15.3.2: Types and Geographic Patterns of Faults
Types of Faults
Faults are distinguished on the basis of the movement of the footwall relative to the hanging wall. Dip slip faults are those in which vertical displacement primarily occurs.
Video : Normal Fault (Courtesy USGS)
A normal fault is one in which the hanging wall falls down relative to the foot wall due to tensional stress . The Teton Mountains in North America are a result of normal faulting. A reverse fault is one in which the hanging wall moves up relative to the foot wall due to compression. A thrust fault is a reverse fault at 45 o angle.
Video : Thrust Fault (a reverse fault at 45 o angle) (Courtesy USGS)
Video : Horst and Graben (Courtesy USGS)
A horst forms between two normal faults when a block of rock in the middle remains stationary and the blocks on either side subside. A graben forms when a block of rock falls between two faults. Huge, relatively flat bottomed valleys, like Death Valley in California, are created in this way. The Basin and Range physiographic province of North America is noted for its faulted mountains (horsts) and flat-bottomed valleys (grabens). Large plateau surfaces form in this fashion. The Basin and Range Province of North America is noted for its faulted mountains and flat-bottomed valleys.
Video : Strike slip / Transform Fault
Strike slip or transform faults are those that primarily exhibit horizontal movement . Such activity can be discerned from surface features like offset streams. The San Andreas fault is a well-known strike slip fault caused by the Pacific Plate sliding past the North American Plate. ( View a segment of the San Andreas fault in Google Earth )
Oblique faults exhibit both horizontal and vertical movement.
Geographic Patterns of Faults and Earthquakes
The fracturing and movement of the Earth's surface have been linked to stresses created by movements of molten rock within the asthenosphere. Convection currents within this subsurface region tug at crustal plates, driving them into or past one another, or pulling them apart. Thus, the patterns of earthquakes and faults are closely associated with the boundaries of plates.
Though faulting and earthquakes are common along plate boundaries, they also occur in zones of weakness within the interior of a plate. The New Madrid Seismic Zone is the site of one of the largest earthquakes to strike North America (1811-1812). Though it had relatively little effect on human populations due to its location in a sparsely populated area at the time, it had a significant impact on the natural environment. The largest earthquakes caused the Mississippi River to flow backwards and low water falls were formed from the fracturing of the earth. 150,000 acres of forest ere destroyed and Reelfoot Lake, KY was formed. Now, several large population centers could be affected by the seismic zone (Note: this links to pdf).