"This must be left to the geodesists. I have no doubt that in the not too distant future we will be successful in making a precise measurement of the drift of North America relative to Europe."-- Alfred Wegener, 1929

When Alfred Wegener proposed the theory of continental drift in 1915, the theory was skeptically received. The idea that continents drifted apart was an old one, rooted in the remarkable fit of the coasts of South America and Africa. Still, without compelling evidence for motion between continents, the idea that such motions were physically impossible prevented most geologists from accepting Wegener's ideas.

Wegener realized that proving continents moved apart was a formidable challenge. Although geodesy - the science of measuring the shape of and distances on the earth - was well established, standard surveying methods offered no hope of measuring slow motions between continents far apart. Wegener thus decided to measure the distance between continents using astronomical observations - an example of what we now call space-based geodesy.

Using an extraterrestrial reference was not new. In about 230 BC, Eratosthenes found the Earth's size from observations of the sun's position at different sites. Since then, navigators found their positions by observing the sun and stars. Measuring continental drift, however, called for measurement accuracies far greater than ever before to show small changes in positions over a few years. Wegener's attempts failed, and the idea of continental drift was largely rejected.

By the 1970's the story was very different. Geologists accepted continental drift, in large part because paleomagnetic measurements, those based on the geometry and history of Earth's magnetic field, showed that continents had in fact moved over millions of years. It thus seemed natural to see if modern space-based technology could accomplish Wegener's dream of measuring continental motions over a few years.

Three basic approaches were attempted. Each faced formidable technical challenges - and all succeeded. Very Long Baseline Interferometry uses the difference in the time when radio signals from distant quasars arrive at different points on earth. Satellite Laser Ranging uses the time required by light from ground-based lasers to bounce off satellites. The third approach relies on the travel time of radio signals between satellites and ground stations. The most popular, convenient, and least costly of such measurements are made using the NAVSTAR satellites of the Global Positioning System (GPS).

Space Geodesy with the Global Positioning System

Space geodesy uses space-based technologies to measure the positions of geodetic monuments to accuracies of better than a centimeter, even for sites thousands of kilometers apart. Hence measurements of positions over time yield relative velocities to precisions almost unimaginable during the early days of plate tectonic studies. Moreover, these studies cover much larger areas than would have been practical with traditional geodesy, which is restricted to sites which are in view of each other.

Although various systems provide similar data, GPS is the system of choice for most tectonic applications. GPS was developed in the late 1970's by the U.S. Department of Defense for real-time positioning, navigation, and time transfer. A constellation of 21 NAVSTAR satellites transmit coded timing signals on a pair of microwave carrier frequencies synchronized to very precise on-board atomic clocks. By determining the ranges to a minimum of four satellites from the signal delays and the broadcast satellite orbit information, a single GPS receiver can determine its 3-dimensional position and time. The position accuracy is 5 to 100 meters and the time accuracy is at the millisecond level or better, depending on the level of signal degradation (selective availability) imposed by the military.

Accuracy can be improved by differential techniques, which use two or more GPS receivers to remove receiver and satellite clock drift errors. For receivers using only the differential-coded signals, the accuracy ranges from 2 to 10 meter for small hand-held units to better than 1 meter for survey-grade receivers.

The scientific community, building on experience with other space geodetic techniques such as Very Long Baseline Interferometry, determined that GPS could also be used to make even more precise geodetic measurements. The improvement to cm-level or better precision is obtained by usingthe phase delays of the microwave carriers. The use of differential signals reduces clock errors. Combining both transmitted frequencies removes delays caused by the passage of the GPS signals through the ionosphere. Tropospheric delays can be estimated to reduce position errors and, in addition, to provide valuable atmospheric data.

The final element for high precision surveys is provided by continuously operating global GPS tracking stations and data centers under the auspices of the International GPS Service (IGS). The IGS provides essential tracking data, high accuracy GPS satellite orbit and clock information, earth rotation parameters, a unified reference frame of station velocities and coordinates, and ionospheric information. As a result, both IGS site positions and those from local GPS studies using IGS solutions can achieve coordinate precision of 5 to 10 mm anywhere on Earth. The worldwide distribution of IGS stations is thus crucial for local GPS studies and provides valuable data for global studies.

