Saturday, October 22, 2011

Hereafter: Tsunami, Natural Disasters

After watching this scene and thinking about the consequences of tsunamis in Thailand and in Japan, I felt the urge to prepare an activity to talk about it. Tsunamis don't take place in Brazil, so it is very shocking and hard to believe that such a tragedy is always about to happen somewhere in the world.

I. Prepare small posters with the information presented in the items below and tape them around the classroom. Give each students (or pair / group of students) the questions below. Ask them to guess what the answers are. Finally, The students look for their answers by reading the information displayed in the posters and read the class what they found out about tsunamis.

1. What is a tsunami?
  • A tsunami is a series of waves with a long wavelength and period (time between crests) generated by a large, impulsive displacement of sea water.
  • Time between crests of the wave can vary from a few minutes to over an hour.
  • Tsunamis are often incorrectly called tidal waves; they have no relation to the daily ocean tides.
2. How are tsunamis generated?
  • Tsunamis are generated by any large, impulsive displacement of the sea level.
  • Tsunamis are also triggered by landslides into or under the water surface, and can be generated by volcanic activity and meteorite impacts.
3. How often do tsunamis occur?
  • On the average, two tsunamis occur per year throughout the world which inflict damage near the source.
  • Approximately every 15 years a destructive, ocean-wide tsunami occurs.
4. Can strike-slip (horizontal motion) earthquakes trigger tsunamis?
  • Yes, approximately 15% of all damaging tsunamis were triggered by strike-slip earthquakes.
  • This type of earthquake is less likely to trigger a tsunami than one with vertical motion.
  • The waves are likely generated by associated landslides or motion of a sloping bathymetric feature.
  • Tsunamis generated by strike-slip earthquakes normally affect regions near the source only.
5. What does the word tsunami mean?
  • Tsunami (soo-NAH-mee) is a Japanese word meaning harbor wave.
6. How fast do tsunamis travel?
  • Tsunami velocity depends on the depth of water through which it travels (velocity equals the square root of the product of the water depth times the acceleration of gravity).
  • Tsunamis travel approximately 475 mph in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph.
  • A tsunami travels from the central Aleutian Is. to Hawaii in about 5 hours and to California in about 6 hours, or from the Portugal coast to North Carolina in about 8.5 hours.
7. How big is a tsunami?
  • Tsunamis range in size from inches to over a hundred feet.
  • In deep water (greater than 600 feet), tsunamis are rarely over 3 feet and will not be noticed by ships due to their long period (time between crests).
  • As tsunamis propagate into shallow water, the wave height can increase by over 10 times.
  • Tsunami heights vary greatly along a coast. The waves can be amplified by shoreline and bathymetric (sea floor) features.
  • A large tsunami can flood low-lying coastal land over a mile from the coast.
8. What does a tsunami look like when it reaches shore?
  • Normally, a tsunami appears as a rapidly advancing or receding tide.
  • It some cases a bore (wall of water) or series of breaking waves may form.
9. How is a tsunami different from a wind-generated wave?
  • Wind-generated waves usually have periods (time between crests) between 5 and 20 seconds. Tsunami periods normally range from 5 to 60 minutes.
  • Wind-generated waves break as they shoal and lose energy offshore. Tsunamis act more like a flooding wave. A twenty foot tsunami is a twenty foot rise in sea level.

Information provided by the informative site:

II. Watch the segment from the movie Hereafter and discuss the questions in pairs:


1. Describe the scene.

2. What did you see in the segment that you learned during the previous task?

3. What happened to the character of the American woman in the segment?

4. What are the most famous tsunamis you have heard of?

5. What do you think should be done in order not to have so many casualties because of tsunamis?

6. Are you afraid of tsunamis where you live? Do you face any natural disasters threats?


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