The activity is part of a tectonics unit looking at crustal stress and faulting. In having the students make maps of crustal stress, they will get experience retrieving, plotting, and interpreting data. The LiveScript framework allows them to adapt the assignment to any part of the world they are interested in. The LiveScript also exposes the students to using code for scientific exploration without the typical frustration of starting code from scratch.

Skills developed:
Gaining familiarity with scientific computing for analysis and visualization

This is aimed at an upper level geoscience undergraduate or first year graduate student. This could easily fit in a seismology, tectonics, or Solid Earth Geophysics course. I use it as a take home activity, but it could be easily adapted for a lab or in-class exercise. Not counting the required visit to globalgmt.org to get the earthquake data, the activity could be done in 30-40 minutes. There could be considerably more time taken by a student interested in exploring earthquake focal mechanisms and/or crustal stresses.

While the activity takes some familiarity with MATLAB and exposure to beach balls and stress, the technical skills are not high. The LiveScript has fully working code that the student can adapt or use as is. The text carefully walks the student through the activity while leaving considerable room for independent exploration.

The LiveScript uses five files. Two are simply for plotting purposes (coastlines and plate boundaries). Two are the workhorse MATLAB functions the Script draws on for visualization of crustal stresses. They do not need to be opened or edited at any point in the activity. The fifth (CMTs.txt) is created by the student. An example file is included here.

StressMap LiveScript (MATLAB Live Script 418kB Oct10 24) 
world coastlines for plotting ( 1.2MB Oct10 24) 
plate boundaries for plotting ( 167kB Oct10 24) 
Function for stress direction determination (Matlab File 2kB Oct10 24) 
Funtion for plotting earthquake beachballs (Matlab File 14kB Oct10 24)

Aim: Students will use focal mechanisms from globalcmt.org to make a crustal stress map of an area. They will be able to view and interact with the MatLab code in the LiveScript that makes the maps.

Background: To fully understand tectonics requires understanding stresses in the Earth's crust. Arguably, the most important dataset available for mapping stresses in the Earth's crust is using moment tensors from earthquake focal mechanisms to find the principle stress directions in a given region. This exercise uses a MatLab LiveScript to guide the student through the process of creating a stress map. Once completed, they will have seen how stress data is determined and how to interpret stresses in a tectonic context. The tool can easily be adapted for any region of the Earth.

The included example is for the Afar Triangle, where the Red Sea, Gulf of Aden and Ethiopian Rift come together as a rift-rift-rift triple junction.

Required materials:

stressmap.mlx (LiveScript, primary working document)

focalmech.m  (MatLab function for plotting beachball focal mechanisms)

stressdirections.m (MatLab function for finding principle horizontal stresses from a moment tensor)

world_coastlines.dat (file for plotting coastlines)

plate_boundaries_global.dat (file for plotting plate boundaries)

CMTs.txt (file containing moment tensor data, created by student)

            The included example file is for the Afar triangle

Outcomes: In the process, the script will create 2 maps of the area of interest: 1) beachball focal mechanisms, 2) the horizontal principle stresses

The directions for creating the moment tensor file (CMTs.txt) is in the LiveScript, but also included here

1. Go to globalcmt.org, click on "CMT catalog web search".
2. Set ending date to today starting date depending on how many earthquakes you want to retrieve (default will give you from 1976 to now). Set a lat-lon bounding box for your region of interest. Output type "GMT psmeca input".
3. You will get a list of all earthquakes in the catalog within your bounding box in your given time frame. The 13 columns in the output are: lon, lat, depth (km), Mrr, Mtt, Mff, Mrt, Mrf, Mtf (the 6 independent elements of the moment tensor), a scientific notation exponent for the tensor, X, Y, event ID.
4. Copy all the lines into a text editor. X & Y are just place holders. Find and replace both X & Y to NaN. Remove all letters at the ends of the last column (find and replace may be useful again).
5. Save the text file you just edited to "CMTs.txt". This is the datafile the LiveScript will access to plot focal mechanisms and stress directions.

The MatLab functions focalmech and stressdirections will both use the information in this file to make the plots.

The LiveScript concludes with several discussion questions that I use for assessment (along with the produced maps).

1. How does SHmax for strike slip events compare to P for reverse events? How does SHmin for strike slip events compare to T for normal events?
2. Write a synopsis of what you see. Does it make sense for your understanding of the area's tectonics? Explain
3. Are there unexpected deviations? What do you make of them?
4. Does the topography seem to influence the stress? (helps to compare with GeoMapApp)
5. Compare the two different data visualizations. What observations are easier to pick up on in the first map than the second? And vice versa?
6. What other interesting observations/interpretations capture your attention?

globalcmt.org (data repository for focal mechanism data).