Structural Geology Stress and Strain

1. 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

2. 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

3. 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

4. Deformation Factors • Heat – Allows “fluid” behavior, > heat = > plastic deformation • Time – Great time allows plastic deformation to occur if force is applied continually. Little time doesn’t allow atoms to adjust so brittle failure is the result • Composition – hard vs soft minerals e.g. qtz vs micas

5. Faulting • Occurs along weaknesses in the Earth’s 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

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

7. FaultsHanging-wall/Foot-wall

8. Type of Faults • Normal – dip-slip, more vertical than horizontal, hanging wall moves down relative to footwall (tensional)

9. 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

10. Normal faults in the Entrada Fm. Grand Staircase-Escalante Nat. Mon., Utah

11. Type of Faults - Reverse • Reverse – dip-slip, hanging wall moves upward relative to footwall (compressional)

12. 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

13. 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

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

15. 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).

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

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

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

19. 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)

20. Fault Scarps

21. 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

22. Minimizing Earthquake Hazards • National EQ Hazard Reduction Program • USGS in coop with state and universities • Develop knowledge base for physical aspects and behavior of EQ’s • 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

23. 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

24. 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

25. 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