What is a fault?

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is the intersection of a fault plane with the ground surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault.

Fault Types

Three main types of faults

Faults are subdivided according to the movement of the two blocks. There are three or four primary fault types:

Normal fault

A dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension. “Occurs when the “hanging wall” moves down relative to the “foot wall””

Reverse fault

A dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, When the dip angle is shallow, a reverse fault is often described as a thrust fault. “Occurs where the “hanging wall” moves up or is thrust over the “foot wall””

Strike-slip fault

A fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.

Types of Strike-slip fault movement

A left-lateral strike-slip fault

If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away

A right-lateral strike-slip fault

If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the right block moves toward you and the left block moves away.

Reference:
USGS: What is a fault and what are the different types?
USGS: Fault
University of Saskatchewan: Fault Types
University of Wisconsin System: Types of Earthquakes & Faults