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6-12

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Earthquakes

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Lesson Summary

Overview

In this lesson, students explore the causes of earthquakes and their impact on the geology of an area and on human societies. They begin by looking at the role tectonic plates play in creating the forces that cause earthquakes, to help them understand why earthquakes occur when and where they do. Hands-on activities illustrate how rocks can withstand a certain amount of stress, but that every material has its breaking point. When rocks break underground, an earthquake occurs. In the last section, students explore the impact earthquakes have on humans and look at the efforts scientists are making to better understand and predict these sometimes deadly events.

Objectives

  • Describe the causes of earthquakes and identify where they are likely to occur
  • Explore the effects earthquakes have on the geology of an area
  • Describe how earthquakes can damage property, and explain why it is important to be able to predict their occurrence

Grade Levels: 6-8, 9-12

Suggested Time

Two to three class periods

Multimedia Resources

Materials

  • A malleable material like Silly Putty© - one piece per student
  • One sugar cube
  • Textbooks (to stack on top of a sugar cube)

Before the Lesson

If possible, arrange computer access for all students to work in pairs.

The Lesson

Part I: Plate Tectonics

1. Ask students to take a few minutes to write their responses to this question: "If someone asked you to describe what's really important about plate tectonics, what two or three things would you tell them?" After a few minutes of independent writing, have students form small groups and compare their ideas. Have groups take turns sharing one of their ideas with the entire class.

Next, show the Plate Tectonics: An Introduction QuickTime Video. After they have watched the video, ask students to compare what they just learned about plate tectonics to what they thought they knew before watching the video. Tell them to jot down their ideas. Then have them share with the class some of what they learned as well as questions they still have. You may also want to show students the Tectonic Plates and Plate Boundaries Flash Interactive to help them visualize plate boundaries on a global scale. They will use these images as reference for later questions.

2. Have students work in pairs as they explore the Mountain Maker, Earth Shaker Flash Interactive. As they work through the activity, have them record their responses to these questions:

  1. What are the three major types of fault boundaries?
  2. How do the plates move relative to each other at each of these boundaries?
  3. Which boundaries do you think produce the most earthquakes?
  4. Which do you think produce the strongest earthquakes, and which produce the weakest?

3. To give students a view of what happens at a fault boundary, show the Earthquakes: The Prehistoric Record QuickTime Video. When finished, ask these questions:

  1. How does the trench help geologists discover where past earthquakes have occurred?
  2. What evidence tells scientists that an earthquake occurred at a particular time?
  3. Where does the force strong enough to break rock come from?
  4. What is the relationship between plate tectonics and earthquakes?

Part II: Stress, Strain, and Vibration

4. Give each student a piece of malleable material like Silly Putty. Ask them to hold their piece in both hands and stretch it by slowly pulling the sides in opposite directions. Point out that they are applying stress to the putty. Have them describe what they observed about the material's response to this action. Next, have students repeat the action, this time pulling as quickly as possible. Again, have them describe what they observed, and ask them to explain why they think the material responded differently to the two actions. Ask them how they would compare this to what they think happens to rocks in an earthquake. How might rocks behave similarly or differently to the putty when force is applied to them? Students may point out some of the obvious differences between rocks and the material. For example, rocks are not nearly as elastic as a material like Silly Putty, and the stresses acting on rocks occur over great periods of time and sometimes at depth.

5. A sugar cube provides a different analogy. Hold up a sugar cube in front of the class, and ask them how they think it will respond to stress and strain. Place the sugar cube on a table where the entire class can see it. Then ask them to predict what might happen if you placed a single textbook on top of it. How about two textbooks? Five textbooks? And so on. Ask them to write down their prediction of how many textbooks it will take to break the sugar cube, how the cube will break, and what else will happen as a result. Test the students' predictions and discuss how the outcome was similar to or different from what they predicted. Ask the students to consider what would happen if rocks underground were put under enough strain to break them.

6. Show the Earthquakes: The Seismograph QuickTime Video for a look at the history of seismology and the development of the seismograph. Ask students these questions:

  1. How do rock fractures cause vibrations?
  2. Which waves arrive first: P-waves or S-waves? Why do you think this is so?
  3. Ask students to demonstrate the motion of each type of wave. Why do you think S-waves tend to do more damage?
  4. How does a seismograph record an earthquake's vibrations?

7. Explain that seismologists can determine how far away an earthquake originated by measuring the difference between the arrival times of the two types of waves. The greater the difference, the greater the distance between the seismograph and the point where the rocks fractured and moved, called the focus. As you describe this scenario, draw a sketch on the board, plotting the position of the seismograph and the location of the earthquake's focus, with seismic waves emanating in all directions from that point.

