Honors Earth Science
Lab: Using GPS to Track Plate Motion
Name_________________________________________
Period _________ Date _____________________
Reading GPS Time Series Plots Worksheet
The Global Positioning System, GPS, is used to study the Earth, how tectonic plates move
and how the Earth deforms. GPS monuments are attached the ground and measure how a particular
location on the surface moves relative to other points.
While the GPS units in a car measure movement in miles per hour, high precision GPS units
used for scientific studies measure how fast or slow the Earth’s plate move and can measure a few
millimeters in a year. Even millimeters can be important because slow moving rock can cause big
earthquakes. GPS stations used for scientific work are more precise for many reasons:
 the unit uses multiple signals coming from satellites
 the clocks within the unit are atomic digital rather than crystal analogue
Anatomy of a GPS used to measure Plate Motion
GPS antenna inside of dome, solidly attached into the ground with
braces. If the ground moves, the station moves.
Solar panel for power
Equipment enclosure
 GPS receiver
 Power/batteries
 Communications/ radio/ modem
 Data storage/ memory
What is needed to locate the receiver in 3D space and time:


3 satellite signals
o Up-Down
o North-South
o East-West
A 4th satellite used for time accuracy
2
1
3
4
1. GPS stations are located at points 1, 2, 3, and 4. Predict how they will move relative to each
other.
Stations
Prediction
1 and 2
2 and 3
3 and 4



SBCC GPS STATION
Located near Mission Viejo, CA
Position data collected every 30 seconds
One position estimate developed for each day
o North = +
o South = - North
o East = +
o West = -East
o Up = + Vertical
o Down = - Vertical
Date
North (mm)
East (mm)
Vertical (mm)
1/1/2004
-37.67
36.57
2.33
1/2/2004
-38.04
35.73
5.63
1/3/2004
-37.16
35.83
4.69
1/4/2004
-37.34
36.34
5.36
1/5/2004
-37.59
36.44
9.11
…
…
…
…
-9.43
9.63
2.36
16.48
-18.09
7.35
45.98
-43.42
-6.43
1/1/2005
What do the graphs show?
 Axes
1/1/2006
o X – measured in mm
1/1/2007
o Y – measured in 0.10 years
 “North” graphs
o Positive slope indicates northward
movement
o Negative slope indicates southward
movement
 “East” graphs
o Positive slope indicates eastward movement
o Negative slope indicates westward
movement
 “Height” graphs
o Positive slope indicates upward movement
o Negative slope indicates downward
movement
Real GPS Data at a Divergent Boundary
So far, we’ve explored the directions a GPS monument can move, but not how fast a monument is
moving. By analyzing multiple GPS Time Series Plots you can determine the directions and rates of
regional deformation. Remember, the GPS stations are permanently cemented to the ground, so if
the monument is moving, the Earth’s plates are moving or deforming. Look at the data from the GPS
monuments below, from Iceland.
2. What direction is the GPS Monument REYK moving? What evidence do you have for that?
3. What direction is the GPS Monument HOFN moving? What evidence do you have for that?
4. Describe the relative motion between REYK and HOFN.
Work through the next steps to construct the vectors for these two stations. Plot them on graph
paper first then plot them on the map of Iceland.
Look at the 1997 to 2007. In 10 years:
5. How far north/south did REYK move? ________ mm to the _______ (North or South)
6. How far east/west did REYK move? ________ mm to the _______ (East or West)
7. Convert your values to mm/yr
Look at 1998 and 2008. In 10 years:
8. How far north/south did HOFN move? __________ to the _______ (North or South)
9. How far east/west did HOFN move? ____________ to the _______ (East or West)
10. Convert your values to mm/yr
This time we’re going to draw two vectors, the north vector and the east vector and then combine
them to create a single vector. Each block = 1mm. A vector shows magnitude (the rate of motion)
and direction (of motion). Remember:
 Tail is the location of the GPS monument
 Length of arrow is the magnitude
 Plotted on a map to show direction
Draw the first vector arrow along the North axis with the tail at 0.
Draw the east vector arrow along the East axis with the tail at 0.
To add the vectors together re-draw the East arrow by placing the tail of the East vector at the
head of the North vector; then draw a new vector from the tail of the North vector to the arrow
head of the East vector.
11. Describe how the vectors are different and how they are the same.
12. In 500 years, how much further apart will the stations moved?
13. Give one possible explanation for the way the ground is moving in Iceland.
14. There are gaps in the data for Monument B. Given what you know about how GPS data are
collected, give two possible causes for the gaps.
Next Up: Convergent Boundaries
The Pacific Northwest is tectonically active, experiencing frequent earthquakes and volcanic
eruptions. These natural phenomena are the result of the subduction of the Juan de Fuca plate
beneath the North American Plate. Using the same technique that you applied to the GPS stations of
Iceland, sketch the vectors of motion for the following stations on the map of the state of
Washington. When you are done, answer the following questions:
15. Describe the motion indicated by each vector.
STATION
DESCRIPTION
SEAT
P020
ALBH
NEAH
P453
16. How is the state of Washington changing because of the subduction zone and convergent
boundary to the west of it?
ALBH
P453
SEAT
NEAH
P020
Sliding on By: Transform Boundaries
Plot the velocity vectors for each station (Time Series Plots are on next pages)
17. How do the velocities at each station change from west to east?
18. Using these extra stations, where would place the location of the plate boundary/the San
Andreas fault?
19. What other types of data might you explore to support your decision for this location?