Unit 7-1: Earthquakes Result From Stress

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Unit 7-1Earthquakes Result From Stress
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What is an Earthquake?

• Earthquake
• The shaking of the Earths crust caused by a
  release of energy.
• Earthquakes are caused by the motion of the
  lithospheric plates.
• Similar to volcano origins.
• Earthquakes are much more destructive, as they
  affect a much larger area.

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What is an Earthquake?

• Small earthquakes are very common.
• There are over a million earthquakes every year.
• This is roughly one earthquake ever 30 seconds.
• Many of these earthquakes are too small to be
  noticed by anything but a sensitive piece of
  scientific equipment called a seismometer.

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What is an Earthquake?

• About 3000 result in moving a section of the
  Earths crust.
• Several hundred result in significant changes.
• About 20 result in severe changes.

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What is an Earthquake?

• An earthquakes danger is directly related to how
  close it is to human settlement.
• The closer and more powerful the earthquake is to
  a densely populated area, the more dangerous it
  is.
• The earthquake itself is only the start of the
  destruction, as many more events are triggered by
  earthquakes.

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Causes of Earthquakes

• There are actually several causes of earthquakes
• Volcanic eruptions
• Cavern collapse
• Meteor impact
• The most common is due to stress that builds up
  in the Earths crust.
• Most scientists believe that stress occurs when
  two lithospheric plates move past each other.

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Causes of Earthquakes

• Fault Plane
• The surface that separates two lithospheric
  plates.
• The friction between the plates normally keeps
  them in place.
• However, every passing moment causes stress to
  slowly build up.

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Causes of Earthquakes

• As stress builds up, it eventually overcomes the
  friction, then
• WHAM!
• Earthquake!
• Elastic-Rebound Theory
• As stress and pressure are building up in the
  plates, they deform.
• When the earthquake occurs, the plates snap back
  to their original shape.

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Depth of Earthquakes

• The depth of the earthquake is dependent on the
  type of plate boundary.
• At a spreading or sliding boundary less than
  30km
• At a subduction boundary can be up to 700km
  deep
• Focus
• The actual location of the earthquake inside of
  the earth.

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Depth of Earthquakes

• Epicenter
• The location on the Earths surface directly
  above the focus.
• The deeper the earthquake is, the more
  wide-spread the damage is.
• A shallow earthquake is localized, while a deep
  earthquake travels far.

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Depth of Earthquakes

• Depth classification
• Shallow 60km or less
• Intermediate 60km-300km
• Deep 300km or deeper
• The discovery that earthquakes start deeper at
  some plate boundaries was one of the first steps
  in the development of the theory of plate
  tectonics.

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Earthquake Waves

• Earthquakes produce three types of wave motion.
• Each of these wave types travels at a different
  speed and through different material.
• The difference in speed is very important in
  determining the epicenter.

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Earthquake Waves

• Primary (P) Waves
• Back and forth wave motion.
• Alternately squeezes and stretches the rock it
  passes through.
• Can travel through any material
• Solid rock
• Magma
• Ocean Water
• Air

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Earthquake Waves

• Secondary (S) Waves
• Vibrate at right angles to the P waves, so they
  oscillate side to side as they move forward.
• S-Waves can travel through solids, but not
  through liquids.
• The rate at which P and S waves move are
  determined by the material they move through.
• Regardless of the material, P waves move about
  twice as fast as S waves.

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Earthquake Waves

• Surface (L) Waves
• The final type of wave generated by an
  earthquake.
• These waves travel forward while rolling like an
  ocean wave.
• These are among the more destructive wave created
  as this causes the most movement of the surface.