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Faults |
| Through the study of faults and their effects, much can be learned about the size and recurrence intervals of earthquakes. Faults also teach us about crustal movements that have produced mountains and changed continents. Initially a section of Earth's crust may merely bend under pressure to a new position. Or slow movement known as seismic creep may continue unhindered along a fault plane. However stresses often continue to build until they exceed the strength of the rock in that section of crust. The rock then breaks, and an earthquake occurs, sometimes releasing massive amounts of energy. The ensuing earth displacement is known as a fault. This slide set describes the mechanism and types of faulting. It illustrates a variety of fault expressions in natural and manmade features. |
Fault block diagram | |
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Diagram showing the three main types of fault motion. |
Aerial view of San Andreas fault | |
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Right-Lateral Strike-Slip Faults |
View of cross section of San Andreas fault | |
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Right-Lateral Strike-Slip Faults View to the east of the Highway 14 road cut that transects the San Andreas fault zone. Since 1994, California Transportation has cut smooth the portion of the San Andreas that bisects Highway 14, creating a spectacular view of the compressional forces in the area. The Northridge, California, earthquake (January 17, 1994) narrowed the San Fernando Valley by 9 cm and raised the top of the hills to the north by approximately 38 cm. The earthquake resulted in increased compression along the San Andreas fault. In this view, a subsidiary fault transects the fold in the right half of the photo. Photo credit: Clifford E. Harwood, Encino, CA |
Second view of cross section of San Andreas fault | |
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Right-Lateral Strike-Slip Faults View to the south of Highway 14 road cut that transects the San Andreas fault zone. In this area, the displacement is not confined to a single fracture but is distributed throughout the zone, and includes many folds as well as several faults. Two subsidiary faults offset the layers and form the two "legs" of a triangle. Photo credit: Clifford E. Harwood, Encino, CA |
Drain offset caused by seismic creep | |
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Right-Lateral Strike-Slip Faults Drain offset produced by seismic creep along the San Andreas fault at the Almaden Winery in central California. The winery is also experiencing offset walls, bent and broken pipes, etc., due to seismic creep. Motion along the fault is mostly strike- slip. Photo credit: University of California, Berkeley |
Fence offset produced by 1906 San Francisco earthquake | |
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Right-Lateral Strike-Slip Faults The fence was offset 2.6 m by the magnitude 8.2 earthquake of April 18, 1906, San Francisco, California. The section of the San Andreas fault shown here is 0.8 km north of Woodville. The photo is looking northeast. The lateral or strike-slip fault offset is large; however, the trace is nearly invisible. This earthquake, together with the fire that followed, resulted in more than 3,000 deaths and 400 million dollars of property damage. Photo credit: G.K. Gilbert, U.S. Geological Survey Photo has been colorized |
En echelon fractures along Imperial fault | |
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Right-Lateral Strike-Slip Faults The Imperial fault east of El Centro, California, shows characteristic right stepping en echelon fractures. The rupture on the fault during the magnitude 6.9 earthquake of October 15, 1979, in Imperial Valley, California, extended from about 4 km north of the international border to about 4 km south of Brawley. Maximum lateral displacement was about 55 cm in the Heber dunes, and maximum vertical displacement was 19 cm southeast of Brawley. The earthquake injured 19 people, and damaged more than 2,000 homes and businesses in the Imperial Valley. Property damage was estimated at $30 million. Photo credit: University of Colorado |
Offset of woodpile along Imperial fault | |
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Right-Lateral Strike-Slip Faults Offset of woodpile shows right-lateral strike-slip offset of fault. The rupture occurred on the fault during the Imperial Valley, California, earthquake of 1979. Photo credit: G. Reagor, U.S. Geological Survey |
Offset of rows in plowed field | |
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Right-Lateral Strike-Slip Faults A fault trace crosses a plowed field. The trace was produced by the October 1979 earthquake in the Imperial Valley. The agricultural industry suffered heavy losses from damage to canals, irrigation ditches, and subsurface drain tiles disturbed by the movement along the Imperial fault. Photo credit: G. Reagor, U.S. Geological Survey |
Offset of cement-lined ditch in Guatemala | |
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Left-Lateral Strike-Slip Faults Offset of a cement-lined ditch by the left-lateral strike-slip Motagua fault resulting from the earthquake of February 4, 1976, in Guatemala. The Motagua fault is part of the transform fault system between the North American and Caribbean plates and comprises the northern boundary of the Caribbean plate. The North American plate is moving 2.2 cm per year to the west along this boundary. The observed surface rupture along the Motagua fault was 320 km. Displacement across the fault was almost entirely horizontal with the strike-slip component ranging up to 3.4 m and averaging 1.1 m. Vertical displacements were less than 30 percent of the horizontal displacement. Faults caused extensive damage to underground facilities as well as to surface structures. Photo credit: U.S. Geological Survey |
