Using GPS to Study the Earth Professor Kristine M. Larson Dept. of Aerospace Engineering Sciences University of Colorado

Outline What did people do before GPS? How does GPS work? How do we use GPS to study the Earth? –Plate Tectonics –Subduction Zones –Extensional Deformation –Strike-Slip Deformation –Vertical Deformation

Before We Get Started 25 mm scientists use the metric system! plate motions of 2.5 cm/yr (25 mm/yr) is ~ 1 inch/yr. a grain of sugar is ~1 mm

How did people navigate before GPS?

5 How did people know where they were before GPS? Stars were used to calculate latitude and longitude.

6 Global Positioning System Designed by the U.S. Department of Defense. Specific goals: position in real-time, anywhere on the globe, works in all kinds of weather. First launch in 1978; operational GPS is the best navigation and positioning system ever developed.

The Future

How does the GPS constellation work?

Similar to triangulation method used to locate earthquakes. S S S How well can you locate an earthquake?

For GPS, we “transmit” signals from the squares The GPS user is located at the diamond.

To make this work: GPS uses really good clocks.

GPS satellites orbit the Earth at 20,000,000 meters A GPS satellite writes down the time on the signal before it sends it to the Earth. When the signal is received, the time sent and time received are compared and multiplied by the speed of light (300,000,000 m/sec). It takes a GPS signal ~0.07 seconds to travel to Earth.

But GPS only works if: You know where the satellite transmitters are. –GPS signals also transmit the satellite locations. And you can tell the transmitters apart. –The signals are made in a way so that you can tell which transmitter sent them. For real positioning and navigation problems, we use the intersection of three spheres, not three circles.

Intersecting Spheres But only 1 point is on the Earth

How well can a GPS receiver calculate latitude, longitude & height?

I use GPS to study how the Earth deforms: plate tectonics, volcanoes, and earthquakes. How fast do these tectonic plates move?

plate speeds - in cm/yr this is about how fast your fingernails grow

How can I measure plate tectonics with an instrument that only calculates latitude and longitude to an accuracy of 1-5 meters? I use a more expensive receiver and the GPS receivers are not moving around (usually).

19 typical GPS setup receiver - $15,000 solar panelsantenna - $5,000 monument- drilled into bedrock Burgess Ranch, Utah

how do we calculate plate motions? one year 50 mm each circle is one day’s position calculated from GPS Latitude Change ~30 mm/yr

Southern California Australia Antarctica We are doing this all over the world

Hawaii Iceland Holland

Plate Boundary Observatory

so now we can compare GPS measurements with predictions from plate tectonics

25 The Rules of Plate Tectonics Interiors of tectonic plates do not deform. Tectonic plates rotate around a fixed point. No vertical motion within plates. All the interesting motion occurs at the plate boundaries! 25

26 The Rules of Plate Tectonics Interiors of tectonic plates do not deform. Tectonic plates rotate around a fixed point. No vertical motion within plates. All the interesting motion occurs at the plate boundaries! 26

fixed rotation speed depends on distance from rotation axis

The North American plate is rotating about the blue triangle

All the plates together Blue lines are the plates boundaries Each red arrow was measured by GPS.

30 The Rules of Plate Tectonics Interiors of tectonic plates do not deform. Tectonic plates rotate around a fixed point. No vertical motion within plates. All the interesting motion occurs at the plate boundaries! 30

31 ChileSan Andreas we’ll use the Wasatch as example of extensional fault

32 subduction zone

34 How much did the Earth deform during the recent earthquake in Chile?

San Andreas Fault

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San Andreas Fault P623 PVRS Can we see the San Andreas fault motion in GPS data?

mm/yr -3.5 mm/yr -2.2 mm/yr 28.8 mm/yr mm/yr -0.7 mm/yr Velocities with respect to North America PVRSP623

41 San Andreas Fault

42 Extensional Deformation

earthquakes and plate boundaries

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45 GPS Sites

46 Wasatch Fault

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P114 P100 P114 P118 P030

P114 P100 P030 P118 P114

P mm/yr

P100 P114 P mm/yr

Complicated Plate Boundary

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54 GPS results for Asia

55 The Rules of Plate Tectonics Interiors of tectonic plates do not deform. Tectonic plates rotate around a fixed point. No vertical motion within plates. All the interesting motion occurs at the plate boundaries! 55

Churchill, Manitoba

Why is Churchill going up? Height Velocity = 10.8 mm/yr

Canada was covered by ice 11,000-14,000 years ago. And ice is very heavy.

P564

Why is P564 going down? -37 mm/yr

63 The Rules of Plate Tectonics Interiors of tectonic plates do not deform. Tectonic plates rotate around a fixed point. No vertical motion within plates. All the interesting motion occurs at the plate boundaries! Rules are correct, except when they’re not... 63

Pacific NW Subduction Zone the green line is the prediction from plate tectonics

65 Final Remarks Being a scientist is a little bit like being a detective. Models are good starting points (for example, plate tectonics), but the world is generally more complicated than our models predict.

66 acknowledgements Seth Stein, Northwestern University Plate Boundary Observatory, UNAVCO NEIC IRIS Education/Outreach USGS Education/Outreach Zuheir Altamimi, ITRF2005 results SOPAC Pacific Geoscience Centre, British Columbia, Canada USGS circular 1182 Gan et al., JGR, 2007 NSF grants to CU/Larson: EAR , AGS

extra slides

Photo: Jim Kauahikaua Volcanoes

55 cm extension 25 cm uplift

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