Teach the Earth > Structural Geology > Structure, Geophysics, and Tectonics 2012 > Teaching Activities > Focal mechanism sphere

Focal mechanism sphere

Sarah Titus, Carleton College

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This see-through sphere helps students visualize how slip on a fault relates to the focal mechanism representation of the same deformation.



The focal mechanism sphere was designed and built by three students in my Tectonics course (Tom Birren, Aurora Roth, and Tyler Schuetz). The course is a 200-level class, where Introductory Geology is the only prerequisite.

Skills and concepts that students must have mastered

For the demonstration to work best, it helps if students have discussed focal mechanisms. By the time I use it, we have spent a week practicing with ideas of focal mechanisms including plotting one by hand as part of a lab, building them out of play-doh, interpreting maps generated in Jules Verne Voyager, and practicing using a variety of paper exercises.

How the activity is situated in the course

Instead of having other students build the device, I use it as demonstration tool.


Content/concepts goals for this activity

This a visualization tool for focal mechanisms.

Higher order thinking skills goals for this activity

Visualization of three-dimensional information and understanding the link to the 2D representation of that 3D information.

Other skills goals for this activity

Description and Teaching Materials

Here are the instructions from the three students:
Equipment needed:

  • Drill and bit set
  • Plastic of fiber glass spherical display case roughly 12" in diameter (two hemispheres that attach to each other)
  • 4 small dowel rods no longer than a foot is necessary
  • 2 small blocks of wood roughly the size of your palm (We used ½" ply cut roughly 2.5'' x 4")
  • 12 springs with loops on each end (3 per quadrant)
  • 24 eye-hooks (2 per spring)
  • Marker or tape (optional)
  • Wood glue
  1. Drill pilot holes for eye hooks: 6 on the widest side of each block of wood, 3 in each quadrant of the display sphere.
  2. Drill holes in the side of the blocks of wood that is of the smallest dimension for a snug fit of the dowel rod. Make sure to put wood glue in the hole to adhere to the dowel.
  3. Drill two pairs of holes on opposite sides of the sphere. Drill pairs about ½" apart and wide enough that the dowels slide freely through them. These holes should be parallel sets.
  4. Assemble each hemisphere separately. First the dowel rod and block sliding mechanisms in place then attach springs to corresponding eye-hooks.
  5. Mark each quadrant (optional).

The 3D Focal Mechanism can be rotated by the viewer to represent any possible fault motion. By simply operating the dowels one can observe which quadrants experience tension and compression by looking at the springs.

Photos and diagrams of the finished focal mechanism sphere (Acrobat (PDF) 6.7MB Apr26 13)
Skill puzzle for practicing focal mechanisms (Acrobat (PDF) 268kB Apr26 13)

Teaching Notes and Tips

This instrument was originally designed by students in Tectonics. They were tasked with designing a device that illustrated a Tectonics concept. Students were given four lab periods to design the device and develop teaching materials to accompany the device.

Now I use the best devices as demos for later classes.


On an exam, students are asked to solve some problem involving focal mechanisms. They are also asked to write about focal mechanisms from a plate boundary system.

References and Resources

I have students plot a focal mechanism by hand in this lab.

Jules Verne Voyager is great for making maps with focal mechanisms, or other geophysical data.

Source for the sphere.

I found Vince Cronin's primer on focal mechanisms very helpful.

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