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Ch8_EQ_students

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Student power point packet notes

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Ch8_EQ_students

  1. 1. CH. 8 – EARTHQUAKES
  2. 2. Earthquakes <ul><li>Vibrations in crust caused by shifting rock masses </li></ul><ul><li>Focus - point w/in crust where EQ originates </li></ul><ul><li>Epicenter – point on Earth’s surface directly above focus </li></ul>
  3. 3. Earthquakes <ul><li>Shallow-focus vs. deep-focus EQ’s </li></ul>
  4. 4. Causes of Earthquakes <ul><li>1) Volcanic eruptions </li></ul><ul><ul><li>used for predictions </li></ul></ul><ul><li>2) Crustal Rebound - weak EQ’s </li></ul><ul><li>- caused by melting of continental ice sheets </li></ul>
  5. 5. Causes of Earthquakes <ul><li>3) Elastic Rebound </li></ul><ul><li>- pressure builds up in rocks (potential energy) </li></ul><ul><li>once rx break, stored energy is released (kinetic energy) </li></ul><ul><li>- rx return to original shape </li></ul>
  6. 6. Fault <ul><li>Fracture in crust where rocks have shifted </li></ul><ul><li>Two categories of faults: </li></ul><ul><li>1) Dip-slip faults (vertical motion) </li></ul><ul><li>2) Strike-slip faults (horizontal motion) </li></ul>
  7. 7. Dip-Slip Faults <ul><li>Vocabulary ( mining terms ): </li></ul><ul><ul><li>1) Fault Plane – fracture surface </li></ul></ul><ul><ul><li>2) Footwall </li></ul></ul><ul><ul><li>3) Hanging wall </li></ul></ul>
  8. 8. Mining along faults
  9. 9. Types of Dip-Slip Faults <ul><li>1) Normal Fault </li></ul><ul><li>- hanging wall moves down relative to footwall </li></ul><ul><li>- caused by tensional forces </li></ul>
  10. 10. Normal Fault
  11. 11. Types of Dip-Slip Faults <ul><li>2) Reverse Fault </li></ul><ul><li>- hanging wall moves up relative to footwall </li></ul><ul><li>- caused by compressional forces </li></ul>
  12. 12. Reverse Fault
  13. 13. Fault-Block Mountains <ul><li>Bounded by normal faults </li></ul><ul><li>Mountain = horst; valley = graben </li></ul><ul><li>Ex: Basin & Range Province in Nevada & Utah </li></ul><ul><li>Ex: Teton Mountains, Wyoming </li></ul>
  14. 14. Basin & Range Province
  15. 15. Teton Mountains, WY
  16. 16. Strike-slip faults <ul><li>Motion is horizontal </li></ul><ul><li>Surface features are displaced: </li></ul><ul><ul><li>Fences, railroad tracks, tree rows, stream channels </li></ul></ul><ul><li>Ex: San Andreas Fault, CA </li></ul>
  17. 18. Types of Seismic Waves <ul><li>1) Body Waves </li></ul><ul><li>- energy travels from focus through Earth’s </li></ul><ul><li>interior </li></ul>
  18. 19. Body Waves: <ul><li>a) Primary Waves (P-waves) </li></ul><ul><li>- fastest </li></ul><ul><li>- energy travels in push-pull motion (linear) </li></ul><ul><li>- travel through solids, liquids, and gases </li></ul>
  19. 20. P-wave w/slinky
  20. 21. Body Waves: <ul><li>b) Secondary waves (S-waves) </li></ul><ul><li>- approx. half as fast as P-waves </li></ul><ul><li>- energy travels at right angles to path of motion </li></ul><ul><li>- only travels through solids </li></ul>
  21. 22. S-wave with rope
  22. 23. Types of Seismic Waves <ul><li>2) Surface waves </li></ul><ul><li>- energy travels across surface </li></ul><ul><li>- causes the most </li></ul><ul><li>structural damage </li></ul>
  23. 24. Locating EQ Epicenters <ul><li>KEY : Difference in arrival times between P- and S-waves </li></ul><ul><li>Farther apart = seismic station is further from epicenter </li></ul>
  24. 25. Seismogram
  25. 26. Locating EQ Epicenters <ul><li>Use multiple seismic stations to determine epicenter’s location </li></ul>
  26. 27. Measuring EQ’s <ul><li>1) Intensity = damage-oriented </li></ul>
  27. 28. EQ Intensity Factors <ul><li>1) Amount of energy released </li></ul><ul><li>2) Distance from epicenter </li></ul><ul><li>3) Type of surface material </li></ul><ul><ul><li>- bedrock vs. sediment </li></ul></ul><ul><li>4) Building design </li></ul><ul><li>5) Population Density </li></ul>
  28. 29. EQ Intensity <ul><li>Measured using Mercalli Scale </li></ul><ul><li>(Giuseppe Mercalli in 1902) </li></ul><ul><li>- scale of I-XII based on damage to location </li></ul><ul><li>not a measure of EQ’s actual energy </li></ul>
  29. 30. Disadvantages of Mercalli Scale <ul><li>1) Subjective </li></ul><ul><li>2) Type of surface material varies </li></ul><ul><li>3) Building design varies </li></ul><ul><li>4) No data for unpopulated areas </li></ul>
  30. 31. Measuring EQ’s <ul><li>2) Magnitude – measures amount of energy released from focus </li></ul>
  31. 32. Richter Magnitude Scale <ul><li>Measures largest amplitude of seismic waves on seismogram </li></ul><ul><li>Amplitude measures the ground shaking </li></ul>
