Stick-Slip Movement

 

This section is intended to help students observe how stick-slip movement works. Instructions on how to build a motorized version of the following experiment as well as data produced from this model are available.

Motorized version

"Can Earthquakes Be Predicted?"
Some background information

Activities page

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What Happens When the Earth Quakes?

In this unit students will move beyond their own personal survival and that of their community, the focus of Unit 1, to the big picture of earthquakes in space and time. Since the Seismic Sleuths curriculum is intended to supplement, and not to replace, your school’s own syllabus, it sketches this big picture without filling in all the basic earth science background. Your preparation to teach these lessons must begin with an assessment of your students’ readiness. If they have no familiarity with rocks and minerals or with faulting and other processes that form landscapes, you may need to provided a brief introduction from the first few chapters of a high school geology or earth science textbook.

Unit 2 begins with a hands-on activity that models what happens when the stresses accumulated at a fault are released in an earthquake. Using a box, a board, sandpaper, and other simple materials, students apply scientific method and basic math skills to measure movement, calculate averages, and plot their information on a graph.

The second lesson includes three activities and an overview of what is now known about Earth’s ever-shifting surface and its layered inner structure. In the first activity, students will reproduce the magnetic evidence for the migration of Earth’s poles in the course of tectonic movement. In the second, they see how this record is written in the rocks at mid-ocean ridges. In the third, they create a map showing the arrangement of the continents 120 million years ago, and compare it with the map of the world today. As students consider several alternative explanations for tectonic plate movement, remind them that earth science, like the Earth it studies, is constantly in motion. Scientific knowledge moves forward through questioning and the development of hypotheses into theories; its goal is never to provide dogmatic answers.

The third lesson begins with an exercise in which students contrast the small scope of historic time with the vastness of geologic time. In the second activity, Paleoseismology, they simulate the techniques seismologists use to read the record of relatively recent earthquakes.

The amount of damage an earthquake causes depends on the strength and duration of the earthquake, on population density, on methods of construction (to be dealt with in Unit 4), and on the geophysical/geological characteristics of the impacted area. Lesson 4 progresses to three of the most potentially destructive earthquake effects: liquefaction, landslides, and tsunami. Each occurs when a seismic shock impacts an area with certain physical characteristics. Lesson 5 underlines the importance of site, as students interpret maps highlighting different features of the landscape. They will draw on their new knowledge to make additions to the local map they began in Unit 1.
 
 

Stick-Slip Movement

Rationale

Students will operate a model to observe the type of motion that occurs at a fault during an earthquake and explore the effects of several variables.

Focus Questions

How much energy will a fault store before it fails?

Is this quantity constant for all faults?

Objectives

Students will:

1. Model the frictional forces involved in the movements of a fault.
2. Measure movement, calculate averages, and plot this information on a graph.
3. Explore the variables of fault strength vs. energy stored.

Materials

For each small group

• 1 copy of Master Stick-Slip Data Sheet
• 4 sandpaper sheets, 60, 120, and 400 grit
• Scissors
• Strapping tape
• 1-lb weight
• 8 thumbtacks
• A box of rubber bands
• 2 large paper clips
• Yardstick or meter stick
• 2 m of string
• Large dowel
• Marking pen
• Scales
• Pine board, approx. 2.5cm x 30cm x 1.8m
• Several books to support one end of the board
• Protractor for measuring angles

 

Procedure

Teacher Preparation

To assure success, construct the model ahead of time and rehearse the activity. Then arrange materials for student models in a convenient place.

1. Introduction
   
   Elicit a definition of fault from the class, supplementing students’ information as necessary until the essential elements have been covered.

   Explain to students that when an earthquake occurs and movement begins on a fault plane, the movement will not proceed smoothly away from the focus. Any change in the amount of friction along the fault will cause the fault movement to be irregular. This includes changes along the length of the fault and with depth, changes in rock type and strength along the fault, and natural barriers to movement, such as changes in the direction of the fault or roughness over the surface of the fault plane.

   Rupture along a fault typically occurs by fits and starts, in a type of sporadic motion that geologists call stick-slip. As energy builds up, the rock on either side of the fault will store the energy until its force exceeds the strength of the fault. When the residual strength of the fault is exceeded, an earthquake will occur. Movement of the fault will continue until the failure reaches an area where the strength of the rock is great enough to prevent further rupture. In this manner, some of the energy stored in the rock, but not all of it, will be released by frictional heating on the fault, the crushing of rock, and the propagation of earthquake waves.

2. Lesson Development
   1. Divide the class into working groups of at least four students each. Distribute one copy of Master 2.1a, Stick-Slip Data Sheet, to each group. Tell students that the are going to model a process, record data for each trial, and then vary the process, changing only one variable at a time.
   2. Allow groups to assemble their materials, then give these directions:
      1. Fold each piece of 120-grit sandpaper in half lengthwise and cut, to produce eight strips of sandpaper, each 11.5 cm x 28 cm (4.5 in. x 11 in.) in size.
      2. Wrap one of the strips around the box and secure it around the sides (not the top and bottom) with two rubber bands. (See diagram.) Weigh and record box mass.
      3. Tape the seven remaining strips of 120-grit sandpaper into one long strip. (Be sure to use tape only on the back of the sandpaper.) Now attach the sandpaper lengthwise down the center of the pine board, using two thumbtacks at each end and being sure the sandpaper is drawn tight.
      4. Attach one paper clip to one of the rubber bands around the box.
      5. Tie one end of the string onto another paper clip and place a mark on the string about 1 cm from the clip. Use one rubber band to join the paper clip on the box with the paper clip on the string. Tie the free end of the string around the dowel or paper towel roll.
      6. Tape the meter stick onto the sandpaper strip on the board.
      7. Position the box at one end of the board so it is centered on the sandpaper. Use books to raise the other end of the board approximately 10 cm (4 in.). Measure and record the height.
      8. Gently roll the string onto the dowel until the string lifts off the paper and becomes taut. Note the location of the mark on the string relative to the meter stick. Take care to keep the dowel in the same position during rolling and measurement.
      9. Continue to roll the string onto the dowel until the box moves. The box should move with a quick, jumping motion. Record the new location of the mark on the string (the distance the box moved) on the data table. Continue rolling up the string and recording jump distance until the box hits the meter stick. The meter stick can be pulled upwards to allow the box to continue to be pulled.
      10. Subtract the beginning measurement from the ending measurement or add up the jump measurements to find out how far the box moved. Divide by the number of jumps to calculate an average jump distance.
   3. Instruct other students in the same group to change one variable, repeat the procedure, and average the distance of the jumps. Students may vary the model by adding one or more rubber bands, adding more books to change the angle of the board, substituting the brick for the box, or using sandpaper of a different grit. If time allows, give every student a chance to operate the model with each of the variations.
   4. Ask students to complete their data sheets.
3. Conclusion

   Ask the class:

   • What might the different variables represent in terms of earthquakes and landscape conditions? (Number of rubber bands – different amounts of energy released; angle of the board – steepness of the fault; sandpaper grit size – differences in the amount of force required for a fault to move – the amount of friction.) Emphasize that different faults can store different amounts of energy before they fail. Some faults have the potential for generating larger earthquakes than others.
   • Do the rubber band and string go totally slack after each movement? (No.) What does this tell you about the release of stored energy on a fault when an earthquake occurs? (No earthquake ver releases all the energy stored in the Earth at a particular point. It is because some stored energy always remains that one quake may have numerous foreshocks and aftershocks, and earthquakes recur frequently in some active areas.)

 

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Created By Dave Love, Chris Durand and John Van Der Kamp

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last modified: 25 April, 2022