Hazards 4 Seismic Hazards

Nature and Distribution:

• Earthquake: as the crust of Earth is constantly moving, there tends to be a slow build up of stress within the rocks. When this pressure is released, parts of the surface experience, for a short period, an intense shaking motion. This is an earthquake.
• Focus: the point at which the pressure is released in the crust
• Epicentre: the point immediately above the focus on the Earths surface
• Constructive plate margins:
  • Shallow focus
  • Often occur at mid ocean ridges so pose no threat to humans
• Collision plate margins (continental/continental)
  • Shallow focus
  • Form fold mountains
• Conservative plate margins:
  • Shallow focus
• Destructive plate boundaries:
  • Deep foci
  • Occur in the Benioff zone (where oceanic crust is being subducted under the continental crust)
• 90% of earthquakes are at plate boundaries, 10% are intraplate
• In 2002 in the UK Midlands a 4.8 EQ occurred, which was caused by movements on an old fault line
• seismometer: the instrument that measures the seismic waves generated by an earthquake

Magnitude and Frequency:

• Richter Scale:
  • The amplitude of the largest recorded seismic wave and the distance between the focus and the seismometer 
  • A 1-10 scale
  • Not accurate when measuring large scale earthquakes
• Moment Magnitude Scale:
  • The total moment released of the EQ, where a moment is the product of the distance the fault  has moved and the force required to move it
  • A 1-10 scale
  • Used globally
  • Up to 8, very similar to Richter scale, but above it, it is more accurate
• Mercalli Scale:
  • Uses observations of people to determine the amount of damage caused by the event
  • A 12 point scale
• The UK experiences 20-30 earthquakes per year

Primary Effects:

• Body waves:
  • P waves:
    • Travel through liquids and solids
    • Travel at 5.5km/s
    • Faster than S waves
  • S waves:
    • Travel through solids
    • Travel at 3km/s
    • Slower than P waves
• Surface waves:
  • Love waves:
    • Shake the ground at right angles to the direction of movement
  • Rayleigh waves:
    • Have a rolling motion

Secondary Effects:

• Liquefaction
  • Soils with high water content lose their mechanical strength and start to behave as a liquid
• Landslides/avalanches
• Tsunamis
• Fires
  • As a result from broken gas pipes and collapsed electricity transmission services
• Effects on people and the built environment
  • Collapsing buildings, destruction of road systems, destruction of communication systems, destruction of service provisions: water, electricity, gas, flooding, food shortages, disruption to the local economy

Short Term Impacts:

• 3/4 deaths are from falling buildings
• Rapid urbanisation has increased the earthquake risk. Low cost and poorly built housing make people is slums disproportionately vulnerable 
• The destruction of infrastructure will hinder the rescue efforts 
• Contamination of the water supply

Long Term Impacts

• The effect on the economy

Prediction:

• Prediction is very difficult
• Monitoring groundwater levels, release of radon gas, unusual animal behaviour
• Fault lines can be monitored and local magnetic fields measured
• A study of the San Andreas fault between 1969…