Unit 1 Review Short Questions and Answers

Unit1 Review_Short Questions & Answers

Topic1

1. Why are people who live on coastal beaches so poorly aware or concerned about hazards in those environments?
* Most of them, including developers, real estate people, and governmental employees, have never experienced a hurricane or major flood. [pp. 1, 9]

2. What kind of natural hazards are not normally insurable?
* landslides, ground settling, or swelling soils [p. 8]

3. Why do many coastal communities not educate visitors and new residents about natural hazards in their areas?
* They view such information as bad for tourism and bad for business. [p. 7] 4. What is the normal relationship between number of occurrences of a particular type of event and the size of such events?
* numerous small events, a few larger events, and only rarely a giant event [p. 6]

5. When is a large event such as a major earthquake not a disaster?
* when it happens in a remote area and does not affect anyone [p.3]

6. When people incur a significant loss from a natural disaster, who is most commonly to blame, and why?
* People themselves are to blame because they place themselves in the environment of such large natural events. [pp. 1-3]

7. What can happen to make a moderate-size event into a large natural disaster?
* overlapping events that amplify the effect [p. 7]

8. If you erect a barrier for protection against some natural event, what detrimental effect can follow?
* It can affect people in other locations (such as downstream). [p. 10]

Topoc2.

1. When did the direction of the Pacific plate change, and what direction was it moving before this change?
* changed at 43 million years.
* Before this change, it was moving north-northwest. [p. 29]

2. If the ocean floor is getting wider, why is the Earth not becoming larger?
* old ocean floor sinks at subduction zones (trenches) [p. 14]

3. Distinguish between Earth’s crust and mantle.
* Crust overlies mantle. It is basalt composition under the ocean basins, granitic composition in the continents. [pp. 17-19]

4. What characteristics of tectonic plates distinguish them from deeper Earth materials?
* They are rigid. [p. 17]

5. Do tectonic plates consist of crust, mantle, or some combination of crust and mantle, and if so, what part of parts of each?
* all of the crust and part of the upper mantle. [p. 15]

6. What keeps the Appalachians standing as a mountain range even though they have been continuously eroding since they formed hundreds of millions of years ago?
* Isostacy. As material is eroded off the range, it floats higher, just as removing ice from the top of an iceberg causes it to float higher. [p. 17]

7. What is the basis for the “scientific method?”
* tentative hypotheses are tested using new observations and experiments [p. 17]

8. Why do many oceanic volcanoes occur as long lines of volcanoes that are active at only one end? How does the process work?
* They are hotspot volcanoes. A hot plume of magma rising through the mantle burns through the overlying lithosphere to erupt as a volcano. Since the lithosphere moves over the deeper mantle, new volcanoes form progressively in a line over the deep plume. [p. 27]

9. Distinguish between Earth’s lithosphere and asthenosphere in terms of both location and properties.
The lithosphere overlies the asthenosphere. It is rigid, and the asthenosphere deforms plastically. [p. 17]

10. What is the difference between the Earth’s lithosphere and asthenosphere?
* lithosphere is more-rigid; asthenosphere is more-plastic/easily deformed.
* It is on top of the asthenosphere. [p. 17]

12. What does the oceanic crust consist of, and how thick is it?
* basalt, about 7 km thick [p. 19]

13. What does the continental crust consist of, and how thick is it?
“granite” composition (whole range of continental rocks, especially granite, gneiss, schist) 30 to 60 or 70 km thick [p. 19]

14. What are the main types of lithospheric plate boundaries, in terms of relative motions? Provide a real example of each (by name or location).
* convergent (continent – ocean plate collision): Cascadia subduction zone (Pacific Ocean floor against North America)
* convergent (continent- continent collision): Himalayas
* divergent (rift or spreading, or extension): mid-oceanic ridge
* transform (lateral motion): San Andreas Fault [p. 20-24]

15. Along which type(s) of lithospheric plate boundary are basalt-flow eruptions abundant? Provide a real example (name or location).
* spreading (Iceland/oceanic ridge) [p. 20]

