Teaching Notes

Example Output

North Time Series Plot. Click the image for a larger view.
Users generate Time Series Plots for two directions of GPS data and add trendlines. The slope of the trendlines indicate station velocity. They use a graphical or mathematical method to add the vectors, then overlay them on a map (not shown) to interpret regional geology.

Grade Level

This chapter is appropriate for high school and undergraduate courses in Earth Science and Geology. It offers a quantitative exercise that complements the descriptive nature of units on plate tectonics.

For maximum effectiveness, students should have been introduced to important key concepts of plate tectonics.

Learning Goals

After completing this chapter, students will be able to:


This chapter introduces students to applying GPS data to the study of tectonic plate motion. The chapter promotes data analysis skills, technology skills in using spreadsheets and online mapping sites, practice in generating and interpreting graphs, and three-dimensional visualization skills.

The activity can be expanded and customized in order to support other science topics and curriculum standards. Some possibilities of further exploration and application are listed below.

  • Compare motion and deformation in the Pacific Northwest to that of the California region.
  • Investigate volcanic deformation near Mount St. Helens and St. Augustine volcanoes.
  • Analyze the motion of the Pacific and North American Plates by generating and studying velocity vectors for Southern California.

Background Information

Before starting this EET chapter, students have been introduced to key concepts of Plate Tectonics. The activity deepens student understanding of plate tectonics by focusing on the the details of plate movement in a particular region.

The Global Positioning System (GPS)

The global positioning system (GPS) is a fleet of about 30 satellites orbiting Earth approximately 20,000 kilometers above the planet's surface. A GPS receiver on the ground picks up signals from these satellites, and processes them to determine its position via a sophisticated form of triangulation. A position can be calculated using signals from three satellites, but at least four satellites are always used to minimize errors due to signal timing and less-than-optimal satellite spacing in the sky. You may already be familiar with handheld GPS units which people use for navigation in cars, recreation (hiking and geo-caching), mapmaking, and land planning. There are several important differences between handheld GPS units and the high-precision ("differential") GPS units that Earth scientists use in their research.

Handheld GPS receivers calculate positions that are known as autonomous solutions. In other words, each handheld GPS receiver is independent from all other receivers and uses only satellites to calculate positions. As a result, handheld GPS receivers are unable to correct for many error sources and their accuracy is on the order of meters.

Differential GPS uses two GPS receivers to calculate a position: a stationary receiver ("base station") whose location is accurately known from surveying, plus a roving ("moving") receiver. Both receivers continuously calculate their positions from the satellites. The base station compares the calculated position with its known location and "differences" between the two measurements to determine the error in the GPS signal. Then, the base station sends the error corrections to the roving receiver. Using differential GPS for navigation allows scientists to minimize the errors associated with measuring positions. To obtain positions such as those described in this chapter, scientists use a more advanced type of differential processing to generate a network solution. In this network processing, a large number of fixed GPS receivers are processed together to achieve very precise relative positions. These relative positions are related to an "absolute" reference frame to give coordinates with respect to Earth's surface itself.

Using GPS to Study Plate Motion and Crustal Deformation

GPS stations used for geodesy are cemented into the ground so that the instrument is tightly coupled with the bedrock. Changes in the location of a GPS station are therefore caused by movement of the Earth's surface. By comparing the motion of several GPS stations in a region over time, scientists can detect motion of tectonic plates and infer deformation of the Earth's crust.

Although slight motion occurs continuously, when an earthquake event happens, the ground on either side of the fault moves instantaneously, sometimes resulting in strong shaking. GPS measurements enable scientists to map the location and distance of these displacements. Although we cannot feel it, the crust on either side of the fault continues to slip even when earthquakes are not occuring. Scientists can record this motion with GPS. By comparing the locations of GPS monuments on different plates, scientists can detect the rate at which the plates are moving.

Analyzing GPS Station Data

UNAVCO's Data for Educators site provides prepared time-series plots and downloadable GPS data, primarily for stations located within the United States. The data show daily positions of the station compared to its reference location. The set of data includes positions measured in the North, East, and vertical directions from the reference location.

Plotting a station's position in North and East directions over time reveals the overall direction and average rate that a station is moving. Trendlines from the North and East plots are used to generate a composite vector that shows the horizontal motion of the station over time.

Instructional Strategies

Once students have learned the basic concepts of plate tectonics, you can invite them to explore the details of this global phenomenon at a regional scale. You might launch a class-wide conversation around the question "How does the subduction of the Juan de Fuca plate impact the Cascadia Region?" To get the conversation started, ask students to describe the general process of subduction, before they apply that understanding to the relatively tiny Juan de Fuca plate. Change their perspective by querying them about the details that they can imagine occurring on the overriding North American plate.

One way to approach this investigation is to split the class into up to 16 small teams. Have each team access and prepare GPS data for one of the stations listed under "Show me a list of stations" at the beginning of Step 1. Each team would generate the time series plots and determine the velocity vector for their station. As a group, the students plot their vectors on a single map to facilitate a discussion of the regional geology. To prepare students to perform the tasks themselves within the limited time that they have access to computers, you may want to use a computer projector to demonstrate some portions of the process ahead of time.

Science Standards

The following National Science Education Standards are supported by this chapter:

Grades 9-12

Grades 5-8

Geography Standards

The following U.S. National Geography Standards are supported by this chapter:

Other Standards

The following standards of the National Council of Teachers of Mathematics are supported by this chapter:

Number and Operations Standards for Grades 9-12:

Geometry Standards for Grades 9-12:

Time Required

This list shows estimates for completing each part

Other Resources

EarthScope is a national Earth Science program funded by the National Science Foundation (NSF) to study the geologic structure and evolution of the North American continent and understand processes controlling earthquakes and volcanoes. The data from these three observatories are unparalleled in terms of scope and resolution, and they are providing a detailed picture of North America. EarthScope includes three observatories:

Further information and resources:

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