Faults and Earthquakes

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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aerial view of joints An aerial view of jointed sandstone in Southern Utah.
jointed rocks These joints along the Eau Claire Dells in Marathon County break the rocks into roughly rectangular blocks
jointed rocks Along Baird Creek. The lower part of the stream bank is scoured smooth by water, but the upper part retreats as blocks break away along joints. Note also the curving trees, evidence of soil creep.
aerial view of joints The right place at the right time. A light snowfall near Little Sturgeon melts where water accumulates along joints, revealing hidden joints in fine detail.
columnar joints Sheets of magma shrink as they cool, developing polygonal fractures called columnar jointing. Here we see columnar jointing in a dike in Scotland.
columnar joints Columnar joints can be almost perfectly regular but they are more often rather irregular like these joints in a lava flow in southern Idaho.
exfoliation in Yosemite As crystalline rocks are uncovered by erosion, the pressure on them lessens, they expand, and frequently split along joints parallel to the surface or to rock contacts. Because they peel something like an onion, this process is called exfoliation.

Note that exfoliation also refers to a quite different process, the spallation of thin layers during spheroidal weathering.

Rock overhangs collapse until they are either gone, or the remaining rock can withstand the stress. The best shape for withstanding stress is an arch, like those just above and right of the road.

exfoliation in Yosemite Half Dome is one of the most famous joint-controlled landforms. It probably did not get cut in half by the glaciers but was more likely originally asymmetrical like the dome in the lower left background.

Normal Faults

normal fault This sequence of volcanic rocks in California is cut by a normal fault slanting down and left. The thick stack of brown layers at the top of the hill has also dropped down and left, in the process undergoing extension.
horst Nevada is covered by a series of fault mountain ranges, or horsts. Here is the East Humboldt Range. The intervening alluvium-filled valleys are grabens. This region is called the Basin and Range Province.
Front Range, Colorado The Front Range of the Colorado Rockies (seen here south of Denver) is defined by a great complex fault. The ridges in front of the mountains are layers of sedimentary rock bent upward as the fault slipped. A few miles further east the same layers are flat and hundreds of feet beneath the surface.

The Owens Valley, California

Sierra Nevada at Manzanar The Sierra Nevada in California is bounded on the east by a great fault scarp that produced a magnitude 8 earthquake in 1872. The scarp rises over 10,000 feet. Mount Whitney, highest point in the conterminous U.S., is just out of the picture to the left.

The Oriental-style buildings mark the former entrance to Manzanar, a Japanese internment camp during World War II. Manzanar is now a National Historic Site. 

Owens Valley One of the most spectacular drives in the U.S. is U.S. 395 in the winter. Here we look north along the highway. On the left the peaks in the Sierra Nevada reach 14,000 feet. On the right in the distance, the White Mountains also reach 14,000 feet. The floor of the valley is at 4,000 feet. This is a 10,000-foot deep trench, or graben, where a strip of crust dropped between two faults. Some of the low hills in the center are cinder cones formed when lava came up along the Sierra Nevada fault.
Owens Valley This is looking south from a point approximately in the center of the previous picture. The White Mountains (left) and Sierra Nevada (right) are capped by clouds, with their bases visible. The clear strip in the center is the floor of the Owens Valley.
Deep Springs Valley A view from east of the Sierra Nevada. The snow-capped Sierra Nevada are visible in the distance, with the Owens Valley in the middle distance. The White mountains are in the foreground. The near valley with the small playa lake is Deep Springs Valley. Its steep eastern (left) side is a fault scarp.

This same view is also used to illustrate a playa lake in the slide show on landscape evolution.

Cross section of grabens Same view with a cross-section superimposed. East of the Sierra Nevada the alternating horsts and grabens of the Basin and Range Province begin. Deep Springs Valley is the westernmost graben in this area.

The San Andreas Fault

Southern California

San Andreas Fault An aerial view looking northwest along the San Andreas Fault. The city of San Bernardino is on the left and the fault is obvious as a prominent boundary in color and topography. In the distance it cuts across the mountains and shows up as a color boundary. The pass where it cuts through the mountains is Cajon Pass.

In the extreme foreground note that there are two lines of vegetation along the base of the mountains. These mark two strands of the fault. Great faults are rarely single breaks but often consist of multiple parallel faults.

The prominent white line running up the mountains at right center is the aqueduct pictured in the groundwater slide set..

San Andreas Fault A surface view of the landscape in the previous picture. The fault runs along the base of the mountains and is marked by a straight line of trees. The trees grow along the fault because the shattered rock in the fault zone provides easy root access and abundant water.
San Andreas Fault An end view of the San Andreas Fault in Cajon Pass near San Bernardino. Brown layered sedimentary rocks (left) abut white granitic rocks (right). Seen from close up, the rocks are shattered over a zone several meters wide along the fault.

