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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 - PowerPoint PPT Presentation
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Chapter 7 Lesson 2Earthquakes
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_width=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=yexLVVeWHb8
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/