Faults and Earthquakes
Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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||An aerial view of jointed sandstone in Southern Utah.
||These joints along the Eau Claire Dells in Marathon
County break the rocks into roughly rectangular blocks
||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.
||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.
||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 can be almost perfectly regular but
they are more often rather irregular like these joints in
a lava flow in southern Idaho.
||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.
||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.
||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.
||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.
||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
||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.
buildings mark the former entrance to Manzanar, a
Japanese internment camp during World War II. Manzanar is
now a National Historic Site.
||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.
||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.
||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
||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
The 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.
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..
||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.
||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
||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.
||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
||To most Americans even today, the North
American earthquake was the San Francisco earthquake of
||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
Hayward Fault Zone
||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.
||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.
||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, Missouri, was the site of some of North
America's greatest earthquakes in 1811 and 1812.
||The New Madrid of 1811 is gone, eroded away by the
Mississippi River over the years. The present town is
protected by a levee.
||The shallow trough where the trees grow is the result
of subsidence during the 1811-1812 earthquakes.
||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.
||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.
||Another melange in western Newfoundland. Note the
blocks of all sizes in the sheared rock.
Earthquakes Don't Kill People, Buildings Kill People
||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
||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?
||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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