Applications for Precision GPS: Seismology, Volcanic Eruptions, Ice Sheet Dynamics, and Soil Moisture Kristine M. Larson Dept. of Aerospace Engineering Sciences University of Colorado

Outline Traditional solid Earth geophysical applications of GPS Space Weather (briefly) Ice Sheets Earthquakes Volcanoes Soil Moisture

Start with 24-hour averaged positions using dual-frequency receivers And then you wait.

Plate Boundary Deformation GSI

ITRF2005 Altamimi et al., 2007 Global Plate Motions

Space Weather Implications Ambiguity resolution is critical. To the extent that better knowledge of TEC can aid ambiguity resolution, solid Earth geophysicists care about space weather. Some new applications described in this talk have obvious real-time potential - for which ambiguity resolution is even more important (and challenging).

Space Weather Implications 2 Many GPS receivers have been installed to catch “once in a career” geophysical signals. Receiver failure because of space weather is always a concern. For example, my dissertation data were collected in campaigns during June 1986, September 1987, March 1988, and March 1989.

Using GPS at time scales less than a day Ice sheet speeds, ~100 m/yr (1-2 cm/hr); are they linear? Earthquakes, cm/sec; large accelerations. Volcanoes, 1-2 cm/hr; not linear, but low accelerations. Soil moisture, non-traditional GPS application.

Ice sheet velocities in Greenland Install poles; measure position. Return following year; remeasure position; compute velocity. Thomas et al., 2000

Installed a GPS receiver on the ice Number of days where temperature was above freezing.

Zwally et al., 2002.

Earthquakes Global plate velocities are based on ~10 million measurements per site. Each Greenland (12-hr) ice sheet velocity is based on ~1000 GPS measurements. For seismic applications, each position is based on 6-10 measurements. Denali, Alaska Earthquake, November 2002

Geodetic Challenges Multipath (reflected signals) is important (and doesn’t difference out). Oscillations in position time series reflect different multipath environments

Good news: multipath looks the “same” from day to day The GPS orbital period is 1/2 sidereal day, shift time should be 1 sidereal day (or one day minus 236 seconds). Is it?

Repeat Times Vary by Satellite Choi et al., 2004

Repeat Times Vary by Day of Year See the GPS Tool Box for code to calculate repeat times.

Does it Really Matter? multipath Badly corrected multipath

GPS Seismograms Miyazaki et al., 2004; Emore et al., BSSA, East

Advantages of GPS seismology

Another advantage of subdaily GPS Earthquake Postseismic Each point is a 24-hr average

Great earthquakes are often more complicated than this M8 M7.4

Past two years: 55 cm extension 25 cm uplift Dike intrusion Deflation

Larson and Miklius, in preparation cm

Multipath/Soil Moisture The frequency of ground multipath (reflections) is determined by the antenna height. The amplitude of ground multipath is determined by ground reflectance, which can be related to soil moisture content.

Soil Moisture day of year Kurc and Small, 2004 PBO Site Marshall, CO Larson et al., GPS Solutions, 2007 The data are free.

And many other networks

Conclusions There are lots of geophysical problems that benefit from high- precision GPS measurements at sub-daily time intervals. Constellation asymmetry, the troposphere and multipath are currently the limiting error sources. Acknowledgements Co-authors NSF, NASA, JSPS IGS & ITRF UNAVCO, NEHRP, NGS, USGS, CORS, GEONET, NRCAN, SOPAC, CDDIS, IRIS, EUREF, SCIGN, GIPSY.