The Four BASIC Types Of Faults
(This does not include growth faults)
The Normal Fault
The normal fault is not necessarily normal in the sense that it is common….because…. it is not the most common of faults. However what is normal about them is that their movement tends to follow the gravitational pull on the fault blocks involved. The fault plane on the normal fault is generally very steep. In a normal fault the two involved blocks are (by gravity) pulling away from one another causing one of the fault blocks to slip upward and the other downward with respect to the fault plane (it is hard to determine whether both or just one block has moved.). The exposed upward block forms a cliff-like feature known as a fault scarp. A scarp may range from a few to hundreds of meters in height and their length may continue for 300 or more kilometers (around 200 miles).
The Reverse Fault
The reverse fault is a normal fault except the general movement of the fault blocks is toward
each other, not away from each other as in the normal fault. This forms a thrust fault type expression on the surface with material overlaying other material.
Transcurrent Fault (Strike-Slip Fault)
Probably the most well known and well studied fault is the transcurrent (strike-slip) fault known as the San Andreas fault of California. This fault marks the margin line between the Pacific and North American Plates. Movement on a strike strip fault is generally horizontal. On the surface, scarps form as hills crossing the fault zone are torn apart by movement over time. Actually anything crossing this fault zone is either slowly torn apart, or offset. Rivers crossing the fault line are called offset streams and are classic signatures of fault activity along the San Andreas. These faults can be very long, the San Andreas is nearly 600 miles long.
In the 1994 Northridge, California event, a deep thrust fault located about 18 km under the city of Los Angeles produced an eartquake that registered a magnitude of 6.7. When thrust faults are exposed on the surface overburnden material lies over the main block. They are normally associated with areas of folded surfaces and or mountaineous regions. The dip angles of thrust faults are normally not as steep as a normal fault. Chief Mountain, in Montana (one of the places we look at using the USGS quads in the principles of geography class) is an example of a thrust mountain.