Download What Are Earthquakes?

Name
CHAPTER 7
Class
Date
Earthquakes
SECTION
1 What Are Earthquakes?
California Science
Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
6.1.a, 6.1.d, 6.1.e, 6.3.a
• Where do most earthquakes happen?
• What makes an earthquake happen?
• What are seismic waves?
What Is an Earthquake?
Have you ever been in an earthquake? An earthquake
is a movement or shaking of the ground. Earthquakes
happen when huge pieces of the Earth’s crust move suddenly and give off energy. This energy travels through the
ground and makes it move.
STUDY TIP
Outline As you read, create
an outline of this section. Use
the headings in the section
to guide you.
Where Do Most Earthquakes Happen?
Most earthquakes happen at places where two tectonic
plates touch. Tectonic plates are always moving. Where
two plates touch, they may be moving toward each other,
moving away from each other, or sliding past each other.
The movements of the plates can break the rocky
crust in many places. A place where the crust is broken is
called a fault. Earthquakes happen when rock breaks and
slides along a fault.
A place in the crust where a lot of faults are found
is called a fault zone. Most fault zones, such as the San
Andreas Fault Zone, are found at plate boundaries. But
some fault zones are in the middle of plates.
READING CHECK
1. Define What is a fault?
Earthquakes and Plate Boundaries
•
Recorded earthquakes
TAKE A LOOK
2. Infer Use the earthquake
locations to help you figure out where the tectonic
plate boundaries are on the
map. Use a colored pen or
marker to draw tectonic plate
boundaries on the map.
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Earthquakes
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What Are Earthquakes? continued
Why Do Earthquakes Happen?
As tectonic plates move, pressure builds up on the
rock near the edges of the plates. When a rock is put
under pressure, it deforms, or changes shape. Rock can
deform in two main ways.
One kind of deformation is called plastic deformation.
Plastic deformation happens when rock bends and folds
like clay. When the pressure is taken away, the rock stays
folded.
TAKE A LOOK
3. Explain How do you
know that the rock layers in
the figure were once under a
lot of pressure?
CALIFORNIA
STANDARDS CHECK
6.1.e Students know major
geologic events, such as
earthquakes, volcanic eruptions,
and mountain building, result
from plate motions.
Word Help: major
of great importance or
large scale
Folded Layers
of Rock
A second kind of deformation is called elastic
deformation. Elastic deformation happens when rock
bends a little bit under pressure but returns to its original
size and shape when the pressure stops.
Earthquakes happen when rock breaks under
pressure. When the rock breaks, it snaps back to almost
the same size and shape as before the earthquake. This
snap back is called elastic rebound. When the rock
breaks and rebounds, it gives off energy. This energy
causes the ground to shake.
Elastic Rebound
4. Explain How does
the movement of tectonic
plates cause earthquakes?
1. Forces push rock in
opposite directions. The
rock deforms elastically.
It does not break.
2. If enough force is placed
on the rock, it breaks.
The rock slips along the
fault. Energy is released.
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What Are Earthquakes? continued
How Do Earthquakes Happen at Divergent
Boundaries?
A divergent boundary is a place where two tectonic
plates are moving away from each other. As the plates
pull apart, the crust stretches. The crust breaks into pieces
along faults. The pieces are called fault blocks because
they are blocks of rock that move along faults.
Most of the crust at divergent boundaries is thin, so
the earthquakes tend to be shallow. Most earthquakes at
divergent boundaries are no more than 20 km deep.
Earthquakes at Divergent Boundaries
Ocean
Tectonic plate
Tectonic plate
READING CHECK
5. Define What is a fault
block?
READING CHECK
6. Explain Why do most
earthquakes at divergent
boundaries happen at
shallow depths?
Earthquake
How Do Earthquakes Happen at Convergent
Boundaries?
A convergent boundary is a place where two tectonic
plates collide. When two plates come together, both plates
may crumple up to form mountains. Or one plate can subduct,
or sink, underneath the other plate and into the mantle.
The earthquakes that happen at convergent boundaries
can be very strong because so much pressure is produced.
When one plate subducts under another, earthquakes can
happen inside the sinking plate at depths of up to 700 km.
Earthquakes at Convergent Boundaries
READING CHECK
7. Explain Why are most
earthquakes at convergent
boundaries very strong?
Ocean
TAKE A LOOK
Earthquake
Tectonic plate
8. Identify Draw arrows
on the figure to show the
directions in which the two
tectonic plates are moving.
Tectonic plate
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What Are Earthquakes? continued
How Do Earthquakes Happen at Transform
Boundaries?
READING CHECK
9. Identify How deep
can earthquakes occur at
transform boundaries?
A transform boundary is a place where two tectonic
plates slide past each other. As the plates move past each
other, pressure builds up on the rock. Eventually, the
rock breaks along a fault.
The rock will break only if it is brittle. Rocks far below
Earth’s surface are generally not brittle. Therefore, most
earthquakes at transform boundaries happen in the upper
50 km of the crust.
Earthquakes at Transform Boundaries
TAKE A LOOK
10. Identify Draw arrows
showing the directions that
the tectonic plates in the
figure are moving.
Tectonic plate
Tectonic plate
Earthquake
How Does Earthquake Energy Travel?
READING CHECK
11. List What are the two
kinds of seismic waves?
When an earthquake occurs, a lot of energy is given
off. This energy travels through the Earth in the form of
waves called seismic waves.
