The Power of a Tsunami

Tsunamis are destructive waves that endanger the lives of coastal residents.

Introduction

Just a few days after Christmas 2004, an undersea earthquake in the Indian Ocean produced a series of gargantuan waves that slammed into the coasts of 11 countries. People were washed out to sea, drowned in their homes, and slammed by water-borne debris. According to the National Oceanic and Atmospheric Administration (NOAA), more than 230,000 people lost their lives. Most of those who survived were left homeless, all of their possessions destroyed by forces never before experienced. The waves responsible for this disaster were generated by a strong earthquake near the west coast of Sumatra, an Indonesian island.

The quake registered 9.0 on the Richter scale. Waves generated by this quake were not ordinary ocean waves, but tsunamis, a Japanese term that translates as “harbor wave.” The Japanese characters that “spell” tsunami are shown in Figure 1. A tsunami is a series of waves created by sudden motions of the ocean floor. The sudden motion could be caused by an earthquake or landslide.

These disturbances displace large volumes of water, giving them a hard, rapid shove and propelling them into motion with tremendous energy. Tsunamis travel out in all directions from the point of disturbance, moving as fast as 600 miles per hour (mph) (700 kilometers per hour [kph]), faster than a Boeing 747. They can reach nearby coastlines in minutes and distant shores within a few hours. As a tsunami moves up on shore, it gains height and runs up on the land.

Tsunamis are very different from ordinary wind-generated ocean waves. Wind waves affect only the upper surface of the ocean and never travel more than 60 mph (96.6 kph). Ordinary waves rarely have a wavelength, the distance from one wave to the next, more than 1,000 feet (ft) (304 meters [m]). However, tsunami wavelengths may be 329,000 ft, (100 kilometers [km]) long. The surges produced by tsunamis may reach up to

Figure 1

How tsunami is written in Japanese characters 100 ft (30 m) and have enough force to pick up cars and trucks, smash homes and businesses, and roll boulders. In this experiment, you will learn more about tsunamis and what can be done to protect coastal residents.

Time Required

45 minutes

Materials

• access to the Internet

• science notebook

Please review and follow the safety guidelines at the beginning of this volume.

Procedure

1. Access the National Oceanic and Atmospheric Association (NOAA) Web site, http://www.tsunami.noaa.gov/tsunami_story.html, mwhich provides information on tsunamis and shows animations of tsunamis in action. Read this Web site and answer Analysis questions 1 through 7.
2. Access the NOAA Web site, http://www.publicaffairs.noaa.gov/ grounders/tsunamis.html. Read the material and answer Analysis questions 8 through 11.
3. Search the Internet for information on five historic tsunamis. Use the information you find to complete the data table.
4. Use what you learned in your Internet searches to write an advertisement for a local newspaper that asks people to support efforts to create and maintain an Indian Ocean tsunami warning system. In your ad, explain why the warning system is needed. Make the ad interesting and attractive.

Analysis

1. What is a tsunami?

2. What are two causes of tsunamis?

3. Give an example of a tsunami produced by each of the causes in question 2.

4. What determines the height of a tsunami?

5. What determines the wavelength of a tsunami?

6. View the animation in Figure 1 of the NOAA Web site. What does it show?

7. View the animation in Figure 2 of the NOAA Web site. What does it show?

8. How are tsunamis different from ordinary ocean waves?

9. In the deep ocean, at what speed can a tsunami travel?

10. Why do tsunami waves become dangerous as they approach land?

11. Where does a tsunami’s force cause the most destruction?

Tsunamis are serious threats to populations living along the coasts. These gigantic waves can inundate communities, sweeping away residents, cars, and buildings within minutes. As protection, a Tsunami Warning System (TWS), supported by 26 countries, was established in 1965. The purpose of the TWS is to monitor seismic activity and wave height throughout the Pacific Ocean.

The TWS evaluates data for tsunami generating activity and provides warning information when appropriate so that coastal residents can react quickly and move to safety. NOAA and the National Weather Service operate two of the stations, one in Alaska and one in Hawaii. If the size and location of seismic activity is such that it could generate a tsunami, a warning goes out to effected coastal regions. Warning includes information on the wave’s estimated arrival time and the potential wave size.

The Pacific Ocean-style warning system is not in place across the world. One reason that the tsunami of 2004 claimed so many lives is that the existing TWS does not include the Indian Ocean, a region that has seen very few tidal waves in the past. Indian officials had no way of knowing that a life-threatening wave had been generated because they lacked the equipment to gather required information. What is more, they do not have the funds to establish such a system.

Most tsunamis occur in the Pacific Ocean, primarily in the region known as the Ring of Fire, a geologically active zone shown in Figure 2. Earthquakes registering 7 or more on the Richter scale occur several times a year in the Pacific. The map shows the occurrence of some historically important earthquakes.

Figure 2 Ring of Fire

The Ring of Fire is located along the borders of the Pacific tectonic plate and several continental plates. Plates, large sections of the Earth’s crust, travel slowly, moving only 1/2 to 1 inch (in.) (about 1 1/2 to 3 centimeters [cm]) each year. The plates are like giant rafts floating on the Earth’s inner layer of molten mantle. As they travel, some plates move apart, but others scrape together or collide. In a collision, one plate is forced down underneath another. The Pacific Ring of Fire is an area where many plates are being forced downward, a process known as subduction.

In these areas, subducted plates are pushed beneath other plates with such tremendous energy and pressure that they melt into magma. Figure 3 shows a plate that is moving from left to right. As it moves, it is pushed beneath another plate. Molten lava created by the subduction rises to the surface as volcanoes. This type of geologic activity to expected to continue generating tsunamis in the Pacific Ocean.

Figure 3