Earthquake

• Earthquake Definition

  Shaking of the earth’s ground due to sudden release of energy in the earth’s lithosphere
• Earthquake occurrence 


  Generally occur along plate boundaries, which contain systems of deep fractures called faults
• Earthquake occur when

  • Rock masses on either side of a fault are pushed by tectonic forces
  • Friction causes them to get locked, and stress build up
  • When the stress exceeds the strength of the fault (rock), the rocks snap or suddenly move to a new position
  • The sudden movement causes seismic waves to be released resulting in ground shaking
• Earthquake concepts

  • The point in the earth’s crust where waves are released is known as focus -> origin of earthquake
  • The point on the earth’s surface directly above the focus is called the epicentre -> shaking is generally felt strongest

  
• How are earthquake measured?

  Measured using: seismometers

  -> sensitive instruments that detect ground vibrations & determine the magnitude of an earthquake

  -> greater the seismic energy released during an earthquake = greater the magnitude
• Richter Scale (ML)

  • Calculates earthquake magnitude using the height of the largest wave recorded on seismometers
  • —> Measured based on the maximum seismic intensity reached
  • —> Scale numbered from 1 to 10
  • —> Scale is logarithmic ( magnitude 6 earthquake is 32 times more energy than magnitude 5 )
• Richter Scale limitation


  Rate an earthquake with a single drastic spike in wave energy as having a higher magnitude than a long earthquake with many large, intense wave

  • underestimates longer earthquakes -> released more overall energy -> could deal more damage
  • no longer commonly used (expect for small, local earthquake)
• Moment Magnitude Scale (Mw)


  Rate earthquake magnitude based on the total energy released during the earthquake

  • estimates total energy instead of largest wave
  • more accurate (especially in magnitude 8 earthquake & above)
  • scale is logarithmic (magnitude 6 earthquake is 32 times more energy than magnitude 5 )