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Fall 2000 Lohrengel Geology 1013 -Earth System Science Chapter 5 Study Instructions: Read each question carefully before answering. Work at a steady pace, and you should have ample time to finish. _____________________________________________ 1. How are seismic waves used to learn about the internal structure of the Earth? 2. Describe how the method of triangulation is used to locate the epicenter of an earthquake. Support you answer with a neat, well- labelled diagram. 3. What does it mean when we say that the lithosphere is in isostatic balance on the asthenosphere? How does glacier ice provide a demonstration of the principle of isostasy? Support your answer with neat, well- labelled diagrams. 4. Explain how you could use a gravimeter to demonstrate the principle of isostasy. 5. What is an S-wave shadow zone? How have S-waves helped scientists to learn about the Earth's core? Support your answer with a neat, well- labelled diagram. 6. Sometimes the largest earthquakes (such as the M 8.4 Good Friday earthquake in Anchorage, Alaska, 1964) cause fewer deaths than smaller earthquakes (such as the M 6.8 earthquake of December, 1988 in Armenia). Why do you think this is? 7. What is isostasy? What does isostasy tell us about the relationship between the lithosphere and the asthenosphere? 8. Long-term earthquake forecasting has been much more successful than short-term prediction and early warning. Why? 9. The outer core is inferred to be molten because it does not transmit seismic waves. 10. When slippage of rock occurs along a fracture in a rock, the fracture is called an earthquake. 11. The epicenter of an earthquake is the point where the earthquake's energy is first released. 12. The point on the Earth's surface directly above the focus is called the epicenter. 13. The point on the Earth's surface directly above the epicenter is called the focus. 14. Seismic waves travel through the Earth at uniform velocities, spreading out in all directions. 15. P waves are compressional/expansional waves that can pass through gases, liquids and solids. 16. S waves travel more slowly than P waves through the same material. 17. Surface waves are the first seismic waves to be recorded by the seismograph in the event of an earthquake. 18. The moho is the seismic discontinuity that marks the boundary between the core and mantle of the Earth. 19. The amount of energy released by a Richter magnitude 7.5 earthquake is approximately _________ times greater than the energy released by a magnitude 5.5 earthquake. a. 30 b. 60 c. 100 d. 900 20. Transform plate boundaries are characterized by a. earthquakes. b. volcanic activity. c. mountain building. d. All of these are true. 21. Earthquakes are a. restricted to plate boundaries. b. most common along plate boundaries. c. evenly distributed across plates. d. most common in ocean basins. 22. The term epicenter refers to a. the focus of an earthquake. b. the point from which an earthquake's energy originates. c. the point on the Earth's surface directly above the focus of an earthquake. d. the point of failure of rocks in the elastic rebound theory. 23. The term seismic gap refers to a. a region where no seismic activity occurs. b. the point of rupture of stressed rocks deep in the Earth (in the elastic rebound theory). c. a method for predicting seismic sea waves (tsunami). d. a segment of a seismically active fault along which no large earthquakes have occurred recently. 24. An instrument that measures ground vibrations is a a. gravimeter. b. seismogram. c. geodolite. d. seismograph. 25. The Richter magnitude of an earthquake a. is measured on an instrument called a Richter scale. b. is measured by the amplitude of seismic waves recorded by seismographs. c. is determined by the area over which it is felt. d. depends on how near the recording instrument is to the epicenter. 26. Which one of the following is not a danger closely associated with earthquakes? a. fires b. seismic sea waves c. quicksand d. surface rupturing e. heating of the ground surface 27. The most intense earthquakes to jolt North America in the past 200 years were centered in a. California. b. New York. c. South Carolina. d. Missouri. 28. Research into the short-term prediction of earthquakes currently focuses on observations of changes that may signal an impending earthquake, such as a. swarms of tiny earthquakes. b. changes in magnetism. c. tilting of the land surface. d. All of these are true. 29. The focus of an earthquake is a. the point on a fault surface where seismic energy is first released. b. the point on the Earth's surface where seismic energy is first released. c. the region of greatest damage during an earthquake. d. the point on the Earth's surface above the epicenter. 30. Which one of the following does not belong with the others? a. gravimeter b. seismogram c. seismograph d. magnetometer 31. The S-wave shadow zone assisted scientists in determining that a. the core of the Earth is composed primarily of metallic iron and nickel. b. the asthenosphere is a weak layer of anomalously low seismic velocity. c. the outer core of the Earth is molten. d. All of these are true. 32. An increase of 1 on the Richter magnitude scale for earthquakes corresponds to an increase in released energy of approximately a. 10-fold. b. 20-fold. c. 30-fold. d. 100-fold. 33. An increase of 1 on the Richter magnitude scale for earthquakes corresponds to an increase in maximum seismic wave amplitude of a. 10-fold. b. 20-fold. c. 30-fold. d. 100-fold. 34. Tsunamis a. are also called "seismic waves." b. are sometimes incorrectly called tidal waves, but have nothing to do with tides. c. travel very slowly across the open ocean, but hit the shore as enormous breakers. d. All of these are true. 35. The asthenosphere a. is a zone of anomalously low seismic velocity. b. underlies the lithosphere. c. is a zone of plastic deformation within the upper part of the mantle. d. All of these are true. 36. Which one of the following is not use d by scientists to predict earthquakes? a. foreshocks b. seismic gaps c. isostasy d. strange animal behavior 37. The flotational balance among segments of the lithosphere is referred to as a. gravity. b. isostasy. c. density. d. seismicity. 