OUR FAULTS – TYPES AND ACTIONS
E.Q. – 17
September 1997
Sources:
Bobbie Callison, Murray Hunt, Jack Tomlin
Objective:
To understand how the earth moves when stress is applied at plate margins.
Science Themes:
Patterns of change, energy, systems and interactions
Process skills:
Observing, comparing, relating
Grade Levels:
4-8
Focus Words:
Collision, subduction, transform, trench, plate tectonics, compression, tension,
shearing, faults.
Materials:
Fault model with masking tape
Map of plate boundaries (upper lid of model case)
Quake watch handout
Spring (Slinky)
Background Information:
See the enclosed Teacher Feature photo copy of fault types and also read about different fault types in
the EV Earthquake Guide.
Movement along plate boundaries is the cause of the majority of earthquakes and volcanoes that occur
globally. Most earthquakes are usually six or more miles deep and usually shake without ever causing
a surface rupture.
There are three kinds of fault types found along plate boundaries; these are: convergent (collision),
divergent (pulling apart), and transform (slide-by). Where collision occurs, the heavier material
subducts under the lighter. Ocean floor material is dense and when it collides with the lighter
continental material, it subducts under the continental material. An example is the Indo-Australian
Plate which collided with the Eurasian Plate and created the Himalayas. The older, denser IndoAustralian Plate continues to squeeze beneath the Eurasian Plate. Because both plates are too light to
sink they have created the highest mountain range in the world. If two ocean plates happen to
converge, the older, cooler (and therefore denser) of the two plates will slide under the younger,
warmer, less dense plate.
Plate movement creates stress which leads to earthquakes. An earthquake occurs when a block of rock
slips relative to an adjacent one along a fault plane. Before an earthquake occurs, stress builds up
between the two faces of the fault boundary. There are three kinds of stress: compression, tension, and
shear.
Compression is a squeezing or pushing action—compress a spring or apply compression to stop
bleeding. Compression shortens. Reverse faults are associated with compression. This occurs when the
overlying block moves up relative to the underlying block. A thrust fault is a reverse fault with a low
angled fault plane—some may be almost horizontal (see diagrams).
Tension is pulling apart such as stretching a rubber band or tightening a guitar string to raise its pitch.
Tension elongates. Normal faults are associated with tension. The overlying block moves down
relative to the underlying block. Normal faults occur where there is seafloor spreading and new
seafloor is created. Tension causes rifts (fissures caused by tension) along short sections of the
boundary. As the rift widens mantle material flows up creating new seafloor on both sides. Transform
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faults form at right angles to this rift, the adjoining sections of the rift are dormant. Earthquakes are
concentrated along these faults.
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Shear is where forces are parallel, but in opposite directions—the blades of scissors shear past each
other. Placing the palms of your hands together and sliding them in opposite directions is shear;
pruning shears shear off branches. Shear takes place along transform faults that accompany and are at
right-angles to seafloor spreading rifts and shear also occurs along the San Andreas fault.
Rifts also take place on land. An example of this is the East African Rift. Mt. Kilimanjaro is part of this
rift. If action along the rift continues, a section of East Africa will drift into the Indian Ocean. About 800
million years ago a large chunk of western North America rifted into the Pacific Ocean creating a new
continental margin near the present western border of Idaho. It has not been determined where that
chunk of North America ended its journey.
The San Andreas Fault System is one of the longest and best known transform faults. This is called a
system because of the many branching and parallel faults that accompany the main fault from the
Pacific Ocean through the Central Valley. The applied stress is largely shear although other stress can
be present because the fault has curves that can cause tension and compression. The Loma Prieta
earthquake had both lateral slip and compression causing one side of the fault to move up over the
other side. The big seaward bend in the San Andreas, just north of Los Angeles, exhibits both shear
and compression. The compression causes thrust faults which squeeze up mountain ranges and create
devastating earthquakes. If the San Andreas bent inward, the two sides would pull apart causing
valleys to sink in between the mountains.
The fault plane of the San Andreas Fault is almost vertical. As one side slides by the other, everything
on the opposite side of the fault appears to move to the right, so it is called a right lateral slip fault.
Earthquakes within plates are influenced by stress of both past or present plate motion. For instance,
earthquakes in the Great Basin, an area which includes most of Nevada, are due to extension (some call
it relaxing) of the crust. For over 150 million years. the North American continent was compressed as it
crashed into the subducting Pacific Ocean floor which also transported mini continents and island arcs
that smashed against North America's western boundary. This pressure (compression) has diminished
and the North American continent is springing back. As a result, the Great Basin has almost doubled
its east-west distance. Over millions of years the distance between Reno and Salt Lake City are has also
almost doubled. As extension continues, valleys drop and mountains rise, and earthquakes happen. A
relatively recent event was the occurrence of the Idaho 7.3 Mt. Borah earthquake in 1983. Hot spots,
such as those found in the Yellowstone area, are also a source of earthquakes created by stress.
Our western coast has grown by several hundreds of miles in the last 100 to 150 million years. At the
time of the extinction of dinosaurs, there were ocean beaches along what is now the Sierra foothills.
The geologic history of California is complicated but fascinating. Its secrets are gradually being
uncovered giving us more pieces to the puzzle. Keeping ourselves up to date about new geological
evidence is critical in presenting a comprehensive Earthquake program.
Activity:
There are four different plate margins indicated within this kit, each with its own map and crank to
indicate the result of plate movement.
Caution:
Don't place anything on top of the kit.
Don't turn any of the cranks when the cover is on.
PLEASE DO NOT LET STUDENTS OVER CRANK,
THE MECHANISM IS FRAGILE!
