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  • earthquakes occur when build up of stresses finally result in rigid rocks on either side of a tectonic fault suddenly fracturing and sliding due to forces caused by either:
    • subduction of an oceanic plate under either a continental plate or another oceanic plate (oceanic basalt is heavier thus goes under a continental plate and in so doing causes an orogeny on the continent forming mountains)
      • these cause both shallow earthquakes and deep focus earthquakes as the subduction plate may be many kilometres deep
    • convergent collision of two continental plates (collision of Indo-Australian plate with Eurasia to form the Himalayas)
    • sliding conservative motion of an oceanic plate sliding in concert with a continental plate (San Andreas fault)
    • rising magma within fault lines
    • intra-plate fault line movements (eg. within Australia despite being a long way from any plate boundaries, there are areas of active mountain building especially SE Australia)
  • these start at a “hypocenter” (or “focus”) below the surface marking epicentre and then seismic waves spread with strength depending upon the rupture speed
  • usually the damage resulting from earthquakes are due to the seismic waves that propagate outwards and these may also cause tsunamis

Seismic waves

  • the two main initial fast body seismic waves are:
    • P Primary waves
      • fast longitudinal compression-decompression waves
      • fastest in low density, highly compressible materials (rebounds faster and transmits more energy onwards)
    • S Secondary waves
      • transverse wave with ~60% of velocity of P waves
  • slower surface waves:
    • these are more destructive to buildings than body waves and arrive later and with more amplitude at the surface on seismographs
    • they mainly arise from shallow focus quakes or nuclear explosions
    • Love waves (horizontal motion)
      • caused by the interference of many shear S waves
      • their amplitude (horizontal/ transverse motion) decays more slowly with distance from epicentre compared to body waves
      • they can travel several times around the Earth before dissipating
    • Raleigh waves (vertical elliptical motion)
      • are a surface acoustic wave, travelling along the surface of solids
      • these have the slowest propagation speed and arrive last
      • circular oscillations means energy is lost quicker with distance from epicentre

Depth of focus

  • those less than 70km deep are regarded as “shallow
  • those 70-300km deep are “intermediate depth
  • those 300-700km deep are “deep focus
  • earthquakes do not occur > 700km deep as the warmer, deeper rocks are not brittle enough to fracture

Determinants of the amount of surface damage

Intensity of surface waves

  • these generally lessen the further from the epicentre in a roughly concentric manner and the distribution of strength as it radiates out can be mapped with isoseismic lines - ie areas with same intensity share the same line on a geographic map

Sub-surface rock deposit type

  • buildings are more likely to be damaged if the subsurface material is easily deformed and cannot readily absorb and transmit the seismic waves such as:
    • sand and gravels which oscillate more and thus result in higher amplitude L waves
    • clay and poorly cemented sandstone deform more readily than granite or limestone
  • granite or limestone generally deform much less and as a result there is less building deformation

Building structural considerations

  • bricks and stonework separate along mortar resulting in potential wall collapse
  • multilevel building floors can separate from walls resulting in them falling and pancaking on top of each other
  • bridges built in sections can separate from their supporting piers
  • gas, water and drainage pipes may rupture and leak
  • damaged electrical cables can then cause gas explosions
  • weakened structures are then more susceptible to further damage from after shocks over the next minutes, hours, days or even weeks

Other considerations

  • very strong waves of magnitude over 10-12 may cause the ground to form waves with cracks opening at the top of waves and cracks closing at the troughs
  • land slips may occur undermining roads and causing ground subsidence
  • a slip of the oceanic floor typically pushes a large amount of water upwards forming a tsunami wave
  • flooding can occur in areas on alluvial deposits as water is squeezed to the surface by “liquefaction” and buildings can then sink

Fatalities are usually indirect consequences

  • fires
  • building collapse
  • landslides
  • tsunamis
  • destruction of infrastructure leading to famine or disease (lack of potable water, etc)
climate/earthquakes.txt · Last modified: 2023/05/29 23:52 by gary1

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