Seismic Hazards

• Earthquake


  The movement of tectonic plates causes pressure to build up in the Earth's crust. When the pressure is released, a series of tremors known as earthquakes occur.
• Global distribution of earthquakes


  Closely related to the location of plate boundaries
• Earthquakes at constructive plate boundaries


  • Shallow focus earthquakes occur as a result of tensional forces in the crust
  • Often occur at mid-ocean ridges and therefore pose little hazard to people
• Earthquakes at collisional plate boundaries

  • Shallow focus earthquakes occur where continental crust is pushing against continental crust to form fold mountain belts
• Earthquakes at conservative margins

  • Shallow focus earthquakes occur where two sections of Earth's crust move laterally
• Earthquakes at destructive plate boundaries

  • Tend to have deeper foci
  • Occur in a narrow zone known as the Benioff zone
  • There are compressional forces as the oceanic crust subducts underneath the continental crust
• Richter Scale


  A formula based on amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer
• Moment Magnitude Scale


  • Based on the total moment release of the earthquake
  • Moment is a product of the distance a fault moved and the force required to move it
  • Derived from modeling recordings of the earthquake at multiple stations
  • Capable of measuring M8 and greater events accurately
• Logarithmic scale (base 10)


  For each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismometer goes up ten times
• Magnitude of earthquakes


  • Magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT
  • Magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT
• Mercalli Scale


  • Uses the observations of the people who experienced the earthquake to estimate its intensity
  • Not as scientific as the Richter scale
• Factors affecting earthquake damage

  • Building designs
  • Distance from the epicenter
  • Type of surface material (rock or dirt) the buildings rest on
• Groundshaking


  • Can severely damage structures like buildings, bridges and dams
  • Can destabilise cliffs and sloping ground
• Groundshaking variation


  • Affected by topography, bedrock type, and the location and orientation of the fault rupture
• Tsunamis


  • Long wavelength oceanic waves generated by the sudden displacement of seawater by a shallow earthquake, volcanic eruption or submarine landslide
  • A number of waves may be produced and they can travel long distances at high speeds to flood far-off shores
  • The height of a tsunami varies and may be affected by the sea floor depth and shape, and other factors
• Liquefaction


  • Occurs when waterlogged sediments are agitated by seismic shaking
  • Separates the grains from each other, reducing their load bearing capacity
  • Buildings and other structures can sink down into the ground or tilt over, whilst underground pipes and tanks may rise up to the surface
  • When the vibrations stop the sediments settle down again, squeezing groundwater out of fissures and holes in the ground to cause flooding
• Landslides and Rockfalls


  • Groundshaking due to earthquakes destabilises cliffs and steep slopes, causing landslides and rockfalls
  • Heavy rain and unconsolidated or fractured rock are exacerbating factors
• Scientists have tried lots of different ways of predicting earthquakes, but none have been successful
• The probability of a future earthquake can be calculated, based on scientific data
• The US Geological Society (USGS) estimate that the probability of a major earthquake occurring in the San Francisco Bay area over the next 30 years is 67%
• Possible signs of earthquake activity

  • Micro quakes before the main tremor
  • Bulging of the ground
  • Decreasing radon gas concentrations in ground water
  • Raised groundwater levels
  • Increased argon gas concentrations in soil
  • Curious animal behaviour
• How to prepare for earthquakes

  • Earthquake drills – Drop, cover and hold on
  • Store supplies
  • Plan how you will communicate with family
  • Check the structure of your home
  • Secure objects in your home to walls
  • Check local news reports for emergency instructions
• Earthquake-proof buildings


  • 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
  • When the pressure is released, it causes a jolting motion in the plates
  • This jolt is responsible for seismic movement spreading throughout the ground in the form of seismic waves (or shock waves)
• 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 plate 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 subjective rate of the destruction caused by an earthquake
• Earthquake magnitude


  • 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
• Earthquake frequency


  • Hundreds of smaller magnitude earthquakes that cannot be felt by humans occur every day
  • Larger earthquakes are less frequent
• Earthquake regularity


  • Earthquakes follow no pattern and are random