Chapter 5 tsunami: the great wave



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CHAPTER 5

TSUNAMI: THE GREAT WAVE




Correct answers are indicated by an asterisk, both in short answer and multiple choice questions.





True or False questions can be easily prepared from multiple choice questions

Web Sites:

http://www.asiantsunamivideos.com/ - Video Clips of the Asian Tsunami

http://www.ngdc.noaa.gov/seg/hazard/tsu.shtml

http://www.pbs.org/wnet/savageearth/tsunami/index.html

http://www.pmel.noaa.gov/tsunami/

http://www.tsunami.noaa.gov/

http://www.tsunami.org/

http://www1.tpgi.com.au/users/tps-seti/spacegd7.html


Videos:

Geologic Hazards Slide Sets: Tsunami: National Geophysical Data Center, NOAA, Dept. 930, 325 Broadway, Boulder, CO 80303




CHAPTER 5: End of Chapter Answers





  1. What are three of the main causes of tsunami?

* sub-sea earthquakes, landslides into a body of water, volcano-flank collapse into a water body, sub-sea volcanic eruptions, asteroid impact into an ocean




  1. Of the three main types of fault movements—strike-slip faults, normal faults, and thrust faults—which can and which cannot cause tsunami? Why?

* strike-slip cannot, normal and thrust faults can

* strike-slip faults do not displace water, normal and thrust do displace water




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

* about 15 meters



  1. How does the height of a tsunami wave change as it enters a bay? Why?

* wave height increases. Since more water of each wave is forced into a narrower area the height must rise to accommodate the volume of water.




  1. How many tsunami waves are generated by one earthquake?

* ten or more


  1. How fast do tsunami waves tend to move in the deep ocean?

* about 760 km/hour (or 475 mi/hour)





  1. Do tsunami speed up or slow down at the coast? Why?

* They slow down because the waves drag on, and are held back by, the shallower bottom.




  1. Why is even the side of an island away from the source earthquake not safe from a tsunami?

* because the waves can refract around the coast of an island to strike the opposite side.




  1. For a subduction-zone earthquake off the coast of Oregon or Washington, how long would it take for a tsunami wave to first reach the coast

* less than 20 minutes




  1. Because 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?

* flank collapse of a volcano in the Canary Islands or landslide of a large section of the continental shelf




  1. What specific evidence is there for tsunami events having struck bays on the coasts of Washington and Oregon?

* flattened and drowned forests, massive sand sheets that cover coastal peat soils.


  1. What is a seiche? (be specific)

* a wave that sloshes back and forth from end to end of an enclosed body of water.



CHAPTER 5: Short answer questions:




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


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 (concisely and in detail) 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.


  1. 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




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

*15 meters is about the largest offset on a fault on the ocean floor so that is also the largest wave height generated.


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

* about 30 meters





  1. In December, 2004, a giant earthquake struck southeastern Asia.

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 1200 km

g. Which country or countries would have been severely damaged by the earthquake?



* Sumatra


  1. 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?]

* a bay

a. Why?

* a bay focuses the wave, forcing the water into a narrower area and causing the wave to rise.


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

* about 100 km/hour


  1. What are the approximate wavelengths of tsunami waves?

* 200 km




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

* 10 to 35 minutes




  1. 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.

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 exactly what tectonic forces led to the ground sinking and later rising, and why? Be brief and to the point.



* 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.

  1. 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


  1. 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.




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

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.


  1. Which wave of a major tsunami is likely to be the highest? 1st, 4th, 10th, 20th?

* 4th


  1. At what depth in the ocean do tsunami waves drag on bottom?


* at all depths because their wavelengths are so large


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


* by collapse of the flank of an oceanic volcano


  1. Why are even good swimmers often killed by tsunami waves?

* impact of debris carried in the waves or abrasion by being dragged along the bottom at high rates of speed


  1. What is often the first indication of the arrival of a tsunami at the coast?

* a rapid drop in sea level


  1. 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.


  1. What causes a seiche?

* strong prolonged wind in one direction across a lake or inland sea, a strong change in atmospheric pressure at one end of a lake or inland sea, a large landslide into one end of a lake or inland sea.

  1. About how many years are there between giant tsunami from the Cascadia subduction zone that inundate the west coast of North America?


* 300 to 900


  1. When was the last giant tsunami event to affect the west coast of Washington and Oregon?


* in the year 1700

  1. 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.

* subduction zone is locked

* last major earthquake was in 1700.

* giant earthquakes every few hundred years

* next major earthquake could come any time




CHAPTER 5: - Multiple choice questions



  1. Two of the main causes of tsunami include:
  1. volcano-flank collapse & sudden degassing of the mantle from a mid-oceanic ridge

  2. sudden changes in ocean-floor lithosphere velocity & major rockfall into the ocean

  3. asteroid impact into the ocean & collapse of a mid-oceanic ridge.