A range of GPS instruments and techniques is available to address a wide variety of research applications (see table below). In some applications, permanent GPS receivers are installed. In others, geodetic monuments are occupied in periodic campaigns using portable receivers. GPS is also used for various mapping applications. The specific approach used depends on the precision requirements and the cost and logistics constraints.

Plate Motions

GPS data are giving a clear picture of present day plate motions, for comparison with global plate motion models which average over the past few million years (2-4). In general, the results are similar, consistent with the idea that although motion at plate boundaries can be episodic, the viscous asthenosphere damps out the transient motions, causing steady motion between plate interiors. There appear, however, to be some intriguing discrepancies. In some cases, such as the Caribbean Plate, GPS data give a picture quite different from the plate motion model, which was suspected to be uncertain due to limitations in the 3-Myr data (5).

Figure 1 - Motions of GPS sites in ITRF-94 reference frame, after (3). Note Hawaiian site motion along the island chain, as expected because these GPS motions essentially correspond to present day absolute plate motions in a fixed hotspot reference frame (2).

The general agreement is important for seismic hazard analysis, because it implies that data on various various time scales can be combined to study earthquake recurrence. GPS data are thus becoming a key part of earthquake hazard studies.

Plate Boundary Zones

GPS data are providing detailed views of the spatial distribution of deformation within plate boundary zones. This is important because the simplest view of plate tectonic implies that all deformation occurs across the boundary between idealized rigid plates. In fact, earthquakes, volcanism, and other deformation occur over broader plate boundary zones, which appear to cover about 15% of Earth's surface (6). Although plate motion models predict only the integrated motion across the boundary, GPS data can show how this deformation varies in space and time. Understanding this deformation is a major geological problem, which also has social relevance because of the resulting geologic hazards to populated areas.

Figure - 2 - Comparison of the idealized rigid plate geometry to the broad boundary zones (red) implied by seismicity, topography, or other evidence of faulting. The precise geometry of these zones, and in some cases their existence, is under investigation. (Figure by T. Shoberg and P. Stoddard, after (6)).

The boundary zone between the large North American and Pacific plates is especially interesting, because the overall motion between the two plates varies from spreading in the Gulf of California, to strike-slip along the San Andreas system, to convergence in Alaska. The zone also includes smaller microplates and a zone of continental rifting in the Great Basin. Motions within this boundary zone are being defined by programs in Alaska (7), California (8), the Pacific Northwest (9), and the Basin and Range (10).

Figure 3 - GPS and VLBI observations across a portion of the North America-Pacific boundary zone, derived by combining data from a variety of sources (11). The site motions relative to stable North America show strike-slip motion along the San Andreas Fault system. Extension occurs in the Basin and Range north of about 36 degrees North, and changes smoothly into the strike-slip motion across a well defined transition zone. South of 36 degrees North, the San Andreas system accommodates most of the plate motion, and little deformation occurs in the Basin and Range. Net motion across the boundary zone is essentially that predicted by global plate motion model NUVEL-1A (2). Seismicity (purple dots) illustrates the San Andreas system, eastern California shear zone, and intermountain and central Nevada seismic belts.

Similar results have been derived for the complex interactions across the southern Eurasia boundary by programs in the Himalaya (12), China (13), Tien Shan (14), Caucasus (15), and Eastern Mediterranean (16) collision zones. Continental rifting is being studied across the East Africa Rift. Site positions and velocities are available on the WWW.

Figure 4 - GPS observations of motion across a portion of the Africa-Arabia-Eurasia plate collision zone, relative to Eurasia. Northern portions of Arabia move approximately North 40 degrees West, consistent with global plate motion model NUVEL-1a. Eastern Turkey shows distributed deformation, whereas Western Turkey and the Aegean rotate as a Anatolian plate about a pole near the Sinai peninsula, causing strike-slip motion along the North Anatolian Fault. Some extension occurs within the Aegean portion of this plate (16).

