Week 2: Faulting, Stress, Earthquakes

• Define strain rate

  • Strain is the amount of deformation that has occurred, it is dimensionless
• Relationship between strain and faulting/earthquakes

  Strain rates are related to faulting and earthquake activity as, faulting and earthquakes are able to deform environments. By using strain rates and inferred stress on these deformation structures we can understand the past events that occurred to cause the deformation, over what time scales they occurred on and the stress regime that caused them
• Example of sinistral strike-slip faulting

  The Australian/Indian Plate experiences sinistral strike slip faulting due to intraplate stress. Although this stress can’t be measured directly, in Australia it has been inferred.

  o   Faulting in Western Australia shows reverse faulting orientated N/S, with E/W compressive stresses

  o   Faulting in Melbourne shows N/S orientated sinistral faults, with NW/SE compressive stresses

  o   These deformation structures show a reflection on the intra plate stresses experienced by the Australian Continent
• • Stress, strain, faulting and earthquakes are all related through the seismic cycle
  • The seismic cycle consists of periods of gradual strain to the release of mass amounts of stress in earthquakes
• The Seismic Cycle

  • Inter-seismic period: Little to no seismic activity,
  • Strain accumulation period: Overtime, tectonic forces lead to stress build up
  • Pre-seismic period: Strain continues to accumulate along faults, as the fault becomes increasingly stressed, its likelihood of rupturing increases
  • Earthquake rupture period: Accumulated stress exceeds the fault’s threshold; it fails and releases the strain in seismic waves
  • Post-seismic deformation phase: After the main failure of the fault there continues to be smaller shocks which cause further deformation
• What does the Seismic Cycle show us

  • This cycle shows how strain is related to the build-up of stress along a fault, and it is through earthquake activity that faulting is related to stress/strain. These dynamics are related though the seismic cycle
• 3 main types of faults

  • Normal Faults
  • Reverse Faults
  • Strike-slip Faults
• Normal Faults

  • Occur mainly in extensional regions where the crust is being pulled apart. In normal faults, the hanging wall is lower than the footwall
  • Listric faults are a sub type of normal faults, these faults have variable dip along the fault plane but are typically steeper at the surface and shallower at depth. Most common in extensional rifts
• Reverse Faults

  • Occur in compressional regions where the crust is being pushed together, and one side becomes upthrown over the other. The hanging wall sits above the footwall.
  • Thrust faults are a sub type of reverse faults, these faults are low angle faults with upthrown hanging walls, they are commonly found in orogenic regions
• Strike-slip Faults

  • Occur in transform regions, where shear stress causing two sides of a fault plane to glide past one another. They are vertical faults with horizontal movement. Strike-slips faults with left lateral movement are known as sinistral faults and right lateral movement are dextral faults
  • Oblique strike-slip faults, are common in regions of tectonic deformation. Oblique faults are common due to the fact that most faults aren’t primarily composed of strictly horizontal or vertical components
• Structural features of faults

  fold axis, hinge, limbs, axial plane, axial surface, hinge line
• Geological significance of Faults

  • Folds have an important geological significance as they are deformation structures that occur under compressive stress regimes
  • Through this folds provide indicators on the tectonic history, strain history and rock mechanics of the region
• Mechanisms of folding

  o   Ductile flow folding: When the rocks in the fold are deformed under high pressure and temperatures, they behave plastically. Commonly with similar beds, and mostly commonly seen type of folding in orogenic zones

  o   Slip folding: Formed by the buckling and bending of layers parallel to the bedding direction, this allows slip in the beds (as a result limbs have different sense of slips) and a retaining of thickness = parallel beds. Common type of folding in sedimentary rocks, and in sedimentary basins
• Earthquake Formulas

  • Each earthquake has a recurrence interval, this the time/number of earthquakes
  • How often a recurrence of the fault will happen
  • Alpine fault – every 300 years
  • Slip rate = displacement/time
  • Single event displacement= How much the fault has been displaced during one event
  • On average the alpine fault experiences 8km of displacement along it, displacement along faults may not always be consistent through the fault
  • The longer the fault the more displacement it can experience in one earthquake