Finding the Moho Under Milwaukee
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 of the activity/assignment
A few PowerPoint or Keynote slides are used to set the historical context of the project and to introduce the general problem (see "Presentation Files" and "Instructors Notes" below). Bouncing a rubber ball in class from differing heights above the floor, and letting students see and hear the effects of differing travel times, helps students understand that longer travel time in a reflection experiment indicates a deeper reflector. The relevant parts of YouTube videos are shown (see links under "Other Materials" below). Have the students measure the time interval between the explosion and the impact a few times while showing the videos. One of the correspondents talks about (and attempts to show) how the bridge vibrated after the explosion.
Pass out the worksheet and the raw seismograms related to the Hoan demolition experiment. Then, with a copy of the seismograms projected onto the screen, hold an initial discussion of how to interpret the graphics: note the time scale, discuss what the different wave amplitudes mean, and so on. Then cluster into groups of 2-4 students and have each group try to "pick" the first arrivals of  the explosion-induced direct wave,  the impact-induced direct wave, and  the corresponding reflected waves. Depending on the type of students involved (intro non-geologists, intro geology/geophysics, geophysics), the teacher can provide more or less assistance in picking the arrivals of the direct and reflected waves.
Work through the quantitative material on the worksheet. Questions about how to handle uncertainty always occur, and if the students do not admit to having questions about this the teacher should ask them how they handle uncertainties. In a nutshell, the resultant uncertainty associated with the sum or difference in two numbers are the sum of the two uncertainties. For example, (23 ± 2) + (14 ± 1) = 37 ± 3. The resultant uncertainty associated with the product of two numbers can be estimated with the sum of the fractional (or percentage) uncertainties. For example, the percentage uncertainty of 23 ± 2 is (2/23) or 8.7% and the percentage uncertainty of 14 ± 1 is (1/14) or 7.1 %, so (23 ± 2) x (14 ± 1) = 322 ± 51 because (8.7% + 7.1%) = 15.8% and 15.8% of 322 is ~51. For a nice summary of simple uncertainty calculations, refer to http://spiff.rit.edu/classes/phys273/uncert/uncert.html or http://webpages.ursinus.edu/lriley/ref/unc/unc.html, or the statistics resources on the SERC website.
When the worksheets are completed, recap the experiment and compare the results with a map of crustal thicknesses for North America (e.g., Braile, 1989, Fig. 23B). Finally, it is nice to have the students evaluate the experience as homework.
Determining whether students have met the goals
Successful completion of the worksheet resulting in a credible estimate of crustal thickness is an initial indication that the student has grasped the material. The immediate goal of having students gain an experience in measuring the thickness of the continental crust should give them a more intimate understanding of contour maps of crustal thickness.
Beyond the immediate goals, one hopes that students might be at the beginning of an understanding of other related ideas. Can students participate in a meaningful discussion of other similar experiments such as the subsurface studies that utilized the implosion of the Seattle Kingdome stadium in 2000 (e.g., http://geopubs.wr.usgs.gov/open-file/of02-123/ and http://earthquake.usgs.gov/scenarios/related/kingdome.php). Can they grasp why geophysicists use reflection seismology to image the subsurface in their search for oil and gas? Can they extend the analogy to understand other forms of geo-sensing that use reflections, such as radar, sonar, lidar, laser rangefinding?
Having students write a brief retrospective assessment of the experience for homework is another way to gauge how successful the exercise has been.
