Seismic hazards

• Shallow-focus earthquakes


  Surface down to approximately 70 km.

  Often occur in brittle rocks.

  Generally release low levels of energy, but high-energy shallow quakes can cause severe impacts.
• Deep-focus earthquakes


  70-700 km.

  Increasing depth leads to high pressure and temperature.

  Less frequent bit very powerful.

  Full understanding of deep-focus earthquakes is evolving; water and change in minerals may be contributing factos.
• Richter scale


  Developed in 1935.

  Uses the amplitude of seismic waves to measure magnitude.

  Logarithmic scale from 1 to 9, with every whole number increase being ten-fold.
• Moment Magnitude Scale


  1.0 to 9.0.

  Measures energy release as related to geology, the area of the fault surface and the amount of movement on the fault.

  Accurate for large earthquakes, but not used for small earthquakes.
• Modified Mercalli Scale


  Measures earthquake intensity and impact.

  Relates to impacts felt and seen by those affected.

  Qualitative not quantitative.
• Rift Valley


  A valley formed by downfaulting between parallel faults.
• Escarpment


  A tilt block forming an extensive upland area, with a short, steep, scarp slope and a long, gentle, dip slop on the other side.
• Liquefaction


  When violently shaken, soils with a high water content lose their mechanical strength and become fluid.
• Landslides and avalanches


  Slope failure as a result of the ground shaking.
• Flooding


  Earthquakes can indirectly cause flooding in a number of ways: triggering tsunamis, destabilising/destroying dams, destroying and/or lowering protective levees.
• Ground shaking and displacement 


  Vertical (Rayleigh waves) and horizontal (Love waves) move of the ground.

  Severity depends in the earthquake magnitude, distance from the epicentre and geology.
• Tsunami


  A giant sea wave generated by shallow-focus underwater earthquakes.

  Have a wavelength of over 100 km and wave height of under 1 m in the open ocean; travel quickly at over 700 km/h; rapid increase in height in shallow water as high as 25 m.

  A water trough forms in the front of the tsunami where sea level is reduced called a drawdown.