slow movement that occurs along a fault that goes without recognition. However, larger movements along a fault result in a tremendous release of energy. This quick release of energy is known as an earthquake. An earthquake is a sudden slip along a fault. The sudden slip causes the ground to shake and send out waves of seismic energy. The area underground where the sudden slip occurred is called the focus, and this is where the seismic waves originate, flowing out from this point. The point on Earth's surface directly above where the sudden slip occurred is called the epicenter. Depending on how large an earthquake is, these seismic waves can cause just a minimal amount of shaking or large amounts of destruction.

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  • 0:02 Definition of Faults
  • 2:00 Types of Faults
  • 4:37 Active vs Inactive Faults
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Types of Faults

There are three different types of faults, which are separated by how the sections of rock move in relation to each other along the fault:

  • Dip-Slip Faults - occur at an angle in comparison to the surface of Earth
  • Strike-Slip Faults - rocks slide past one another
  • Oblique-Slip Faults - combine dip-slip and strike-slip movements

Scientists classify types of faults based on the angle of the fault as it relates to the surface of Earth and the direction of the movement along the fault.

Dip-Slip Fault

A dip-slip fault is a fault where one side is inclined above the higher due to vertical movement. A dip-slip fault is caused by one of two types of stresses on the rock. A dip is the angle of the fault's incline, while the slip is the displacement of adjacent points on two sections of rock, typically on either side of a fault. There are two types of dip-slip faults: normal and reverse faults. A normal fault is one in which the footwall, or the side of rock above the inclined fault, moves upward. The other side of the rock is called the hanging wall, and it moves downward in a normal fault. This occurs because of tensional stress, which pulls two sides of rock away from one another, causing one side of the rock to move downward.

Plate Movement In A Normal Fault

Normal Fault

A reverse fault is one in which the footwall moves downward and the hanging wall moves upward. A reverse fault is created by compressional stress, which pushes two plates of rock toward one another. Eventually, one side of rock begins to move above the other rock as a result of the pressure. A reverse fault can also be referred to as a thrust fault. The thrust fault is a reverse fault that has a dip less than 45 degrees. There is also the possibility of a blind thrust fault occurring. A blind thrust fault is a type of fault that does not cause the surface of Earth to crack, so there is no evidence of the existence of the fault.

Plate Movement In A Reverse Fault

Reverse Fault

Strike-Slip Faults

A strike-slip fault is a fault where the pieces of rock on either side of the fault move horizontally along the fault in opposite directions of each other. Strike-slip faults are created by a type of stress known as shear stress, which pushes the two sides of rock past one another. Strike-slip faults are classified as either right-lateral or left-lateral, depending on the direction in which a section of rock is moving. A right-lateral strike-slip fault occurs when the block of rock on the other side of the fault moves to the right. The San Andreas Fault is an extremely well-known fault that is located in California. It is a right-lateral strike-slip fault. A left-lateral strike-slip fault occurs when the block of rock on the other side of the fault moves to the left.

Plate Movement In A Strike Slip Fault

Strike Slip Fault

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Frequently Asked Questions

What are the 3 main types of faults?

Dip-Slip faults occur at an angle in comparison to the surface of Earth, Strike-Slip faults have rocks that slide past one another, and Oblique-Slip faults combine dip-slip and strike-slip movements.

What is a fault in the earth?

The geological fault definition states that a fault is a crack in the lithosphere that occurs between two pieces of rock.

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