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Chapter 7 Lesson 2
Earthquakes
What is an Earthquake?
• A sudden trembling of the ground
caused by movement happening in
the crust
Where Do Earthquakes Happen?
• Most earthquakes happen at the edges of plates,
where plates meet
• About 80 percent of all earthquakes happen
along the edge of the Pacific Plate
• Along the edges, plates can collide, they may pull
apart, or they may slide past each other
• These motions can cause the rocks to bend and
stretch until they break
• As a result, faults form at or below the surface of
the crust
Tectonic Plates
Earthquake Locations
What is a Fault?
• Fault- huge cracks in
the Earth’s crust
Three Kinds of Faults
• 1. Normal Fault
– Produced at divergent
boundaries
– Rocks above the fault
surface move down
— http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Nor
mal+Fault&flash_file=normalfault&flash_width=220&flash_height=320
Three Kinds of Faults
• 2. Reverse Fault
– Produced at convergent
boundaries
– Rocks above the fault
move upward
– http://earthquake.usgs.gov/learn/animations/animation.php?flash_tit
le=Thrust+Fault+and+Reverse+Fault&flash_file=thrustfault&flash_widt
h=220&flash_height=320
Three Kinds of Faults
• 3. Strike Slip Fault
– Produced at transform
boundaries
– Rocks slide past each other in
different directions
– http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=S
trike-Slip+Fault&flash_file=strikeslip&flash_width=240&flash_height=310
How Earthquakes Start
• Earthquakes start the moment rocks begin to
scrape past each other along a fault.
How Earthquakes Start
• Focus
– The point where the earthquake starts, where
rocks begin to slide past each other
– It is usually below the surface
– The sudden movement causes vibrations to
spread out from the focus
– These vibrations travel through the crust in the
form of waves
How Earthquakes Start
• Seismic Waves
– A vibration that spreads out away from a focus
when an earthquake starts
– They soon reach the Earth’s surface at a point
directly above the focus
How Earthquakes Start
• Epicenter
– The point on the Earth’s surface directly above
the focus of an earthquake
– People can first feel the ground shaking at the
epicenter
How Earthquakes Start
• Epicenter
– This shaking is what causes most
earthquake damage
– Damage is usually greatest
at the epicenter because
it is so close to the focus
– As waves travel away from
the focus they get weaker
Aftershocks
• After the first shaking, it may be quiet, and
then there are aftershocks
• This is the shaking of the Earth’s crust after
the initial shaking of an earthquake
Earthquake Waves
• There are different kinds of seismic waves
which travel differently and at different
speeds.
Earthquake Waves
• Primary Waves, or P Waves
– Accordion-like motion shaking
– Rock material squeezes together and spreads
apart repeatedly
– This motion produces seismic waves that move in
the same direction that the rock is shaking
– They are the fastest seismic wave- the first to
reach any faraway location
– http://www.youtube.com/watch?v=2rYjlVPU9U4&feature=
player_embedded
Earthquake Waves
• Secondary Waves, or S waves
– Shaking like a ruler held off the edge of a desk and
“twanged”
– This motion in rocks creates seismic waves that move
in a perpendicular direction from the vibration
– These waves travel slower than primary waves
– They are second to arrive at any given faraway
location.
– http://www.youtube.com/watch?v=en4HptC0mQ
4&feature=player_embedded
Earthquake Waves
• Surface Waves
– This shaking causes Earths surface to heave up
and down like an ocean wave, or sway from side
to side
– Tears apart structures built on the surface
Earthquake Waves
• Seismographs
– Scientists study and identify waves using a
seismograph
– A sensitive device that detects the shaking of the
Earth’s crust during an earthquake
– http://www.youtube.com/watch?feature=player_
embedded&v=DX5VXGmdnAg
Seismograph
Seismogram
Tsunamis
• If the focus of the earthquake is beneath the
sea floor, the seismic waves can travel through
the ocean, producing huge ocean waves called
tsunamis
• http://www.youtube.com/watch?v=yexLVVeW
Hb8
See Mrs. Balducci’s Website….
• …..For more information on earthquakes such
as earthquake current events and earthquake
simulations.
• Look under Chapter 7 Restless Earth LINKS
http://earthquake.usgs.gov/learn/kids/