Such data are being used to define the kinematics of the boundary zones, and (in conjunction with other geological and geophysical data) provide constraints that can be used to develop and test models of their mechanics (17). For example, GPS data show the full variation of the motion across the Andes, from the interior of the oceanic Nazca plate to the interior of stable South American continent, and provide a detailed look at the process of ocean-continent convergence and continental mountain building (18). Site positions and velocities are available on the WWW. GPS programs are providing similar data at other ocean-continent and ocean-ocean convergent boundaries, and so are significantly improving our knowledge of the complex processes there (7,9,19).

Figures 5 & 6 - GPS data and an interpretation across the Nazca-South America plate boundary zone in Peru and Bolivia. About 30-40 mm/yr of slip, roughly half of the overall convergence rate, is accumulating on the locked plate boundary thrust fault and should be released in future great earthquakes. This estimate avoids some of the difficulties inherent in previous aseismic slip estimates based on the earthquake history. About 10-15 mm/yr of crustal shortening occurs inland at the sub-Andean foreland fold and thrust belt, indicating that the Andes are continuing to build. This shortening rate is significantly greater than inferred from seismic moments, suggesting that the shortening is largely aseismic. Little along-trench motion of coastal forearc slivers is observed, despite the oblique convergence geometry (18). Pink dots show shallow seismicity (<60 km depth).

Plate Rigidity and Intraplate Deformation

The rigidity of major plates, a key premise of plate tectonic theory untestable two decades ago, is being quantified with GPS data. The stable interiors of plates appear surprisingly rigid, to better than a few mm/yr.

Figure 7 - Comparison of GPS-derived velocities (with 95% confidence ellipses) for permanent GPS sites in North America east of the Rocky Mountains to velocities predicted by modeling these sites as being on a single rigid plate (arrows with no ellipse). The rigid plate model fits the GPS velocity field well, indicating that the interior of the North American plate is rigid at least to the level of the average velocity residual, less than 2 mm/yr. These data place bounds on continental scale differential motion east and west of the New Madrid seismic zone (20).

Deviations from plate rigidity, in areas of intraplate seismicity such as the Wabash Valley and New Madrid seismic zones (21), are being investigated. The results can be surprising. For example, GPS data in the New Madrid zone show very small deformation, suggesting that the recurrence time for large earthquakes may be much longer than previously assumed, and implying that the seismic hazard at New Madrid may be considerably less than often assumed. Such studies of intraplate seismic zones should shed light into the stresses within continental interiors and how they cause deformation and earthquakes.

Figure 8 - The expected recurrence interval for a great earthquake in the New Madrid seismic zone can be estimated from GPS observations showing at most a few mm/yr of fault-parallel motion, assuming the horizontal slip in the great 1811-1812 earthquakes was a few meters, as inferred from historical accounts.

Earthquake Mechanics

GPS data are providing important information about earthquake mechanics. Data from many regions show that significant afterslip, not detectable seismologically, is a common phenomenon (22, 23). These observations offer insight into the effects of the coseismic stress changes, and provide constraints on the rheology of the lithosphere and asthenosphere. Such data are of special importance given the emerging view that stress transfer between faults may be very important and may contribute to earthquake triggering.

Figure 9 - GPS-observed residual motions after the 1989 Loma Prieta earthquake, after removal of interseismic motions occurring prior to the earthquake. Note contraction normal to the San Andreas (22), interpreted as evidence for aseismic slip on nearby faults triggered by the earthquake. Within a few years, these sites resume their interseismic motion approximately parallel to the San Andreas.

GPS data also permit comparison of geodetic, seismological, and geologic estimates of the rates and directions of deformation within active regions. Initial data from around the world suggest that these rates can be quite different, and should lead to an improved understanding of the partitioning between seismic and aseismic deformation. The issue of this partioning is crucial for seismic hazard assessment.