38. The asthenosphere is the zone in which a. seismic wave velocities are highest. b. rocks are very ductile. c. all earthquakes originate. d. All of these are true. 39. Vibrations sent out by earthquakes are measured with an instrument called a ________________. 40. Energy released at an earthquake's focus radiates outward as two kinds of body waves: _______________ waves and _______________ waves. 41. The pronounced seismic discontinuity that marks the mantle-crust interface is called the ____________________. 42. Seismic waves that do not pass through the body of the Earth are called _______________. 43. The asthenosphere is a zone of anomalously ______________ seismic-wave speed. 44. The _______________ scale is used to calculate the amount of energy released during an earthquake, on the basis of the recorded amplitude of the seismic waves. 45. The Earth's core has a high density and is inferred to consist primarily of _______________ and a small amount of ________________. 46. The flotational balance maintained by large segments of the lithosphere that "float" on the underlying asthenosphere is referred to as __________________. 47. The scale that measures earthquake intensity on the basis of felt vibration and extent of damage to buildings is called the __________________ scale. 48. Rocks can be elastically deformed by seismic body waves in two ways: (1) by a change in ___________________ or (2) by a change in __________________. 49. What are six different ways earthquakes cause damage? 50. What is a seismic gap? Why are seismic gaps so important in earthquake prediction? 51. How did S-waves reveal that the Earth's outer core is liquid? 52. What does the elastic rebound theory suggest? 53. Explain how a bent piece of wood can be an analogy for an earthquake. 54. What is the difference between the focus and epicenter of an earthquake? When new reporters give the "location" of an earthquake, which one are they actually referring toƒƒthe epicenter or the focus? 55. What are the most important differences between S waves and P waves? 56. What are the main differences between seismic body waves and surface waves? 57. What, exactly, is the moho? 58. A single earthquake has only one magnitude on the Richter scale, but it may have numerous magnitudes on the modified Mercalli intensity scale. How can that be? _____________________________________________ Fall 2000 Lohrengel Geology 1013 -Earth System Science Answer Key: Chapter 5 Study 1. * not available * 2. * not available * 3. * not available * 4. * not available * 5. * not available * 6. * not available * 7. * not available * 8. * not available * 9. F 10. F 11. F 12. T 13. F 14. F 15. T 16. T 17. F 18. F 19. D 20. A 21. B 22. C 23. D 24. D 25. B 26. E 27. D 28. D 29. A 30. B 31. C 32. C 33. A 34. B 35. D 36. C 37. B 38. B 39. seismograph 40. (P...S) OR (primary...secondary) OR (compressional...shear) 41. (Mohorovicic discontinuity) OR (M-discontinuity) OR (moho) 42. surface waves 43. (low) OR (slow) 44. Richter magnitude 45. iron...nickel 46. (isostasy) OR (isostatic equilibrium) OR (isostatic balance) 47. (modified Mercalli) OR (modified Mercalli intensity) 48. shape...volume 49. Earthquakes cause damage by (1) ground motion; (2) faulting and surface rupturing; (3) fires; (4) land movement and slope collapse; (5) liquefaction; and (6) tsunami. 50. Seismic gaps are places along seismically active faults where, for one reason or another, earthquakes have not occurred for a long time and where elastic strain is increasing. Seismic gaps receive a lot of attention because they are considered the place most likely to experience large earthquakes. 51. S-waves cannot be transmitted through liquids. S-waves are transmitted through the crust and mantle (solid) but not through the outer core (liquid). 52. The elastic rebound theory suggests that, if fault surfaces lock rather than slip easily past one another, the rocks on either side of the fault will bend and in bending they will store elastic strain energy. When the fault finally does slip and the bent rocks rebound to their original shapes, an enormous amount of energy is released as an earthquake. 53. When you bend a thin piece of wood, the energy from your muscles is stored in the wood in the form of elastic strain energy. If you release the wood, this stored energy will return it to its former shape. However, if you bend the wood too far it will break, and the stored elastic energy will be suddenly converted to heat energy, sound energy (the snapping of the wood), and vibrations in the wood. The same is true of rocks: when they are deformed by movement along a fault, they build up elastic strain energy. When the rock finally breaks and slippage occurs along the fault, the stored energy will be released all at once in the form of an earthquake (elastic rebound theory). 54. The focus is the point (or, sometimes, the area) where energy is first released, which generally lies at some depth below the surface. The epicenter is the location on the surface of the Earth that immediately overlies the focus. When the "location" of an earthquake is reported on the news, it is with reference to the epicenter of the quake, not the focus. 55. P waves travel more quickly (through the same material) than S waves. S waves cannot be transmitted through fluids. P waves are compression/expansion waves; S waves are shear waves. 56. Surface waves travel more slowly than body waves, and they travel along or closely to the surface of the Earth, rather than through it; they are the last waves to be detected by a seismograph. Surface waves tend to have very long wavelengths and high amplitudes; therefore, they often cause significant damage. 57. "Moho" is short for "Mohorovicic discontinuity (or M-discontinuity). It is a pronounced seismic discontinuity that marks the base of the crust (i.e., the top of the mantle, or the crust-mantle boundary). 58. The calculation of Richter magnitude takes into account the distance of the seismic station from the earthquake's epicenter; the modified Mercalli scale does not. In other words, the strength of an earthquake on the Mercalli scale depends on distance from the epicenter: the closer you are, the stronger the shaking and the higher the magnitude on the Mercalli scale.