Earthquakes – Comp. Sci. 2
7-1 Forces in Earth’s Crust
7-3 Monitoring Earthquakes
Vocabulary 7.1
Stress
Compression
Shearing
Normal Fault
Reverse Fault
Strike-slip Fault
Plateau
3 Kinds of Stress in Earth’s Crust pg. 223
As Earth’s plates move, they can bend or fold rock. Forces created
by movement of the Earth’s plates are examples of stress. Stress
adds energy to rock until the rock changes shape or breaks.
Three kinds of stress can occur in the Earth’s crust –
tension, compression, and shearing.
All three work over millions of years to change the shape and
volume of rock.
Tension, compression, and shearing work over millions of
years to change the shape and volume of rock
How Does Stress Change Earth’s Crust? Pg. 223
TensionTension is the stress force that pulls on the crust and
thins the rock in the middle. It occurs where two plates
pull apart.
1st kind of stress can occur in
the crust.
How Does Stress Change Earth’s Crust? Pg. 223
CompressionCompression is the
stress force that
squeezes rock until it
folds or breaks. It occurs
where two plates come
together and push
against each other.
2nd kind of stress can occur in
the crust.
How Does Stress Change Earth’s Crust? Pg. 223
Stress can push, pull, or
squeeze rock in Earth’s
3rd kind of stress can occur in
the crust.
crust.
Shearing-Shearing is the stress
force that pushes rock in two
opposite directions. It occurs
where two plates slip past each
other.
Stress in Earth’s Crust
Stress can push, pull, or squeeze rock in
Earth’s crust. Three kinds of stress can occur
in the crust.
APPLY CONCEPTS
Look at the pair of arrows in the second
diagram.
These arrows show how tension affects rock.
Draw a pair of arrows on the third diagram to
show how compression affects rock.
Then draw a pair of arrows on the bottom
diagram to show how shearing acts on rock
Assess Your Understanding pg. 223
I get it! Now I know that stress changes Earth’s crust by
changing the ____________________________________
________________________________________________
How Do Faults Form? Pg. 224
When enough stress builds up in rock, the rock
breaks and makes a fault.
The three main types of faults are:
normal faults
reverse faults
strike-slip faults
How Do Faults Form?
Normal Faults
Normal faults form where rock is pulled apart by tension in Earth’s crust. The
block above is angled fault called the hanging wall. The rock below the fault
is called the footwall. The hanging wall slips downward when rock moves
along the fault.
Reverse Faults
A reverse fault has the same structures as a normal fault, but the hanging
wall moves up and the footwall moves down. Reverse faults form where
compression pushes the rock of the crust together.
Strike-Slip Faults
Shearing produces strike-slip faults. The rocks on either side of a strike-slip
fault slip past each other and have little up or down motion. A strike-slip fault
that forms the boundary between two plates is called a transform boundary.
Faults pgs. 224-225
The three main types of faults are defined by the
direction in which rock moves along the fault.
Apply It! Pg. 225
The low angle of a thrust fault allows rock in the hanging wall to be pushed
great distances. For example, over millions of years, rock along the Lewis
thrust fault in Glacier National Park has moved 80 kilometers.
1. Identify- Based on the arrows showing fault movements in the diagram, a
thrust fault is a type of (normal fault, reverse fault).
2. Challenge – Why might the type of rock in the hanging wall of the Lewis
thrust fault be different from the type of rock in the footwall?
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
Assess Your Understanding pg. 225
1a. Review – When enough stress builds up in brittle rock, the rock
breaks causing a ____________________.
b. Infer- A geologist sees a fault along which blocks of rock in the
footwall have moved higher relative to blocks of rock in the hanging
wall. What type of fault is this?______________________________
I got it! Now I know that faults form when ____________________
_______________________________________________________
_______________________________________________________
_____________________________________________
How Does Plate Movement Create New
Landforms? Pg. 226
Over millions of years, the forces of plate movement can
change a flat plain into features such as anticlines and
synclines, folded mountains, fault-block mountains, and
plateaus.
Folding pgs. 226-227
Folding Earth’s Crust
Folds are bends in rock that form when Earth’s crust is
compressed and gets shorter and thicker. A fold in rock that
bends upward into an arch is an anticline. A fold that bends
downward in a V shape is a syncline.
Anticlines and Synclines
Compression can cause folds in the crust.
Two types of folding are anticlines, which
arch up, and synclines, which dip down.
Fault Block Mountain
Stretching Earth’s Crust
Tension forces create normal faults where two plates move away
from each other. A fault-block mountain forms when the hanging
walls of two normal faults drop down on either side of the footwall.
Tension and Normal Faults
As tension forces pull the crust apart, two
normal faults can form a fault-block
mountain range.
Forces in Earth’s Crust – Figure 2
Tension and Normal Faults
As tension forces pull the crust apart,
two normal faults can form a faultblock mountain range, as you can see
in the diagram.
