How to Do the Earthquake Demonstration in Class
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SERC Pedagogic Service Project
Initial Publication Date: October 3, 2005
The earthquake machine can be used in less than 5 minutes for a simple, show-and-tell demonstration by the demonstration of how elastic energy is stored (as the winch is cranked) and then released in a sudden slip event that is analogous to an earthquake. Cranking the winch slowly in silence is very effective, so students hear the grinding of brick upon sand paper as the slip event takes place. Often, small amounts of slip occur and the sound of brick on sand grit can be heard in staccato. If more time can be devoted to the activity, a full interactive lecture demonstration can be developed.
Guidelines for Interactive Lecture Demonstration
Sketching the Apparatus
--Before beginning the exercise, ask students to make a sketch of the apparatus. Start with the apparatus in extended position, with the brick(s) at the far end of the board, and the tubing between the bricks and winch extended and slack. Point out various parts of the earthquake machine (bricks, winch, sandpaper, etc). Tell students that the apparatus has strong parallels to real Earth processes and features, and that these will be identified during a post-demonstration discussion.
Predicting What will Happen --Once students have completed their sketches, ask them to predict what might happen when you crank the winch. Typical responses in class have been as follows: "The brick will start to slide." "The brick will not slide for a while, then it will suddenly jump." It is helpful to get students to make their predictions more exact by prompting them further. Ask them, for example, if it will start to slide right away, and why or why not? Ask if it will slide only once, or more than once, and why or why not? Prompt students to discuss whether the brick will slide in repeated similar amounts at regular time intervals, or in varying amounts at irregular time intervals.
Simulating Fault Slip -Mark the position of one edge of the brick before cranking the winch. If the apparatus is on a table, the marks can be made in chalk on the table alongside the board, or with pieces of tape. Have a student stand beside the apparatus and ask him/her to mark every place to which the brick slides and stops. Later, these marks will provide estimates of "fault slip". Ask a second student to begin cranking, and emphasize that it is important to crank at a steady rate (i.e., constant number of wheel revolutions/minute). A third student can be asked to record time, making note of the time when cranking begins and the each time when the brick slides at least a given amount (e.g., 3 cm). Ask the class to be very quiet and to listen and observe carefully. As everyone waits for something to happen, the students become quite engaged and mesmerized. This is one of the most effective demonstrations I have done in this regard. Most likely, everyone will hear at least one small gritting noise as a small event ("foreshock") occurs, then the brick will slide several cm or more along the board, releasing accumulated elastic strain energy. Remind the student helpers to keep cranking steadily and to mark the positions of the brick, as they might stop after this first event.
Follow-up to Demonstration -Measure the amount of slip that occurred during each event, and ask the time-keeper for the times that each event occurred (starting from time 0 at the beginning of the demonstration. Discuss what happened with students and ask them to draw parallels between the earthquake machine and real-Earth features and processes. The sandpaper and brick, for example, simulate crust on each side of a fault. Discuss the type of motion along the "fault"-it's purely horizontal, as with a strike-slip fault. The crank is an analog for steady plate motion, and the rubber tubing for elasticity of rocks.
An interesting parallel to discuss is the recurrent slip on the San Andreas at Parkfield, California, which had its most recent earthquake event in late 2004. USGS scientists refer to their research at Parkfield as the Parkfield Experiment (more info) . You can draw parallels between that experiment and the one done with the earthquake machine, and compare Parkfield earthquake data (recurrence times and slip amounts) to those of the class demonstration. I've even had students make slip-time diagrams (time on x-axis and slip on y-axis) for Parkfield and the earthquake machine and compare them to one another. These ideas can be explored further as discussed below in the section on Hypotehses.
Longer Demonstrations
Another alternative is to ask students what might happen if another brick is placed on top of the brick that slides. After cranking the winch for a while and observing this scenario, students will realize that adding more normal force requires even greater shear force to get the brick to slide. A dramatic demonstration can be done by letting a substantial amount of strain accumulate when the two bricks are stacked, and then asking what will happen if the upper brick is suddenly lifted by a student. After discussing this prediction, a student removes the upper brick and the lower brick is released, sliding rapidly along the sandpaper.
