Focal mechanism sphere
Skills and concepts that students must have mastered
How the activity is situated in the course
Content/concepts goals for this activity
Higher order thinking skills goals for this activity
Other skills goals for this activity
Description and Teaching Materials
Here are the instructions from the three students:
- 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
- Drill pilot holes for eye hooks: 6 on the widest side of each block of wood, 3 in each quadrant of the display sphere.
- 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.
- 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.
- Assemble each hemisphere separately. First the dowel rod and block sliding mechanisms in place then attach springs to corresponding eye-hooks.
- 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.
References and Resources
I have students plot a focal mechanism by hand in this lab.
I found Vince Cronin's primer on focal mechanisms very helpful.