16. In what tectonic environment that is not at a plate boundary are major volcanoes found? Identify one of these in an ocean basin and one on a continent.
* hotspots or mantle plumes
* in ocean basin: Hawaii
* on continent: Yellowstone [p. 28]

17. List the three main tectonic environments for large earthquakes in western North America and name a specific example of each. Tectonic environment Name or specific location
* subduction zone Pacific Coast trench (Cascadia trench)
* spreading zone Basin and Range
* transform fault San Andreas Fault

18. The lithosphere is composed of what two layers?
* crust and upper mantle [p. 17]

19. With which type or types of boundary are active volcanoes generally associated?
* divergent (or spreading), convergent (or subduction) [p. 20]

20. What was the primary reason why Alfred Wegener’s theory of continental drift rejected?
* He had no mechanism for moving continental crust through strong basaltic oceanic crust. [p. 13]

21. The “Ring of Fire” refers to what, and where is it?
* the ring of active volcanoes surrounding the Pacific Ocean [p. 22]

22. Convergent plate boundaries are especially noted for which two types of natural hazards?
* earthquakes and volcanoes +/- tsunami [p. 20]

23. In the line of arc volcanoes, like the Cascades of Washington and Oregon, over an active subduction zone, a series of events leads to the magmas that erupt at the surface. What magma forms first, and where and how does it form?
* Basalt magma forms at (just above) the subduction zone below the volcano, when water formed by dehydration of hydrous minerals in the descending slab causes melting of the mantle peridotite above. [p. 23]

24. How is the magma formed that feeds the volcanoes in Hawaii?
* The lithosphere carrying Hawaii slowly moves over a mantle hotspot causing melting to form basalt magma. [pp. 28-29]

Topic 3.

1. Where a tall building is right next to a short building, why is the tall building often damaged? Why does the damage occur and where in the building?
* Tall buildings sway at a lower frequency than short buildings so the buildings bang into one another. The tall building breaks at the top of the short building. [p. 58]

2. Name the three main types of earthquake waves.
* primary, secondary, surface (or) P, S, surface [pp. 44-45]

3. What is the approximate P-wave velocity through the Earth? Indicate whether your answer refers to the earth’s crust or mantle.
* 5-6 km/sec in continental crust; 8 km/sec in mantle [p. 44]

4. Which type or types of earthquake waves arrive at a distant seismograph most quickly by traveling through the mantle of the earth?
* P- and S-waves [pp. 44, 47]

5. Which types of earthquake waves do the most damage?
* Surface waves [p. 45]

6. Which earthquake waves arrive first?
* P-waves [p. 44]

7. Which earthquake waves arrive last?
* Surface waves [p. 45]

8. Which of the three main types of earthquake waves tend to be most destructive to buildings?
* Surface waves [p. 45]

9. Why are surface waves the most destructive type of earthquake waves (two different reasons)?
* larger amplitude of shaking (greatest ground motion)
* closer to surface of earth and thus closer to building [p. 45]

10. Which waves have the lowest frequencies?
* Surface waves [p. 45]

11. What does the Mercalli Intensity Scale depend on?
* how much damage occurred and how strongly people feel the earthquake [p. 47]

12. How much more energy is released in a magnitude 7 earthquake than in a magnitude 6 earthquake?
* about 32 times [p. 49]

13. What are the three main factors that affect moment magnitude?
* shear strength of the rocks displaced
* total surface area of rocks ruptured by the fault movement
* average slip distance on the fault [p. 50]

14. Between magnitude 7 and magnitude 8, about how much harder is the shaking during an earthquake? Or, how much increase is there in the acceleration of the ground?
* not much [pp. 51-52]

15. In addition to the amount of damage, increases in what factors go along with an increase in earthquake magnitude? List several.
* fault offset
* length of fault ruptured
* acceleration of the ground
* time of shaking
* velocity of motion of the ground [pp. 35-36, 50]

