Plate Tectonics and Earthquakes

Introduction

Many of the world’s most powerful earthquakes are generated at subduction zones where tectonic plates converge. Visually introduce the theory behind these “megaquakes” by simulating the forces involved with subducting plates.

Concepts

• Convergent plate boundary
• Subduction zone
• Elastic rebound
• Static versus dynamic friction

Background

The purpose of this activity is to model the fault movement of the Earth’s crust at convergent plate boundaries by stretching a rubber band chain attached to two wooden blocks in tandem. Friction between the blocks and a sandpaper surface will simulate the forces that occur at subduction zones.

Materials

Safety Precautions

Wear safety glasses for protection from eye injury in case a rubber band breaks during the activity.

Prelab Preparation

1. Obtain the sandpaper strip with adhesive backing and cut one 1-m-long piece.
2. Making sure the demonstration table is clean, remove the adhesive backing from the 1-m-long piece of sandpaper and press the strip down along one edge of the table (see Figure 1).
   {12159_Preparation_Figure_1}
3. From the remaining strip of sandpaper, cut one 3⅞" x 3" piece. Remove the backing from the sandpaper and press onto one face of one of the wood blocks.
4. Place two meter sticks end to end next to the sandpaper strip on the demonstration table so the zero mark of the first meter stick lines up with the left end of the strip (see Figure 1).
5. Obtain seven rubber bands. Attach one rubber band to the screw eye of the plain wood block by looping one end of the rubber band through the eyehole and then through the other end of the band (see Figure 2). Pull gently to tighten.
   {12159_Preparation_Figure_2}
6. Attach a second rubber band to the first one in the same manner as the first rubber band was attached to the screw eye. Repeat with the remaining five rubber bands in sequence to form a chain.
7. Obtain three more rubber bands. Repeat steps 5 and 6 with the sandpaper block and three rubber bands.
8. Loop the third rubber band attached to the sandpaper block around the perimeter of the plain block so the two blocks are in tandem (see Figure 3).
   {12159_Preparation_Figure_3}

Procedure

1. Place the wood blocks on top of the sandpaper strip so the end of the block with the sandpaper is lined up with the zero end of the meter stick and the sandpaper side is down. The plain block should be placed so the rubber band chain between the blocks is taut, but not stretched (see Figure 4).
   {12159_Procedure_Figure_4}
2. Position the chain of rubber bands attached to the plain block in a straight line. Place a pencil through the loop of the last rubber band so the tip of the pencil points toward the meter stick. Extend the chain without causing any tension force on the wood block.
3. Instruct students to record the position of the pencil tip in centimeters in the data table on the Plate Tectonics and Earthquakes Worksheet.
4. Instruct students to also record the positions of the leading edge of the plain block and the sandpaper block in centimeters.
5. Begin to stretch the rubber band chain by moving the pencil 1 cm to the right. Pause and observe whether or not the plain block moves.
6. If the block does not move, repeat step 5, moving the pencil and stretching the rubber band chain in 1-cm intervals. Make sure the pencil is moved 1 cm at a time and always pause 2–3 seconds before moving the pencil again.
7. When the plain block moves for the first time, note and record the current position of the pencil in the second row of the data table.
8. Note and record the new position of the plain block to the nearest 0.1 cm, as well as the position of the sandpaper block, even if the sandpaper block did not move (see Sample Data).
9. Repeat steps 5–8 until the sandpaper block moves. This will most likely be a very dramatic change in position. Note and record the positions of the pencil, the plain block, and the sandpaper block, respectively.
10. Repeat step 9 until the plain block has reached the end of the sandpaper strip.

Student Worksheet PDF

12159_Student.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration at least seven times: two wood blocks with screw eye, 8 feet of adhesive-backed sandpaper and 75 rubber bands.
• The rubber bands may stretch out. Use fresh rubber bands for each demonstration.
• The sandpaper may become less abrasive with repeated use. Replace the sandpaper on the wood block as well as the sandpaper strip as needed.
• Visit the U.S. Geological Survey website http://earthquake.usgs.gov/ (accessed November 2010) to learn more about earthquakes and plate tectonic boundaries and to see where earthquakes have occurred recently around the globe.
• Use Flinn Scientific’s Exploring Earthquakes Activity Stations Kit (Catalog No. AP7406) to further explore earthquakes and their effects.

Sample Data

Answers to Questions

1. Analyze the data for the plain block. (a) How many times did the plain block slip? (b) Determine how many centimeters the block moved for each slip and find the average distance per slip.
   1. The plain block moved 10 times.
   2. The average distance the block moved was

      (1.6 + 2.0 + 0.2 + 1.3 + 3.3 + 19.2 + 0.8 + 19.9 + 1.7 + 7.9) cm/10 slips = 57.9 cm/10 slips = 5.8 cm/slip

2. Repeat the analysis in Question 1 for the sandpaper block data.
   1. The sandpaper block moved three times.
   2. The average distance the sandpaper block moved was

      (20.5 + 22.7+ 9.8) cm/3 slips = 53 cm/3 slips = 17.7 cm/slip

3. How much did the leading rubber band chain stretch before the plain block slipped the first time? Before the sandpaper block slipped?

