Engineering Geology - Chapter 16 - Earthquakes

  1. What causes earthquakes?
    According to the elastic rebound theory, earthquakes happen when there is strain in the ground rock that builds up and causes the rock to bend/deform. As the strain continues to build up, eventually the rock reaches its limit and breaks, releasing energy and moving along a fault line. The released energy causes the ground to shake and roll. 
  2. Compare and contrast the concepts of intensity and magnitude of earthquakes.
    • Intensity and Magnitude both measure how powerful an earthquake is.
    • The intensity of an earthquake is a measure of the effects that an earthquake produces on both structures and people.
    • The magnitude of an earthquake is a measure of the amount of energy released by an earthquake.
  3. Describe several ways that earthquakes cause damage.
    • Common effects are the following:
    • -Ground motion, involving trembling and shaking of the land during an earthquake.
    • -Fires, resulting from broken gas and water mains; and fallen electrical wires.
    • -Landslides, triggered by ground shaking.
    • -Liquefaction, occurring when water-saturated soil or sediment temporarily get the consistency of liquids due to sudden increase in water pressure.
    • -Permanent displacements, leaving fractures and scars on the land surface.
  4. How do earthquakes cause tsunami?
    The seafloor is disturbed from movement caused by earthquakes. As the floor rises or sinks, the water over these areas is lifted or dropped, respectively, and creates a long, low wave spreading rapidly as the water returns to sea level.
  5. The elastic rebound theory

    A. involves the sudden release of progressively stored strain in rocks, causing movement along a fault
  6. The point within Earth where seismic waves originate is called the

    C. focus
  7. What is the difference and/or relationship between the focus and the epicenter of an Earthquake?
    • The focus is the point of initial breakage and movement along a fult. It is within the Earth and is the point where seismic waves originate. From the focus, seismic waves travel out in all directions.
    • The epicenter is the point on the surface of the Earth directly above the focus. Surface waves radiate outwards horizontally from the epicenter.
  8. Name the 2 types of seismic waves. 
    Body waves and Surface waves
  9. What are Body Waves?
    Body waves are a type of seismic wave that travel outward from the focus in all directions through Earth's interior. 
  10. Name the 2 types of body waves. 
    Primary waves (P-waves) and Secondary waves (S-waves)
  11. What are P-waves?
    • Also known as Primary Waves, P-waves are:
    • - compressional / longitudinal
    • - fast (4-7 kM/sec)
    • - waves that rock back and forth, parallel to the direction of the wave movement. Deformation moves with wave (slinky example)
    • - waves that can pass through both liquids and solids
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  12. What are S-waves?
    • Also known as Seconday Waves, S-waves are:
    • - shearing / transverse
    • - waves where ground moves up and down perpendicular to the direction of wave movement. (string example)
    • - slower than p-waves (2-5 kM/sec)
    • - waves that can only pass through solids.
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  13. What are Surface Waves?
    Surface waves are a type of seismic waves that travel along the Earth's surface, away from the epicenter. 
  14. Name the 2 types of surface waves.
    Love waves and Rayleigh Waves
  15. What are Love Waves? 
    • Also known as Q-waves, Love waves are: 
    • -waves that move side to side perpendicular to wave motion (think z-axis) 
    • -waves that can only pass through solids.
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  16. What are Rayleigh Waves?
    • Rayleigh Waves are:
    • -Similar to Ocean waves, except they move through the ground instead of water.
    • -Waves that cause a "rolling" motion on Earth's surface.
    • -Particle motion is elliptical and opposite to the direction of wave motion. 
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  17. What is the minimum number of seismic stations needed to determine the location of the epicenter of an earthquake?

    D. 3
  18. The Richter scale measures

    A. magnitude
  19. Benioff zones are found near

    C. oceanic trenches
  20. what is the elastic rebound theory?
    The elastic rebound theory describes how Earthquakes are caused. Rocks along fault lines are bent, and strain builds up, until the rocks reach a point where they finally break, move, and release the stored energy.
  21. What is the difference and/or relationship between Seismometers, Seismographs, and Seismograms?
    • All three are tools used to measure earthquakes.
    • Seismometers: devices used to measure seismic waves (the seismic measuring device normally involves a pendulum or other form of inertial system)
    • Seismographs: recording devices used to produce a permanent record of the motion detected by seismometers (seismographs are used nowadays to signify the entire equipment that measures and record the seismic waves, i.e., seismographs refer to seismometers too):
    • Seismograms: permanent paper (or digital) records of the earthquake vibrations (seismograms are used to measure the strength of earthquakes)
  22. What is a travel-time curve used for?
    A travel-time curve is used first to determine the distance distance of the focus (from the seismograph) based on time elapsed between the first P-wave and the first S-wave arrival

    (P- and S-waves leave earthquake focus at the same time but arrive at different times due to the different velocity; P-wave gets farther and farther ahead of the S- wave with distance and time from the earthquake).
  23. What is the difference and/or relationship between magnitude and moment magnitude?
    • Magnitude measures the amount of energy released by an earthquake, obtained from the analysis of seismograms.
    • Moment Magnitude of an earthquake is a more objective measure of energy released by a major earthquake. In addition to seismogram analysis, the moment magnitude uses uses rock strength, surface area of fault rupture, and amount of movement.
  24. What is a Benioff Zone?
    An active seismic zone located deep within a subduction zone.
  25. Which seismic wave can pass through both liquids and solids?
  26. Can earthquakes occur in Minnesota?
    Yes, they can. However, since we aren't near any active fault zones (Minnesota earthquakes are thought to be reactions of ancient fault zones to new stresses), the earthquakes are not very frequent and not very large or destructive. They only occur about once every 10-20 years and are usually less than magnitude 5. 
Card Set
Engineering Geology - Chapter 16 - Earthquakes
CHAPTER 16: Earthquakes