  4. subduction-zone earthquakes and transform-fault earthquakes

  5. * subduction-zone earthquakes & landslide into the ocean





  1. What was the first of two catastrophic events in Sumatra in December 2004 that killed tens of thousands of people. That first event was:

  1. a giant volcanic eruption

  2. a giant tidal surge accompanying a late-season typhoon

  3. the largest landslide in historic time

  4. a continent-continent collision earthquake

  5. * a subduction zone earthquake


  1. What was the second of two catastrophic events in Sumatra in December 2004 that killed many tens of thousands of people. That second event was:

  1. * a series of giant tsunami waves

  2. a giant volcanic eruption

  3. a catastrophic mudflow from the flank of the volcano

  4. a giant landslide triggered by the first event

  5. the largest south-Asian flood on record




  1. The catastrophic secondary event following the December 2004 earthquake in Sumatra killed thousands of people in India about:

  1. 10 minutes after the earthquake

  2. 1 hour after the earthquake

  3. * 3 hours after the earthquake

  4. 24 hours after the earthquake

  5. 10 days after the earthquake



  1. The following submarine faults cannot cause tsunami waves.

  1. * strike-slip faults

  2. normal faults

  3. subduction-zone faults

  4. thrust faults

  5. breakaway faults of major landslides




  1. The largest earthquake-caused tsunami waves in the open ocean are about:

  1. * about 15 meters high

  2. about 1 meter high

  3. about 50 meters high

  4. about 300 meters high

  5. about 1 kilometer high





  1. As it enters a bay, the height of a tsunami wave:
  1. * rises because more water of each wave is forced into less space

  2. rises primarily because the bay narrows

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

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

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




  1. The number of tsunami waves generated by one earthquake is generally:

  1. one

  2. * 10 or 12

  3. about 50

  4. about 100-120

  5. 1000 or more.



  1. The speed of tsunami waves in the deep ocean is generally about:

  1. 7-8 km/hour

  2. 70-80 km/hour

  3. * 700-800 km/hour

  4. the speed of an Olympic sprinter

  5. the speed of light.





  1. Tsunami waves nearing the open coast:
  1. slow down because they don’t want to hit the coast too fast

  2. slow down because they finally begin to feel bottom

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

  4. speed up because the shallowing water forces the leading edge to squirt out ahead

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




  1. For a subduction-zone earthquake off the coast of Oregon or Washington, how long would it take for a tsunami wave to first reach the coast?

  1. about 20 seconds

  2. about 2 minutes

  3. * less than 20 minutes

  4. about 2 hours

  5. about 20 hours.




  1. 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?

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

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

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

  4. * flank collapse of a volcano in the Canary Islands

  5. flank collapse of Mt. Vesuvius.




  1. What specific evidence is there for multiple tsunami events having struck coastal bays of Washington and Oregon?

  1. debris from houses buried in sand

  2. remains of houses partly buried in sand at the head of coastal bays

  3. coastal terraces show a series of landslides, all of which happened at the same time

  4. well-layered sands a few meters thick and containing fragments of shattered trees .

  5. * beaches in bays with stumps of big trees covered by unlayered beach sand



  1. No very large thrust-motion earthquakes have occurred along the coast of Washington

and Oregon in hundreds of years because.

  1. that part of the coast does not get large earthquakes

  2. * the fault zone offshore has been locked the whole time

  3. that region is subjected to strike-slip faults

  4. motion of faults in that region does not generate earthquakes because they are lubricated by seawater


  5. magma in the active volcanoes dominates tectonic activity in the Pacific Northwest and

  6. fault motion is insignificant.



  1. What is a seiche?

  1. * a weather-controlled wave that sloshes back and forth in an enclosed body of water

  2. a tsunami generated by landslide in to a body of water

  3. a tsunami wave that sweeps up onto a coastal hillside and back down with the debris that it stripped off the hill

  4. the process that creates high- and low-tide shoreline, in a large lake

  5. a “near-field” tsunami wave that reaches shore within minutes of the earthquake that generated it




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

  1. about 3 meters

  2. * about 30 meters

  3. about 300 meters

  4. about 3000 meters

  5. about 30,000 meters.





  1. In the open sea, tsunami:

  1. are giant breaking waves that are dangerous to ships

  2. are slow-moving waves, slower than the swell

  3. * are long-period waves, often of 15-20 minutes

  4. have short wavelengths of 100-200 feet, similar to wind waves




  1. In the open sea, tsunami reach an average wave height of about:

  1. * 1 meter

  2. 10 meters

  3. 100 meters

  4. 1,000 meters




  1. Which is the most dangerous location for a tsunami hazard?

  1. a straight stretch of coast directly exposed to the open sea

  2. a rocky point protruding into the ocean in deep water

  3. * a bay with a nice sandy beach at its end

  4. a small boat in the deep ocean

  5. a large boat 300 kilometers offshore.



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

  1. about 1 km/hour.

  2. about 10 km/hour

  3. * about 100 km/hour

  4. about 700-800 km/hour

  5. about 2000 km/hour




  1. What are the approximate wavelengths of tsunami waves?

  1. 10 meters.