   The rubber band chain stretched 21 cm before the plain block slipped and stretched 59 cm before the sandpaper block slipped.

4. What caused the blocks to slip? Describe the forces involved and the transfer of energy that took place from one slip of each block to the next.

   The motion of the pencil stretched the chain of rubber bands, increasing their elastic potential energy. The last rubber band connected to the plain block exerted a force on the block, but the force of static friction between the block and the sandpaper strip opposed any forward motion of the block. Eventually the rubber band chain exerted enough force on the block to overcome the force of static friction and the block slipped forward with kinetic energy. The kinetic energy of the block was converted to heat energy as dynamic friction opposed its forward motion and the block stopped. Some energy was also transferred to the air and table as vibrations and waves. When the plain block slipped forward, the tension on the rubber band chain was eased, but the rubber bands between the two blocks were stretched, pulling on the sandpaper block. A greater static friction force kept the sandpaper block in place. The cycle of tension and release of the plain block continued until the pulling force on the sandpaper block was great enough to overcome the static friction. This large force caused a large slip of the sandpaper block.

5. The motion of the rubber bands and blocks in this activity provides a model for the movement and forces involved in subduction zone earthquakes. What does the movement of the chain of rubber bands represent? What does each block of wood represent? Refer to Figure A.

   The movement of the rubber band chain represents the continuous movement of the subducting tectonic plate. The plain block represents the transition subduction zone where smaller earthquakes occur and the sandpaper block represents the locked zone where major earthquakes may be generated.

6. How is this model similar to the subduction zones created by converging tectonic plates? How is it different?

   The rubber band chain moved at a relatively constant slow speed, much like the Earth’s tectonic plates. As the force of friction was exceeded, the plain block moved suddenly and tension on the rubber bands relaxed momentarily. This slippage created greater tension on the sandpaper block. The sandpaper block remained “locked” in place until enough tension built up to overcome friction and the block slipped a great distance. In a similar way, rocks along a fault shift suddenly and an earthquake occurs. Larger earthquakes occur when movement in the transition zone increases tension on the locked zone until the stored energy in the locked zone is suddenly released. The forces involved with tectonic plate movement are more complex and the energy comes from the internal heat of the earth. The force of friction within a fault comes from the plates grinding against each other, and the rocks along a fault store energy until they suddenly shift or break.

Discussion

The rocky plates that make up the Earth’s crust are in constant motion. The interactions of these plates create faults, or cracks, that offset the Earth’s crust. Continuous movement of the plates builds up pressure until the rocks along a fault shift or break, releasing energy that causes an earthquake. The cycle of gradual buildup and release of stress along a fault is known as the elastic rebound theory, first proposed by American geologist Henry Fielding Reid (1859–1944). Reid was part of a task force commissioned by the state of California to investigate the 1906 San Francisco earthquake. Reid closely examined the surface ground displacement caused by the 1906 earthquake. By investigating data from surveying records, he realized that some ground displacement occurred away from the fault before the earthquake. He concluded that stress built up slowly along the fault until the strain was suddenly released by slippage of the fault. Reid compared the energy released by the rebound of the fault to that of a rubber band breaking when it was stretched too far. Even though the theory of continental drift was proposed by German scientist Alfred Wegener (1880–1930) shortly after Reid’s theory, it would be more than a half century later before the movement of the Earth’s plates would be connected to earthquakes along fault lines.

Convergent boundaries that create subduction zones occur where tectonic plates moving toward one another meet and one plate sinks or subducts under the other. One example is the Cascadia subduction zone that separates the Juan de Fuca Plate and the North American Plate. The Pacific oceanic crust sinks below the North American continent about 4 cm per year. One model, researched by Canadian geologists Hyndman and Wang, indicates that deep within a subduction zone high pressure and temperature result in low dynamic friction between the plates causing rocks to slowly deform as the plates continually move. At the shallow edge of the subduction zone greater friction results in the plates “locking” together, creating deformations in the overlying crust (see Figure 5). Between the high- and low-friction zones is a transition zone of intermediate friction with cycles of tension and compression resulting in relatively small quakes. As the converging plates continue to move, the pressure builds in the locked zone until the force of static friction is exceeded. Slippage of several meters may occur, resulting in a major earthquake.

The greatest earthquake ever recorded (magnitude 9.5) occurred on May 22, 1960. It originated in the subduction zone created by the convergence of the Nazca and South American plates. The epicenter was 900 km south of Santiago, Chile. This great earthquake was preceded by a smaller quake the day before and the focus was at a relatively shallow depth of 33 km. These observations support the model presented in this demonstration. In this simulation, the leading rubber band chain represents the deep low-friction zone with continual steady movement. The plain block represents the transition zone where smaller quakes occur and the sandpaper block simulates the locked, high-friction zone.

References

Geodynamics. Natural Resources Canada. www.nrcan.gc.ca (accessed November 2010).

Introduction to Plate Tectonics and Earthquakes. Teachers on the Leading Edge, Guiding Pacific Northwest Teachers from Subduction to Eruption. http://orgs.up.edu/totle/ (accessed November 2010).