16. If you find a well exposed fault that moved before seismographs were available, how can you infer the approximate magnitude of earthquakes generated by the movement? (two different ways)
* measure the offset distance (distance moved) during the event
* measure the total length of fault break during the event [p. 50]

17. List three characteristics of the ground that would increase or amplify the ground shaking and destruction of a building during an earthquake.
* soft mud amplifies shaking relative to bedrock
* loose sand filled with water causing liquefaction [pp. 52-53]

18. Since most deaths in earthquakes are caused by buildings and other structures falling on people, what building materials are most susceptible to collapse?
* masonry (brick, stone), adobe [p. 54]

19. Since most deaths in earthquakes are caused by buildings and other structures, what types of construction or design are most likely to cause building collapse?
* unbraced (weak) ground-floor garages [p. 54]

20. Since most deaths in earthquakes are caused by buildings and other structures, what sometimes causes collapse of a mid-floor of a building (e.g., 3rd floor of a 7-story building) when the other floors remain standing?
* building oscillates at same frequency as the ground during an earthquake [p. 57]

21. In some buildings, often a single floor will collapse. What characteristics of a building will lead to a single-floor collapse?
* weak floors, such as garages or storefronts on the ground floor [p. 54]

22. If a building is to be built on soft sediment, is a short or tall building safer? Why?
* A short building. Soft sediment shakes at a low frequency, and a short building shakes at a high frequency so they do not shake in resonance during an earthquake. [p. 57]

23. Why are structures on soft sand or mud often destroyed in an earthquake, when nearby structures on bedrock remain essentially undamaged?
* Bedrock shakes with small amplitude vibrations.
* Soft sediment shakes with stronger (large amplitude) vibrations, meaning greater back and forth distances. [pp. 52-53, 57]

24. Which is more likely to be damaged during an earthquake, a building down on low, flat ground next to a bay or on top of a hill nearby (ignore the possibility of any landsliding) and why?
* On low, flat ground next to a bay, soft sediment would shake more violently (larger amplitudes) during an earthquake than harder rock on a hilltop nearby. [pp. 52-53, 57]

25. Seismic waves travel at different velocities. [pp. 44-45]
a. Which waves travel fastest and how fast do they move through the Earth’s mantle?
* P-waves. They travel at about 8 km/sec.
b. Which waves travel next fastest and how fast?
* S-waves. They travel at about 5 or 6 km/sec.
c. Which waves come next and how fast do they travel?
* Surface waves. They travel at about 2 or 3 km/sec.

26. Which seismic waves do not travel through liquids?
* S-waves [p. 44]

27. What is the motion of P-waves?
* compressional; compressing and stretching particles in the direction of wave travel [p. 44]

28. What is the motion of S-waves?
* up and down shear motion of particles / wiggling [p. 44]

29. What is the motion of surface waves?
* rolling like water particles in a water wave [p. 45]

30. What are the main types of fault motion? Provide the name and type of motion.
* strike-slip: lateral slip of one block past the other
* normal: steep fault with the block above the fault moving down
* reverse: steep fault with the block above the fault moving up
* thrust: gently-sloping fault with the block above the fault moving up [p. 33]

31. Extension of the Earth’s crust generally causes what type of fault or faults?
* normal faults [p. 33]
a. What type of plate boundary would produce such a fault or faults?
* rift (or spreading)
b. Compression of the Earth’s crust generally causes what type of fault or faults?
* reverse or thrust faults
c. What type of plate boundary would produce such a fault or faults?
* ocean-continent or continent-continent collision

32. What is the difference between elastic deformation and plastic deformation?
* Rocks subjected to stress involving elastic deformation return to their original form after the stress is relieved; the deformation is reversible.
* Rocks subjected to plastic deformation do not return to their original shape but remain deformed after the stress is relieved. [p. 35]

33. How do seismologists determine how far away an earthquake was from their seismograph?
* They determine the time lag between the P- and S-wave arrival and knowing the different velocities of those waves, calculate the distance. The time lag increases the farther they are from the earthquake. [p. 46]

