Geology lecture 11


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  • Color codes are related to how deep the earthquakes occur Blue= super deep (where the trenches are) concentrated where there are subduction zones
  • The larger the surface area of the fault plane that moves, the bigger the earthquake- on the big principle faults that define the boundaries
  • Intraplate EQs tend not to get that large- no greater than 5 Highest magnitude EQs only occur on subduction zones
  • Can look at a landscape to see where faults exist
  • Geology lecture 11

    1. 1. Earthquakes Chapter 10
    2. 2. Outline• Earthquakes and seismicity -Basics• Faulting and earthquakes -Hypocenter and epicenter -Fault motion and initiation -Fault types• Seismic waves -Body waves (P & S), surface waves (Love & Raleigh) -Seismology, seismographs• Earthquake further details -Locating them, size, frequency, depths -Tectonic settings and occurrences -Damage and prediction Chapter 10 Chapter 10
    3. 3. What is an Earthquake?• Earth shaking caused by rapid energy release. • Tectonic/other stresses cause rocks to break. • Energy moves outward as waves. • Waves can be measured.• Earthquakes (EQs) are destructive. • ~3.5 million deaths in the last 2,000 years.• VERY common. Chapter 10
    4. 4. Seismicity• Seismicity (earthquake activity) occurs due to… • Motion along a new fracture (fault) in the crust • Motion on existing fault • Sudden change in mineral structure • Magma movement at depth • Volcanic eruption • Giant landslides • Nuclear detonations Chapter 10
    5. 5. Earthquake Concepts• Hypocenter (focus) - Spot where earthquake waves originate • Usually occurs on a fault plane waves expand outward from hypocenter• Epicenter – Land surface spot above hypocenter Chapter 10
    6. 6. Faults and Earthquakes• Most earthquakes (EQs) occur along faults. • Faults are fractures along which rocks move • Movement termed displacement, offset, or slip • Markers may reveal amount/direction of offset Chapter 10
    7. 7. Faults and Fault Motion• Faults are common. • Active faults – ongoing stresses producing motion • Inactive faults – motion occurred in the geologic past• Displacement can be visible. • Fault trace – a surface tear • Fault scarp – a small cliff• Blind faults don’t reach the surface (no trace/scarp)] Chapter 10
    8. 8. Fault Motion• Faults move in jumps.• Once motion starts, it quickly stops due to friction• Builds up again, finally causing failure• Behavior is termed “stick-slip”. • Stick – friction prevents motion • Slip – friction briefly exceeded by motion Chapter 10
    9. 9. Faults and Fault Motion• Most faults slope (although some are vertical)• On sloping fault, crustal blocks are classified as: • Footwall (block below the fault) • Hanging wall (block above the fault) Chapter 10
    10. 10. Fault Types• Fault type based on relative block motion.Normal fault: Chapter 10
    11. 11. Fault Types• Fault type based on relative block motion.Reverse fault: Chapter 10
    12. 12. Fault Types• Fault type based on relative block motion.Thrust fault • Low angle reverse fault: Chapter 10
    13. 13. Fault Types• Fault type based on relative block motion.Strike-slip fault: Chapter 10
    14. 14. Fault Types• Fault type based on relative block motion.Oblique fault • A combo of vertical (dip) slip and horizontal (strike) slip. Chapter 10
    15. 15. Outline• Earthquakes and seismicity -Basics• Faulting and earthquakes -Hypocenter and epicenter -Fault motion and initiation -Fault types• Seismic waves -Body waves (P & S), surface waves (Love & Raleigh) -Seismology, seismographs• Earthquake further details -Locating them, size, frequency, depths -Tectonic settings and occurrences -Damage and prediction Chapter 10 Chapter 10
    16. 16. Seismic Waves• Body waves – travel through Earth’s interior • Compressional or Primary (P) waves: Push-pull (compress and expand- volume change) motion • Travel through solids, liquids, gases • Tend to travel very fast Chapter 10
    17. 17. Seismic Waves• Body waves- pass through Earth’s interior • Shear or Secondary (S) waves: • Change in position, not volume • “shaking motion” • Only travel through solids; not liquids • Slower than P waves Chapter 10