Predict
Label the hanging wall and the two
footwalls in diagram A.
in diagram B, draw the new position of the
hanging wall after movement occurs.
Describe what happens.
_________________________________
_________________________________
_________________________________
_________________________________
_________________________________
Plateau
Uplifting Earth’s Crust
Forces can also raise plateaus. A plateau is a large area of flat land
that was lifted up high above sea level.
The Kaibab Plateau
Look at the sequence of drawings. In your own words, describe what happens in
the last two diagrams.
Assess Your Understanding pg. 229
2a. Review- Normal Faults often occur when two plates (come
together / putt apart)
b. Interpret Diagrams – Look at the diagram that accompanies the
photograph in Figure 5. Does the block of rock in the middle move up
as a result of movement along the normal faults? Explain.
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
I get it! Now I know that plate movements create new features by
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
7-2 Earthquakes and Seismic Waves
Vocabulary 7.2
1. Earthquake
2. Focus
3. Epicenter
4. P wave
5. S wave
6. Surface wave
7. Seismograph
8. Modified Mercalli scale
9. Magnitude
10. Richter Scale
11. Moment magnitude scale
Earthquakes and Seismic Waves
Earthquakes start below the surface of the Earth.
An earthquake's seismic waves carry energy up
toward the surface and down through the interior.
Seismic waves are vibrations that are
similar to sound waves. They travel
through Earth carrying energy
released by an earthquake
1.



Look at the drawing showing Earth’s interior.
At which point(s) can seismic waves be
detected?
A only
A and B
A, B, and C
2. Infer – At which point do you think the seismic
waves will have the most energy? Why?
________________________________________
________________________________________
________________________________________
What Are Seismic Waves? Pg. 231
Plate movement increases the stress along a fault until the rock slips
or breaks and an earthquake begins. An earthquake is the shaking
and trembling that happens when rock under Earth’s surface moves.
The focus is the place under Earth’s surface where rock starts to
move. The point on the surface directly above the focus is called the
epicenter.
Some of the energy released during
an earthquake travels through Earth
in the form of seismic waves. Seismic
waves travel out in every direction
from the focus.
Types of Seismic Waves
P – Waves (A)
The first waves to arrive, also known as primary waves. P
waves compress and expand the ground like an accordion.
P waves can damage buildings. P waves travel through
solids and liquids.
S – Waves (B)
After primary waves come secondary waves or S waves. S
waves are seismic waves that can vibrate from side to side
or up and down. Their vibrations are at an angle of 90
degrees to the direction that they travel. When S waves
reach the surface, they shake structures violently. S waves
cannot travel through liquids.
Surface Waves (C)
When P and S waves reach the surface, they become
surface waves. Surface waves move more slowly than P and
S waves but they can produce severe ground movements.
Surface waves can make the ground roll like water waves in
an almost circular pattern or shake buildings from side to
side.
Types of Seismic Waves pg. 233
P, S, and Surface Waves
Earthquakes release stored energy
as seismic waves.
Identify – Draw a line from each
type of seismic wave to the
movement it causes.
How Are Earthquakes Measured? Pg. 234
Modified Mercalli Scale
The Modified Mercalli
scale uses Roman
numerals to rate the
damage and shaking at
any given location,
usually close to the
earthquake.
How Are Earthquakes Measured? Pg. 234
Earthquake Magnitude
The table gives the moment magnitudes of some recent earthquakes.
Earthquakes and Seismic Waves pg. 236
Seismic Wave
Speeds
Seismographs at five
observation stations
recorded the arrival times
of the P and S waves
produced by an
earthquake. These data
were used to draw the
graph.
Earthquakes and Seismic Waves pg. 237
An Earthquake’s Epicenter
The map shows how to find the epicenter of an earthquake using
data from three seismographic stations.
Assess your Understanding
Monitoring Earthquakes
7.3
Vocabulary
Seismogram
Monitoring Earthquakes
Seismic Data From the USArray Project
In 2004, scientists in the USArray project placed 400 seismographs across
the western United States. Every month, 18 seismographs are picked up
and moved east, “leapfrogging” the other seismographs.
Monitoring Earthquakes
Recording
Seismic
Waves
In a simple
seismograph,
a pen
attached to a
suspended
weight records
an
earthquake’s
seismic
waves.
Monitoring Earthquakes
Seismograms
When an earthquake’s seismic waves reach a simple
seismograph, the seismograph’s drum vibrates. The vibrations are
recorded by the seismograph’s pen, producing a seismogram.
Monitoring Earthquakes
Earthquake Risk
The map shows
areas where serious earthquakes
are likely to occur, based on the
location of past earthquakes across
the United States.
Monitoring Earthquakes
Earthquakes Around the World
Earthquakes are closely linked to plate tectonics.
The map shows where past earthquakes have
occurred in relation to plate boundaries.