Figure 10 - Comparison of geodetic and seismological evidence for crustal shortening in the Tien Shan (14). GPS data indicate that this intracontinental mountain belt, 1000-2000 km north of the Himalaya, accommodates about half the net convergence between India and Eurasia. This shortening rate is approximately twice that inferred from seismic moments. Note that focal mechanisms reflect the local strike of structures, despite the coherent shortening direction shown by the GPS data.

Volcano Monitoring

GPS is proving a powerful tool for volcano monitoring. It provides a rapid and remote (hence safe) way of measuring surface deformation associated with volcanic processes. GPS measurements are underway at sites including Monserrat (24), Long Valley (25), Hawaii (26), Popocepatl (near Mexico City), Arenal (Costa Rica), Misti (near Arequipa, Peru).

Figure 11 - Continuous GPS site on Monserrat, 2.5 km away from the erupting Soufriere Hills volcano, seen in the background. This station was subsequently destroyed by a pyroclastic flow and surge. (Photo courtesy G. Mattioli).

Figure 12 - Cross section through Long Valley Caldera in eastern California, showing seismicity and surface deformation for two time periods. As the seismicity pattern changed the positions of two continuous GPS sites changed significantly (red arrows) (27).

These efforts should benefit from a low-cost single frequency GPS array being developed at the UNAVCO Boulder facility, to be deployed in California, Mexico, and the Phillipines. Detailed geodetic data should significantly improve our understanding of volcanic processes and ultimately aid in eruption forecasting. These goals are more than purely academic, as illustrated by the fact that an eruption of Popocepatl could impact up to 20 million people within 70 km of the volcano.

Deep Earth Dynamics

GPS data, though taken at Earth's surface, can give important insight into deep Earth dynamics. For example, GPS data from the Yellowstone area, one of the world's largest volcanic systems, are being integrated with seismological and other data to provide important new insight into the interaction between the mantle plume and continental lithosphere (28).

Figure 13 - Three-dimensional site velocities of the Yellowstone caldera estimated from GPS surveys. The results show subsidence of up to 17 mm/yr accompanied by approximately 5 mm/yr contraction across the caldera. The period shown (1987-1995) followed a 50-year period of uplift. Most recent results suggest a return to uplift, illustrating the complex time-dependant deformation.

GPS measurements of motions from postglacial rebound in North America and Fennoscandia should provide important information about mantle rheology.

Figure 14 - GPS and tide gauge measurements of vertical uplift rates along the U.S. East coast can be used to constrain models of mantle viscosity, by comparing data to the predicted uplift rates for different mantle models. (Figure by T. van Dam and M. Schenewerk.)

Deep Earth Dynamics

GPS data are increasingly being used in tectonic and geological studies with smaller spatial scales than earlier studies. For example, UNAVCO is supporting programs looking at block motions in island arcs, coastal motion due to sealevel changes, fault-related terraces, and processes in peatland bogs.

Polar Studies

GPS is being used extensively in polar and glaciological studies in Antarctica, Alaska, and Greenland. UNAVCO is assisting projects to determine the pattern and rate of ice-sheet thickening, to assess the mass balance of ice sheets, to measure ice stream motions, and to measure ice-surface roughness (29).

Figure 15 - GPS measurements at site on the Antarctic Ice Sheet. The position of a steel pole is monitored as part of a project to measure the rate of thickening or thinning of the ice sheet. (Photo courtesy I. Whillans).

Atmospheric Science

Following the dictum that "one scientist's noise is another's signal", GPS is becoming a powerful tool in the atmospheric sciences. Because atmospheric moisture limits GPS accuracy for geodesy, considerable effort has been invested in trying to measure this moisture. Hence GPS receivers provide a powerful method to directly measure atmospheric moisture, a major factor in weather forecasting, and ionospheric electron density (30). UNAVCO is working with the GPS Science and Technology division of the University Corporation for Atmospheric Research to explore this area of overlapping interest.

