Types of Faults

Faults are distinguished on the basis of the movement of the footwall relative to the hanging wall (Figures TL.23a-d). Dip slip faults are those in which vertical displacement primarily occurs. 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 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. A reverse fault is one in which the hanging wall moves up relative to the foot wall due to compression . If the hanging wall is pushed up and then over the foot wall at a low angle it is called a thrust fault. A horst is formed when a block of rock is pushed up between two faults. 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. Imagine That! -- "Himalayan Thrust Fault" : NSF/Finger Lakes Productions International

normal fault

Figure TL.23a Normal Fault

reverse fault

Figure TL.23b Reverse Fault

transform fault

Figure TL.23c Transform Fault

thrust fault

Figure TL.23d Thrust Fault

basin and range

Figure TL.25 The Great Basin located in the upper center of this shaded relief map is composed of parallel mountain ranges with intervening basins caused by faulting (Courtesy USGS)

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. 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.

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Figure TL.26 Cumulative global earthquakes 1960 -1995
Courtesy NASA (Source)

Map of New Madrid Seismic Zone

Figure TL.27 New Madrid Seismic Zone
Courtesy USGS (Source)

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. The seismic zone can be seen in Figure TL.26.

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