Deformation of rows by 1976 earthquake | |
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Left-Lateral Strike-Slip Faults Rows in the cultivated field west of El Progresso, Guatemala, deformed by the earthquake of February 4, 1976. The thick, saturated, unconsolidated deposits have yielded by plastic deformation rather than rupture along the left-lateral strike-slip fault. This quake resulted in the deaths of 23,000 people and $1.1 billion dollars in property damage. Photo credit: U.S. Geological Survey |
Tree bisected by fault in Guatemala | |
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Left-Lateral Strike-Slip Faults This tree was killed by movement along a strike-slip fault through its center, during the same Guatemala earthquake. This was the largest strike-slip event in North America since 1906. Photo credit: U.S. Geological Survey |
Fault scarp near Hebgen Lake, Montana | |
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Normal Faults This fault scarp near Hebgen Lake, Montana, after the magnitude 7.1 earthquake of August 18, 1959, shows a dip-slip movement of 5.5 to 6.0 m. Since the earth above the fracture moved down in respect to the earth below the fracture, the fault is known as a normal fault. Such faults result from extension or stretching of the rock layers. Maximum vertical displacement was 6.4 m observed near Red Canyon Creek. Considerable cracking and shifting of roadways, and the destruction of much timber, caused damage exceeding $11 million. In Yellowstone National Park new geysers started erupting, and large steaming cracks, resulting from massive slumping, were observed after the quake. Photo credit: National Geophysical Data Center |
Fault scarp caused by Borah Peak, Idaho earthquake | |
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Normal Faults This section of the normal fault scarp was produced by the earthquake of October 28, 1983, at Borah Peak, Idaho. It is 678.8 m northwest of the alluvial fan head on Rock Creek. Nearly vertical slickensides on the fault surface are visible on the lower half of the scarp. The scarp is 2.5 m high at this location. The earthquake killed two children in Challis, Idaho, and caused $12.5 million in property damage. Photo credit: R.C. Bucknam, U.S. Geological Survey |
Fault scarp and horizontal offset near Dickey, Idaho | |
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Oblique-Slip Faulting The magnitude 7.2 earthquake of October 28, 1983, near Borah Peak, Idaho, produced this fault. The scarp and fracture zone are located on Rock Creek, near Dickey, Idaho, and Double Springs Pass Road. The wooden pole is 1.9 m high. This normal fault shows characteristics of oblique-slip, where there is both vertical and horizontal displacement. The horizontal movement is left-lateral. The fault scarp extended for more than 35 km, with vertical displacements up to 2.5 m observed between MacKay and Challis, Idaho. Photo credit: G. Reagor, U.S. Geological Survey |
Oblique-slip fault scarp near Pleasant Valley, Nevada | |
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Oblique-Slip Faulting Oblique-slip fault produced during the 7.7 magnitude earthquake of 1915, Pleasant Valley, Nevada. The strike-slip component is shown by the offset of the fence. Photo credit: U.S. Geological Survey |
Oblique-slip fault scarp at Flamingo Heights, California | |
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Oblique-Slip Faulting Oblique-slip fault scarp at Flamingo Heights produced by the June 28, 1992, Landers, California earthquake. Sections of the fault slipped a maximum of 5.5 m horizontally and 1.8 m vertically. The surface rupture was almost 70 km long. In the Landers area, there was about 3 m of right-lateral strike-slip movement. Photo credit: Lindie R. Brewer, U.S. Geological Survey |
Reverse fault near Clark County, Montana | |
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Reverse and Thrust Faults A view of the reverse fault in a roadcut on the west side of French Gulch just south of the Sun River in Lewis and Clark County, Montana, as it appeared in 1966. This reverse fault places the lower beds of the Castle Reef Dolomite (light gray) onto the Flood Shale member of the Blackleaf Formation (dark gray). The fault dips 60 degrees W, and the overlaying strata dip about 50 degrees W. The black shales beneath the fault are badly crumbled, whereas the carbonate beds above it are undisturbed. Photo credit: M.R. Mudge, U.S. Geological Survey |
Blind thrust fault scarp at El Asnam, Algeria | |
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Reverse and Thrust Faults The fault scarp from the earthquake of October 10, 1980, at El Asnam, Algeria, shows a 3-m vertical offset on a blind thrust fault. The movement along this fault results from compression. When the thrust layers encounter opposition at depth, they are forced up vertically, producing a vertical scarp. The scarp extends for kilometers along the landscape of El Asnam. This 7.3 magnitude earthquake killed 5,000 people and caused severe damage. Photo credit: H.C. Shah, Stanford University |
Under-thrust fault from nuclear test | |
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Reverse and Thrust Faults Under-thrust fault resulting from the Gnome explosion, a nuclear test, in Eddy County, New Mexico, 1962. The fault parallels bedding of the lower plate, following a thin clay seam that normally lies 1.6 m above basal clay-like halite at left. The cavity is to the right. Scale is .9 m long. Photo credit: L.M. Gard, Jr., U.S. Geological Survey |