34. How do seismologists determine the location of an earthquake?
* They determine the distance of the earthquake from at least three seismographs of quite different locations and draw circles of distance from each seismograph. The earthquake was at the intersection of the three distance circles. [pp. 46-47]

35. What is the difference between the epicenter and the focus of an earthquake?
* The focus is the point of origination of the earthquake (generally below the Earth’s surface). The epicenter is the point on the Earth’s surface directly above the focus. [p. 44]

36. What does earthquake wave frequency mean?
* the number of wave crests (or cycles) to pass a location per second [p. 45]

37. How is the Richter Magnitude Scale measured?
* the logarithm (to the base 10) of the amplitude of a set of earthquake waves on a seismograph [p. 48]

38. How much greater amplitude of earthquake waves is a magnitude 6 earthquake than a magnitude 5 earthquake?
* 10 times [p. 48]

39. How much greater energy is released by a magnitude 6 earthquake than a magnitude 5 earthquake?
* about 32 times [p. 49] 40. How much greater energy is released by a magnitude 7 earthquake than a magnitude 5 earthquake?
* 32 x 32 = 1024 times (more than 1000 times) [p. 50]

41. Which waves are used to determine earthquake magnitude?
* Any of them, P-, S-, or surface waves. The numbers are only a little different. [p. 49]

42. What can you conclude from the Gutenberg-Richter Frequency-Magnitude relationship?
* There are frequent small earthquakes, fewer moderate-size, and only rarely large earthquakes. [p. 50]
a. What is the largest possible earthquake magnitude? Why?
* about magnitude 10 because the longest fault length that could be broken is the circumference of the Earth, about 40,000 km. [p. 50]
b. What is the largest earthquake ever recorded since modern seismographs were invented – or in the last 100 years?
* about 9.5 [p. 51, 55]
c. Where and when was the largest earthquake ever recorded?
* in Chile, 1960 [p. 51, 55]

43. Consider a magnitude 7 earthquake, near its source. [p. 52] a. What is the likely amount of damage? Provide examples.
* most masonry structures destroyed.
b. What is the approximate rate of acceleration during the earthquake, in terms of percentage of the acceleration of gravity or as a multiple of the acceleration of gravity?
* generally > 80 percent of g
c. What is the approximate time of shaking?
* 20-30 seconds (almost a half minute)
d. What is the approximate distance of offset?
* 2-3 meters
e. What is the approximate length of fault broken during the earthquake?
* 50-80 km

44. What is the relationship between the magnitude of an earthquake and the displacement or offset on the fault causing the earthquake?
* A larger magnitude earthquake is generated by a larger fault offset. [pp. 51-52]

45. Provide an example of the amount of fault offset and the magnitude of earthquake generated.
* A 2-3 meter offset would generate a magnitude 7 earthquake. [p. 52]

46. What causes formation of a sand boil? Be specific.
* Liquefaction and compaction of sand at depth expels the excess water to the surface, carrying sand with it. [p. 35]

47. What is liquefaction?
* Earthquake shaking of loose water-bearing sand causes settling and compaction of the sand and expulsion of the water. [p. 52]

48. What hazard does liquefaction pose, and for who or what?
* Differential settling of the ground can collapse or topple buildings on people. [p. 52]

49. Sometimes a single floor of a tall building (above ground floor) collapses in an earthquake even though the floors have identical construction. Why?
* The frequency of shaking of the ground matches the frequency of shaking of the building. [p. 57]

50. What aspect of building design can help keep an intermediate floor of a building from falling over or collapsing in a strong earthquake?
* concrete tightly wrapped with steel [p. 54]

51. If a building is to be built on bedrock, it that good or bad? Why?
* Good. Bedrock shakes at a high frequency, and a tall building flexes at a low frequency so they do not shake in resonance during an earthquake. [p. 57]

