Transcript Slide 1 - Cloudfront.net

7.1
What Are Earthquakes?
1
Describe what happens during an
earthquake.
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Earthquakes
•The movements or shaking of the ground
•Mostly take place at plate boundaries
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Earthquakes
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clips\1.SeismicityGlobal_Low.mov
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Earthquakes
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Joke of the day


What do you call a cow during an
earthquake?
A milkshake.
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Earthquakes at Divergent Boundaries
•Two tectonic plates pull away from one another
•Tension
•Normal fault
•Happens at mid-ocean ridge
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Earthquakes at Convergent Boundaries
•Two tectonic plates collide.
•This causes a mountain or subduction (one plate moves over
another).
•Compression
•Reverse fault
•Plates thrust over one another
•Happens at subduction zones
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Earthquakes at Transform Boundaries
•Two plates move past one another horizontally
•Shearing
•Strike-slip fault
•Happens mostly between plates made of oceanic
lithosphere, but does happen between continental
lithosphere.
•Occur at shallow depths
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Why do earthquakes happen at transform
boundaries?
At transform boundaries, shear stress causes
the rock to break into a series of blocks. The
blocks form a series of strike-slip faults.
Earthquakes happen along the strike-slip faults
when stress becomes too great.
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Boundaries
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Fault Zones
•Places along plate boundaries where large numbers of
interconnected faults are located
•Example: San Andreas fault zone
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Joke Of the Day


Here in California, when a bridge falls down,
who’s fault is it?
San Andrea’s fault
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Fault Zones
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Why Earthquakes Happen
•Plastic deformation – deform like clay being molded
•Elastic deformation – rock is stretched like a rubber
band
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Elastic rebound
(when elastic deformation returns to its original shape)
•Causes fault to jerk and break
•Leads to earthquakes
•Energy is released
• seismic waves- energy travels through rock
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How does elastic rebound cause
earthquakes?
Tectonic forces push rock on either side of a
fault in opposite directions. Because the
rock is locked together, it does not move
and deforms in an elastic manner. When
enough stress builds up, the rock along the
fault fails, the blocks of rock on either side of
the fault move, and energy is released as an
earthquake.
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Earthquake Waves
Body Waves (Seismic waves) –
waves that travel through Earth’s
interior
•P Waves
•Pressure waves or primary waves
•First waves to be detected
•Travel through solids, liquids, and
gases
•S Waves
•Shear waves or secondary waves
•Move side to side
• Cannot travel through liquids
•Slower than P waves
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How do P waves and S waves differ in
the way that they transfer energy?
P waves transfer energy by squeezing
and stretching rock. S waves transfer
energy by shearing rock horizontally form
side to side.
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P Waves
..\..\..\Desktop\Power point
clips\1.P-Wave.mov
S Waves
..\..\..\Desktop\Power point
clips\2.S-Wave.mov
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Surface Waves
•Move only along the Earth’s surface.
•Cause the most damage
•Travels slower than body waves
•One type is rolling, up and down
motion
•Another type is back and forth
motion
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Explain why surface waves are the most
destructive type of seismic waves.
The energy of surface waves is
concentrated at the surface of Earth.
Therefore, surface waves tend to cause a
lot of damage.
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Seismic Waves
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earthquake\Investigating_Earthquakes__Movement_in_the_Earth_s_
Crust.asf
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Review Questions
1. Describe the three types of plate motion and the
faults that are characteristic of each motion.
Normal faults are characteristic of divergent motion,
where plates pull apart. Reverse faults are
characteristic of convergent motion, where plates
collide. Strike-slip faults are characteristic of
transform motion, where plates slide past each other
horizontally.
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2. How do the locations of earthquakes support the
theory of plate tectonics?
Most earthquakes occur at tectonic plate
boundaries rather than in the interior of tectonic
plates. This indicates that most active faults occur
at tectonic plate boundaries as a result of plate
motion.
3. Why are surface waves more destructive to
buildings than P waves or S waves are?
Surface waves are more destructive to buildings
than P waves and S waves are because the energy
of surface waves is concentrated at Earth’s
surface.
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4. Imagine that you are splashing water in a pool.
Explain how you could model each type of seismic
wave. Then, try to explain why S waves do not travel
through liquids.
You could model P waves in a swimming pool by
pushing water out in front and away from you. You
could not model S waves in a swimming pool, and S
waves do not move through liquid, because you cannot
create a shearing motion by pushing liquid back and
forth. You could model surface waves in a swimming
pool by bobbin up and down in the pool, which would
create an up and down motion in the water.
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Review 7.1
•Earthquake
•Body waves
•P wave
•S wave
•Surface waves
•Seismic waves
•Elastic deformation
•Elastic rebound
•Normal fault
•Reverse fault
•Strike-slip fault
•Stress on Earth because of plate movement
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