Natural Hazards And Disasters 4th Edition by Donald Hyndman – Test Bank

 

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Chapter 5

TSUNAMI

 

MULTIPLE-CHOICE QUESTIONS

1. What are two main causes of tsunami?

a.       volcano flank collapse and sudden degassing of the mantle from a mid-oceanic ridge

b.      sudden changes in ocean-floor lithosphere velocity and major rockfall into the ocean

c.       asteroid impact into the ocean and collapse of a mid-oceanic ridge

d.      subduction-zone earthquakes and transform-fault earthquakes

e.       subduction-zone earthquakes and landslide into the ocean

 

ANSWER: e

 

2.      Which of the following submarine faults CANNOT cause tsunami waves?

a.       strike-slip faults

b.      normal faults

c.       subduction-zone faults

d.      thrust faults

e.       breakaway faults of major landslides

 

ANSWER: a

 

3. As it enters a bay, the height of a tsunami wave:

a.       rises because more water from each wave is forced into less space.

b.      rises primarily because the bay narrows.

c.       rises because of thermal expansion in the warm water of the bay.

d.      drops because the wave must spread out and slow as it approaches the shore.

e.       drops because as the wave breaks it rushes ahead as a fast-moving swash.

 

ANSWER: a

 

4.      What is the average speed of tsunami waves in the deep ocean?

a.       8-9 km/hour

b.      80-90 km/hour

c.       800-900 km/hour

d.      the speed of an Olympic sprinter

e.       the speed of light

 

ANSWER: c

 

 

5. Tsunami waves nearing the open coast:

a.       slow down because they don’t want to hit the coast too fast.

b.      speed up because they finally begin to feel bottom.

c.       slow down because they drag more on the shallower bottom than in the deep ocean.

d.      speed up because the shallower water forces the leading edge to squirt out ahead.

e.       speed up because the wave launches forward as it breaks.

 

ANSWER: c

 

6.      Although the Atlantic coast of North America experiences few large earthquakes, what specific other event could generate a large tsunami wave that could cause catastrophic damage there?

a.       a giant subduction-zone earthquake near the coast of Africa

b.      a giant subduction-zone earthquake along the eastern edge of North America

c.       a giant transform-fault earthquake on the Atlantic Ocean floor

d.      flank collapse of a volcano in the Canary Islands

e.       flank collapse of Mt. Vesuvius

 

ANSWER: d

 

7.      Why do ships in the open ocean NOT notice passage of a tsunami wave?

a.       The wave goes by so fast that it is gone before anyone notices.

b.      The long time between wave crests dictates that the wave flanks have almost no slope.

c.       Ships are carried on the crest of the wave, so they don’t feel any up or down motion.

d.      Although the wave crests are several hundred meters high, the time between them is several hours, so no one notices.

e.       The wave hits the ship so hard and fast that no one survives to tell about it.

 

ANSWER: b

 

8.      What type of event has repeatedly generated very high tsunami waves in coastal fjords of southeastern Alaska?

a.       a large rockfall into the fjord

b.      a giant subduction-zone earthquake

c.       a massive stratovolcano eruption

d.      an undersea landslide caused by liquefaction of beach sediments

e.       the toe of a fast-moving glacier suddenly surging forward

 

ANSWER: a

 

 

9.      How are tsunami waves in the Atlantic Ocean likely to be generated?

1. by sudden rise of the Mid-Atlantic Ridge
2. by collapse of the flank of an oceanic island volcano
3. by a huge iceberg suddenly grounding on the coast of Newfoundland
4. a subduction-zone earthquake along the plate boundary just off eastern North America
5. a large-magnitude earthquake on a transform fault on the ocean floor

 

ANSWER: b

 

10. What is often the first indication of the arrival of a tsunami at the coast?
11. a giant 100-meter-high wave looming on the horizon
12. a sudden rise in sea level to a crest within about 20 seconds
13. a rapid drop in sea level
14. sea turtles running up the beach
15. people running down to the beach to see the action

 

ANSWER: c

 

11.  What are the approximate wavelengths of tsunami waves?

a.       36 meters

b.      360 meters

c.       3.6 kilometer

d.      36 kilometers

e.       360 kilometers

ANSWER: e

 

