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Chapter 6: Earthquakes and Plate Boundaries
Ch 6 Journal: Have you ever felt an earthquake?
Or, was there ever a time when you felt
unsteady, like the ground was going to slip out
from under you? Write a description of your
experience.
Lesson 6.1- What is an earthquake?
1. An earthquake is the rupture and
sudden movement of rocks along a
fault.
a. A fault is a fracture surface
along which rocks can slip.
2. Energy stored as a change in shape
is called elastic strain.
a. Heat and matter move through
Earth’s mantle by convection. Some
heat energy from Earth’s interior is
transformed into kinetic energy.
b. Some of the kinetic energy in Earth’s
mantle creates elastic strain in rocks.
3. Elastic strain builds up in rocks along
faults as the rocks move past each
other. When rocks cannot stretch to
add more elastic strain, they release
energy by breaking and slipping.
a. When rocks are so strained that
they can no longer stretch or
flow a fault ruptures.
b. A stretched rubber band best
models elastic strain.
4. When rocks along a fault suddenly
break or slip, elastic strain is released.
Complex waves radiate in all
directions, carrying some of the
released energy.
a. The energy flow leading to an
earthquake would be heat energy,
kinetic energy, elastic energy, to
seismic waves.
5. Plate boundaries are often fault zones
that are 40-200 km wide, rather than a
single fault.
6. The focus of an earthquake is the
place on a fault where the rupture and
movement of the earthquake begins.
B. Plate Boundaries and Earthquakes
1. Most, but not all, earthquakes occur at plate
boundaries. The three different types of plate
boundaries have different usual patterns of
earthquakes.
a. Most earthquakes at divergent plate
boundaries occur at relatively shallow depths
and are relatively weak earthquakes.
b. At transform plate boundaries, most
earthquakes occur on strike-slip faults and at
relatively shallow depths. Earthquakes with a
shallow focus can produce severe shaking, and
so dangerous earthquakes can occur at
transform plate boundaries.
c. Earthquakes at convergent plate boundaries
tend to be the deepest earthquakes. They have
been some of the most destructive earthquakes
in human history.
2. A small percentage of earthquakes occur
away from plate boundaries. These earthquakes
can be destructive because people are often
unprepared for them.
Discussion Question:
Not all the energy stored as elastic strain by
rocks is carried away from a fault by complex
waves when an earthquake occurs. What
happens to the rest of the energy?
The total amount of energy before and after the
earthquake must be the same, but energy will be
transferred from one place or form to another.
A large amount of energy is carried away by
complex waves through the ground, but some
energy is transferred in other forms. The noise
of rocks breaking is caused by sound energy,
some energy is used to break rocks themselves,
and other energy is used to heat areas around
the breaking rocks slightly.
Earthquake video (20 min)
Earthquake video (20 min)
Lesson 6.2- Earthquakes and Seismic Waves
A. What are seismic waves?
1. Seismic waves are waves of energy
produced at the focus of an
earthquake as elastic strain is
released.
a. Seismic waves travel outward in all
directions from the focus of an
earthquake.
b. The amount of energy carried by
seismic waves decreases as the waves
move away from the focus. This is
because rocks absorb some energy as
the seismic waves pass through them.
2. The epicenter of an earthquake is the
point on Earth’s surface directly
above the focus of the earthquake.
B. Types of Seismic Waves
1. There are three different types of seismic
waves: primary waves, secondary waves,
and surface waves.
a. Primary waves are compressional
waves that move rocks and other
matter parallel to the direction the
wave is traveling. Primary waves are
the fastest seismic waves.
b. Secondary waves are shear waves
that move rocks and other matter
back and forth perpendicular to the
direction the waves are traveling.
Secondary waves travel about 60
percent as fast as P-waves.
c. A spring toy illustrates the motion of
an S-wave.
d. Some of the energy from P-waves and
S-waves that reach Earth’s surface can
be trapped in the upper areas of Earth’s
crust to form surface waves. Surface
waves are the slowest type of seismic
wave.
e. Surface waves usually cause stronger
shaking than P-waves or S-waves, and
they are often the most destructive type
of seismic wave.
C. Using Seismic Wave Data
1. A seismologist is a scientist who studies
earthquakes.
2. The farther you are from the focus of an
earthquake, the farther S- waves will be
behind the P- waves.
3. Seismic waves change speed and
direction when the material through which
they are traveling changes.
4. Observing the paths of seismic waves can
help scientists to understand what kind of
materials make up Earth’s interior.
5. The shadow zone is a portion of Earth that
does not receive any seismic waves from a
particular earthquake.
6. Because S-waves cannot move through
liquid and P-waves would be bent by a
liquid, scientists think Earth’s outer core is
liquid and causes the shadow zone.
7. If a seismograph is located a great
distance from the epicenter, the P-waves
would arrive earlier than the S-waves.
Discussion Question:
Where is the first place on Earth’s surface
that an earthquake will be felt, and why?
If an earthquake is noticeable on Earth’s
surface at all, it will always be felt first at
the epicenter. This is because the epicenter
is directly above the focus, which means it is
the shortest straight-line distance from the
focus of the earthquake to the surface. Even
though P- and S- waves move out from the
focus in all directions, each type of wave
moves at the same speed. Because they have
to travel the shortest distance to reach the
epicenter, P-waves will be felt there first.