    18. 18. Seismic Waves• Surface waves- travel along Earth’s surface 1. Love waves – S-waves that intersect the surface Back and forth motion 2. Rayleigh waves – P-waves at the surface move like ripples on a pond These waves- slowest and more destructive Chapter 10
    19. 19. Seismology Instruments• Seismographs – instruments that record seismicity • Detect EQs anywhere on Earth• Reveals size and location of EQs Chapter 10
    20. 20. Seismograph Operation• Waves always arrive in sequence. • P-waves 1st • S-waves 2nd • Surface waves last• Arrivals captured by• seismograph Chapter 10
    21. 21. How a Seismograph Works How a Seismograph WorksSeismologists use two basic configurations of seismographs, onefor measuring horizontal ground motion, like the one shown inthis animation, and the other for measuring vertical groundmotion. Both work on the principle of inertia as described byNewton’s law, which states that an object at rest tends to remainat rest unless acted on by an outside force. Thus, during anearthquake, vibrations cause the frame of the seismograph tomove. The pendulum apparatus remains fixed as the papercylinder moves back and forth beneath it. For more information,see “Seismographs and the Record of an Earthquake” starting onp. 315 and Figure 10.13 in your textbook. Chapter 10
    22. 22. Outline• Earthquakes and seismicity -Basics• Faulting and earthquakes -Hypocenter and epicenter -Fault motion and initiation -Fault types• Seismic waves -Body waves (P & S), surface waves (Love & Raleigh) -Seismology, seismographs• Earthquake further details -Locating them, size, frequency, depths -Tectonic settings and occurrences -Damage and prediction Chapter 10 Chapter 10
    23. 23. Locating an Epicenter• P- & S-waves travel at different velocities.• 1st arrivals of P- and S-wave varies with distance• Travel-time graph plots distance of each station to the epicenter Chapter 10
    24. 24. Locating an Epicenter• 3 stations can pinpoint epicenter. • A circle is drawn around each station • radius= distance to epicenter • Circles around 3 (or more) station will intersect • Intersection> epicenter Chapter 10
    25. 25. Earthquake Size• Two means of describing earthquake size: 1. Intensity. 2. Magnitude.1. Mercalli Intensity Scale. • Intensity – degree of shaking based on damage. • Roman numerals assigned to different damage levels. • Damage occurs in zones. • Intensity decreases with distance. Chapter 10
    26. 26. Earthquake Size• Magnitude – amount of energy released. • Max amplitude of motion from • a seismograph • Value normalized for • seismogrpahic distance• Several magnitude scales: • Richter • Moment• Scales are logarithmic. • 1 unit increase= 10x increase • in size Chapter 10
    27. 27. Measuring Earthquake Size• Energy released can be calculated. • M 6.0= energy of the Hiroshima bomb • M 8.9= annual energy released y all other earthquakes Chapter 10
    28. 28. Earthquake Size & FrequencySmall EQs are frequent ~100,000 M 3 EQs/yearLarge EQs are rare ~32 M 7 EQs/year Chapter 10
    29. 29. Earthquake Occurrence• EQs linked to tectonic plate boundaries• Shallow depth- convergent/transform boundaries• Intermediate/deep- convergent boundaries Chapter 10
    30. 30. Earthquake Depths• Shallow EQs – 0-20 km. • Along mid-ocean ridges • Transform boundaries • Shallow part of trenches • Continental crust Chapter 10
    31. 31. Earthquake Depths• Intermediate/deep EQs – along subduction trace (Wadati- Benioff Zones) • Intermediate- 20-300 km- down-going plate still brittle • Deep- 300-670 km- mineral transformations• Earthquakes rare below 670 km (mantle is ductile) Chapter 10
    32. 32. Convergent Boundaries• Cities near subduction zones have to contend with frequent & occasionally large EQs. Chapter 10
    33. 33. Continental Earthquakes• EQs in continental crust. • Continental transform faults (San Andreas) Continental rifts (Basin and range, East African Rift) Collision zones (Himalayas, Andes, Alps) Intraplate settings (ancient crustal weaknesses) Chapter 10
    34. 34. San Andreas Fault• Pacific plate passes by North American plate.• San Andreas is an active strike-slip fault. • Very dangerous; 100s of EQs/ year Examples: San Francisco- 1906 (burnt down) Loma Prietà- 1989 (world series) Chapter 10