GPS Technology and Data Archiving

UNAVCO is working actively to promote advances in GPS technology, data analysis, and archiving, via work at the Boulder facility and elsewhere. A variety of approaches are being used to provide training and documentation for investigators and their overseas collaborators. Data processing software is being improved, and different processing software packages are being compared to understand the cause of discrepancies. Hardware efforts include assessment of the effects of GPS antennas and other system features on the accuracy of GPS measurements, and development of the low-cost single frequency GPS array for volcano monitoring, earthquake engineering, and atmospheric studies. UNAVCO is also assisting the development of seafloor GPS, a system which combines seafloor acoustic transponders, shipboard GPS, and a nearby on-land reference GPS system (31).

A major UNAVCO program is archiving GPS data, to ensure its availability for long term research, and developing "seamless" archiving between a number of archiving centers to ensure that GPS data can be located by investigators without consideration of where the data physically reside.

Education and Outreach

UNAVCO shares the general commitment by earth scientists to convey better both the content and excitement of our science to students. Students have heard about GPS, and are drawn both to its high-technology aspects - students like the idea of being involved with space technology - and its application to exciting phenomena like earthquakes, volcanoes, and mountain building. They are also intrigued with learning how GPS is used in activities ranging from pure science to locating cars, boats, and aircraft.

Already, GPS has significantly impacted graduate programs in geoscience. Some outstanding young scientists are being trained as specialists interested primarily in GPS, and a much larger group are learning to use GPS data as an integral part of the diverse datasets for their studies of tectonic processes. In addition, GPS projects form part of undergraduate research projects at many institutions, and undergraduates often participate in field projects.

Figure 16 - A high school student in Alaska measuring the height of a GPS antenna as part of a program to assess earthquake hazards associated with the subduction process (32).

GPS is also appearing in elementary and secondary education, via participation of students in field projects (32) and and a variety of other channels. For example, UNAVCO supplies small hand-held receivers to the Global Learning and Observations Benefit the Environment (GLOBE) program, which has trained 8000 U.S. and foreign teachers to use GPS in their courses. The GPS coordinates are used to locate study areas for which GLOBE provides remote sensing images for studies in biology, geology, atmospheric science and hydrology.

Figure 17 - Students in the GLOBE program learning to use a GPS receiver.

References and Notes

Geological applications of GPS, and space geodesy in general, have become so widespread in recent years that a full listing of this literature is impractical in a short brochure. Selected references are listed here, and the review papers cited provide further information and references.

1. T. Dixon, Rev. Geophys. 29, 249 (1991); B. Hager, R. King, M. Murray, Ann. Rev. Earth Planet. Sci. 19, 351 (1991); G. Blewitt, in Contributions of Space Geodesy to Geodynamics: Technology, Geodynamics Ser. 25, D. Smith and D. Turcotte, Eds. (AGU, Washington, D. C.), 195-213 (1993); P. Segall and J. Davis, Ann. Rev. Earth Planet. Sci. 25, 301 (1997).

2. Plate motions are specified by Euler (angular velocity) vectors giving either the relative motion between a plate pair, or the absolute motion of an individual plate with respect to a fixed hotspot reference frame. These vectors are now derived from space geodetic data, and have previously been derived from data recording plate motions averaged over the past few million years. The ITRF, a reference frame for space geodetic data (C. Boucher, IERS Technical Note 20 (1996)), which is updated periodically (e.g. ITRF-94, ITRF-96), is designed to agree on average with absolute plate motion model NNR NUVEL-1A, termed NNR-A. The latter is a revision of model NNR, derived by combining the assumption of no net torque on the lithosphere (D. Argus and R. Gordon, Geophys. Res. Lett. 18, 2039 (1991)), with global relative plate motion model NUVEL-1 (C. DeMets et al., Geophys. J. Int. 101, 425 (1990)), reflecting a change in the magnetic anomaly timescale subsequent to the publication of NUVEL-1. NUVEL-1A and NNR-A predict plate motion directions identical to those for NUVEL-1 and NNR NUVEL-1, but 4% slower (C. DeMets et al., Geophys. Res. Lett. 21, 2191 (1994)). Because rate data in the models come from ridges, the predicted rates across subduction zones are derived indirectly by the closure of plate circuits. Space geodetic velocities in ITRF-94 can be compared to plate motions predicted by NNR-A, and relative motions are typically compared to NUVEL-1A.

