Structural Geology Stress and Strain

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Structural GeologyStress and Strain

• Structural Geology the study of crustal
  deformation and basin/mountain development
• Stress force applied to a rock
• Strain change in shape and/or volume induced by
  stress

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Types of Stress

• Compression convergent plate boundaries
• Crumpled, thickening vertically and shortening
  laterally
• Creates folds, reverse and thrust faults
• Himalayas, NW coast of N.A., western coast of
  S.A.
• Tension divergent plate boundaries
• Extends crust, thins vertically and lengthens
  laterally
• Creates basins, normal faults, grabens
• Mid-ocean ridges, Red Sea, Gulf of California
• Shear opposing forces along a plane
• Forms parallel blocks, pull-apart basins,
  transform faults, folds and rotational structures
• Gulf of California, San Andreas fault

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Types of Deformation

• Elastic returns to original state
• Temporary, not permanent
• Yield point point of deformation beyond which
  change is permanent
• Plastic irreversible change in shape or volume
  that occurs without the rock breaking
• Usually under conditions of high temp and press
• Usually a slow process giving atoms time to shift
  in response to force applied
• Brittle irreversible change that penetrates
  mineral bonding
• Usually under conditions of low temp and press
• Force applied suddenly not allowing atoms time to
  shift or move in response to force

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Deformation Factors

• Heat Allows fluid behavior, gt heat gt
  plastic deformation
• Time Great time allows plastic deformation to
  occur if force is applied continually. Little
  time doesnt allow atoms to adjust so brittle
  failure is the result
• Composition hard vs soft minerals e.g. qtz vs
  micas

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Faulting

• Occurs along weaknesses in the Earths crust
• Tectonic forces (stress) result in the build-up
  of deformation (strain) in rocks
• Once the yield point of a rock is reached brittle
  failure causes fractures or faulting
• Fault plane usually curved, planar surface of
  movement between two adjacent crustal blocks
• Slip rate long-term movement along fault plane
• Production of seismic waves that move outward
  from focus of EQ
• Two main types of faults
• Dip-slip
• Vertical motion
• Normal and reverse
• Strike-slip
• Horizontal motion
• Left- or right-lateral

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Fault Terminology

• Fault plane
• Strike azimuth or trend direction
• Dip angle from horizontal of fault plane
• Footwall block
• Hanging wall block

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FaultsHanging-wall/Foot-wall
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Type of Faults

• Normal dip-slip, more vertical than horizontal,
  hanging wall moves down relative to footwall
  (tensional)

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Normal Faults

• Normal faults in felsic volcanic rock, Death
  Valley, California
• Note the back-tilted layering
• Footwall on right of fault trace
• Hanging wall on left of fault trace and has moved
  down relative to footwall

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Normal faults in the Entrada Fm. Grand
Staircase-Escalante Nat. Mon., Utah
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Type of Faults - Reverse

• Reverse dip-slip, hanging wall moves upward
  relative to footwall (compressional)

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Reverse Faults

• Reverse faults in Entrada Fm., Grand
  Staircase-Escalante Nat. Mon., Utah
• Footwall on right of fault traces
• Hanging wall on left that has moved up relative
  to footwall

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Reverse Faults

• Basalt dike, now exposed at surface, cutting the
  Moenkopi Fm. Capitol Reef Nat. Park, Utah
• Note alteration zone in Moenkopi
• Note reverse faults below dike
• Footwall on right of fault trace

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Type of Faults

• Thrust special type of reverse, block moves at
  low angle (45 degrees) (compressional)

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Thrust Faults

• This fault, at the leading edge of the Sevier
  Fold-Thrust Belt in southern Nevada, places
  Cambrian Bonanza King Formation (gray) over
  Jurassic Aztec Sandstone (pink).

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Thrust Faults

• This thrust fault is approximately horizontal
• Note the anticline in the hanging wall and the
  syncline in the footwall

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Type of Faults

• Strike-slip Horizontal slip of adjacent blocks,
  right and left

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Strike-slip Faults

• Right-lateral SS along the Las Vegas Shear Zone,
  Nevada
• Part of a large-scale displacement feature
  trending NW-SE

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Strike-slip Faults

• San Andreas fault and Pt. Reyes Peninsula,
  California
• The San Andreas trends northwestward up the
  narrow Tomales Bay
• Elongated transform fault between spreading
  centers in the Gulf of California (East Pacific
  Rise) and of the coast of NW U.S. (Juan De Fuca
  Ridge)

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Fault Scarps
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Man-made Earthquakes

• Water reservoirs
• Crustal loading
• Seepage caused lubrication
• Deep waste disposal
• Lubrication of preexisting fault zones through
  subsurface injection of waste fluids
• Nuclear explosions
• Above and below ground nuclear testing
• Mining
• Weakening of rock through subsurface mining
• Blasting initiated seismic events

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Minimizing Earthquake Hazards

• National EQ Hazard Reduction Program
• USGS in coop with state and universities
• Develop knowledge base for physical aspects and
  behavior of EQs
• Determine EQ potential
• Predict consequences
• Application of lessons learned
• Estimation of seismic risk
• Compilation of hazard maps
• Seismic gaps
• Deformation maps
• Micro-quake monitoring
• Classification of faults
• Estimation of maximum moment magnitude
• Slip rate

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Minimizing Earthquake Hazards

• Short-term prediction
• Precursor based
• Specifies a shorter time period of occurrence,
  est. magnitude, localized area of event and
  probability
• Developing area of research with only limited
  success
• EQ warning systems
• Of such short advanced notice not a feasible tool
  at this time
• Japan does use a system of limited success

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Prediction

• Seismic gaps accumulation of pent-up strain as
  opposed to tectonic creep
• Micro-quake swarms micro cracks cause rocks to
  dilate
• Tilt or Bulges can be measured by tiltmeters or
  lasers
• Change is seismic velocity due to air pockets
  in micro-cracks
• Variations in electrical conductivity air
  lowers, water increases
• Changes in ground water level and chemistry
• Lunar/solar alignments
• Animal behavior

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Perception of Hazard

• Human nature
• Denial
• Micro-zonation
• Location of critical facilities and equipment in
  safe areas
• Critical transport and communications lines must
  be fortified so they remain open after event
• Building codes
• Effective construction oversight that adopts new
  technology
• Education
• Both the general populace and critical local and
  higher government employees
• Personal response
• Know where you live
• What to expect
• How to react