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Structural Geology Stress and Strain


... consequences Application of lessons learned Estimation of seismic risk Compilation of hazard maps Seismic gaps Deformation maps Micro-quake monitoring ... – PowerPoint PPT presentation

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Title: Structural Geology Stress and Strain

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

Types of Stress
  • Compression convergent plate boundaries
  • Crumpled, thickening vertically and shortening
  • Creates folds, reverse and thrust faults
  • Himalayas, NW coast of N.A., western coast of
  • Tension divergent plate boundaries
  • Extends crust, thins vertically and lengthens
  • 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

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

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

  • 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

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

Type of Faults
  • Normal dip-slip, more vertical than horizontal,
    hanging wall moves down relative to footwall

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

Normal faults in the Entrada Fm. Grand
Staircase-Escalante Nat. Mon., Utah
Type of Faults - Reverse
  • Reverse dip-slip, hanging wall moves upward
    relative to footwall (compressional)

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

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

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

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).

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

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

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

Strike-slip Faults
  • San Andreas fault and Pt. Reyes Peninsula,
  • 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

Fault Scarps
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

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

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

  • Seismic gaps accumulation of pent-up strain as
    opposed to tectonic creep
  • Micro-quake swarms micro cracks cause rocks to
  • Tilt or Bulges can be measured by tiltmeters or
  • 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

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
  • Education
  • Both the general populace and critical local and
    higher government employees
  • Personal response
  • Know where you live
  • What to expect
  • How to react
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