1. Use the map inside the lid to give an overview of all the earthquakes that have occurred in the Bay
Area between 1972 and 1989. In addition to this information, you can include the Northridge quake
in Southern California that occurred in 1994. Discuss the large faults in our area: San Andreas
Fault, Hayward and Calaveras.
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Ask students — What is a fault? (An underground area where the earth's crust has fractured, where
movement has occurred, resulting in relative displacement of rock)
Ask students — How was the San Andreas fault formed? (About 30 million years ago as the Pacific
plate subducted and pushed new material onto the existing North American plate, a fractured area 5 to 10
miles wide formed).
Show them the copy of siginificant California quakes since 1800 and the Quake Watch sample that
is printed in the newspaper every week (Tuesday, in the San Jose Mercury). Tell students they can
check the newspaper themselves for this weekly report.
2. Present a brief overview of plate tectonics: The earth's crust is made up of large plates that move
slowly causing three general types of action (divergent, convergent or transform) along plate
boundaries or margins. Check to make sure students understand these terms. Have them use their
hands as you demonstrate this action to them. This movement began about 200 million years ago
and is referred to as "continental drift". Most earthquakes occur along plate boundaries (show plate
boundary map). The EV Fault Model will help demonstrate the different kinds of fault action that
occurs in California.
Earthquakes (ground shaking) happen when stress builds along a plate margins and causes the
ground (rocks) to rupture or break. Let's look at the different types of plate boundaries and see
what happens as stress is applied. Only transform and convergent margins are demonstrated on
the EV Fault Model.
Transform Plate Boundaries
Strike-slip faults:
Explain to students that one type of earthquake occurs along a strike-slip fault. (indicated with a
map of Point Reyes area showing the San Andreas fault).
The Pacific plate moves 1– 2 inches per year, Northwest, past the North American plate, creating
horizontal, tension between rocks on each side of the plates. As stress increases, bonds holding the
rocks together break causing an earthquake!
1. Have children shove a foot down and forward on the floor until it slips (friction then slippageearthquake).
Or place palms of hands touching and facing each other (left palm is Pacific plate and the right
palm is the North American plate, the crack in between is the San Andreas fault). Slowly move left
hand forward past the right hand.
Does it move smoothly or stick as more pressure is added and then jerk forward?
2. Break off a piece of masking tape about 3 inches in length and place it so that it lays across the San
Andreas fault line.
Ask a student to turn the crank slowly to apply stress. PLEASE DO NOT LET STUDENTS OVER
CRANK, THE MECHANISM IS FRAGILE! Watch what happens as the Pacific plate slowly moves
past the North American plate (You may get wrinkling of the tape, followed by a snap).
3. While waiting for the break, use the rubber band to demonstrate the elastic rebound of rocks Pull
on the rubber band and let it snap back (This is what happens to rocks as they are stressed apart and
settle back, creating aftershocks).
Thrust faulting
This type of fault action occurs along transform and convergent margins as indicated by the map
area of Loma Prieta (far left section). (See Teacher Feature copy of reverse and thrust faults.) Instead
of plates slipping past horizontally, one section moves up with respect to the other plate usually
due to compressional forces, such as bends in the San Andreas fault. Blind thrust faults are
common in California; these faults never break the surface of the crust.
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They are difficult to locate until they slip during an earthquake, then the after shocks may help
define the area of fracture. An example is along the western edge of the Santa Clara valley from Los
Gatos to Los Altos Hills. Other thrust faults will be visible on the surface; Mount Diablo was
formed by thrust faulting.
There was extensive damage to homes during the Loma Prieta quake as land west of the San
Andreas fault slipped over the eastern side. Also the Northridge, Los Angeles quake in 1994, was
the result of a blind thrust fault that slipped. Blind thrust faults have been discovered along Cache
Creek in Rumsey Canyon near Clear Lake.
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1. Ask students to use their hands to demonstrate this type of movement.
2. Ask a student to use the spring to demonstrate tension and compression.
3. Ask a student to slowly turn the crank to observe what happens as stress is applied to the one
plate.
4. Ask students if they can explain how mountains get higher? (Loma Prieta mountain on the Pacific
plate went 4 feet higher as a result of he 1989 earthquake).
Convergent Boundaries
Subduction:
1.
Subduction action is indicated by the map of Juan de Fuca plate off California, Oregon and
Washington that subducts beneath the North American plate (see right section of model).
2. Ask children to slide one hand under the other hand. The heavier plate (oceanic plate) subducts
under the lighter plate (continental plate).
3. If the subducting hand were actually oceanic crust, would the top part be scraped off? (Yes, the
friction between the plates causes heat that melts the seafloor. A trench is formed at the subduction site
that eventually gets filled in with lighter sediments that can't be subducted. This is how our California
coast range mountains were formed - scraped off the ocean floor by a subducting Farallones plate and
pushed onto the existing North American plate).
4. Ask a student to slowly turn the crank and watch a trench form as the plate bends under.
Imagine this trench filling with sediments.
Collision:
The collision or bumping together of two continental plates is responsible for mountain building as
continental crust buckles and is pushed upward. Or mountains may be formed by subduction with
initial volcanic action followed by intrusion of granitic material (Aleutians, Sierra Nevada).
The map of Mt. Diablo is used to exemplify mountain building, although this mountain may not
have been formed entirely due to collision of two plate boundaries. It is believed that Mt. Diablo
was formed primarily by thrust faulting over 80 million years ago.
1. Ask students to use their hands again to represent two colliding plates. Ask what other
mountains in the world were formed by this action? (Alps, Himalayas)
2. Ask a student to slowly turn the crank to watch the lifting of continental crust as the plates
collide.
Many, many thanks to Murray Hunt and Eric Tiegel for all the time and effort spent on creating and building this
beautiful fault demonstration model.
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