GPS, Strain, and Earthquakes

Summary

Understanding how the Earth's crust deforms is crucial in a variety of geoscience disciplines, including structural geology, tectonics, and hazards assessment (earthquake, volcano, landslide). With the installation of numerous high precision Global Positioning System (GPS) stations, our ability to measure this deformation (strain) has increased dramatically, but GPS data are still only rarely included in undergraduate courses, even for geoscience majors. In this module students analyze GPS velocity data from triangles of adjacent GPS stations to determine the local strain. Students learn about strain, strain ellipses, GPS, and how to tie these to regional geology and ongoing societal hazards. A case study from the 2014 South Napa earthquake helps students make connections between interseismic strain and earthquake displacements.

Strengths of the Module

Most structural geology courses only cover finite strain (generally through the analysis of deformed fossils), missing the rich opportunity to investigate ongoing strain (infinitesimal strain) now measurable through methods such as GPS. This module introduces geoscience majors to Plate Boundary Observatory (PBO) GPS data in order to study infinitesimal strain and connect it to broader tectonic settings and hazards.

The earlier units help ground the students in the societal impact of earthquakes and give them an opportunity to explore strain and deformation through a variety of physical models ranging from Silly Putty® to drywall compound. This affective and intuitive introduction to strain provides a firm foundation from which to learn about strain quantitatively.

Students work with GPS velocity data in sets of three stations in the same region that form an acute triangle. By investigating how the ellipse inscribed within this triangle deforms, students learn about strain, strain ellipses, GPS data, and how to connect these to regional geology and hazards. The calculations can be done using provided Excel or Matlab calculators (most typical) or students can be asked to do the calculations or coding themselves.

The Unit 6 summative assessment has the students select a set of three GPS stations in an area of interest to themselves. For many students this is their first experience with "research" and possibly their first time giving an oral presentation. Despite initially being daunted by this idea, students generally end up doing well on it and gaining notable confidence with GPS data.