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Earthquakes
Types of Seismic Waves
Seismic waves carry energy from an earthquake away from
the focus, through Earth’s interior, and across the surface.
-Earthquakes
.
*Focus: Point beneath the earth’s surface where
the earth moves
*Epicenter: The point on the surface directly
above the focus. It is where the strength of the
earthquake is strongest.
Earthquakes
Types of Seismic Waves
P waves are seismic waves that compress and expand the ground like
an accordion.
S waves are seismic waves that vibrate from side to side as well as up
and down.
Earthquakes -
Types of Seismic Waves
Surface waves move more slowly than P waves and S waves,
but they produce the most severe ground movements.
Earthquakes - Earthquakes and Seismic Waves
Seismic Waves Activity
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Webcode: cfp-1022
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fp&wcsuffix=1022&area=view&x=8&y=7ss
Earthquakes - Forces in Earth’s Crust
Types of Stress
The stress force called tension pulls on the crust, stretching
rock so that it becomes thinner in the middle.
Earthquakes - Forces in Earth’s Crust
Types of Stress
The stress force called compression squeezes rock until it
folds or breaks.
Earthquakes - Forces in Earth’s Crust
Types of Stress
Stress that pushes a mass of rock in two opposite directions
is called shearing.
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
Tension in Earth’s crust pulls rock apart, causing normal
faults.
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
A reverse fault has the same structure as a normal fault, but
the blocks move in the opposite direction.
Earthquakes - Forces in Earth’s Crust
Kinds of Faults
In a strike-slip fault, the rocks on either side of the fault slip
past each other sideways, with little up and down motion.
Earthquakes - Forces in Earth’s Crust
Changing Earth’s Surface
Over millions of years, the forces of plate movement can
change a flat plain into landforms such as anticlines and
synclines, folded mountains, fault-block mountains, and
plateaus.
Earthquakes - Forces in Earth’s Crust
Changing Earth’s Surface
Over millions of years, the forces of plate movement can
change a flat plain into landforms such as anticlines and
synclines, folded mountains, fault-block mountains, and
plateaus.
Earthquakes - Forces in Earth’s Crust
Building Vocabulary
A definition states the meaning of a word or phrase. As you
read, write a definition of each Key Term in your own words.
Key Terms:
stress
hanging wall
tension
footwall
strike-slip fault
compression
anticline
shearing
syncline
normal fault
plateau
reverse fault
Examples:
Stress
is aofforce
on rocka to
change
itsisshape
The block
rock that
that acts
lies above
normal
fault
or
volume.
called
the hanging wall.
The
force
on the crust,
The stress
rock that
liescalled
belowtension
is calledpulls
the footwall.
stretching rock so that it becomes thinner in the
In
a strike-slip fault, the rocks on either side of the
middle.
fault slip past each other sideways, with little up or
The stress force called compression squeezes rock
down motion.
until it folds or breaks.
A
fold inthat
rock
that bends
upward
into
is an
Stress
pushes
a mass
of rock
in an
twoarch
opposite
anticline.
directions is called shearing.
A
fold in in
rock
that bends
downward
to form
a valley is
Tension
Earth’s
crust pulls
rock apart,
causing
a
syncline.
normal
faults.
A plateau
a large
of flatstructure
land elevated
high
reverse is
fault
has area
the same
as a normal
above
seathe
level.
fault, but
blocks move in the opposite direction.
Earthquakes - Earthquakes and Seismic Waves
Measuring Earthquakes
The Mercalli scale was developed to rate earthquakes
according to the amount of damage at a given place.
Earthquakes
Richter Scale
The Richter Scale for measures the 'size' or 'strength' of an
earthquake. This scale is quantitative and based on the
amount of energy released by an earthquake.
The energy of a quake is a function of both the amplitude
and the duration of a single wave.
The seismogram below shows waves with a wide range of
amplitude and duration.
This would be magnitude 8 quake!
Earthquakes - Earthquakes and Seismic Waves
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
Seismographs at five
observation stations recorded
the arrival times of the P and S
waves produced by an
earthquake. These data are
shown in the graph.
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
Reading Graphs:
What variable is shown on the
x-axis of the graph? The yaxis?
X-axis––distance from the
epicenter; y-axis––arrival
time.
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
Reading Graphs:
How long did it take the S
waves to travel 2,000 km?
7 minutes
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
Estimating:
How long did it take the P
waves to travel 2,000 km?
4 minutes
Earthquakes - Earthquakes and Seismic Waves
Seismic Wave Speeds
Calculating:
What is the difference in the
arrival times of the P waves
and the S waves at 2,000
km? At 4,000 km?
2,000 = 3.5 minutes
4,000 = 4.5 minutes
Earthquakes - Earthquakes and Seismic Waves
Locating the Epicenter
Geologists use seismic waves to locate an earthquake’s
epicenter.
Earthquakes - Monitoring Earthquakes
The Modern Seismograph
Seismic waves cause the seismograph’s drum to vibrate. But
the suspended weight with the pen attached moves very
little. Therefore, the pen stays in place and records the
drum’s vibrations.
Earthquakes - Monitoring Earthquakes
Instruments That Monitor Faults
In trying to predict earthquakes, geologists have developed
instruments to measure changes in elevation, tilting of the
land surface, and ground movements along faults.
Earthquakes - Monitoring Earthquakes
Using Seismographic Data
The map shows the probability of a strong earthquake along
the San Andreas fault. A high percent probability means that
a quake is more likely to occur.
Earthquakes - Earthquake Safety
Earthquake Risk
Geologists can determine earthquake risk by locating where
faults are active and where past earthquakes have occurred.
Earthquakes - Earthquake Safety
How Earthquakes Cause Damage
A tsunami spreads out from an earthquake's epicenter and
speeds across the ocean.
Earthquakes - Earthquake Safety
Designing Safer Buildings
To reduce
earthquake
damage, new
buildings must be
made stronger and
more flexible.
Earthquakes - Earthquake Safety
Asking Questions
Before you read, preview the red headings and ask a what,
how, or where question for each heading. As you read, write
answers to your questions.
Question
Answer
Where is the quake risk highest?
Earthquake risk is the highest along
faults and where past earthquakes
have occurred.
How do earthquakes cause damage?
Earthquake damage occurs as a result
of shaking, liquefaction, aftershocks,
and tsunamis.
How can you stay safe during an
earthquake?
The best way to stay safe during an
earthquake is to drop, cover, and hold.
What makes buildings safe from
earthquakes?
Buildings can be made safer by being
built stronger and with greater flexibility.