San Francisco Bay Area

San Andreas Fault Looking north along the San Andreas Fault at Crystal Springs Reservoir, south of San Francisco. The Pacific Plate is on the left. The fault is marked for much of its length by a valley because the broken rocks along the fault are easily weathered and eroded. The dam that impounds the reservoir is off the fault out of the picture to the right.
San Andreas Fault Taken just outside the previous picture, on the right. Rocks in fault zones often look like this. Shattered rock like this is called fault gouge. Polish created by fault slippage is called slickensides
Newspaper, 1906 To most Americans even today, the North American earthquake was the San Francisco earthquake of 1906.
Poster, Pier 39 At one time it was taboo to discuss the San Francisco Earthquake. One spoke of the "San Francisco Fire" (you can't put out an earthquake). But times have changed and Californians are a lot more casual about seismic hazards. The poster describes how well engineered Pier 39 is for a large earthquake but notes that the nearby shoreline might liquefy, leaving customers stranded. But, it concludes, there are so many fine eateries on the Pier they'll have a great time waiting for a boat to come get them!

Hayward Fault Zone

Hayward Fault Near San Francisco, the San Andreas Fault system splits into several branches. The Hayward Fault runs east of San Francisco Bay. Here we look south along the fault in Fremont, California. The hills are a horst, uplifted between the Hayward Fault and the next fault to the east, the Calaveras Fault.
Hayward Fault Parts of the Hayward Fault are creeping aseismically, offsetting curbs as shown here. Note that the offset is right-lateral, typical of the whole San Andreas System.
Hayward Fault Not far away, fault slip opened cracks in this street. The three staggered (or en echelon) cracks indicate right-lateral motion. They would close up if the motion were left-lateral. This slip was evidently a brief episode; when revisited 8 years later, the cracks were unchanged. By 1998 the street had been repaved and the cracks were no longer visible.

New Madrid

New Madrid, MO New Madrid, Missouri, was the site of some of North America's greatest earthquakes in 1811 and 1812.
New Madrid, MO The New Madrid of 1811 is gone, eroded away by the Mississippi River over the years. The present town is protected by a levee.
New Madrid, MO The shallow trough where the trees grow is the result of subsidence during the 1811-1812 earthquakes.
New Madrid, MO Thanks to the incredibly flat terrain, relics of the 1811-1812 earthquakes are still evident. Here are some sand blows, visible because the sand is much lighter than the surrounding soil.

Thrust Faults

Thrust Fault, Antarctica The diagonal contrast boundary slanting down and right is a thrust fault on Gibbs Island, Antarctica. Dunite to the right is thrust over schist, to the left.
Thrust Fault, Antarctica Close-up of highly sheared serpentine in the Gibbs Island thrust fault zone.
Map of Appalachians On the west side of the Appalachians are many thrust faults. They have very irregular traces on the map because they are almost horizontal. The mountains consist of a stack of thin slices of rock separated by thrust faults. These are termed imbricate thrust faults. The thin sheets of rock are termed nappes.
Thrust Faults in Snow How imbricate thrust faults form is clearly evident in this miniature example in snow. Note that the next fault is forming at the base of the pile. The wedge of snow thickens until it is strong enough to withstand the stress, and fractures in a new place at the base of the pile.
Moine Thrust, Scotland The student's hand is resting on barely-metamorphosed Paleozoic limestone. Above his head are Precambrian gneisses. The boundary between them is the Moine Thrust Fault in Scotland.

The only way to explain barely-metamorphosed rocks beneath highly metamorphosed rocks is faulting. The rocks above the student's head moved many kilometers over the rocks he has his hand on.

Thrust Fault, Scotland In the previous picture the fault zone was narrow enough to cover with your hand. In this picture the fault zone occupies the entire height of the hill, which consists of intensely-stretched and deformed rocks.


Melange, Newfoundland The greatest faults of all are those between plates. Where plates converge, there is often a zone of bashed and mixed rock called melange (French for mixture). This is the Dunnage Melange near Gander, Newfoundland, which marks where North America and part of Europe collided during the formation of the Appalachians.
Melange, Newfoundland Another melange in western Newfoundland. Note the blocks of all sizes in the sheared rock.

Earthquakes Don't Kill People, Buildings Kill People

Unsafe Earthquake Construction This building in Bosnia combines all the worst features of masonry and wood construction. The wood frame is not strong enough to support the masonry fill in an earthquake, and the masonry is both poorly-supported and heavy.
Unsafe Earthquake Construction A building under construction in southern Turkey. Will those forms stay in place as the concrete cures? How much re-bar is there in the concrete?
Unsafe Earthquake Construction Examine these buildings in southern Turkey closely and you will see that the walls are not perfectly plumb and walls in neighboring buildings are not perfectly parallel. Poor construction is one reason why an earthquake in California might kill 10 people, and one exactly as strong in Peru or Iran might kill 10,000.

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Created 13 January 1998, Last Update 6 November 1998

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