Faults and Faulting      

Geologic faults or fault lines are planar rock fractures that show evidence of relative movement (Wikipedia).   Because faults do not usually consist of a single fracture, the term fault zone is used to refer to a zone of complex fracturing.   Faults have been classified based on the relative movement of the blocks of rock that are involved (Leet, pp. 350-353).   The names of these blocks of rock first came from miners who found themselves standing in a tunnel that ran along or crossed a fault.   They called the block above the fault the hanging wall, and the block below it the footwall.   Types of faults include the following.

Normal Faults

Normal faults (also known as gravity faults) are faults in which the hanging wall has moved downward relative to the footwall.   A normal fault is illustrated in the geologic cross-section on the right.

Reverse Faults

Reverse faults (also known as thrust faults) are faults in which the hanging wall has moved upward relative to the footwall.   A reverse fault is illustrated in the geologic cross-section on the left.

Strike-Slip Faults

A fault in which the movement has been horizontal is called a strike slip fault because the slipping has been parallel to the strike of the fault.   A strike-slip fault is illustrated in the map view on the right. 

 Grabens

Grabens are structures that suggest some settling of the earth.   They include two parallet normal faults as shown in the geologic cross-section on the left.   If the resulting valley is extensive, it may be called a rift valley.

Some Useful Links

The above discussion provides basic information and illustrations for faults land faulting.   To further explore this subject, excellent publications or the resources of the internet can be explored.   Following are some hyperlinks that may help with the evaluation and description of faults:

• What is a Fault? (Berkley Seismological Laboratory):  Website describing the various types of faults and providing a number of useful informational links.
• Faults (University of Leeds, School of Earth and Environment, UK):  Material describing faults and faulting with pictures and drawings.   Based on information presented in their first year course in structural geology.
• A Model of Three Faults (USGS Website):   Educational information about faults, faulting, and tectonics with many hyperlinks to descriptive information.

• USGS maps, products, and publications - Online information resource on the U.S. Geological Survey website.