Seismic Hazards

• Plates do not perfectly fit into each other, meaning they do not move in fluid motions
• At all boundaries, plates can become stuck due to the friction between plates
• When the plates are stuck

  The convection currents in the asthenosphere continue to push, which builds the pressure
• The pressure builds so much that it cannot be sustained


  The plates eventually give way
• The pressure is released in a sudden movement


  Causing a jolting motion in the plates
• Focus


  The point underground where the earthquake originates from
• Epicentre


  The area above ground that is directly above the focus
• Spatial Distribution of Earthquakes

  • Along all boundaries
  • The Ring of Fire accounts for 90% of the world's Earthquakes
  • The Alpine-Himalayan belt accounts for 5-6% of the world's earthquakes
• Richter Scale

  A logarithmic measure of the strength of seismic waves
• Modified Mercalli Intensity Scale

  A rate of the destruction caused (originally the Mercalli scale when developed in 1884, but the name was changed after 1931 when it was modified)
• The Mercalli scale is subjective, meaning sometimes it is disputed as it is dependent on human development being present rather than the strength of the seismic waves
• The magnitude of the earthquake

  Is dependent on the depth of focus
• Conservative boundaries have the shallowest boundaries, meaning they are closer to the epicentre and the seismic waves are stronger
• Destructive boundaries usually have deeper focuses, meaning the seismic waves are spread over a larger area before they reach the epicentre
• Earthquakes are frequent around the world and occur every day at boundaries
• Hundreds of smaller magnitude earthquakes that cannot be felt by humans occur every day, whereas the larger earthquakes are less frequent
• Earthquakes follow no pattern and are random so there is irregularity between events
• Earthquakes are almost impossible to predict
• Microquakes may give some indication but the magnitude cannot be predicted as how strong they are is random
• Shockwaves (seismic waves)

  1. Friction builds up and pressure increases

  2. Pressure becomes too much, the plates eventually move

  3. Energy is transferred into kinetic energy, which is released and vibrates throughout the ground

  4. The further away from the focus, the weaker the shockwaves, as the energy is transferred into the surroundings
• Tsunamis

  1. Oceanic crust is jolted during an earthquake, displacing the water above

  2. The water travels fast but with a low amplitude (height)

  3. As it gets closer to the coast, the sea level decreases so there is friction between the sea bed and the waves

  4. This causes the waves to slow down and gain height, creating a wall of water that is on average 10 feet high, but can reach 100 feet
• Liquefaction

  When soil is saturated, the vibrations of an earthquake cause it to act like a liquid
• Soil becomes weaker and more likely to subside when it has large weight on it
• Movement in soil or snow will cause it to become unstable, leading to landslides and avalanches
• Type of Seismic Hazard

  • Primary
  • Secondary
• Effects of Seismic Hazards

  • Environmental
  • Economic
  • Social
  • Political