52. Where, in the United States and southern Canada, are the largest and most frequent earthquakes and on what fault?
* in western California along the San Andreas Fault [p. 70] 53. What is the type of motion shown by the San Andreas Fault, and why is it there?
* strike slip (or lateral) motion; It is a plate boundary between the Pacific and North American plates. [pp. 70, 72]

54. Where in the Rocky Mountains are the largest and most-frequent earthquakes?
* Utah to eastern Idaho to western Montana [p. 69]

55. Western Washington State sees damaging earthquakes every few years. These earthquakes are 100-200 km inland from the oceanic trench. What types of faults are causing these earthquakes and why are they there?
* Blind thrust faults over the descending subduction zone [p. 68]

56. Where in North America, east of the Rocky Mountains, is the area that has seen earthquakes as large as the largest recorded on the San Andreas Fault in historic time?
* The New Madrid area of southeastern Missouri and adjacent Tennessee [p. 67]

57. There has been at least one highly successful prediction of a major earthquake that saved a very large number of lives. Where and when was that earthquake? What information led to the prediction?
* Haicheng, China, in 1975
* numerous foreshocks and very strange animal behavior [pp. 61-62]

58. For a very long fault such as the San Andreas in California and the subduction zone fault off the west coast of Mexico, what can be used to identify the specific areas that are most likely to generate a large earthquake in the next few decades? Explain briefly how this works.
* Seismic gaps, the gaps in recent earthquake activity, are most likely to be areas of new fault slip and therefore earthquakes. Motion between the two tectonic plates continues; other areas along the boundary have slipped so those that have not slipped recently are most likely to go next. [pp. 66-67]

59. Freeway overpasses often collapse in a strong earthquake, even though their supports are concrete and heavy duty steel reinforcing bars. Why?
* Shaking in the earthquake cracks and crumbles the concrete. That leaves the rebar without lateral support so it bends and the freeway collapses. [p. 76]

60. For years before the 1989 Loma Prieta earthquake south of San Francisco, earthquake seismologists had been concerned about the possibility of a significant earthquake on the San Andreas Fault in that particular area. Other than the fact that it was on the San Andreas Fault, an area that has seen earthquakes in historic time, what lead them to that conclusion?
* It was a seismic gap, an area of fault that has not seen a major earthquake in a long time, even though areas on either side of the gap have had earthquakes. [pp. 66, 85]

61. Some major faults show migration with time (e.g. over the past few hundred or a thousand years), of earthquakes along the fault. Name one such fault – or indicate exactly where it is. When was the last major earthquake on that fault?
* North San Anatolian fault in Turkey [pp. 67-68]
* 1999 [pp. 68, 87]

62. Where is the safest place to be in an earthquake?
* outside (with nothing overhead) [pp. 75, 79, 82]

63. What kinds of structural materials make dangerously weak walls during an earthquake?
* bricks, concrete blocks, stone, or adobe (mud) [pp. 75- 76, 79, 93]

64. What type of feature is commonly used to prevent a building from shaking so much during an earthquake?
* base isolation, for example, rubber pads between the building and its foundation [p. 81]

65. What type of wall strengthening is commonly used to prevent a building from being pushed over laterally during an earthquake?
* diagonal beams [pp. 76-78, 87]

66. Why do the floor or deck beams of parking garages and bridges sometimes fail and fall during an earthquake?
* The horizontal beams, resting on a surface designed to permit thermal expansion and contraction, shake completely off their ledges. [p. 76]

67. Why are building fires so hard to fight after an earthquake?
* broken water mains [pp. 75, 88]

68. Why do so many people in certain regions get sick and often die from sickness after an earthquake?
* broken sewer lines contaminate water supplies [p. 75]

69. Since most deaths in earthquakes are caused by buildings and other structures what types of construction or design are most likely to cause building collapse?
* unbraced (weak) ground-floor garages (or store-fronts)
* too many windows on any floor
* walls not well secured to floors [pp. 76-78, 88]

