Geology Ch. 8 Earthquakes

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Geology Ch. 8 Earthquakes
2011-10-29 18:11:44

Earthquake Chapter
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  1. Earthquake:
    the shaking or trembling of the ground caused by the sudden release of energy, usually as a result of faulting, which involves displacement of rocks along fractures.
  2. Aftershock:
    continuing adjustments along faults caused by an earthquake. Usually smaller than the main EQ but can still cause considerable damage.
  3. Elastic Rebound Theory:
    • was proposed by H.F. Reid to explain how energy is released during EQ’s. Rocks on opposite sides of the San Andreas fault had been storing up energy and bending slightly for at least 50 years. Eventually the strength of the rocks was exceeded and they snapped back to their former shape and the energy that was
    • stored was released.

    • The tighter a spring is wound, the more energy
    • is stored. If it is wound so tightly that it breaks, the stored energy is released as the spring rapidly unwinds and partially regains its former shape.
  4. Seismology:
    The study of EQ's
  5. Seismographs:
    • are instruments that detect, record, and measure
    • vibrations produced by an EQ.

    Seismogram - the record made by a seismograph
  6. Seismic Waves:
    when an EQ occurs, energy (seismic waves) radiates out from the point of release
  7. When do most EQs occur?
    when movement occurs along faults, which are related to plate movements.
  8. Remember:
    The longer the fracture along which movement occurs, the more time it takes for the stored energy to be released, and therefore the longer the ground will shake.
  9. Focus:
    or hypocenter, is the location within Earth's lithosphere where fracturing begins (the point where energy is first released)
  10. Epicenter:
    The point on Earth's surface directly above the focus
  11. 3 Categories of EQ's based on focus depth:
    • Shallow focus – focal depths less than 70 km from the surface
    • Intermediate focus – depths between 70 and 300 km
    • Deep focus – more than 300 km deep
  12. Remember:
    EQ’s that happen along divergent or transform boundaries are usually shallow focus

    • Many shallow focus, and nearly all intermediate
    • and deep focus EQ’s occur along convergent margins.
  13. Where EQ's occur:
    • 95% take place in seismic belts corresponding to
    • plate boundaries where plates converge, diverge, and slide past eachother.

    80% of all EQ’s occur in the circum-pacific belt, a zone of seismic activity nearly encircling the the Pacific Ocean basin.

    15% of all EQ’s occur in the Mediterranean-Asiatic belt.

    • 5% of the EQ’s occur mostly in the interiors of
    • plates and along oceanic spreading-ridge systems.

    • More than 900,000 EQ’s are recorded annually.
    • 31,000 are strong enough to be felt
  14. Body Waves:
    an EQ generates two types of waves: P-waves and S-waves
  15. P-waves:
    primary waves, are the fasted seismic waves and can travel through solids, liquids, and gases.
  16. Also known as push-pull waves, they are similar to sound waves because they move material forward and backward along a line in the same direction that the waves themselves are moving.
  17. S-waves:
    secondary waves, are somewhat slower than P-waves and can travel only through solids. S-waves are shear waves because they move the material perpendicular to the direction of travel, causing shear stresses in the material.
  18. Why can't S-waves move through liquids?
    Because liquids (and gases) are not rigid, they have no shear strength and S-waves cannot be transmitted through them.
  19. Remember:
    The velocities of P and S-waves are determined by the density and elasticity of the materials through which they travel.
  20. Ex. of P and S-wave velocity:
    Seismic waves travel more slowly through rocks of greater density, but more rapidly through rocks with greater elasticity.
  21. Elasiticity:
    a property of solids, such as rocks, and means that once they have been deformed by an applied force, they return to their original shape when the force is no longer present.
  22. Remember:
    Because P-wave velocity is greater than S-wave velocity, P-waves always arrive at seismic stations first.
  23. Surface Waves:
    travel along the surface of the ground, or just below it, and are slower than body waves.