There are two kinds of seismic waves. Body waves are
seismic waves that travel through the inside of Earth to
the surface. Surface waves are seismic waves that travel
through the top part of Earth’s crust.
BODY WAVES
There are two kinds of body waves: P waves and
S waves. P waves are the fastest kind of seismic wave.
P waves cause rock to move back and forth. P waves can
move through solids, liquids, and gases.
Another kind of wave, an S wave, can cause the rock
to move horizontally from side to side. S waves can also
cause the rock to move up and down. S waves travel
more slowly than P waves. They can travel only through
solids.
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What Are Earthquakes? continued
SURFACE WAVES
Surface waves travel along the top of Earth’s crust.
Only the very top part of the crust moves when a surface
wave passes. Surface waves travel much more slowly
than body waves, but they cause a lot more damage.
READING CHECK
12. Compare Which kind
of seismic waves travel the
most slowly?
Contraction Expansion Contraction
Direction P wave is
traveling
Wave 2
Wave 1
P waves are body waves that squeeze and stretch rock.
Side-to-side
motion
Direction S wave
is traveling
TAKE A LOOK
13. Compare How are the
motions of P waves and
S waves different?
S waves are body waves that can move rock from side
to side.
Critical Thinking
Up-and-down
rolling motion
Direction surface
wave is traveling
14. Infer What do you think
is the reason that surface
waves usually cause the most
damage?
Surface waves can move the ground up and down in a
circular motion.
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Section 1 Review
Date
6.1.a, 6.1.d, 6.1.e, 6.3.a
SECTION VOCABULARY
earthquake a movement or trembling of the
ground that is caused by a sudden release of
energy when rocks along a fault move
elastic rebound the sudden return of elastically
deformed rock to its undeformed shape
seismic wave a wave of energy that travels
through the Earth, away from an earthquake in
all directions
1. Compare What is the difference between an earthquake and a fault?
2. Compare Give two ways that P waves are different from S waves.
3. Identify Where do most earthquakes happen?
4. Describe What causes an earthquake to happen?
5. Explain What is the main difference between body waves and surface waves?
6. Apply Concepts Why are some earthquakes stronger than others?
7. Infer Why do few earthquakes happen in Earth’s mantle?
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Earthquakes
Earth Science Answer Key continued
6. weathering and erosion have removed the
4. The movement of the plates causes pressure
overlying rock
7. Sierra Nevada, Cascades, Transverse Range,
Central Range
8. Type of
How it forms
Examples in
California
5.
6.
7.
Magma forms
at a convergent
plate boundary.
It erupts to form
volcanoes or
cools to form
batholiths.
Mount Shasta,
Lassen Peak,
Sierra Nevadas
8.
Pieces of a
subducting plate
are scraped off
and attached to
a continent.
Coast Ranges,
Transverse
Ranges
mountain
Volcanic
Accreted terrane
9.
10.
11.
12.
13.
9. North American and Pacific
10. There are many faults along which plate
14.
movement takes place.
Review
1. a piece of a tectonic plate that is scraped off
2.
3.
4.
5.
6.
to build up. When the pressure is released,
an earthquake occurs.
a piece of rock surrounded by faults
The crust is thin at divergent boundaries.
A lot of pressure builds up before the rock
breaks.
Arrows should be perpendicular to the plate
boundary, pointing toward each other.
50 km
Arrows should be parallel to the fault,
pointing in opposite directions, left side
away, right side toward.
body waves and surface waves
surface waves
P waves move particles back and forth.
S waves move them from side to side.
Possible answer: All of their energy is
released at the surface.
Review
1. An earthquake is a shaking or movement of
and added to another tectonic plate during
convergence
transform
Lassen Peak and Mount Shasta are part
of the Cascade Mountains. They formed
above a subduction zone when melted rock
erupted onto Earth’s surface.
Strike-slip faults are probably most common because the San Andreas is a transform
boundary. Strike-slip faults are common at
transform boundaries.
The southern part has more compression
than the northern part.
Until about 10 million years ago, most of
California’s coast was located on a convergent boundary. Today, most of California’s
coast is located on a transform boundary.
Only a small part of it is found on a convergent boundary.
2.
3.
4.
5.
6.
Chapter 7 Earthquakes
SECTION 1 WHAT ARE EARTHQUAKES?
7.
1. A fault is a break in the crust.
2. Plate boundaries should follow the lines
defined by earthquake locations.
the Earth. A fault is a crack in the crust that
rocks can slide along. That is, earthquakes
happen along faults.
P waves move particles back-and-forth, and
S waves move them side-to-side. P waves
travel more quickly than S waves. P waves
travel through solids, liquids, and gases, but
S waves travel only through solids.
Most earthquakes happen at tectonic plate
boundaries.
Earthquakes happen when pressure builds
up on a rock and it breaks. That is, elastic
rebound of rocks causes earthquakes.
Body waves travel through the interior of
the Earth, but surface waves travel only on
its surface.
The strength of an earthquake is directly
related to the amount of pressure that builds
up on the rock before it breaks. Some rocks
are stronger than others, so more pressure
builds up before the rock breaks. When lots of
pressure builds up, larger earthquakes happen.
Almost all earthquakes happen when rock
breaks. The rock in the crust is brittle and
breaks in response to stress. Rock in the
mantle flows in response to stress.
3. The rocks are folded.
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Interactive Reader and Study Guide Answer Key
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Earth Science