70. Are wood-frame houses safe or unsafe for the occupants in a moderately strong earthquake? Why?
* Safe for the occupants though the house may be severely damaged.
* Wood structures are flexible and will not generally collapse. [p. 79]

71. In some buildings, often a single floor will collapse. What characteristics of a building will lead to such a single-floor collapse?
* Weak floors; too many windows on a certain floor; garages or storefronts on the ground floor [pp. 76-78, 88]

72. In many old brick or stone buildings how are the floors held up?
* The wood floor beams rest loosely in notches in brick or stone wall. [p. 79]

73. What is unsafe about old brick or stone construction in an earthquake?
* Flexing of the walls can permit the beams to pull out and the floors to fall. [pp. 75-76]

74. If the electricity goes out at night during an earthquake, why should matches or candles not be used to provide light?
* They can ignite gas in the air. [pp. 74-75]

75. Why did the double-deck freeway at the east edge of San Francisco Bay, collapse in the 1989 Loma Prieta earthquake, while nearby structures in low hills to the east suffered little damage?
* sediment shakes more violently (larger amplitudes) during an earthquake than bedrock [pp. 85-86]

76. Why are strong concrete structures sometimes destroyed in a large earthquake, whereas nearby homes built with frame construction (wood lumber) are little damaged?
* concrete is rigid so it will crack and break rather than bending or flexing during earthquake shaking
* wood is flexible and will bend or flex but not break during earthquake shaking [pp. 79-80]

77. List the characteristics of brick walls in buildings 100 to 200 years old that make them vulnerable to collapse during an earthquake. Assume the bricks and the mortar between them are still of good quality.
* Brick walls are rigid and will crack and break rather than flex during an earthquake.
* Old brick walls are structural brick with no internal reinforcing (e.g.: rebar) and no internal frame structure (structural wood walls). [p. 79]

78. Why is it advisable to keep well away from an old brick building during an earthquake, even if the walls do not collapse or crumble and the windows do not break?
* Many have overhanging brick parapets (brick roof overhangs) that break off and fall, crushing anything below. [p. 76]

79. If you were sitting in a parked car next to an old brick building in Boston, St. Louis, or Seattle, what would you do if an earthquake begins shaking violently – and why? Assume that you cannot merely drive away.
* Immediately get out of the car and away from the building because brick walls or parapets can fall and crush the car and any occupants. [pp. 75-76]

80. Apartment buildings with ground-floor garages are often heavily damaged during a strong earthquake. What is the main damage and why?
* Ground-floor garages lack much lateral support because of their wide openings for car entry so that floor falls over laterally. [p. 76]

81. Sometimes a single floor of a building (above the ground floor) collapses in an earthquake, even though the rest of the building is relatively undamaged. What aspect of the design of the building often leads to that collapse?
* a floor with too many windows lacks lateral (or diagonal) support [p. 88]

82. What aspect of building design can help keep an intermediate floor of a building from falling over or collapsing in a strong earthquake?
* diagonal braces or shear walls [pp. 76-78]

83. Although single-story frame (wood construction) houses don’t often collapse in an earthquake, they are sometimes severely damaged. What is the most common cause of such damage, and how could it have been prevented?
* They shake off their foundations.
* The house should be well bolted to the foundation. [pp. 79-80]

84. What are some inexpensive ways to minimize earthquake damage to a single-story frame (wood construction) house?
* Anchor the floor to the foundation.
* Install diagonal braces or sheets of plywood along walls.
* Anchor loose, heavy objects to the floor and walls – for example, water heaters, refrigerators, book cabinets, TV sets. [pp. 76-78, 80-81]

85. Why do the wood floors in old brick buildings sometimes collapse in a strong earthquake, even though the brick walls do not?
* The joists (horizontal wood floor beams) often rest loosely in slots in the brick walls. Lateral shaking of the walls can shake the floor beams out of the slots and permit them to fall. [p. 79]