  32. 33. Seismogram
  33. 34. Richter Magnitude Scale <ul><li>Logarithmic scale (not linear!) </li></ul><ul><li>Each higher number on Richter Scale represents: </li></ul><ul><li>a) 10 times more ground shaking </li></ul><ul><li>b) 32 times more energy released </li></ul>
  34. 35. Richter Magnitude Scale <ul><li>2.0 Felt only by seismograph </li></ul><ul><li>3.0 ____ x more energy than a 2.0 </li></ul><ul><li>4.0 ____ x more energy than a 2.0 </li></ul><ul><li>5.0 ____ x more energy than a 2.0 </li></ul>
  35. 36. Richter Magnitude Scale <ul><li>4.0-4.9 Felt by most people </li></ul><ul><li>6.0-6.9 Destructive in populated areas </li></ul><ul><li>>8.0 Truly catastrophic EQ’s </li></ul>
  36. 37. Richter Magnitude Scale <ul><li>Largest recorded U.S. EQ </li></ul><ul><li>1964 Alaskan “Good Friday” EQ 9.2 </li></ul><ul><li>Largest recorded EQ in world </li></ul><ul><li>1960 Chile 9.6 </li></ul>
  37. 38. Benefits of Richter Scale <ul><li>1) Objective measurements from seismogram </li></ul><ul><li>2) Data available for unpopulated areas </li></ul>
  38. 39. Effects of EQ’s <ul><li>1) Ground Shaking </li></ul><ul><li>2) Ground Ruptures – cracks occur in soil (not bedrock!) </li></ul>
  39. 40. Effects of EQ’s <ul><li>3) Fault Scarp – cliff exposed by vertical movement along dip-slip fault </li></ul><ul><li>- used to measure growth rate of mountains </li></ul>
  40. 41. Effects of EQ’s <ul><li>4) Liquefaction </li></ul><ul><li>- occurs in sediment </li></ul><ul><li>- vibrations release water trapped between sediment grains </li></ul><ul><ul><li>“ quick sand” or “quick clay” </li></ul></ul>
  41. 42. Liquefaction <ul><li>Foundations under buildings fail </li></ul><ul><li>Ex: Marina District is San Francisco </li></ul>
  42. 43. Effects of EQ’s <ul><li>5) Landslides </li></ul><ul><li>- more prevalent if pre-existing weakness exists in rx </li></ul><ul><li>Ex: EQ Lake in Montana 1959 </li></ul>
  43. 44. Effects of EQ’s <ul><li>6) Tsunami – “harbor wave” </li></ul><ul><li>- seismic sea wave (not tidal wave) </li></ul><ul><li>- caused by dip-slip fault movement in ocean crust </li></ul>
  44. 45. Tsumani diagram
  45. 46. Tsunami <ul><li>Danger is from speed: between 500-950 km/hr </li></ul><ul><li>Ex: December 26, 2004 </li></ul><ul><li>- Indonesia: EQ of 9.0 offshore triggered tsunami </li></ul><ul><li>- over 200,000 people killed </li></ul>
  46. 47. Tsunami Warning System
  47. 48. Effects of EQ’s <ul><li>7) Fire – causes most property damage </li></ul><ul><li>- liquefaction breaks buried gas & water pipelines </li></ul><ul><li>Ex: Marina District in 1906 & 1989 </li></ul>
  48. 49. EQ Prediction <ul><li>Based on seismic “gaps” </li></ul><ul><li>- look for areas on fault where no EQ’s have occurred </li></ul>
  49. 50. EQ Prediction <ul><li>EQ Prediction along San Andreas Fault </li></ul><ul><li>- best guess is 30-year estimate of likely EQ activity </li></ul><ul><li>- note Parkfield, CA </li></ul>
  50. 51. San Andreas EQ Probability
  51. 52. Earth’s Interior <ul><li>Layers defined by physical properties & composition </li></ul><ul><li>Clues (indirect evidence): </li></ul><ul><li>Seismic waves - variations in speed & direction </li></ul>
  52. 53. Clues: seismic waves <ul><li>1) P-waves speed up rapidly below 50 km depth within Earth </li></ul><ul><li>- marks a change in composition &/or physical state of the rocks </li></ul><ul><li>1909: Mohorovicic identified crust-mantle boundary </li></ul>
  53. 54. Clues: seismic waves <ul><li>Within mantle: </li></ul><ul><li>P-wave velocities slow back down (100-660 km deep) </li></ul><ul><li>- slower velocity zone is known as the __________________ </li></ul>
  54. 55. Asthenosphere <ul><li>Weak layer in upper mantle </li></ul><ul><li>- partially molten (~1-5%) which slows P-waves down </li></ul><ul><li>- not completely molten b/c S-waves travel through it </li></ul>
  55. 56. Clues: seismic waves <ul><li>Lithosphere – rigid rx of crust and uppermost mantle </li></ul><ul><li>Asthenosphere – weaker rocks (partially molten) in upper mantle </li></ul>
  56. 57. Asthenosphere
  57. 58. Clues: seismic waves <ul><li>2) P-wave shadow zone </li></ul><ul><li>- P-waves bend as they enter the outer core </li></ul><ul><li>3) S-wave shadow zone </li></ul><ul><li>S-waves disappear in outer core </li></ul><ul><li>* Proof of liquid outer core </li></ul>
  58. 59. P-wave shadow zone
  59. 60. Clues: seismic waves <ul><li>Outer core is molten </li></ul><ul><li>Inner core is solid </li></ul><ul><li>- pressure in center is too great to allow minerals to expand & melt </li></ul>

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