Nature’s Fury video (44 min)
Lesson 6.3- Measuring Earthquakes
A. How are Earthquakes measured?
1. Scientists determine the size of the large
earthquake that occurred in the Indian
Ocean on December 26, 2004, by
measuring how much the rock moved
along the fault where the earthquake
started.
2. Because the earthquake occurred
underwater, the movement of rock
caused a huge ocean wave in the Indian
Ocean.
B. Recording Seismic Waves
1. A seismograph is an instrument used
to record and measure movements of
the ground caused by seismic waves.
It measures the magnitude of an
earthquake.
a. Seismographs record the size,
direction, and time of the movements
caused by different types of seismic
waves.
b. Seismographs record ground motion
in two orientations: horizontal, or
back-and-forth, and vertical, or upand-down.
2. The record of the seismic waves created
by a seismograph is called a
seismogram.
a. The x-axis of a seismogram represents
time.
b. Heights of waves on a seismogram
show the relative size of ground
motion caused by seismic waves.
c. P-waves are the type of seismic
wave that is recorded first on a
seismograph.
C. Locating an Epicenter
1. With readings from at least three
seismographs, you can use
triangulation to find the location of
the epicenter of an earthquake.
a. Use a seismogram to measure the number
of seconds between the arrival of P-waves
and the arrival of S-waves.
b. Use a graph showing the time difference
between P- and S-waves plotted against
distance to determine how far the waves
traveled.
c. On a map, draw a circle with a radius
equal to the distance the waves have
traveled around the location of the
seismograph.
d. Draw the same kind of circles for
additional seismograms. The location of the
epicenter will be shown by the intersection
of the circles.
D. Measuring Earthquake Size
1. The magnitude scale is based on a
seismogram’s record of the amplitude of
ground motion.
a. Numerical measurements of
magnitude vary from about 0.0 to 9.0,
but each one number represents ten
times the amount of ground motion.
b. Magnitude can also be used to
understand the amount of energy
released by an earthquake. An increase
of one unit on the magnitude scale
represents 30 times as much energy
being released.
2. The Richter magnitude scale was the
first magnitude scale, but is not as
accurate as more modern magnitude
scales. With the Richter scale, the
magnitude values are not accurate for
small or large earthquakes.
3. Today, the most commonly used scale
for measuring earthquakes is the
moment magnitude scale.
E. Earthquake Intensity
1. Besides size, earthquakes can also be
compared by their intensity, or the amount
of damage they cause.
2. Intensity tends to decrease as you move
from the epicenter, but it can vary depending
on the types of rocks or sediments in an
area.
3. The most widely used intensity scale in
the U.S. is the modified Mercalli scale.
Discussion Question:
Would you personally rather experience an
earthquake with a large magnitude but low
local intensity, or a smaller-magnitude
earthquake with a higher intensity?
Standard Deviant video (25 min)
Lesson 6.4- Earthquake Hazards and Safety
A. Earthquake Hazards
1. Most injuries from an earthquake
are caused by the collapse of
buildings and other structures, not
directly by the ground shaking.
2. Fires caused by broken gas pipes or
electrical lines are the most common
hazard following an earthquake.
3. A landslide is the sudden movement
of soil and rocks down a slope.
4. Earthquakes sometimes cause
liquefaction, a process by which
shaking makes loose sediment
behave like a liquid. Liquefaction
near buildings can cause them to
sink into the ground.
5. An ocean wave caused by an
earthquake is called a seismic sea
wave, or a tsunami.
a. Like seismic waves, tsunamis
carry energy, but they can also
cause flooding and carry objects
in the water they move.
b. The towering wave on a
tsunami forms where the wave
hits shallow water.
c. A warning sign just before a
tsunami strikes, water along a
shoreline sometimes moves
quickly toward the sea,
exposing areas that are usually
underwater.
B. Avoiding Earthquake Hazards
1. The chance of earthquake damage is
greatest close to faults and also
increases in areas where Earth’s
surface is made of loose sediments
rather than solid rocks.
2. Maps showing where the chance of
earthquake damage is greatest, such
as in areas along the San Andreas
Fault, can help in the planning of
safe ways to use land.
C. Earthquakes and Structures
1. Tall buildings or structures and
buildings made of brittle materials
usually suffer the most damage in an
earthquake. Structures made from
flexible building materials, such as
wood, usually suffer less damage.
2. Some buildings in areas threatened
by earthquakes are supported by
flexible, circular moorings placed
under the buildings. These moorings
act like shock absorbers for the
buildings.
D. Earthquake Safety
1. To be as safe as possible during and
after an earthquake, it is important
to prepare ahead of time.
a. With your family, create, review,
and practice an earthquake
disaster plan.
b. Make an earthquake supply kit
9with canned food, water, a
battery-powered radio, a
flashlight, and first aid supplies.
c. Move heavy objects close to the
ground, and learn how to shut off
gas, water, and electricity in your
home.
2. During an earthquake, stay indoors,
get away from anything that could
break or fall on you, and find
something sturdy to hide
underneath.
Discussion Question:
If you feel the ground shake during an
earthquake, does that mean it is safe to
check around your house to see if everything
is all right?
No. P-waves move the fastest, so if you are
some distance away from the epicenter of
the earthquake, s-waves and surface waves
might not yet have reached you. Surface
waves are the slowest waves, but they often
cause the most damage. When you feel the
first movement of an earthquake, it is
important to move to a safe place and stay
there until all the waves have passed. Even
then, it is important to be careful and
observant before moving to another safe
place, because weakened parts of structures
might not fall immediately.
Natural Disasters video (55 min)