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.
After completing this chapter, students will be able to:
- Locate and download GPS station data from the UNAVCO Data for Educators site.
- Create time series plots of GPS station data.
- Describe the motion of a GPS station by interpreting time series plots.
- Calculate a vector from the time series plot that represents the motion of a GPS station.
- Draw conclusions about plate motion in the Pacific Northwest by comparing the motion of GPS stations.
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.
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.
The following National Science Education Standards are supported by this chapter:
- 12DESS1.2 The outward transfer of Earth's internal heat drives convection circulation in the mantle that propels the plates comprising Earth's surface across the face of the globe.
- 12DESS3.3 Interactions among the solid Earth, the oceans, the atmosphere, and organisms have resulted in the ongoing evolution of the Earth system. We can observe some changes such as earthquakes and volcanic eruptions on a human time scale, but many processes such as mountain building and plate movements take place over hundreds of millions of years.
- 12EST2.2 Science often advances with the introduction of new technologies. Solving technological problems often results in new scientific knowledge. New technologies often extend the current levels of scientific understanding and introduce new areas of research.
- 2ASI1.3 Use technology and mathematics to improve investigations and communications. A variety of technologies, such as hand tools, measuring instruments, and calculators, should be an integral component of scientific investigations. The use of computers for the collection, analysis, and display of data is also a part of this standard. Mathematics plays an essential role in all aspects of an inquiry. For example, measurement is used for posing questions, formulas are used for developing explanations, and charts and graphs are used for communicating results.
- 8ASI1.3 Use appropriate tools and techniques to gather, analyze, and interpret data. The use of tools and techniques, including mathematics, will be guided by the question asked and the investigations students design. The use of computers for the collection, summary, and display of evidence is part of this standard. Students should be able to access, gather, store, retrieve, and organize data, using hardware and software designed for these purposes.
- 8ASI2.4 Technology used to gather data enhances accuracy and allows scientists to analyze and quantify results of investigations.
- 8DESS1.1 The solid Earth is layered with a lithosphere, a hot, convecting mantle, and a dense, metallic core.
- 8DESS1.2 Lithospheric plates on the scales of continents and oceans constantly move at rates of centimeters per year in response to movements in the mantle. Major geological events, such as earthquakes, volcanic eruptions, and mountain building, result from these plate motions.
The following U.S. National Geography Standards are supported by this chapter:
- Standard 1: The world in spatial terms: How to use maps and other geographic representations, tools, and technologies to acquire, process, and report information.
- Standard 3: The world in spatial terms: How to analyze the spatial organization of people, places, and environments on Earth's surface.
- Standard 4: Places and Regions: The physical and human characteristics of places.
- Standard 7: Physical Systems: The physical processes that shape the patterns of Earth's surface.
- Standard 18: The uses of geography: How to apply geography to interpret the present and plan for the future.
The following standards of the National Council of Teachers of Mathematics are supported by this chapter:
Number and Operations Standards for Grades 9-12:
- Understand vectors and matrices as systems that have some of the properties of the real-number system.
- Understand meanings of operations and how they relate to one another.
- Develop an understanding of properties of, and representations for, the addition and multiplication of vectors and matrices.
- Compute fluently and make reasonable estimates.
- Develop fluency in operations with real numbers, vectors, and matrices, using mental computation or paper-and-pencil calculations for simple cases and technology for more-complicated cases.
Geometry Standards for Grades 9-12:
- Understand and represent translations, reflections, rotations, and dilations of objects in the plane by using sketches, coordinates, vectors, function notation, and matrices.
This list shows estimates for completing each part
- Case Study: 10 minutes for reading and discussion
- Part 1: 10-20 minutes
- Part 2: 10 minutes
- Part 3: 10-20 minutes
- Part 4: 15 minutes
- Part 5: 20 minutes
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:
- The Plate Boundary Observatory (PBO) is installed and maintained by UNAVCO with funding from NSF and the National Aeronautics and Space Administration (NASA). The PBO provides data for study of surface deformation caused by the interactions between tectonic plates. Data come from a network of high precision Global Positioning System (GPS) monuments, borehole strainmeters, and tiltmeters throughout the western United States.
- The USArray is installed and maintained by the Incorporated Research Institutions for Seismology (IRIS). This project collects data from a transportable array of seismometers. The seismometers are sited in a 70-km grid to collect data for 24 months at time. After two years, the seismometers are removed and set up in another grid in a new region of the U.S.
- The San Andreas Fault Observatory at Depth (SAFOD) is installed and operated by Stanford University. SAFOD drilled across the San Andreas fault in Parkfield, California and collected a core of rock material from a depth of 2.3 miles below Earth's surface.
Further information and resources: