JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B5, PAGES 9961-9981, MAY 10, 1997 Global plate velocities from the Global Positioning System Kristine M. Larson Department of AerospaceEngineering Sciences, University of Colorado, Boulder Jeffrey T. Freymueller GeophysicalInstitute, University of Alaska, Fairbanks Steven Philipsen Department of AerospaceEngineering Sciences, University of Colorado, Boulder Abstract. We haveanalyzed 204 daysof GlobalPositioning System (GPS) data fromthe global GPS network spanningJanuary 1991 through March 1996. On the basisof these GPS coordinate solutions,we have estimated velocities for 38 sites, mostly located on the interiors of the Africa, Antarctica, Australia, Eurasia, Nazca, North America, Pacific, and South America plates. The uncertaintiesof the horizontal velocity componentsrange from 1.2 to 5.0 mm/yr. With the exceptionof siteson the Pacific and Nazca plates, the GPS velocitiesagree with absolute plate model predictions within 95% confidence. For most of the sites in North America, Antarctica, and Eurasia,the agreementis better than 2 mm/yr. We find no persuasiveevidence for significantvertical motions(< 3 standarddeviations), except at four sites. Three of these four were sites constrainedto geodetic referenceframe velocities. The GPS velocities were then used to estimate angular velocities for eight tectonic plates. Absolute angular velocitiesderived from the GPS data agree with the no net rotation (NNR) NUVEL-1A model within 95% confidenceexcept for the Pacific plate. Our pole of rotation for the Pacific plate lies 11.5ø west of the NNR NUVEL-1A pole, with an angularspeed 10% faster. Our relative angularvelocities agree with NUVEL-1A except for some involving the Pacific plate. While our Pacific-North America angular velocity differs significantlyfrom NUVEL-1A, our model and NUVEL-1A predict very small differencesin relative motion along the Pacific-North America plate boundary itself. Our Pacific-Australia and Pacific- Eurasia angular velocitiesare significantlyfaster than NUVEL-1A, predicting more rapid convergenceat thesetwo plate boundaries.Along the East Pacific Rise, our Pacific-Nazcaangular velocity agrees in both rate and azimuthwith NUVEL-1A. Introduction 3.16m.y.). Thesemodels have been tremendously suc- cessfulin explainingthe large-scalefeatures of plate For almost 20 years, models of current plate mo- kinematics.Global plate modelshave shownplate in- tions have been determined using spreading rates at teriors to be rigid over geologictimescales. The vari- mid-ocean ridges, transform fault azimuths, and plate ousgeologic data all giveconsistent measures of global boundary earthquakeslip vectors [LePichon, 1968; plate motions, although earthquakeslip vectors have Chase, 1972; Minster et al., 1974; Minster and Jor- beenfound to be biasedin somecases due to the par- dan, 1978; Chase,1978; DeMets et al., 1990]. With the tition of slip at certain marginswhere subductionoc- exceptionof earthquakeslip vectors,these data repre- curs obliquely[DeMets et al., 1990; Argus and Gor- sent an averageover a substantialperiod of time (for don, 1990]. Absoluteplate motionshave been com- the NUVEL-1 model of DeMets et a/.[1990],the last puted based on the NUVEL-1 relative plate motions and the assumptionof no net rotation (no net torque on the lithosphere),resulting in the absolutemotion Copyright 1997 by the American Geophysical Union. modelno net rotation (NNR) NUVEL-1 [Argusand Paper number 97JB00514. Gordon,1991]. A recentrevision of the magnetictime 0148-0227/97/97JB-00514509.00 scaleled to the rescaledNUVEL-1 models,NUVEL-1A 9961 9962 LARSON ET AL.: GPS GLOBAL PLATE MOTIONS and NNR NUVEL-1A (hereinafterreferred to as NNR- workof Argusand Heftin [1995]by includingmore sites, A) [DeMetset al., 1994]. more tectonic plates, and a longertime seriesand by us- Spacegeodetic data collectedover the last two decades ing only a subsetof the availableGPS data. This allows have made it possible to measure plate motions over us to analyze the entire data set using the same mod- the time scale of years rather than millions of years els and techniques.We also include selectedtemporary [e.g.,Robbins et al., 1993].The abilityto makeglobal- occupationsof sites rather than restricting ourselvesto scalegeodetic measurements was made possible through only permanent station data, improving the distribu- the developmentof highly sophisticatedspace geode- tion of sites on some of the plates. tic techniquessuch as satellitelaser ranging (SLR) and very long baselineinterferometry (VLBI). One of the Measurements primary scientificgoals when these techniques were be- ing developedwas to measureglobal plate motions. While some global GPS tracking sites have existed Both SLR and VLBI achieved