Using GPS data to analyze crustal strain -or- Where is the boundary between the Pacific and North American plates?

Authored by Laurel Goodell of Princeton University, this activity is an adaption of the UNAVCO educational module on crustal strain. It also borrows from an adaption of the module by Anne Egger at Stanford. Princeton graduate student Jon Husson contributed additions and modifications to the UNAVCO module Matlab code used in the exercise.
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The San Andreas fault in California is typically called upon to mark the boundary between the Pacific and North American plates. This activity tests this assumption by investigating crustal strain in southern California. Part A has students analyze by hand GPS time-series data from three stations in southern California, and plot a strain ellipse that characterizes the crustal strain in the triangular area defined by the three stations. Part B introduces students to Matlab code that does the same thing, and sets them loose to plot numerous strain ellipses to in the larger study area. Part C has students interpret their results in the context of the boundary between the Pacific and North American plates, and compare their results to other data such as seismicity and recently active faults.

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Learning Goals

Students get very practical experience with strain and an understanding of how strain operates on a crustal scale. Students also must separate their observations (the strain data), from their interpretations of the strain data in the context of motion along the boundary between the Pacific and North American plates in southern California. They also gain experience with Matlab, an important tool, which we are using in other lab exercises and problem sets – mostly we give the students codes to use, although they are encouraged to play around with the code on their own, and learn how to use Matlab to plot data, for example.

Context for Use

We used this activity as the very first lab for our Spring 2013 GEO373 structural geology class. It would, of course, also be valuable later in the course after covering strain in lecture. But in this case, it was preceded by only one lecture, so students essentially went into it "cold." It worked, though, and as we get to topics such as strain, stress, and faulting we have the experience of the exercise to refer to. We are considering having students re-visit their write-ups later on in the course, especially, after our spring break field trip to the area in question!

Students worked on it in the 3-hour lab session, which included an introduction to Matlab, and then completed the report on their own for submission the following week. This was an excellent application of Matlab which some students had used before, and others with no Matlab experience got a manageable dose. Materials include graph paper, tracing paper, and computer access (student laptops).

Description and Teaching Materials

The lab session starts out with an brief introduction to strain and the plate tectonic setting of southern California.

Students then work through Part A, which has step-by-step instructions on how to choose three appropriate GPS stations, interpret and plot the GPS data, and sketch a strain ellipse describing the strain within the triangular area defined by the three stations. Excellent resources for this portion are UNAVCO module's PowerPoint presentations on "Intro to GPS" and "Using triangle of GPS velocities to determine strain," available at:

For Part B, students are given an introduction to Matlab, and then instructed how to use the Matlab code to draw multiple strain ellipses across the study area.

Part C has students interpret their results in relationship to the plate boundary setting, and also compare their results to trends of recent seismicity, active faults in the area, and long-term average plate motion in the area. Some of these these are included in the handout, as well as in a kmz file to explore with Google Earth.

Teaching Materials

Teaching Notes and Tips


Students can themselves check the strain ellipse they determine by hand, by checking results with the strain calculator. The instructor can easily assess the strain ellipse map produced by each student, and responses to discussion questions at the end assess the level of student understanding, as well as their ability to integrate other types of data into their interpretation.

References and Resources