86. Buildings of different heights shake back and forth at different frequencies. Which shake at higher frequencies, short buildings or tall buildings?
* Short buildings [p. 82]
a. What is the approximate frequency of shaking, or back and forth sway, of a typical 2-story building?
* 5-10 back and forth motions per second (= 5-10 Hz) or 1/5 -1/10 second per back and forth motion.
b. What is the approximate frequency of shaking, or back and forth sway, of a typical 20-story building?
* 0.2 back and forth motions per second (= 0.2 Hz) or 5 seconds per back and forth motion. [p. 82]

87. What can be done to a building, either during construction, or after, to reduce the shaking of a building during an earthquake, and therefore reduce the possibility of severe damage? How does this work?
* Use base isolation pads between the building and its foundation.
* The base isolation pads act as shock absorbers to minimize transfer of ground shaking to the building. [p. 81]

88. Having dug a trench across an active fault, what kinds of information should be collected to determine when the fault last moved, how much it moved, and how frequently?
* amount of offset of layers
* age of soil layers from organic material in the layers - ± sand blows - ± compaction zones [pp. 65-66]

89. Where, in the United States and Canada, is the most seismically active zone? Be specific.
* western fringe of California (onshore), Oregon, Washington, and southwestern Canada (just offshore). [pp. 68-69]

90. What is the nature of the major fault boundary or boundaries in the westernmost United States?
* San Andreas Fault, a strike-slip fault in most of California
* Cascadia subduction zone just offshore of Oregon, Washington, and southwestern Canada [pp. 68, 72]

91. Where are two other significant and active earthquake zones in the continental United States and southern Canada?
* “Intermountain Seismic Belt” (or east edge of the Basin and Range of Utah, southeastern Idaho, to southern Montana) [p. 69]
* New Madrid fault zone (or NE Arkansas, SE Missouri, western Tennessee and western Kentucky.

Unit 4

1. In December, 2004, a pair of closely related natural disasters killed tens of thousands of people. [pp. 97, 116]
a. What was the initial cause of the event, not the one that apparently killed most of the people (the general name for the type of event)?
* a major earthquake
b. Where, specifically, was that initial event?
* off the southwest coast of Sumatra
c. What was the specific kind of that initial event (the variety of the general event noted in part “a”) and the exact nature of it?
* earthquake on a subduction zone; thrust-fault movement
d. What secondary event (the one that killed most of the people) was caused by the initial event noted above (the general name for that type of event)?
* tsunami
e. Explain exactly how the initial event was related to the secondary event.
* The earthquake caused the tsunami when fault movement on the ocean floor suddenly pushed up a large mass of water.

2. Along the San Andreas Fault in California, how many magnitude 7 earthquakes would it take to relieve the same stress in the rocks as one magnitude 8 earthquake?
* about 32 [p. 104]

3. Why are tsunami waves in the open ocean limited in height?
*Fifteen meters is about the largest offset on a fault on the ocean floor so that is also the largest wave height generated. [pp. 99, 102]

4. About how high are the largest earthquake-caused tsunami waves in bays?
* about 30 meters [pp. 103-104]

5. In December, 2004, a giant earthquake struck southeastern Asia. [p. 116]
a. Exactly where was that earthquake?
* just off the southwest coast of Sumatra
b. Why was it there? Explain clearly.
* collision between two tectonic plates
c. What was the magnitude (give the number) of that earthquake?
* 9.0
d. What type of fault movement caused the earthquake?
* thrust-fault movement or subduction-zone movement
e. What was the nature of the boundary marked by that fault?
* a subduction zone
f. How long was the break along that fault?
* approximately 1,200 km
6. Which is the more dangerous location for a tsunami hazard, a straight stretch of open coast, a rocky point, or a bay? Or what is the most hazardous area along a coast? Why?
* a bay [p. 103]
* A bay focuses the wave, forcing the water into a narrower area and causing the wave to rise.

