Earthquake Machine - Block & Sandpaper

Previous Animations of the Month

Other Animations of the Month can be found on the archive page.

Block & Sandpaper Activities

See Hypothesis, Concepts, and Objectives below.

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Introduction

The outer hard shell of the Earth is broken into about dozen major plates. Large earthquakes are generated at the contact between the plates. The plates can glide along at a few centimeters per year over the hot materials below them. Earthquakes are not generated at this lower boundary because at 80-km depth the high temperature and pressure allows the rocks to slowly and permanently deform (think Big Hunk candy bar: brittle when cold, ductile when warm). At shallow depths, however, along the faults that form the boundary between the plates, the rocks deform and rebound (stick and slip) as the plates move past one another. They store elastic energy where they are stuck together that is eventually released during an earthquake.

About the Animations

This block-and-sandpaper model can be used to teach the concept of elastic rebound and how energy is stored and released. Earthquakes can provide a useful context for teaching or reviewing many basic physics concepts, such as sliding and static friction, forms of energy and conversion from one form to another, and the elastic properties of materials. Read the "Concepts" below.

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Blocks EQ Machine Basic

On the graph, the yellow line shows the movement of the hand over time, thus a steady line. The blue line shows the movement of the block during slip on "earthquakes" thus the jumps in distance over time. There are variations of this activity described in the "Activities" links below.

Direct Link to Blocks EQ Machine Basic (Small 170k)

Direct Link to Blocks EQ Machine Basic (Larger 514k)

Blocks Slow Slip Time / Strain

This animation shows the "Earthquake Machine" using two blocks with different grit sandpaper, thus different friction. Animation illustrates the build up and release of strain in locked and slow slip zones. The first block, in red, has fine sandpaper on its bottom and simulates the slow slip zone between tectonic plates. The second block with coarse sandpaper on its bottom side, simulates the locked zone of two plates. As the first rubber band is slowly pulled, strain builds up. Once enough energy is stored in the rubber band to overcome the friction of the sandpaper under the red block, the red block slips slightly. (Note: This is similar to the slow earthquakes or episodic tremor and slip events in the Pacific Northwest.) Once the red block slips, the rubber band between the red and blue wood block stretches slightly. As this rubber band continues to stretch, the strain between the blocks builds until the blue block finally moves. The movement of the blue block would be equivalent to a major earthquake.

The yellow line plots the steady displacement of the hand. The red line shows the strain on the rubber band between the hand and the red block. The strain drops suddenly to a lower level each time the red block slips (earthquake slip). There is low friction between the red block and the surface, so "earthquakes" tend to be more frequent and smaller. The blue line shows the strain on the rubber band between the blocks. The blue block has higher friction with the surface, so it tends to slip in larger "earthquakes."

Blocks Slow Slip Time / Strain (Small 500k)

Blocks Slow Slip Time / Strain (Larger 1.4mb)

Blocks Slow Slip Time / Distance

This animation shows the change in distance over time. Observe that the red line steps up in small increments, while the blue line has large sudden movements, similar to the large movement expected during a catastrophic earthquake along a subduction zone. The change in strain over time is illustrated in Blocks Slow Slip Time / Strain. Observe that the strain in the red line constantly builds and is then released, while in the blue line, the strain builds in sudden steps until a catastrophic release of energy occurs.

Blocks Slow Slip Time / Distance (Small 500k)

Blocks Slow Slip Time / Distance (Larger 1.4mb)

Brief Description of the Model

The sandpaper contact represents the contact between two plates where a fault occurs; sandpaper creates the friction between the blocks. The blocks are the mass that is going to be moved in an earthquake. The movement of the hand represents the motion of a plate interior as the tectonic plates movie around at a steady rate. But they are stuck at the plate boundaries. These boundaries can break and skid along with many small earthquakes, or they can build up stress and let go with a great earthquake.

The strain is building up on the rubber band, and eventually it overcomes the frictional resistance (stress) at the fault contact. When the stress overcomes the resistance we have an earthquake. Even though the hand is moving at a steady rate, we still get stick slip along the fault and the exact second that the friction will be overcome cannot be predicted. With the two-block model one section of the fault has frequent smaller earthquakes, whereas the back block has long periods with no earthquakes punctuated by larger quakes.

Video Descriptions of the Model

John Lahr describes and demonstrates the elastic rebound and block models.

Elastic Rebound

Direct Link to Elastic Rebound (900k)

Single Block Model

Direct Link to Single Block Model (1.75mb)

Two Block Model

Direct Link to Two Block Model (1.84mb)

Earthquake Hypotheses that Can Be Explored with the Model

Hypothesis 1

Earthquakes are periodic (in other words, all of the same slip, and all separated by the same amount of time). There is some evidence for this, particularly among very small earthquakes on creeping faults.

Hypothesis 2

Earthquakes are 'time-predictable' (this means that the larger the slip in the last earthquake, the longer the wait until the next one.) This idea was formulated in the 1980's by Shimazaki and Nakata in Japan, and has been widely used.

Hypothesis 3

Earthquakes occur randomly in time and and have randomly varying size. (This 'Poisson' hypothesis is also widely used, particularly when little information about a fault and its past earthquakes is available).

Block & Sandpaper Activity

Or for a more mechanical demonstration:

Content Objective

Students will be able to:

  • Define earthquakes and model their occurrence using the earthquake machine.
  • Describe the role of inquiry in the process of science
  • Give examples of why models are important in science
  • Use and explain the Earthquake Machine allowing exploration of additional concepts in future lesson phases

Concepts

Students work cooperatively to develop and test a hypothesis, make measurements, and write a short report on the results with graphs. They learn the concept of elastic rebound: energy is first stored and later released using a block that is pulled by rubber bands to create an "earthquake". Elastic energy is stored in the rubber bands and released during "earthquakes." Earthquakes can provide a useful context for teaching or reviewing many basic physics concepts, such as sliding and static friction, forms of energy and conversion from one form to another, and the elastic properties of materials. This lab could be part of an Earth science course, but could also be used in general science or physics. This lesson could be made more "inquiry based" than what is presented in PDF file. One possibility would be to spend one period in inquiry with the rubber bands and blocks of wood, and then a second one doing this more directed experiment.