3. K. Larson, J. Freymueller, S. Philipsen, J. Geophys. Res. 102, 9961 (1997).

4. D. Argus and M. Heflin, Geophys. Res. Lett. 22, 1973 (1995).

5. T. Dixon et al., J. Geophys. Res. 103, 15,157 (1998).

6. R. Gordon and S. Stein, Science 256, 333 (1992); S. Stein, in Space Geodesy and Geodynamics, Geodynamics Ser. 23, D. Smith and D. Turcotte, Eds. (AGU, Washington, D. C.), 5-20 (1993). (Figure shown drafted by T. Shoberg and P. Stoddard).

7. K. Larson and M. Lisowski, Geophys. Res. Lett. 21, 489 (1994); J. Sauber, S. McClusky, R. King, ibid. 24, 2853 (1997).

8. T. Dixon et. al., Geophys. Res. Lett. 18, 861 (1991); K. Feigl et al., J. Geophys. Res. 98, 21,677 (1993); A. Donnellan et al., ibid. 98, 21,727 (1993); R. Snay et al., ibid. 101, 3173 (1996); R. Bennett, W. Rodi, R. E. Reilinger, ibid. 101, 21,943 (1996) Z. Shen et al., EOS 78, 477-482, (1997).

9. H. Dragert and R. Hyndman, Geophys. Res. Lett. 22, 755 (1995). These sites form part of the larger PANGA network.

10. T. Dixon et. al., Tectonics 14, 755 (1995); R. Bennett et al., Geophys. Res. Lett. 24, 3073 (1997); R. Bennett, B. Wernicke, J. Davis, ibid. 25, 563 (1998); L. Martinez, C. Meertens, R. Smith, ibid. 25, 567 (1998); B. Wernicke et al., Science 279, 2096 (1998).

11. Results from submitted manuscript by R. Bennett, J. Davis, B. Wernicke. GPS data from the NBAR and Death Valley networks of Caltech/SAO were combined with data products from the BARD, CORS, IGS, SCIGN, and STRC networks provided by the SCEC and SOPAC facilities. VLBI data from NASA GSFC. The combined velocity field thus reflects contributions from many individuals and institutions contributing to the establishment and day-to-day operation of all the networks used, including IGS, NOAA, the Canadian Geological Survey and Canadian Geodetic Survey, JPL, MIT, Caltech, the STRC consortium, the Yucca Mountain-Death Valley consortium, UNAVCO, Scripps Institution of Oceanography, the USGS, the Seismological Laboratory of UC Berkeley, Trimble Navigation, Inc., and the Smithsonian Astrophysical Observatory. Site positions and velocities are available via anonymous ftp from ftp://cfa-ftp.harvard.edu/pub/rbennett/WUSC

12. J. Freymueller et al., Geophys. Res. Lett. 23, 3107 (1996); R. Bilham et al., Nature 386, 61 (1997).

13. R. King et al., Geology 25, 179 (1997).

14. K. Abdrakhmatov et al., Nature 384, 450 (1996); S. Ghose, M. Hamburger, C. Ammon, Geophys. Res. Lett. 25, 3181 (1998).