7. What are the approximate tsunami-wave velocities over the continental shelf or near-shore?
* 150-300 km/hour [p. 102]

8. What are the approximate wavelengths of tsunami waves?
* 360 km [p. 102]

9. What are the approximate times between tsunami wave crests?
*15 to 30 minutes [pp. 102, 104-105]

10. On low-lying coastal flats near Anchorage, Alaska, the foundations of buildings dropped below sea level during the giant 1964 earthquake. Decades later, their foundations are again above sea level. [p. 119]
a. What type of tectonic boundary caused the earthquake, and what plate motions are involved?
* subduction zone boundary; the Pacific Ocean floor is descending under the continental margin
b. Explain what tectonic forces led to the ground sinking and later rising, and why? Be specific.
* While the subduction zone fault was locked, descent of the ocean floor pulled down the edge of the continental margin, causing it to slowly buckle up.
* Release of the boundary during the earthquake permitted the ground to drop suddenly and the sea move in.
* Decades later the edge of the continent is again rising since the subduction zone fault is again locked.

11. What type of event has repeatedly generated high tsunami waves in coastal fjords of southeastern Alaska? Be specific.
* large rockfall (or landslide) triggered by a nearby earthquake [p. 100]

12. Why do ships in the open ocean not notice passage of a tsunami wave?
* The time between wave crests is 10 to 30 minutes, and the wave rises and falls less than 15 meters in that time. [p. 102]

13. How dangerous are tsunami in the open ocean, and why?
* They are not dangerous at all. The wave height is so much smaller than the wave length, and the time between wave crests is so long, that you would not even notice them. [p. 102]

14. Which wave of a major tsunami is likely to be the highest—first, fourth, tenth, twentieth?
* fourth [p. 105]

15. At what depth in the ocean do tsunami waves drag on the bottom?
* at all depths because their wavelengths are so large [p. 102]

16. How are tsunami waves in the Atlantic Ocean likely to be generated?
* by collapse of the flank of an oceanic volcano [p. 100]

17. What is often the first indication of the arrival of a tsunami at the coast?
* a rapid drop in sea level [p. 104]

18. If you are at the beach and feel a large earthquake, what should you do, and when should you do it?
* run up slope immediately as far as possible – at least 100 meters [p. 108]

19. When was the last giant tsunami event to affect the west coast of Washington and Oregon?
* in the year 1700 [p. 108]

20. There have not been any very large earthquakes on the subduction zone of the coast of Washington and Oregon in hundreds of years. Explain why not, and what are the implications based on records of past events.
* The subduction zone is locked. The last major earthquake was in 1700.
* Giant earthquakes happen every few hundred years, and the next major earthquake could come any time. [pp. 108-110]

21. Explain how volcanoes can lead to tsunamis.
*Volcanic processes displace large volumes of water; water is driven upward or outward by fast-moving flows of hot volcanic ash or submarine volcanic explosions into a large body of water; volcanoes can also collapse in a giant landslide and spill volcanic material into the ocean. [p. 99]

22. What caused the volcanic explosion in Krakatau in 1883 which lead to a tsunami?
* Studies of pyroclastic flow deposits and seafloor materials suggest that seawater seeping into the volcano interacted with the molten magma to generate huge underwater explosions and upward displacement of a large volume of seawater. [p. 99]

23. What is the average time between each tsunami wave?
*more than 30 minutes [p. 105]

24. What are some measurements which can be taken to minimize damages from tsunami waves?
*Streets and buildings of coastal developments survive better if they run perpendicular to the shore rather than parallel. This limits debris and lets waves penetrate. Building higher, landscaping with vegetation capable of resisting wave erosion and scour, planting trees that permit water to flow between them but slow the waves, creating large ditches or reinforcing concrete walls in front of houses can reduce the impact of the first wave. [p. 106]

25. Explain the two levels of the Pacific Tsunami Warning System and explain how they work.
*Tsunami watch – issued when an earthquake of magnitude 7 or grater is detected somewhere around the Pacific Ocean
Tsunami warning – when a significant tsunami is identified from the buoy system, civil defense officials order evacuation of low-lying areas that are in jeopardy [p. 108]