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CHAPTER 9 Earthquakes
1. Earthquakes are vibrations of the Earth caused by the sudden release of energy, usually along a fault.
2. Define an earthquake.
3. Hooke’s law is:
a. Stress is proportional to strain.
b. Strain is proportional to stress.
4. The elastic rebound theory states that pressure builds in rocks on opposite sides of a fault until
the inherent strength of the rocks is exceeded and rupture occurs. When the rocks rupture, stored
energy is released as they snap back to their original position.
5. How does the elastic rebound theory explain how energy is released during an earthquake?
6. According to the elastic rebound theory:
a. earthquakes originate deep within the Earth;
b. earthquakes originate in the asthenosphere where rocks are plastic;
c. earthquakes occur where the strength of the rock is exceeded;
d. rocks are elastic and do not rebound to their former position;
e. none of these.
7. Seismology is the study of earthquakes. Earthquakes are recorded on seismographs, and the record of an
earthquake is a seismogram.
8. Describe how a seismograph works.
9. A seismogram is:
a. an instrument that records earthquake waves; b. the record made by a seismograph;
c. the slowest of the seismic waves; d. a unit of energy released by an earthquake;
e. none of these.
10. The two types of body waves are P-waves and S-waves. Both travel through the Earth,
although S-waves do not travel through liquids. P-waves are the fastest waves and are compressional,
while S-waves are shear.
11. What is the difference between body waves and surface waves?
12. Body waves are:
a. P-waves; b. S-waves; c. Rayleigh waves;
d. answers (b) and (c); e. answers (a) and (b).
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13. Surface waves travel along or just below the Earth's surface. The two types of surface waves
are Rayleigh and Love waves. Rayleigh waves behave like water waves, and Love waves are
similar to S-waves, but are directed horizontally.
14. How do P-waves differ from S-waves? How do Rayleigh waves differ from Love waves?
15. The fastest of the four seismic waves are:
a. P; b. S; c. Rayleigh; d. Love; e. tsunami.
16. An epicenter is:
a. the location where rupture begins;
b. the point on the Earth's surface vertically above the focus;
c. the same as the hypocenter;
d. the location where energy is released;
e. none of these.
17. The epicenter of an earthquake can be located by the use of a time-distance graph of the P- and
S-waves from any given distance. Three seismographs are needed to locate the epicenter of an earthquake.
18. What is the minimum number of seismographs needed to determine an earthquake's epicenter?
a. 1; b. 2; c. 3; d. 4; e. 5.
19. How is the epicenter of an earthquake determined?
20. The focus of an earthquake is the point where energy is released. Vertically above the focus
on the Earth's surface is the epicenter.
21. What is the difference between the focus and the epicenter of an earthquake?
22. Most earthquakes occur within seismic belts. Approximately 80% of all earthquakes occur in
the circum-Pacific belt, 15% within the Mediterranean-Asiatic belt, and the remaining 5% mostly
in the interior of plates or along oceanic spreading ridge systems.
23. The majority of all earthquakes occur in the:
a. Mediterranean-Asiatic belt; b. interior of plates; c. circum-Atlantic belt; d. circum-Pacific belt;
e. along spreading ridges.
24. What is the relationship between plate boundaries and earthquakes?
25. The vast majority of all earthquake foci occur at a depth of less than _______ kilometers.
a. 20; b. 40; c. 60; d. 80; e. 100.
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26. What is the relationship between plate boundaries and focal depth?
27. With few exceptions, the most destructive earthquakes are:
a. shallow focus; b. intermediate focus; c. deep focus;
d. answers (a) and (b); e. answers (b) and (c).
28. Magnitude measures the amount of energy released by an earthquake and is expressed in the
Richter Magnitude Scale. Each increase in the magnitude number represents about a 30-fold
increase in energy released.
29. How much more energy is released by a magnitude 5 earthquake than by one of magnitude 2?
a. 2.5 times: b. 3 times; c. 30 times; d. 1,000 times; e. 27,000 times.
30. Ground shaking is the most destructive of all earthquake hazards. The amount of damage done by an
earthquake depends upon the geology of the area, the type of building construction, the magnitude of the
earthquake, and the duration of shaking.
31. Intensity is a measure of the kind of damage done by an earthquake and is expressed by values
from I to XII in the Modified Mercalli Intensity Scale.
32. A qualitative assessment of the kinds of damage done by an earthquake is expressed by:
a. seismicity; b. dilatancy: c. magnitude; d. intensity;
e. none of these.
33. Explain the difference between intensity and magnitude and between the Modified Mercalli
Intensity Scale and the Richter Magnitude Scale.
34. Why is ground shaking so destructive during an earthquake?
35. Which of the following usually causes the greatest amount of damage and loss of life?
a. fire; b. tsunami; c. ground shaking; d. liquefaction; e. landslides.
36. Tsunami are seismic sea waves that are usually produced by earthquakes. They can do a tremendous
amount of damage to coastlines, even thousands of kilometers away from the earthquake epicenter.
37. Explain how tsunami are produced and why they are so destructive.
38. A tsunami is a:
a. measure of the energy released by an earthquake; b. seismic sea wave;
c. precursor to an earthquake; d. locked portion of a fault; e. seismic gap.
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39. Seismic risk maps are helpful in making long-term predictions about the severity of
earthquakes based on past occurrences.
40. Why are seismic risk maps useful to planners?
41. Earthquake precursors are any changes preceding an earthquake that can be used to predict when an
earthquake will occur. Precursors include seismic gaps, changes in surface elevation, tilting, fluctuations in
water well levels, and anomalous animal behavior.
42. How can earthquake precursors be used to predict earthquakes?
43. What is the dilatancy model? How does it help explain how earthquake precursors are related?
44. A variety of earthquake research programs are underway in the United States, Japan, and China.
However, studies indicate that most people would probably not heed a short-term earthquake warning.
45. Fluid injection into locked segments of an active fault holds great promise as a means of
possible earthquake control.
46. Explain how fluid injection may be useful in controlling earthquakes.
Benioff zone
precursor
dilatancy model
P-wave
earthquake
Rayleigh wave
elastic rebound theory
Richter Magnitude Scale
elasticity
seismic gap
epicenter
seismic risk map
focus
seismogram
intensity
seismograph
liquefaction
seismology
Love wave
S-wave
magnitude
time-distance graph
Modified Mercalli Intensity Scale
tsunami