12.  What are the approximate times between tsunami wave crests?

a.       30 seconds to 1 minute

b.      1 to 3 minutes

c.       10 to 35 minutes

d.      2 to 3 hours

e.       10 to 12 hours

 

ANSWER: c

 

13.  Which wave of a major tsunami is most likely to be the highest?

a.       first

b.      third or fourth

c.       second

d.      tenth

1. twentieth

 

ANSWER: b

 

 

14. Which of the following is LEAST effective in mitigating the impact of tsunami?

1. Limit building in places potentially flooded by tsunami.
2. Design structures to resist erosion and scour.
3. Orient streets and buildings parallel to waves.
4. Plant trees with deep roots.
5. Build concrete walls in front of houses.

 

ANSWER: c

 

15. If you are at the beach and feel a large earthquake, what should you do, and when should you do it?
16. Wait until the commotion dies down, and turn on the TV to find out where it happened.
17. Immediately call the nearest civil defense office to find out where it happened.
18. Carefully scan the horizon to see if a tsunami wave formed. If you see one, run inland.
19. Immediately run up slope as far as possible.
20. Immediately get into the nearest house for protection.

 

ANSWER: d

 

16. About how high are the largest earthquake-caused tsunami waves in bays?

a.       about 3 meters

b.      about 30 meters

c.       about 300 meters

d.      about 3000 meters

e.       about 30,000 meters

 

ANSWER: b

 

17. In the open sea, tsunami reach an average wave height of about:
18. 1 or 2 meters.
19. 10 meters.
20. 100 meters.
21. 1,000 meters.
22. 10,000 meters.

 

ANSWER: a

 

18. What is the most dangerous location in a tsunami hazard?
19. a straight stretch of coast directly exposed to the open sea
20. a rocky point protruding into the ocean in deep water
21. a bay with a nice sandy beach at its end
22. a small boat in the deep ocean
23. a large boat 300 kilometers offshore

 

ANSWER: c

 

19.  What are the approximate tsunami-wave velocities over the continental shelf or near shore?

a.       about 1 km/hour

b.      about 10 km/hour

c.       about 150-300 km/hour

d.      about 700-800 km/hour

e.       about 2,000 km/hour

 

ANSWER: c

 

20.  About how high are the largest earthquake-caused tsunami waves in the open ocean?

a.       about 15 meters high

b.      about 1 meter high

c.       about 50 meters high

d.      about 300 meters high

e.       about 1 kilometer high

 

ANSWER: a

 

SHORT ANSWER QUESTIONS

1.      In December 2004, a pair of closely related natural disasters killed tens of thousands of people.

1. 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)?

ANSWER: A major earthquake.

1. Where, specifically, was that initial event?

ANSWER: Northern Sumatra.

1. 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?

ANSWER: Earthquake on a subduction zone; thrust-fault movement.

1. 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)?

ANSWER: Tsunami.

1. Explain exactly how the initial event was related to the secondary event.

ANSWER: The earthquake caused the tsunami when fault movement on the ocean floor suddenly pushed up a large mass of water.

1. What was the magnitude (give the number) of that earthquake?

ANSWER: Magnitude 9.15

1. What was the nature of the boundary marked by that fault?

ANSWER: A subduction zone.

1. How long was the break along that fault?

ANSWER: Approximately 1,200 km.

2.      Why are tsunami waves in the open ocean limited in height?

ANSWER: Fifteen meters is about the largest offset on a fault on the ocean floor, so that is also the largest wave height generated.

3.      About how high are the largest earthquake-caused tsunami waves in bays?

ANSWER: About 30 meters.

4.      Which is the more dangerous location in a tsunami hazard: a straight stretch of open coast, a rocky point, or a bay? Why?

ANSWER: A bay, because it focuses the wave, forcing the water into a narrower area and causing the wave to rise.

5. When are tsunami warning systems ineffective?

ANSWER: When tsunami are generated by nearby earthquakes, there is not enough time for a warning system to work effectively.

6.      Why do ships in the open ocean not notice passage of a tsunami wave?

ANSWER: The time between wave crests is 10 to 30 minutes, and the wave rises and falls less than 15 meters in that time.

7.      How dangerous are tsunami in the open ocean, and why?

ANSWER: 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.