sufficient accuracy that sincethe late 1980s, the operation and archiving of the they couldbe usedto measureboth globalplate motions network were not globally coordinated,and the distri- and plate boundarydeformation [Clark et al., 1987; bution of stationswas too sparseto supportglobal G PS Smith et al., 1990; Robbinset al., 1993; Ryan et al., studies. This changed in January-February 1991 with 1993],but both sufferedfrom the disadvantageof the the InternationalAssociation of Geodesy(IAG) spon- high costand nonportabilityof the systems,which lim- sored global GPS densificationexperiment, the first ited the number and distribution of sites worldwide. At Global International Earth Rotation Service(IERS) roughlythe sametime SLR and VLBI werebeing devel- and Geodynamicscampaign (hereinafter referred to as oped and tested, the Department of Defensebegan de- GIG). Over 100 GPS receiverswere deployedin this ploymentof the GlobalPositioning System (GPS). Its campaign, although many were of insufiqcientquality primary missionwas and is to provide real-time nav- to be truly useful [Melbourneet al., 1993]. Analysis igation and positioningassistance. Scientistsquickly of a subset of the GIG data provided direct evidence realized that GPS could also be used for positioning of the potential of global GPS: I cm positioningaccu- with a precisionapproaching that of VLBI and SLR. racy [Blewitt et al., 1992] and submilliarcsecondpole GPS analysis softwareshas been developedover the positionestimates [Herring et al., 1991]. Following past decadefor this purpose[e.g. Dong and Bock,1989; the successof GIG, the IAG sponsoredthe develop- Beutler et al., 1987; Blewitt, 1989]. The GPS constel- ment of the International GPS Servicefor Geodynam- lation has now been completed and a global tracking ics (IGS), whichprovides timely accessto high-accuracy network is operating under international cooperation. GPS ephemeridesbased on data from a globalnetwork The focus of this paper will be to use GPS and the of permanent GPS receivers. The IGS has coordinated global tracking network to study plate tectonics. developmentof the global GPS network, which is now Why is geodeticanalysis of global plate motionsim- generallyreferred to as the IGS network. portant when geologicmodels have been so successful? Data from sites participating in the IGS network are Geodetic techniques have become increasingly promi- downloadedby the agencieswhich operate them, and nent in studiesof plate boundary deformation and have transferredvia internet to the IGS global data centers. approachedthe level of precisionof globalplate models. The number of GPS receiversparticipating in the IGS Geodetic techniquesalso measure plate motion over a network continues to expand. At present, there are period of a few years rather than a few million years, more than 70 globalsites in the IGS network,excluding so it is important to find discrepanciesthat would indi- denseregional clusters in California. The most signifi- cate if there have been any very recent changesin plate cant changeover the last few yearshas beenthe increase motions. Today, regional tectonic studiesin California in the number of IGS sites in the southernhemisphere, and elsewhereare attempting to characterizefault slip from four in 1992 to more than 5 times that number rates in plate boundary zones so completely that the today. Receiver,antenna, and softwaredescriptions for entire slip budget comparesto a global plate model to the IGS network are documented at the IGS Central within a few millimeters per year or better. Such stud- Bureau(http://igscb.jpl.nasa.gov). ies are only feasible if the rates of plate motions are For this paper, we have analyzed data for the period known to the same level of precisionand if the rates of between January 1991 and March 1996. The sites we motion over the last few years (or few hundredyears) have chosenfor this study are listed in Table I and are describedadequately by a plate motion model aver- shownin Figure 1. Of these 38 sites, 16 were first ob- agedover the last few millionyears. Early spacegeode- servedduring the 3 week GIG campaign. There is then tic studies have shown a high correlation between ob- a 1 year gap in our time series,until enoughpermanent servedrelative site velocitiesand the predictionsof the stations had been deployedfor the IGS network. Be- NUVEL-1 model[Smith et al., 1990].In this studywe ginningin March 1992, we have analyzeddata from the will compareangular velocities for eight plates and var- IGS network on a weekly basis. An analysisof IGS data ious plate pairs derived from our GPS data with the from January 1991 through November 1993 was previ- NUVEL-1A model. This study differs from the similar ouslypresented by Larsonand Freymueller[1995]. In LARSON