Earthquakes

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Chapter 21

• Earthquakes

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Section 1

• What are earthquakes?

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History of earthquakes

• First recorded earthquake was in China in 1177
  B.C.
• Greek mythology stated that they were caused by
  Posseidon, God of the Sea who had the power of
  earth shaker
• a tsunami often followed, so it was an
  appropriate power for a sea god
• The 1811-1812 earthquakes in New Madrid, Missouri
  were so widespread that they were felt in Boston,
  Massachusetts.

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• Seismology
• Where do they happen?

• Study of earthquakes
• Occur near tectonic plate boundaries
• Tectonic platelarge pieces of Earths crust that
  move on top of the softer rock beneath it
• Movement causes plates to rub against each other
• Faultoccurs where there is a break in the
  Earths crust along the area where the plates are
  rubbing against each other

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• Fault

• Earthquakes can happen because those blocks of
  crust move suddenly along the faults
• Regions where a large number of these earthquakes
  occur are called earthquake zones

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World map of tectonic plates
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• Tectonic plates push against each other and build
  up potential energy
• Potential energy energy an object has because of
  its position and shape created by stress
• When stored energy is released, it becomes
  kinetic energy (the energy of motion)
• An earthquake is kinetic energy traveling through
  the Earth

• What causes Earthquakes?

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• Elastic rebound

• Rocks can be deformed by the stress at a plate
  boundary
• Similar to a rubber band stretchingonce it
  breaks, the pieces return to their original shape
• This is called elastic rebounddeformed rock
  returns to its original shape
• This causes a huge amount of energy to be
  released and the ground shakes

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• Are all earthquakes the same?

• Earthquakes vary depending on where they
  originate
• Some earthquakes occur along faults in the middle
  of tectonic plates
• 3 types of boundaries that can create
  earthquakes
• Transformmake strike-slip fault
• Convergentreverse fault
• Divergentnormal fault

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• Transform
• Convergent
• Divergent

• Causes moderate to shallow earthquakes
• Crust slides horizontally
• Form strike-slip faults
• Strong, deep earthquakes
• Crust slides vertically
• Create reverse faults
• Weak, shallow earthquakes
• Crust is pulled away from each other slide
  vertically
• Creates normal faults

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Hanging wall
Foot wall
Strike slipthe plates slip past each other
Reversethe hanging wall moves upward relative to
the foot wall
Foot wall
Hanging wall
Normalthe hanging wall moves down relative to
the foot wall
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Strike slip example
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Reverse fault example
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Dixie Valley-Fairview Peaks, Nevada earthquake
December 16, 1954
Normal fault example
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San Andreas Fault
All photos courtesy of USGS
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Bigger faults make bigger earthquakes
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• How earthquakes travel

• Kinetic energy is released in the form of seismic
  waves
• Seismic waves waves of energy that travel
  through the Earth, away from an earthquake in all
  directions
• 2 types of waves
• Surface wavestravel only on Earths surface
• Body wavestravel through Earths interior
  include p waves and s waves
• Each type travels thru Earths layers at
  different speeds due to the intensity of the wave
  and the material they go thur (ex. Rock, soil,
  liquid)

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• P for primary
• S for secondary

• P waves travel through solids, liquids and gases
• Fastest seismic waves
• Called primary waves because they are the first
  to be detected
• Move rock particles back and forth by squeezing
  and stretching the rock
• S waves are second fastest
• Stretches rock back and forth
• Also called secondary waves and shear waves
• Cannot travel thru gas or liquid b/c they cannot
  spring back to their original shape

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• Surface waves

• Move the ground surface up and down as the wave
  travels
• Does not affect the rock deep below the surface
• Can feel like a roller coaster ride
• Travel more slowly than body waves
• Most destructive type of wave

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Section 2

• Earthquake measurement

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• Locating earthquakes

• Seismologists use instruments called seismographs
  to locate earthquakes
• Seismographinstruments that record seismic waves
  and are located near or on the surface of the
  Earth
• When the waves move the seismograph, it creates a
  seismograma tracing of ground movement

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Seismograph
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• Where did it happen?

• Seismograms are used to find an earthquakes
  epicenter
• epicenterthe point on the Earths surface
  directly above the earthquakes starting point
• Focusthe point inside the Earth where the
  earthquake began

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• S-P time method

• Seismologist find the epicenter using the S-P
  time method
• They collect seismograms from different locations
  put them on a graph
• They can see the distance each location is from
  the beginning of the earthquake
• This helps them locate the epicenter

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• To find the distance from one location to the
  epicenter
• Record time between p wave and s wave
• Measure amplitude
• Plot them on the graph
• That will give you the distance to the epicenter
• If you have many of these, you can plot them on a
  map and find the epicenter

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• Measuring strength
• Richter scale
• Earthquake energy

• Seismograms can be used to find earthquake
  strength
• Measures earthquake strength
• Developed by Charles Richter in 1930s
• As the magnitude increases, the amount of energy
  released increases
• Each time magnitude increases, the amount of
  energy released is 31.7 times greater
• A m 4.0 is 31.7 times stronger than a m 3.0!

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Richter Scale
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Section 3

• Earthquakes and society

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• Earthquake hazard

• Earthquake-hazard levela measure of how likely
  an area is to experience an earthquake
• This is determined by past and present seismic
  activity
• Different areas of the country and the world have
  varied hazard levels

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                            The red zones are
the ones where seismic activity is most likely to
occur, and the white zones show areas where
seismic activity is least likely (but still
possible).
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• Forecasting
• Strength and frequency

• Seismologists use patterns to help predict
  seismic activity
• The strength of an earthquake is related to it
  frequency (how often the occur)
• There are many more weak earthquakes than there
  are stronger earthquakes
• The number strength of the earthquakes are
  recorded and this helps predict future
  earthquakes

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• Gap hypothesis

• States that areas along active faults that have
  had few earthquakes are the places where strong
  earthquakes will happen in the future
• Those areas with few earthquakes are called
  seismic gapsthere is a gap in seismic activity

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• Earthquakes buildings

• Buildings are often damaged or destroyed due to
  the force of an earthquake

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Earthquake Effects - Ground Shaking
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Result of ground shaking
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Surface fault
Landslide resulting from earthquake
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• Are you prepared?
• Before shaking
• During shaking
• After shaking stops

• Preparation for earthquakes helps to save lives
  and property
• Have an earthquake-safety plan for you and your
  family
• Be prepared with medical and food supplies
• Get under table or desk and protect your head
• Stay away from power lines and buildings
• Move away from any dangers
• Be prepared for aftershocks

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What Controls the Level of Shaking?

• Distance

• Shaking decays with distance

• Magnitude

• More energy released

• Local soils

• amplify the shaking

-aftershocks can continue for days, weeks or even
years!