15. R. Reilinger et al., Geophys. Res. Lett. 24, 1815 (1997).

16. R. Reilinger et al., J. Geophys. Res. 102, 9983 (1997).

17. B. Shen-Tu, W. Holt, A. Haines, J. Geophys. Res. 103, 18,087 (1998); S. Ward, Geophys. J. Int. 134, 172 (1998).

18. E. Norabuena et al., Science 279, 358, (1998). Site positions and velocities are available on the WWW at http://www.earth.nwu.edu/research/snapp.html

19. J. Freymueller, J. Kellogg, V. Vega, J. Geophys. Res. 98, 21,853 (1993); T. Dixon, Geophys. Res. Lett. 20, 2167 (1993); M. Bevis et al., Nature 374, 249 (1995); L. Prawirodirdjo et al., Geophys. Res. Lett. 24, 2601 (1997); C. Pearson, ibid. 25, 3158 (1998); X. Le Pichon et al., Geophys. J. Int. 134, 501 (1998).

20. T. Dixon, A. Mao, S. Stein, Geophys. Res. Lett. 23, 3035 (1996).

21. L. Liu, M. Zoback, P. Segall, Science 257, 1666 (1992); R. Snay, J. Ni, H. Neugebauer, in Investigations of the New Madrid Seismic Zone, USGS Professional Paper 1538-F, 1994; J. Weber, S. Stein, J. Engeln, Tectonics 17, 250 (1998).

22. R. Bürgmann et al., J. Geophys. Res. 102, 4933 (1997).

23. Z. Shen et al., Bull. Seismol. Soc. Am. 84, 780 (1994).

24. G. Mattioli et al., Geophys. Res. Lett. 25, 3417 (1998).

25. T. Dixon et al., J. Geophys. Res. 102, 12,017 (1997).

26. S. Owen et al., Science 267, 1328 (1996).

27. Results from submitted manuscript by A. Newmann, T. Dixon, and J. Dixon.

28. C. Meertens and R. Smith, Geophys. Res. Lett. 18, 1763 (1991); F. Arnet et al., ibid. 24, 2741 (1997).

29. G. Hamilton and I. Whillans, Antarctic J. U.S. 31, 86 (1996).

30. GPS for the Geosciences, National Academy of Sciences Press, 1997.

31. F. Spiess et al., Phys. Earth Planet. Inter. 108, 101 (1997).

32. J. Sauber, S. Stockman, T. Clark, EOS 79, 393 (1998).

33. Many figures were created with the GMT software kindly distributed by P. Wessel and W. Smith. E. Klosko provided graphic assistance.

UNAVCO

UNAVCO, the University NAVSTAR Consortium, is an international organization of more than 90 universities and other research institutions, representing scientists doing geoscience with GPS. Its funding derives primarily from the National Science Foundation's Division of Earth Sciences Instrumentation and Facilities Program, and NASA's Solid Earth and Natural Hazards Program.

Much of UNAVCO's work is conducted by the its Boulder Facility, which assists NSF- and NASA- funded principal investigators. Support is provided at various levels, depending on the project's needs. This can include GPS equipment, field engineering, technology development, training, technology transfer, data management and archiving. In many cases, UNAVCO provides assistance to universities which have their own GPS receivers, often acquired via a major equipment grant organized under UNAVCO auspices. The projects supported include GPS campaigns, where sites are occupied for short periods, and local and regional networks of continuously recording GPS receivers. UNAVCO also provides technical and operational support to the permanent GPS sites in NASA's Global GPS network, many of which contribute data to the IGS global network.

UNAVCO also works collaboratively with other research institutions in the areas of data processing, technology development, and data archiving. In addition, through the Boulder Facility, UNAVCO supports scientific interchange among investigators doing GPS-related science, both from UNAVCO and from other institutions, via an annual community meeting, scientific working groups, and other forums.

1998 UNAVCO community meeting participants.

This booklet gives an overview of some of the exciting results emerging from GPS geodesy, which will help shape research and thinking about fundamental geological processes in years to come.

BACK COVER

The data, from a large number of different programs conducted by university and government investigators, provide a valuable resource for studies of a variety of Earth processes including plate motions, plate boundary deformation, intraplate tectonics, and volcanism. The data can be accessed on the UCAR UNAVCO Web site at http://www.unavco.org/facility/data/data.html

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