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=en4HptC0m

Q4&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/