earthquakes

see also:

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• fires
• building collapse
• landslides
• tsunamis
• destruction of infrastructure leading to famine or disease (lack of potable water, etc)