Download teaching materials and tips
- Worksheet to find crustal thickness under Milwaukee (Microsoft Word 114kB Mar19 10)
- Raw 3-component seismograms from partial demolition of Hoan Bridge, Milwaukee (Acrobat (PDF) 335kB Feb25 10)
Presentation Files (Keynote and PowerPoint)
- Moho Under Milwaukee Keynote slides ( 2.1MB Mar23 10)
- Moho Under Milwaukee PowerPoint slides (PowerPoint 2.6MB Mar23 10)
- Teacher notes for "Finding the Moho under Milwaukee" (Acrobat (PDF) 108kB Mar19 10)
- Phase picks on seismogram from partial demolition of Hoan Bridge, Milwaukee (Acrobat (PDF) 407kB Feb25 10)
- Approximate answers for worksheet (Microsoft Word 116kB Mar19 10)
- YouTube Videos
...about the partial demolition of the Hoan Bridge in Milwaukee:
- Video from Milwaukee TV station WISN 12 News: http://www.youtube.com/watch?v=pS_HdhOBik8
- Video from Milwaukee TV station FOX 6 News (the demolition occurs 3 minutes 27 seconds into this clip): http://www.youtube.com/watch?v=vRz797o_l-k
- Video from Milwaukee TV station FOX 6 News: http://www.youtube.com/watch?v=x6AxV9WbvIk
- Photos of Andrija Mohorovicic provided without restriction by Marijan Herak of the University of Zagreb: http://serc.carleton.edu/details/images/21592.html, http://serc.carleton.edu/details/images/21593.html, http://serc.carleton.edu/details/images/21594.html; see Herak and Herak (2007) and http://www.gfz.hr/sobe-en/discontinuity.htm
- Photo of Hoan Bridge: http://en.wikipedia.org/wiki/File:Hoan_Bridge.jpg
- Photo of Hoan Bridge: http://en.wikipedia.org/wiki/File:HoanBridge12042005.jpg
- Photo of Hoan Bridge in 2010 by Keith Sverdrup (no copyright restrictions): http://serc.carleton.edu/details/images/21293.html
- Photo of rebuilt part of Hoan Bridge in 2010 by Keith Sverdrup (no copyright restrictions):http://serc.carleton.edu/details/images/21294.html
- Annotated photo of rebuilt part of Hoan Bridge in 2010 by Keith Sverdrup (no copyright restrictions):http://serc.carleton.edu/details/images/21295.html
- UWM seismogram from partial demolition of Hoan Bridge: http://www4.uwm.edu/letsci/geosciences/seismic_center/other_recordings/hoan.cfm
- Copy of sound print of Cronin saying "Moho," resulting in a "seismogram" that looks remarkably like the seismogram recorded during the partial demolition of the Hoan Bridge. JPEG file, 72 KB: http://web.archive.org/web/20100701081706/http://bearspace.baylor.edu/Vince_Cronin/www/EarthInterior/mohovoiceprint.jpg
- Sound of Cronin saying "Moho," MP3 file, 128 KB: http://web.archive.org/web/20100626205207/http://bearspace.baylor.edu/Vince_Cronin/www/EarthInterior/Moho.mp3
Braile, L.W., 1989, Crustal structure of the continental interior, in Pakiser, L.C., and Mooney, W.D., Geophysical framework of the continental United States: Boulder, Colorado, Geological Society of America Memoir 172, p. 285-315.
Christensen, N.I., and Mooney, W.D., 1995, Seismic velocity structure and composition of the continental crust -- A global review: Journal of Geophysical Research, v. 100, no. B7, p. 9761-9788.
Dix, C.H., 1955, Seismic velocities from surface measurements: Geophysics, v. 20, p. 68-86.
Herak, M., Web resource describing Andrija Mohorovicic's work in discovering the seismic discontinuity at the base of the crust: http://www.gfz.hr/sobe-en/discontinuity.htm accessed 23 March 2010
Herak, D., and Herak, M., 2007, Andrija Mohorovicic (1857-1936) --On the occasion of the 150th anniversary of his birth: Seismological Research Letters, v. 78, no. 6, p. 671-674.
Herak, D., and Herak, M., 2010, The Kupa Valley (Croatia) Earthquake of 8 October 1909 -- 100 Years Later: Seismological Research Letters, v. 81, no. 1, p. 30-36.
Lichtenstein Consulting Engineers archives on Hoan Bridge: http://web.archive.org/web/20070311050511/www.lce.us/Hoan/
Shah, P.M., and Levin, F. K., 1973, Gross properties of time-distance curves: Geophysics, v. 38, p. 643-656.
Wisconsin Highways description of Hoan Bridge: http://www.wisconsinhighways.org/indepth/hoan_bridge.html