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Earthquakes
Chapter 2 book F page 44
Vocabulary for section 1 page 44
book F
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Stress
Tension
Compression
Shearing
Normal fault
Reverse fault
Strike-slip fault
Earthquakes
• Earthquakes: the shaking that results from
the movement of rock beneath the Earth’s
surface.
– An Earthquake is a release of tension at a
high stress point causing the shaking.
• There three different types of stress.
– Tension
– Compression
– shearing
Tension
• Tension : this type of stress pulls on the
crust, stretching rock so that it becomes
thinner in the middle.
– This type of stress is found where twp plates
are being pulled apart.
Compression
• Compression : this type of stress
squeezes rock until it folds or breaks.
– This type happens at plate boundary's where
one plate pushes against another.
Shearing
• Shearing : stress that pushes rock in two
different directions.
– Shearing can cause rock to break and slip
apart or to change its shape.
Faults
• Fault : a break in Earths crust where
masses of rock slip past each other.
– Most faults occur along plate boundaries,
where the forces of plate motion push or pull
the crust so much that the crust brakes. There
are three main types of faults
• Normal
• Reverse
• Strike-slip
Normal
• Normal fault: is when the crust is pulls
apart.
• Reverse fault: is when the rock is pushed
together compression causes these kind
of faults.
• Strike-slip fault: when plates move past
each other, shearing causes this type of
fault.
Section 2 (Earthquake and seismic
waves)
• Vocabulary for section 2
– Epicenter
– P wave
– S wave
– Surface wave
– Magnitude
– Richter scale
– Seismograph
Where Earthquakes happen
• Most Earthquakes happen in the
lithosphere within about 100 kilometers of
Earths surface.
– The focus: is the area beneath Earths surface
where rock that is under stress breaks,
triggering an earthquake.
• Epicenter: The point on the surface directly above
the focus,
Types of waves
• Seismic waves carry energy out from the
focus in all directions during an
Earthquake.
P waves
• P waves : a type of seismic wave that
compresses and expands the ground.
– P waves or primary waves are so named
because they are the first waves to arrive
S waves
• S waves : a type of seismic wave that
moves the ground side to side, up and
down.
– When S waves reach the surface they shake
structures violently, causing damage and
destruction.
Surface waves
• Surface waves : when P&S waves reach
the surface they can become surface
waves.
– So what makes them different?
– these type of waves move more slowly than
the other two but they can cause Sevier
ground movements causing the ground to roll
like ocean waves and to move from side to
side.
Measuring an Earthquake
• When an Earth quake happens everybody
wants to know how big it was.
– To determine this there are about 20 different
measures for rating Earthquakes. Each with
its strengths and weaknesses. The one used
most often is the Richter scale.
• Richter scale: is a rating of the Earthquakes
magnitude based on the size of the seismic waves.
Instrument for measuring
• Seismograph : a device that measures
and records ground movement caused by
seismic waves as they move through
Earth.
Where to look
• How do we find where the Earthquake
originated from?
– Geologist use seismic waves to locate an
Earthquakes epicenter. P waves arrive at the
seismograph first followed by S waves, to tell
how far away the Earthquake is from the
seismograph scientist measure the difference
in arrival times.
Animation
• http://geologyindy.byu.edu/faculty/rah/slide
s/Rock%20Canyon/Wasatch%20Fault/Wa
satch%20Fault%20Home.htm
Unit Quiz
1. What is an Earthquake?
2. What is an epicenter?
3. Why are P waves called P waves?
4. What does a Richter scale measure?
Unit Quiz
• 5. How do scientists determine where an
epicenter is?
• 6. What kind of movement is produced by
each of the three waves?
• 7. What is a fault?
• 8. What are the different types of faults?