8. Explain how volcanoes can lead to tsunamis.

ANSWER: 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.

9. What are some measures that can be taken to minimize damages from tsunami waves?

ANSWER: 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, and creating large ditches or reinforcing concrete walls in front of houses can reduce the impact of the first wave.

10. Explain the two levels of the Pacific Tsunami Warning System and how they work.

ANSWER: Tsunami watch: issued when an earthquake of magnitude 7 or greater 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.

 

CRITICAL THINKING ESSAY QUESTIONS

1. Discuss options to mitigate the effects of a tsunami on an area.

ANSWER: Tsunami hazards can be mitigated by land-use zoning that limits building to elevations above those that would potentially be flooded by a tsunami. If lower elevations are to be developed, potential tsunami impact should be taken into account during planning.  Coastal developments that orient streets and buildings perpendicular to waves tend to survive better than those aligned parallel to the shore. This layout allows waves to penetrate farther and dissipate as they flow through open streets, limiting debris impact. Structures should also be designed to resist erosion and scour. Landscaping with vegetation capable of resisting wave erosion and scour can help. Trees can slow waves while permitting water to flow between them, but they need to be well rooted or they can themselves become projectiles. A large ditch or reinforced concrete wall placed in front of houses can help reduce the impact of the first wave and may provide a little extra evacuation time.

2. Explain why swimming during a tsunami is not the best option to save your life. Discuss waves and effects of a tsunami on its surroundings.

ANSWER: In addition to drowning in an incoming wave, tsunami dangers include death or injury from being thrown against solid objects or hit by debris, severe abrasions from dragging along the ground at high speeds, and being carried out to sea when a wave recedes. Hazardous fragments can include boards and other remains of houses, trees, and cars. Even when a wave slows to 55 km/hour as it drags on shallow bottom, it is much too fast to outrun.

3. Decide which natural occurrence would create the most catastrophic tsunami and why. Describe the location that would be the most devastating.

ANSWER: The impact of a large asteroid into the ocean would generate huge tsunami radiating outward from the impact site—much as happens with any other tsunami. The average frequency of such events is low, but a 1-km asteroid falling into a 5-km-deep ocean might generate a 3-km-deep cavity. Cavity walls would collapse at speeds up to supersonic, sending a plume high into the atmosphere. Initial kilometer-high waves would crest, break, and interfere with one another. Waves with widely varying frequencies would radiate outward. The behavior of such complex waves is not well understood, but they are thought to decrease fairly rapidly in size away from the impact site. The different wave frequencies would, however, interfere and locally pile up on one another to cause immense run-ups at the shore.

4. In the case of the Sumatra Tsunami in December 2004, what was the initial event that caused the tsunami? How did certain people’s reactions save lives? What can education do to help further people’s knowledge about this natural disaster?

ANSWER: The subduction boundary had been locked for hundreds of years, causing the overriding Burma Plate to slowly bulge like a bent stick; it finally slipped to cause a magnitude 9.15 earthquake on December 26, 2004. Given the size of the earthquake, offset on the thrust plane was some 15 meters, with the seafloor rising several meters. The subduction zone broke suddenly, extending north over approximately 1,200 kilometers of its length, shaking violently for as long as eight minutes. A ten-year-old girl who had recently learned about tsunami in school saw the sea recede before the first wave and yelled to those around her to run uphill. A dock worker on a remote Indian island had seen a television special on tsunami, felt the earthquake, and ran to warn those in a nearby community that giant waves were coming. Together, these two saved more than 1,500 people. Knowledge of hazard processes can save lives. As with all hazards, public education could have done much to save lives. There was a lack of knowledge, even among officials, that a large earthquake could generate large tsunami.

5. Discuss the well-known tsunami that are supposed to occur in the future and the effects of each.

ANSWER: The largest tsunami that are likely to impact people are caused by giant earthquakes in subduction zones and, less frequently, flank collapse of an island shield volcano. Three cases of known major hazards that almost certainly will affect North America in the future include: (1) giant tsunami following a huge subduction-zone earthquake in the Pacific Northwest, (2) catastrophic flank collapse of a shield volcano on the big island of Hawaii, and (3) catastrophic flank collapse of a shield volcano on the Canary Islands in the Atlantic Ocean.