After describing what you've drawn, ask students to work in pairs to figure out how seismologists determine the location of an earthquake's focus, given that seismograph say nothing about the direction in which seismic waves are moving. When they are finished, have students share their ideas with the class. If students struggle to solve this problem, ask them if they think it would help to use more than one seismic station. How many do they think would be sufficient to determine the location?

8. If students are interested in the seismograph itself and how it works, show the Making a Seismometer QuickTime Video. Although this video may seem below the grade level of your students and does not delve into the science of seismographs, it does cover some of the basic concepts. If you would like your students to pursue this further, there are some Web sites with detailed information on making a seismograph. Here are a couple of examples: Rough Science and Build Your Own Seismograph.

Part III: Earthquake Damage and Prediction

9. The effects of earthquakes on humans can be devastating. As a result, scientists have made predicting their occurrence a priority, especially in heavily populated areas. The Earthquake Prediction QuickTime Video shows some early history of earthquake prediction and some of the destruction earthquakes can cause. Before they watch the video, have students determine which type of plate boundary might be responsible for an earthquake in Japan. Then show the video and have students answer these questions:

  1. Why do you think the older scientist disregarded the younger scientist's prediction?
  2. Do you think the younger scientist had enough evidence for his prediction? What was it?
  3. In both the Japanese and San Francisco earthquakes, fire killed more people than collapsing buildings. Why do you think this was the case?
  4. Do you think fire is always the greatest threat posed by earthquakes? Why or why not?
  5. Should building codes be stricter in areas with such a high probability of earthquakes, to keep people from living in houses like those shown in the video?

10. Earthquake prediction depends a great deal on the history of earthquakes in an area. Perhaps the best-studied region in the world with respect to historical earthquake data is California, and particularly the San Francisco Bay area, where a 1906 earthquake nearly leveled the city. Records showing that large quakes have occurred in approximately 100-year intervals have prompted intensified study of the area. Before showing the Earthquakes: San Francisco QuickTime Video, have students look at the Tectonic Plates and Plate Boundaries Flash Interactive again to determine the type of boundary responsible for earthquakes in San Francisco. Then have students watch the video and answer these questions:

  1. Why do scientists try to find ways to predict earthquakes?
  2. What earthquake pattern have scientists observed in California?
  3. What proactive steps can people in those areas take to prevent loss of life and property in the event of a very strong earthquake?

11. The next resource, Earthquakes: Los Angeles QuickTime Video, shows in detail the unique plate motion in the Los Angeles area. This should give students more insight on the San Andreas Fault system and the effects of the topography on the amount of damage caused in the area. Before showing the video, have students determine which plates are responsible for earthquakes in Los Angeles. Then show the video and ask students these questions:

  1. Describe the plate motion of the Pacific plate and the North American plate along California.
  2. In how many years do scientists expect Los Angeles and San Francisco to cross paths?
  3. Why are there so many small, fractured thrust faults near Los Angeles? What does this mean in terms of earthquake probability for this city?

12. Earthquakes can do more than just shake buildings. When they occur on the ocean floor, they can also cause enormous tsunamis like the one that slammed into the island of Sumatra in the Indian Ocean on December 26, 2004. Have students work in pairs as they explore the Anatomy of a Tsunami Flash Interactive. As they explore, have students record answers to the following questions in their notebooks:

  1. What event caused the tsunami on December 26, 2004? Give details.
  2. What type of plate boundary created the movement that caused this earthquake?
  3. The tsunami caused a great deal of destruction, both to human life and to structures in the coastal areas. What might have been done to minimize the damage caused by the tsunami? How can a government help people cope with disasters such as this?
  4. Do all undersea earthquakes cause tsunamis? What other factors might be important?

13. Volcanoes can also produce earthquakes and tsunamis. Earthquakes produced inside a volcano can actually save lives, since they can warn scientists that the volcano is active and may erupt. Have students continue to work in pairs as they explore the Seismic Signals Flash Interactive. Tell them to record answers to the following in their notebooks:

  1. What is the importance of the volcano-triggered earthquakes?
  2. How do the four different types of volcano-triggered earthquakes help scientists understand what is happening inside the volcano?

Check for Understanding

Have students discuss the following:

  1. Explain why some types of tectonic plate boundaries are more likely than others to generate earthquakes.
  2. Describe how rocks respond to stress and strain and how their reaction relates to earthquake occurrence.
  3. What is the current state of earthquake prediction? What does the future hold for our ability to predict when and where an earthquake will occur?

The Digital Library for Earth System Education (www.dlese.org) offers access to additional resources on this topic.

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