Earthquakes

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Earthquakes
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Earthquakes

• Earthquakes are natural vibrations of the ground,
  caused by
• Fractures in earths crust
• Volcanic eruptions

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Stress and Strain

• Stress causes fractures in rock
• Three types of stress
• Compression
• Tension
• Shear
• Deformation caused by stress is known as strain

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Compression

• Compression is stress that decreases the volume
  of material

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Tension

• Tension is stress that pulls a material apart

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Shear

• Shear is stress that causes a material to twist

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Effects of Strain and Stress

• Stress and strain can cause deformation
• Low stresses cause elastic strain
• Elastic strain causes a material to bend and
  stretch
• Can return to normal, like a rubber band
• High stresses cause ductile deformation
• Material is permanently deformed
• Can cause breaks or failure

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Faults

• When stress causes rock to fail, a fault occurs
• Fault Fracture(s) along which movement occurs
• Three main types of faults
• Reverse
• Normal
• Strike-Slip (Transform)

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Reverse Fault

• Reverse Faults are fractures that result from
  horizontal compression
• Shortens crust horizontally

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Normal Fault

• Normal Faults are fractures caused by horizontal
  tension
• Lengthens crust horizontally

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Strike-Slip Fault

• Strike-Slip Faults are fractures caused by
  horizontal shear
• Motion is mainly horizontal

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Earthquake Waves

• Vibrations of the ground during earthquakes are
  called Seismic Waves
• Three types
• Primary Wave (P-Wave)
• Secondary Wave (S-Wave)
• Surface Waves

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P-Waves

• P-Waves squeeze and pull rocks in the same
  direction as the wave travels

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S-Waves

• S-Waves cause rocks to move at right angles to
  the direction of the wave

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Surface Waves

• Surface Waves move rocks up and down, as well as
  side-to-side
• Motion is similar to an ocean wave

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Focus

• Earthquakes originate at a point called a Focus
• Focus is usually below the surface
• The Epicenter is the point on Earths surface
  directly above the focus

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