Chapter 7 Lesson 2 Earthquakes

Chapter 7 Lesson 2
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
– Rocks above the fault
surface move down
Three Kinds of Faults
• 2. Reverse Fault
– Produced at convergent
– Rocks above the fault
move upward
Three Kinds of Faults
• 3. Strike Slip Fault
– Produced at transform
– Rocks slide past each other in
different directions
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
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
• 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
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
Earthquake Waves
• Secondary Waves, or S waves
– Shaking like a ruler held off the edge of a desk and
– 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
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
– A sensitive device that detects the shaking of the
Earth’s crust during an earthquake
• If the focus of the earthquake is beneath the
sea floor, the seismic waves can travel through
the ocean, producing huge ocean waves called
See Mrs. Balducci’s Website….
• …..For more information on earthquakes such
as earthquake current events and earthquake
• Look under Chapter 7 Restless Earth LINKS

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