Download Earthquakes

Chapter 7 Lesson 2 Earthquakes
How Do Earthquakes Happen?
• An earthquake is defined as a sudden trembling
in the ground caused by something happening in
the crust.
• Most happen at the edges of plates, or plate
boundaries. Very few happen in the center of
plates.
• About 80% happen along the edges of the Pacific
plate.
• Caused by plates colliding, sliding, or pulling
apart. The rocks bend and stretch.
When plates move…
• Great forces are exerted on the rocks.
– First, the rocks bend and stretch.
– Once they reach their limit, they break.
– Faults form at or below the surface of the
crust.
Faults
• When rocks in the crust reach their breaking
limit and crack.
San Andres Fault
Earthquakes in NJ
Why do most earthquakes occur in the midnorthern section of the state?
Normal Fault
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Happens at divergent boundaries
Where plates pull apart
Rocks above the fault surface move down.
Ex: Sierra Nevada in CA
Reverse Fault
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Happens at convergent boundaries
Where plates push together
Rocks above the fault move upward
Ex: Himalayas in India
Strike-slip Fault
• Happens at transform boundaries
• Plates slide past each other without moving
up or down.
• Where rocks slide past each other in different
directions.
• Ex: San Andreas Fault in CA
How Do Earthquakes Make Waves?
• An earthquake starts the moment rocks begin to
scrape past each other along a fault.
• It may be a new fault that forms at that moment
or an old fault that has already been formed.
Focus
• The point where the earthquake starts, where
rocks begin to slide past each other
• It is usually below the surface
• The sudden motion causes vibrations to spread
out from the focus
• These vibrations travel through the crust in the
form of waves
Seismic Waves
• The vibrations travel through the crust.
• They reach Earth’s surface at a point directly
above the focus called the epicenter.
Epicenter
• Definition: The point on Earth’s surface directly
above the focus of an earthquake
• People can first feel the ground shaking.
• This shaking is what causes most earthquake
damage 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
aftershocks occur.
• Aftershocks continue the damage of an
earthquake.
• Additional shaking and damage after the first
shaking and relative quiet.
Earthquake Waves
• Vibrations that are given off by an earthquake and
travel through the crust are called seismic waves.
• A seismic wave starts with shaking caused by rocks
scraping against each other.
• This shaking results in several kinds of seismic
waves.
• The waves travel at different speeds.
• Three kinds:
– Primary Waves – P waves
– Secondary Waves – S waves
– Surface Waves
Primary Waves – P waves
• http://www.youtube.com/watch?v=2rYjlVPU9U
4&feature=player_embedded
• Accordion-like motion where rock squeezes
together and spreads apart repeatedly.
• Produces seismic waves that move in the same
direction that the rock is shaking.
• The fastest seismic wave
• The first to reach any faraway location
Secondary Waves – S Waves
• http://www.youtube.com/watch?v=en4HptC0m
Q4&feature=player_embedded
• Shaking like a ruler held off the edge of a desk
and “twanged”
• Produces seismic waves that move in a different
direction from the vibration
• Travel slower than primary waves
• They are the second to arrive at any given
faraway location.
Surface Waves
• Causes Earth’s surface to heave up and down
like an ocean wave, or sway from side to side.
• Tears apart structures built on the surface as
the land stretches and compresses.
When a 7.0-magnitude earthquake rocked the
south island of New Zealand one train track was
permanently altered in what looks more like a
Photoshopped image or a cartoon gag than an
actual photograph.
Seismograph
• Definition: A sensitive device
that detects the shaking of the
Earth’s crust during an earthquake.
• Used by scientists to study earthquake waves
• Shows patterns in the waves.
• Can identify the P waves, S waves, and surface
waves from studying the readings of the
seismograph.
Seismograph
• http://www.youtube.com/watch?v=83GOKn7
kWXM&feature=player_embedded
• http://www.youtube.com/watch?feature=play
er_embedded&v=DX5VXGmdnAg
Ocean Waves
• When an earthquake’s focus is beneath the
ocean floor, the seismic waves can travel
through the ocean.
• They can produce huge ocean waves called
tsunamis
Tsunami Formation
http://www.youtube.com/watch?v=aHljDIDf6js
• When tsunamis reach a shoreline, the top of
the wave travels faster than the bottom and
they can reach heights of more than 50 ft.
• They destroy everything in their path.
• Can reach speeds up to 560 mph.
Richter and Mercalli Scales
• Pg. D25 in textbook
• http://www.iknowthat.com/mhscience/Earth
quakes/earthquake_movie.html
Latest Earthquakes
• Latest Earthquakes in the World - Past 7 days
• http://earthquake.usgs.gov/earthquakes/rece
nteqsww/
• Earthquakes around the world
• http://www.guardian.co.uk/news/datablog/in
teractive/2011/feb/22/earthquake-map-world