  2. 100 meters

  3. 1 kilometer

  4. 10 kilometers

  5. * 200 kilometers




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

  1. 30 seconds to 1 minute

  2. 1 to 3 minutes

  3. * 10 to 35 minutes

  4. 2 to 3 hours

  5. 10 to 12 hours.




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

  1. the wave goes by so fast that is gone before anyone notices

  2. * the long time between wave crests dictates that the wave flanks have almost no slope

  3. ships are carried on the crest of the wave so they don’t feel any up or down motion

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

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




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

  1. 1st

  2. * 3rd or 4th

  3. 8th

  4. 25th




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

  1. * a large rockfall into the fjord

  2. a giant subduction zone earthquake

  3. a massive stratovolcano eruption

  4. an undersea landslide caused by liquefaction of beach sediments

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



  1. 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




  1. Why are even good swimmers often killed by tsunami waves?

  1. they freak out on seeing the giant wave and die of a heart attack

  2. they are carried hundreds of kilometers offshore in the receding wave and can’t swim that far

  3. * they are impacted by debris carried in the waves

  4. they are held down by the incredible undertow and drown

  5. they are pushed so deep under the water surface that they get the bends




  1. What is often the first indication of the arrival of a tsunami at the coast?

  1. a giant 100-meter-high wave looming on the horizon

  2. a sudden rise in sea level to a crest within about 20 seconds

  3. * a rapid drop in sea level

  4. sea turtles running up the beach

  5. people running down to the beach to see the action




  1. If you are at the beach and feel a large earthquake, what should you do and when should you do it?

  1. wait until the commotion dies down, turn on the TV to find out where it happened

  2. immediately call the nearest civil defense office to find out where it happened

  3. carefully scan the horizon to see if a tsunami wave formed. If you see one run inland

  4. * immediately run up slope as far as possible

  5. immediately get into the nearest house for protection




  1. Which of the following would not cause a seiche?

  1. * strong prolonged wind in one direction across a lake or inland sea,

  2. a strong change in atmospheric pressure at one end of a lake or inland sea,

  3. a large landslide into one end of a lake or inland sea

  4. a magnitude 8 earthquake only 100 km away from a lake of inland sea

  5. a massive volcanic explosion under one end of a lake or inland sea.


  1. What specific evidence is there for major subduction-zone earthquakes along the coast of Washington and Oregon?

  1. Coastal explorers recorded their experiences

  2. Early dwellings along the continental margin have been found submerged in coastal bays

  3. Some of the earliest seismographs used by Charles Richter recorded them

  4. Huge landslide blocks from coastal cliffs crushed old forests


e. * Sand from ancient tsunamis is preserved in coastal bays

  1. About how many years are there between giant tsunami from the Cascadia subduction zone that inundates the west coast of North America?


  1. generally between 30 and 50

  2. always close to 300

  3. * between 300 and 900

  4. we don’t know since we know of only one about 1300 years ago

  5. about 600,000, on average



  1. When was the last giant tsunami event to affect the west coast of Washington and Oregon?


  1. in 1963

  2. * in 1700

  3. about 1300 years ago

  4. about 6800 years ago

  5. about 600,000 years ago



  1. Why does the ground along the coast commonly drop in elevation during a major subduction-zone earthquake?

  1. The coast is suddenly pulled down by the subduction zone.

  2. The oceanic crust suddenly moved, dragging the continental edge with it.

  3. Giant earthquake-caused landslides drop large sections of coast into the ocean.

  4. * The coastal part of the continental crust that was flexed up snaps down.

  5. Liquefaction of coastal sands caused sudden settling of the beach sands.





  1. Why did the coastal flats at sea level suddenly drop below sea level during the 1964 Anchorage earthquake, then slowly rose above sea level in the next few decades?

  1. it was dragged down by the subduction zone when it slipped

  2. * the coastal bulge created when the subduction zone was locked then slipped

  3. the stationary oceanic crust suddenly moved down the subduction zone

  4. a giant landslide loosened by the earthquake dropped the coast a few feet

  5. wet coastal sands liquefied during the earthquake



  1. What factors lead to the occasional collapse of giant slices of Hawaiian volcanoes?

  1. * Heavy basalt lava flows above sea level overlie weak, rubbly basalt below sea level; magma injection provides a weak zone that marks the slide surface.

  2. Large magnitude earthquakes shake so hard that some of the basalt pile collapses

  3. Rising basalt magma pushes up a giant bulge that collapses

  4. The solid prong of the rising mantle plume pushes a large slice of the volcano off to the side

  5. Waves on the windward side of the volcano undercut a giant cliff that collapses into the sea



  1. Aside from being atop the collapsing slab, what other major hazard or hazards can accompany collapse of such a giant slab?

  1. a giant tsunami that would destroy coastal communities in western North America; Hawaii would be okay since the wave is moving away from it

  2. * a giant tsunami and a major earthquake that would destroy coastal communities on Hawaii and elsewhere

  3. the slab would creep down slowly enough that people would have ample time to move.

  4. removal of the load of rock decreases pressure on the magma below, releasing the steam to cause a massive and explosive eruption


the landslide surface cuts into the magma chamber, permitting it to erupt in huge volume

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