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Unit 7-1: Earthquakes Result From Stress

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Title: Unit 7-1: Earthquakes Result From Stress


1
Unit 7-1Earthquakes Result From Stress
2
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.

3
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.

4
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.

5
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.

6
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.

7
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.

8
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.

9
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.

10
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.

11
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.

12
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.

13
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

14
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.

15
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.
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