1. Sally McGill California State University, San Bernardino
Using GPS to monitor tectonic plate motions across the San Andreas fault in southern California
Sally McGill
California State University, San Bernardino
This material is based upon work supported by the National Science Foundation under Grant Number Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

2. Note to teachers:
I generally introduce this lesson with a review of plate tectonics including:
The three types of plate boundaries and the types of motion associate with each
A world map of the ocean floors or a world map of tectonic plates, on which I point out that the Pacific Plate
is created at a mid-ocean ridge in the southeastern Pacific ocean
It travels northwestward until it is consumed at subduction zones off the coasts of Alaska, Japan, etc.
And along the way it slides horizontally past California, which is why we have a transform plate boundary in California.

3. Pacific-North American Plate Boundary
What type of motion occurs along the San Andreas fault?
This map zooms in on the San Andreas fault, showing it as a transform fault, linking up the divergent boundary segments in the Gulf of California with those off the coast of Oregon and Washington.

4. Pacific-North American Plate Boundary
What direction is the Pacific Plate moving relative to North America?

5. Pacific-North American Plate Boundary
What direction is the Pacific Plate moving relative to North America?
Toward the northwest
How do we know?

6. P612
LiDAR image of the San Andreas fault just northeast of campus of Cal State University, San Bernardino, showing right-laterally offset drainages. This is one way that we can tell that the Pacific plate (located southwest of the San Andreas fault) is moving northwestward. The other way we can tell is because GPS stations on the Pacific Plate, like station P612, shown here (in photograph and labeled on the LiDAR image) record northwestard motion.

7. Portable GPS antenna mounted on a tripod and centered exactly over a benchmark
Global Positioning System (GPS).

8. GPS has been used all over the Earth to measure the rate of motion of the tectonic plates.

9. POINT65 Oak Hills H.S. MT. LUNA ARROWHEAD 6106 NORCO San Bernardino
Mountains
ARROWHEAD
6106
Here in southern California, a group of undergraduate students, high school teachers and high school students have helped to monitor motion of the Pacific plate relative to the North American plate across the San Andreas fault, using GPS. This map shows 5 of the stations from which we have collected GPS data over a 10-year period, from
NORCO

10. Sites on the North American Plate:
Sites on the Pacific Plate:
POINT65
Oak
Hills
H.S.
MT. LUNA
San Bernardino
Mountains
ARROWHEAD
6106
Teacher: Ask students to name which sites are located on the North American plate (Point 65, Mt. Luna, and Arrowhead) and which are located on the Pacific Plate (6106 and NORCO). For the purpose of this exercise, we think of the San Andreas fault as being the boundary between the two faults. In reality, the plate boundary is a broad zone that includes active faults that extend almost the whole width of California.
NORCO

11. Active Faults of Southern California
The red box shows the location of the map in the previous slide, and the 5 GPS sites, within the broader context of southern California. Each yellow line is an active fault.
Active Faults of Southern California

12. Which site do you expect to be moving the fastest (relative to the North American Plate)?
The slowest?
NORTH AMERICAN PLATE
POINT65
Oak
Hills
H.S.
MT. LUNA
San Bernardino
Mountains
ARROWHEAD
6106
Students will often expect the sites closest to the fault to be moving fastest. Let them say what they think at this point in the lesson, without correcting them, and then return to this question at the end, after they have plotted the data that show that NORCO is the fastest moving site (relative to North America).
NORCO
PACIFIC PLATE

13. Site--POINT65
Date
North position (cm)
East position (cm)
1993.6
0.00
1995.6
3.40
-5.92
1999.8
9.01
-10.44
2002.5
13.80
-16.62
2002.9
14.39
-16.73
2003.5
14.92
-16.76
2004.5
16.72
-18.88
2005.5
17.36
-20.30
2007.8
20.51
-22.64
2008.5
21.35
-23.79
2009.6
22.89
-24.66
2010.5
23.83
-25.97
Demonstrate how to plot the north and east positions as a function of time.
I project this onto a white board and demonstrate plotting the points. One graph will show the north coordinate of Point 65 as a function of time, and the other graph will show its east coordinate.