Testing Hypotheses --The earthquake machine can be used for longer demonstrations to illustrate the nature of recurrent fault slip and to explore alternative hypotheses of earthquake occurrence, as suggested at the USGS Earthquake Machine website:
Back to Earthquake Demonstration page
Guidelines for Interactive Lecture Demonstration
Sketching the Apparatus
--Before beginning the exercise, ask students to make a sketch of the apparatus. Start with the apparatus in extended position, with the brick(s) at the far end of the board, and the tubing between the bricks and winch extended and slack. Point out various parts of the earthquake machine (bricks, winch, sandpaper, etc). Tell students that the apparatus has strong parallels to real Earth processes and features, and that these will be identified during a post-demonstration discussion. Predicting What will Happen --Once students have completed their sketches, ask them to predict what might happen when you crank the winch. Typical responses in class have been as follows: "The brick will start to slide." "The brick will not slide for a while, then it will suddenly jump." It is helpful to get students to make their predictions more exact by prompting them further. Ask them, for example, if it will start to slide right away, and why or why not? Ask if it will slide only once, or more than once, and why or why not? Prompt students to discuss whether the brick will slide in repeated similar amounts at regular time intervals, or in varying amounts at irregular time intervals.
Simulating Fault Slip -Mark the position of one edge of the brick before cranking the winch. If the apparatus is on a table, the marks can be made in chalk on the table alongside the board, or with pieces of tape. Have a student stand beside the apparatus and ask him/her to mark every place to which the brick slides and stops. Later, these marks will provide estimates of "fault slip". Ask a second student to begin cranking, and emphasize that it is important to crank at a steady rate (i.e., constant number of wheel revolutions/minute). A third student can be asked to record time, making note of the time when cranking begins and the each time when the brick slides at least a given amount (e.g., 3 cm). Ask the class to be very quiet and to listen and observe carefully. As everyone waits for something to happen, the students become quite engaged and mesmerized. This is one of the most effective demonstrations I have done in this regard. Most likely, everyone will hear at least one small gritting noise as a small event ("foreshock") occurs, then the brick will slide several cm or more along the board, releasing accumulated elastic strain energy. Remind the student helpers to keep cranking steadily and to mark the positions of the brick, as they might stop after this first event.
Follow-up to Demonstration -Measure the amount of slip that occurred during each event, and ask the time-keeper for the times that each event occurred (starting from time 0 at the beginning of the demonstration. Discuss what happened with students and ask them to draw parallels between the earthquake machine and real-Earth features and processes. The sandpaper and brick, for example, simulate crust on each side of a fault. Discuss the type of motion along the "fault"-it's purely horizontal, as with a strike-slip fault. The crank is an analog for steady plate motion, and the rubber tubing for elasticity of rocks.
An interesting parallel to discuss is the recurrent slip on the San Andreas at Parkfield, California, which had its most recent earthquake event in late 2004. USGS scientists refer to their research at Parkfield as the Parkfield Experiment (more info) . You can draw parallels between that experiment and the one done with the earthquake machine, and compare Parkfield earthquake data (recurrence times and slip amounts) to those of the class demonstration. I've even had students make slip-time diagrams (time on x-axis and slip on y-axis) for Parkfield and the earthquake machine and compare them to one another. These ideas can be explored further as discussed below in the section on Hypotehses.
Longer Demonstrations
Another alternative is to ask students what might happen if another brick is placed on top of the brick that slides. After cranking the winch for a while and observing this scenario, students will realize that adding more normal force requires even greater shear force to get the brick to slide. A dramatic demonstration can be done by letting a substantial amount of strain accumulate when the two bricks are stacked, and then asking what will happen if the upper brick is suddenly lifted by a student. After discussing this prediction, a student removes the upper brick and the lower brick is released, sliding rapidly along the sandpaper. Testing Hypotheses --The earthquake machine can be used for longer demonstrations to illustrate the nature of recurrent fault slip and to explore alternative hypotheses of earthquake occurrence, as suggested at the USGS Earthquake Machine website:
- 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 have randomly varying size. (This 'Poisson' hypothesis is also widely used, particularly when little information about a fault and its past earthquakes is available).
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