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Purpose: Demonstrates the principle of elastic rebound for the cause and recurrence of earthquakes.
Supplies: Wood blocks, sandpaper, springs, weights (rocks), fishing reel (optional)
Note: Here is an updated, improved version of the apparatus: Directions for Building the Earthquake Machine
Background and Demonstration:
Under the Elastic Rebound Theory, the continuous motion of the Earth's plates causes stress to build up at the boundaries between the plates, where friction keeps the boundaries locked. Once the frictional stress (or the strength of the rocks) is exceeded, the two sides move (along a fault) causing an earthquake. This principal applies away from plate boundaries as well, since the locked plate boundaries causes the stress to extend to the plate interior. In this case, earthquakes occur anywhere the local strength of the rock is less than the regional stress level. The key point of the Elastic Rebound Theory is that the stress is continually building up, and that earthquakes act to relieve that stress. For a constant rate of stress increase due to plate motion, the greater the time between earthquakes, the greater the stress release when the earthquake occurs (larger magnitude).
This experiment is based on a simple, yet elegant laboratory model suggested by Burridge and Knopoff in 1967, and is excellent for hands-on experiments by students. The two sides of a fault are represented by sandpaper-covered blocks resting on a sandpaper-covered board. A weight rests on the block (I use a rock) to apply a chosen amount of pressure on the sandpaper (defining the frictional strength of the fault). A spring is attached to the block and to a string, which is pulled by a fishing reel (or any other take-up device), by a weight suspended off the end of the board by a pulley, or by hand (although it is difficult to maintain a constant pull by hand). As the string is pulled, the spring is stretched, and the length of the spring is proportional to the shear stress applied to the block. When that stress becomes greater than the static frictional stress, the block slides relieving part of the stress, and shortening the spring. In theory for such a simple system, the block should stop when the shear stress equals the dynamic frictional stress, although partial stress-drop and friction overshoot have been interpreted for actual earthquakes.
If you continue to pull on the string (either by turning the fishing reel or because the weight is still attached), the spring will continue to stretch until the static frictional stress is reached, and movement of the block will occur again. If this sequence is continued for several cycles, conclusions on earthquake recurrence (seismicity) may be drawn. There may be cases in which the local friction under the block is greater than usual, requiring more shear stress to accumulate before the block moves. By measuring the time between movements, the shear stress (the length of the spring), and the amount of motion of the block, it can be shown that small motions occur after short times and large motions occur after longer periods of "quiescence". Averages and standard deviations may be determined for the time between earthquakes and the amount of slip of earthquakes. Does this pattern change if the rate of stress accumulation is varied (turn the fishing reel faster or increase the suspended weight), or if the frictional stress is increased (add weight to the block)?
Of course, faults are more complicated than a sandpaper-covered block. During an earthquake a length of fault (a segment) moves, but neighboring segments do not. We can simulate this effect by attaching another sandpaper-covered block with a spring to the first block. Now when the first block moves, it stretches the spring between the two blocks, causing a resisting stress on the first block (it doesn't move as far now), and increasing the shear stress on the second block. If you pull on the string for several cycles, an equilibrium will be established such that sometimes the first block moves (small earthquake), sometimes the second block moves (small earthquake) and sometimes both move simultaneously (large earthquake). Sometimes a large motion of one block or a simultaneous motion of both blocks will trigger a small motion after a relatively short time (an aftershock). The amount of motion and shear stress (length of the spring) for each block may be plotted through time. Also the amount of slip and time of occurrence of large and small "earthquakes" may be analyzed. The problem has now become quite complicated! Of course an even more complicated model can be tested by connecting three, four, or more sandpaper-covered blocks with springs and measuring the movement and shear stress on each.
Reference: R. Burridge and L. Knopoff, "Model of Theoretical Seismicity", Bulletin of the Seismological Society of America, 57: 341-371, 1967.
Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.
Questions or comments: jbarker@binghamton.edu
Last modified: March 18, 1996 (content), November 1, 2001 (improved apparatus directions), June 6, 2021 (reformatted and moved to Google sites)
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