    Instead of the sharp jolting or shaking that body waves cause, surface waves generally produce a rolling or swaying motion.
  24. What are the two most important surface waves?
    Rayleigh waves and Love Waves
  25. Rayleigh Waves:
    (R-waves) generally slower than love waves and behave like water waves in that they move forward while the individual particles of material move in an elliptical path within a vertical plane oriented in the direction of the wave movement.
  26. Love Waves:
    (L-waves) the motion is similar to that of an S-wave, but the individual particles of the material move only back and forth in a horizontalplane perpendicular to the direction of wave travel.
  27. Time-Distance Graph:
    graphs that illustrate the difference between the arrival times of the two waves as a function of the distance between a seismograph and an earthquake's focus.
  28. P-S Time Interval:
    the time difference between the arrival of P- and S-waves.
  29. Remember:
    If P- and S-wave time intervals are known from at least 3 seismograph stations, then the epicenter of any EQ can be determined.
  30. What are the two measures that are commonly used to measure EQ's?
    Intensity and magnitude
  31. Intensity:
    the subjective or qualitative measure of the kind of damage done by an EQ as well as people's reaction to it.

    Used by geologists to get a rough approximation of the size and strength of an EQ.
  32. Modified Mercalli Intensity Scale:
    the most common intensity scale, which has values ranging from I to XII
  33. What are the factors of the intensity of an EQ?
    • Amount of energy released
    • Distance from the epicenter
    • Focal depth
    • Population density
    • Geology of the area
    • Types of building construction
    • Duration of shaking
  34. Richter Magnitude Scale:
    scaled that measures an EQ's magnitude.
  35. Magnitude:
    the total amount of energy released by an EQ at its source.
  36. What was the largest magnitude EQ ever recorded?
    a magnitude 9.5 EQ in Chile on May 22, 1960.
  37. How do scientists determine the magnitude of an EQ?
    by measuring the amplitude of the largest seismic wave as recorded on a seismogram.
  38. Remember:
    Each whole number increase in magnitude represents a 10-fold increase in wave amplitude.
  39. Seismic-Moment Magnitude Scale:
    scale that takes into account the strength of the rocks, the area of a fault along which rupture occurs, and the amount of movement of rocksadjacent to the fault.
  40. What are the destructive effects of EQ's?
    • Ground shaking
    • Fire
    • Tsunami
    • Ground failure
  41. Ground Shaking:
    the most obvious and immediate effect of an EQ, and varies depending on the EQ's magnitude, distance from the epicenter, and type of underlying materials in the area.
  42. Which EQ hazard causes the most loss of lives and injuries?
    Ground shaking - effects of ground shaking are collapsing buildings, falling building facades and window glass, and toppling monuments and statues.
  43. Liquefaction:
    a process in which water-saturated sediments tend to liquefy, or behave as fluid. When shaken, the individual grains lose cohesion and the ground flows.
  44. EQ Fires:
    Ground shaking severs electrical and gas lines, causing fires.
  45. Tsunamis:
    when an EQ strikes in the ocean, it causes deadly Tsunamis. Also known as seismic sea waves.
  46. Ground Failure:
    Earthquake-triggered landslides, particularly dangerous in mountainous regions and are responsible for large amounts of damage and many deaths.
  47. Seismic Risk Maps:
    maps that indicate the likelihood and potential severity of future earthquakes based on the intensity of past EQ's.
  48. Precursors:
    most earthquakes are preceded by both short term and long term changes within Earth.
  49. Wave Rays:
    lines showing the direction of movement of small parts of wave fronts.
  50. Refraction:
    as seismic waves travel from one material into another of different density and elasticity, its velocity and direction of travel change. The wave is bent.
  51. Reflection:
    when seismic rays encounter a boundary separating materials of different density or elasticity, some of a wave's energy is reflected back to the surface.
  52. Discontinuity:
    a boundary across which a significant change in Earth materials or their properties occurs.
  53. P-wave Shadow Zone:
    an area in which little P-wave activity is recorded by seismographs.
  54. S-Wave Shadow Zone:
    larger and more complete than the P-zone
  55. The Mohorovicic Discontinuity (Moho)
    the boundary that separates the crust from the mantle. It is present everywhere except spreading ridges. Beneath continents, it ranges from 20 to 90 km, beneath the seafloor it ranges between 5 and 10 km.
  56. Low Velocity Zone:
    a layer in which the rocks are close to their melting point and are less elastic, accounting for the observed decrease in seismic wave velocity.
  57. Seismic Tomography:
    a technique that allows geophysicists to develop more accurate models of Earth's interior.
  58. Geothermal Gradient:
    temperature increase with depth. 25 degreese celsius near Earths surface.
  59. What generates most of Earth's internal heat?
    radioactive decay