14. Site--POINT65
Date
North position (cm)
East position (cm)
1993.6
0.00
1995.6
3.40
-5.92
1999.8
9.01
-10.44
2002.5
13.80
-16.62
2002.9
14.39
-16.73
2003.5
14.92
-16.76
2004.5
16.72
-18.88
2005.5
17.36
-20.30
2007.8
20.51
-22.64
2008.5
21.35
-23.79
2009.6
22.89
-24.66
2010.5
23.83
-25.97
When students draw a best-fit line to the data, encourage them to draw one straight line that passes as close as possible to as many points as possible. They should NOT draw a jagged line that passes through the center of each point. Keep in mind that there is some error or uncertainty associated with each GPS measurement. The best-fit line represents the most likely guess of where the station actually was at any given point in time.

15. After student groups have calculated the north and east velocities of their station, they should plot a vector diagram like this, using a ruler to get they length of each arrow exactly right. They can use 1 cm on the ruler = 1 mm/yr of velocity. Then draw the connecting arrow (red above) and measure its length with the ruler. This is the overall horizontal velocity of the station.

16. Which site is actually moving the fastest (relative to the North American Plate)?
The slowest?
NORTH AMERICAN PLATE
1.6 cm/yr
POINT65
Oak
Hills
H.S.
1.7 cm/yr
MT. LUNA
2.4 cm/yr
ARROWHEAD
3.0 cm/yr
3.3 cm/yr
6106
After student groups have calculated the overall horizontal velocity of each station, you can plot them on a projection of slide 10, or show this slide. Next you should discuss why the results look like this. Why do the site velocities gradually increase as you move from the North American plate onto the Pacific Plate? Why are the sites on the North American plate moving at all, given that our data represent the velocity of each site, relative to North America? Why is there only a gradual (not abrupt) change in the velocities across the San Andreas fault? To answer these questions, we need to understand the theory of elastic rebound.
NORCO
PACIFIC PLATE

17. Elastic Rebound
1. A road is constructed across the fault shortly after stress on the fault has been released by the previous earthquake.

18. Elastic Rebound
2a. During the following decades or centuries, the tectonic plates on which the road is built continue to move even though the fault itself is locked by friction and is not slipping at the surface or in the upper part of the Earth's crust. This causes the plates to bend elastically within km on either side of the fault. The more the plates bend, the more the shear stress builds up on the fault.

19. Elastic Rebound
2b. We can use GPS to measure how fast the plates are bending. The blue arrows illustrate the speed and direction of motion of survey benchmarks located in a line across the fault. Note that during this interseismic period (in between earthquakes) the velocities of the stations change gradually, rather than abruptly, across the fault.

20. Elastic Rebound
3. Eventually the shear stress on the fault becomes stronger than the friction that is keeping the fault stuck, and the two sides of the fault suddenly slide past each other, resulting an earthquake. After the earthquake we can see an abrupt shift in the road across the fault, and the road is no longer bent.

22. Which site is actually moving the fastest (relative to the North American Plate)?
The slowest?
NORTH AMERICAN PLATE
1.6 cm/yr
POINT65
Oak
Hills
H.S.
1.7 cm/yr
MT. LUNA
2.4 cm/yr
ARROWHEAD
3.0 cm/yr
3.3 cm/yr
6106
Now we can understand that these GPS vectors look very much like diagram 2b, shown in slide 18. The San Andreas fault only slips during large earthquakes, and we haven’t had a major earthquake on this part of the San Andreas fault since Instead, the fault is locked and the plates are bending across the fault. The western edge of the North American plate is getting dragged along by the Pacific Plate, and that is why the sites on the North American plate are moving, rather than staying still. That is also why NORCO is the fasted moving site (it is farthest onto the Pacific Plate), and why there is only a gradual (not abrupt) change in the velocity of the stations as you move across the San Andreas fault.
NORCO
PACIFIC PLATE