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What Are Earthquakes?
Chapter 5 Section 1
p. 130 - 136
Vocabulary:
1. seismology
2. deformation
3. elastic rebound
4. seismic waves
5. P waves
6. S waves
The word earthquake defines itself fairly well, but there is
more to earthquakes than just the shaking of the ground. In
fact, an entire branch of Earth science, called seismology, is
dedicated to studying earthquakes.
Most earthquakes take place near the edges of tectonic plates.
As a result of the movements of the plates, numerous features
called faults exist in the Earth’s crust. A fault is a break in the
Earth’s crust along which blocks of the crust slide relative to one
another. Earthquakes occur along faults because of this
sliding.
What Causes Earthquakes? p. 131
As tectonic plates push, pull, or slip past each other, stress increases
along faults near the plates’ edges. In response to the stress, rock in the
plates deforms. Rock along the plates can deform in two main ways:
Plastic Deformation and Elastic Deformation.
Plastic deformation causes the rock to bend like a piece of clay. Earthquakes
do not result from plastic deformation.
The road cut is next to the San Andreas
Fault in southern California. The rocks
in the cut have undergone deformation
because of the continous motion of the
fault.
Elastic deformation does lead to earthquakes. Think of elastically
deformed rock as a rubber band. You can stretch a rubber band only so
far before it breaks. When the rubber band breaks, it releases energy.
Then the broken pieces return to their original unstretched shape.
The sudden return of elastically deformed rock to its original shape is
called elastic rebound. It’s like the return of the broken rubber band pieces
to their unstretched shape. Elastic rebound occurs when more stress is
applied to rock than the rock can withstand. During elastic rebound, energy
is released. Some of this energy travels as seismic waves. These
seismic waves can cause an earthquake.
Plate Motion and Fault Type
Plate Motion
Major Fault Type
Transform
Strike-slip fault
Convergent
reverse fault
Divergent
normal fault
A specific type of plate motion takes place at
different tectonic plate boundaries. Each type
of motion creates a particular kind of fault that
can produce earthquakes.
Earthquake Zones p. 133
Earthquakes can happen both near Earth’s surface or far below it. Most
earthquakes happen in the earthquake zones along tectonic plate boundaries.
Earthquake zones are places where a large number of faults are located.
The San Andreas Fault Zone in California is an example of an earthquake
zone. Not all faults are located at tectonic plate boundaries. Sometimes,
earthquakes happen along faults in the middle of tectonic plates.
San Andreas Fault in California
The three major tremors of the 1811-1812 earthquakes in New
Madrid, Mo, were so intense that according to reports, they
altered the Mississippi River! In the late 1970’s, scientists
found a series of faults deep beneath sediment deposited in the
Mississippi River. The area is still seismically active, and large
earthquakes may still occur.
How Do Earthquake Waves Travel? p. 134
Waves of energy that travel though the Earth are called seismic waves.
Seismic waves that travel through the Earth’s interior are called body waves.
There are two types of body waves: P waves and S waves.
Seismic waves that travel along Earth’s surface are called surface waves.
Each wave travels through Earth’s layers in a different way and at different
speeds. The speed of the seismic wave depends on the kind of material it
travels through.
Waves that travel through solids, liquids, and gases are called P waves
(pressure waves). They are the fastest seismic waves, so P waves always
travel ahead of other seismic waves. They can also be called primary waves
because they are always the first waves of an earthquake to be detected.
If you were to tap a gelatin cube, you would notice its elastic-like movement.
Tapping it changes the pressure inside the cube and deforms it for only a
minute. Then it springs back to its original shape. This process is how P
waves affect rock.
Rock can also be deformed from side to side. After being deformed from
side to side, the rock springs back to its original position and S waves are
created. S waves are shear waves. They are the second fastest seismic
waves. They stretch rock side to side. Unlike P waves, they cannot
travel through the parts of the Earth that are completely liquid. They are
slower than P waves and always arrive later. Another name for S waves
are secondary waves.
Surface Waves p. 135
Surface waves move along the Earth’s surface and produce motion
mostly in the upper few kilometers of Earth’s crust. There are two types
of surface waves. One type produces motion up, down and around. The
other type produces back and forth motion like the motion produced by
S waves. Surface waves are different from body waves in that surface
waves travel more slowly and are more destructive.
Earthquakes – Rock Moves Aside Song
Quiz Time!
1. What is a fault?
a break in the Earth’s crust along which blocks of the crust
slide relative to one another.
2. How does rock that is along a fault deform in response to a
decrease in stress?
It deforms in an elastic manner, like a rubber band, snapping
back to its original shape.
3. _________ is the change in shape of rock due to stress.
deformation
4. _________ always travel ahead of other waves.
P waves
5. Seismic waves that shear rock side to side are called
A. surface waves
B. S waves
C. P waves
D. both B and C
B
6. Where do earthquakes occur?
in earthquake zones along tectonic plate boundaries
Science Humor…
What did one mountain say to the other mountain after the
earthquake?
“It wasn’t my fault!”