    35. 35. Intraplate Earthquakes• 5% of EQs not near plate boundaries.• “Intraplate” EQs poorly understood • Remnant crustal weakness in failed rifts or shear zones? • Stress transmitted inboard? Transmitted far into the plate • Isostatic adjustments? • Gravitational balance• Clusters • New Madrid, Mo • Charleston, S.C. • VA seismic zone! Chapter 10
    36. 36. Earthquake Damage• Ground shaking and displacement. • EQ waves arrive in distinct sequence • Different waves cause different motion• P-waves are 1st to arrive • Produce rapid up and down motion Chapter 10
    37. 37. Earthquake Damage• S-waves arrive next. • Produce back and forth motion • Motion usually much stronger than P-waves • S-waves cause extensive damage Chapter 10
    38. 38. Earthquake Damage• Surface waves lag behind S-waves. • Love waves are the first to follow • Ground moves like a snake Chapter 10
    39. 39. Earthquake Damage• Raleigh waves are last to arrive. • Land surface- ripples in a pond • May last longer than others • Causes extensive damage Chapter 10
    40. 40. Earthquake Damage• Severity of shaking & damage depends on… • Magnitude (energy) of EQ • Ditance from hypoenter • Intensity/duration of vibrations • Subsurface material • Bedrock transmit waves quickly= less damage • Sediments bounce waves= amplified damage Chapter 10
    41. 41. Earthquake Damage• Landslides & avalanches. • Shaking causes slope failures • Rockslides/snow avalanches follow EQs in uplands • An EQ stated the landslide the uncorked Mt. St. Helens on May 18, 1980 Chapter 10
    42. 42. Earthquake Damage• Liquefaction – waves liquefy H2O-filled sediments. • High pore pressure force grains apart reducing friction • Liquefied sediments flow as a slurry • Sand becomes “quicksand” from solid Chapter 10
    43. 43. Earthquake Damage• Tsunamis or seismic sea waves (not tidal waves). • Result of EQ displacing seafloor • Instantly displaces overlying water • May be enormous (up to 10,000 mi2 area) • Occur ~1/year Chapter 10
    44. 44. Tsunami Behavior• Move at jetliner speed across ocean.• May be imperceptible in deep water. • Low wave high (amplitude) • Long wavelength (frequency)• As water shallows, waves• slow from frictional drag • Waves grow in height, • reaching 10-15 or more Chapter 10
    45. 45. Tsunami Reality• Indian Ocean Tsunami • Dec 26, 2004, strong trust EQ (M 9.0+) originated in trench near Sumatra • Largest EQ in 40 years • Slip exceeded 15 m; fault rapture > 1100 km long • Killed ~283,000 people (10 countries around Indian Ocean) Chapter 10
    46. 46. The Indian Ocean Tsunami• Destroyed coastlines around the Indian Ocean.• Death tolls in: • Northern Sumatra • Thailand • Malaysia • Sri Lanka Banda Aceh Chapter 10
    47. 47. Tsunami Prediction• Scientific modeling predicts tsunami behavior.• Tsunami detection: • Detectors placed on seafloor • Sense pressure changes due to sea thickness change• Prediction/detection can save lives Chapter 10
    48. 48. Earthquake Prediction• Prediction would help reduce catastrophic losses.• Can we predict earthquakes? Yes and No • CAN be estimated long-term (10-100s of years) • CANNOT be predicted short-term (hours-months)• Seismic hazards are mapped to assess risk Chapter 10
    49. 49. Earthquake Prediction• Long-term: • Probability of a certain magnitude EQ occurring on a time scale of ~30 to 100 years • Based on idea that EQs are repetitive (tend to move multiple times over a long period of time) Chapter 10
    50. 50. Earthquake Prediction• Long-term: • Require determination of seismic zones by: • Mapping historical epicenters (after ~1950) • Evidence of ancient EQs (before seismographs) • Evidence of seismicity- fault scarps, sand volcanoes, etc. • Historical records Chapter 10
    51. 51. Earthquake Prediction• Long-term: • Estimate recurrence interval- average time between EQs• Historical Records• Geologic evidence- requires dating of events • Sand volcanoes • Offset strata • Drowned forests Chapter 10
    52. 52. Earthquake Prediction• Long-term: • Seismic gaps: places that haven’t slipped in a while • More likely candidates to slip next Chapter 10
    53. 53. Earthquake Prediction• Short-term: • Goal: location and magnitude of a large EQ • No reliable short-range predictions • BUT, EQs do have precursors • Clustered foreshocks • Stress triggering • And, possibly… • Water level changes in wells • Gases (Rn, He) in wells • Unusual animal behavior Chapter 10