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 AerospaceEngineeringSciences,University of Colorado, Boulder
Jeffrey T. Freymueller GeophysicalInstitute, University of Alaska, Fairbanks
Steven Philipsen Department of AerospaceEngineeringSciences,University of Colorado, Boulder Abstract.
We haveanalyzed204daysof GlobalPositioningSystem(GPS) data fromthe global GPS network spanningJanuary 1991 through March 1996. On the basisof these GPS coordinate solutions,we have estimated velocitiesfor 38 sites, mostly located on the interiorsof the Africa, Antarctica, Australia, Eurasia, Nazca, North America, Pacific, and South America plates. The uncertaintiesof the horizontal velocity
components rangefrom 1.2 to 5.0 mm/yr. With the exceptionof siteson the Pacific and Nazca plates, the GPS velocitiesagree with absoluteplate 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),exceptat four sites. Three of these four were sites constrainedto geodetic referenceframe velocities. The GPS velocities were then used to estimate angular velocitiesfor eight tectonic plates. Absolute angular velocitiesderived from the GPS data agree with the
no net rotation (NNR) NUVEL-1A modelwithin 95% confidence exceptfor the Pacific plate. Our pole of rotation for the Pacific plate lies 11.5ø west of the NNR
NUVEL-1A pole, with an angularspeed10% faster. Our relativeangularvelocities agree with NUVEL-1A except for some involvingthe 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 PacificEurasia angular velocitiesare significantlyfaster than NUVEL-1A, predicting more rapid convergence at thesetwo plate boundaries.Along the East Pacific Rise, our
Pacific-Nazcaangularvelocityagreesin both rate and azimuthwith NUVEL-1A. Introduction
For almost 20 years, models of current plate motions have been determined using spreading rates at mid-ocean ridges, transform fault azimuths, and plate
3.16m.y.). Thesemodelshavebeentremendously successfulin explainingthe large-scalefeaturesof plate kinematics.Global plate modelshave shownplate interiors to be rigid over geologictimescales. The vari-
ousgeologic data all giveconsistent measures of global boundary earthquakeslip vectors [LePichon,1968; plate motions, although earthquakeslip vectorshave Chase, 1972; Minster et al., 1974; Minster and Jor- beenfoundto be biasedin somecasesdue to the pardan,1978;Chase,1978;DeMetset al., 1990].With the tition of slip at certain marginswhere subductionocexceptionof earthquakeslip vectors,thesedata repre- curs obliquely[DeMets et al., 1990; Argusand Gorsent an averageover a substantialperiod of time (for don, 1990]. Absoluteplate motionshave been comthe NUVEL-1 model of DeMets et a/.[1990],the last puted based on the NUVEL-1 relative plate motions and the assumption of no net rotation (no net torque on the lithosphere),resultingin the absolutemotion Copyright 1997 by the American Geophysical Union. modelno net rotation(NNR) NUVEL-1 [Argusand Gordon,1991].A recentrevisionof the magnetictime Paper number 97JB00514. 0148-0227/97/97JB-00514509.00 scaleled to the rescaledNUVEL-1 models,NUVEL-1A 9961
9962
LARSON
ET AL.: GPS GLOBAL
and NNR NUVEL-1A (hereinafterreferredto as NNR-
PLATE
MOTIONS
workof ArgusandHeftin [1995]by includingmoresites,
A) [DeMetset al., 1994].
more tectonicplates, and a longertime seriesand by usSpacegeodeticdata collectedoverthe lasttwo 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 modthe time scale of years rather than millions of years els and techniques.We alsoincludeselectedtemporary [e.g.,Robbinset al., 1993].The abilityto makeglobal- occupationsof sitesrather than restrictingourselvesto scalegeodeticmeasurements wasmadepossiblethrough only permanent station data, improving the distributhe developmentof highly sophisticatedspace geode- tion of sites on some of the plates.
tic techniques suchas satellitelaserranging(SLR) and very long baselineinterferometry(VLBI). One of the Measurements primary scientificgoalswhenthesetechniqueswerebeWhile some global GPS tracking sites have existed ing developedwas to measureglobal plate motions. 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 distriand plate boundarydeformation[Clark et al., 1987; bution of stationswastoo sparseto supportglobalGPS Smith et al., 1990; Robbinset al., 1993; Ryan et al., studies. This changed in January-February 1991 with 1993],but both sufferedfrom the disadvantage of the the InternationalAssociationof Geodesy(IAG) sponhigh costand nonportabilityof the systems,whichlim- 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 werebeingdevel- and Geodynamicscampaign(hereinafterreferredto as oped and tested, the Department of Defensebegande- GIG). Over 100 GPS receiverswere deployedin this ploymentof the GlobalPositioningSystem(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 positioningaccupole with a precisionapproachingthat of VLBI and SLR. racy [Blewittet al., 1992] and submilliarcsecond GPS analysis softwareshas been developedover the positionestimates[Herring et al., 1991]. Following pastdecadefor this purpose[e.g. DongandBock,1989; the successof GIG, the IAG sponsoredthe developBeutler et al., 1987;Blewitt, 1989]. The GPS constel- ment of the International GPS Servicefor Geodynamto high-accuracy lation has now been completed and a global tracking ics(IGS), whichprovidestimely access network is operating under international cooperation. GPS ephemeridesbasedon data from a globalnetwork The focus of this paper will be to use GPS and the of permanent GPS receivers. The IGS has coordinated developmentof the global GPS network, which is now globaltracking networkto study plate tectonics. Why is geodeticanalysisof global plate motionsim- generallyreferred to as the IGS network. portant when geologicmodelshave beenso successful? Data from sitesparticipating in the IGS network are Geodetic techniques have become increasinglypromi- downloadedby the agencieswhich operate them, and nent in studiesof plate boundary deformationand 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, morethan 70 globalsitesin the IGS network,excluding so it is important to find discrepancies that would indi- denseregionalclustersin California. The most significate if there have been any very recent changesin plate cant changeoverthe last few yearshas beenthe increase motions. Today, regionaltectonic studiesin California in the numberof IGS sitesin 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 completelythat the today. Receiver,antenna, and softwaredescriptionsfor entire slip budget comparesto a global plate model to the IGS network are documented at the IGS Central within a few millimetersper year or better. Suchstud- Bureau(http://igscb.jpl.nasa.gov). For this paper, we have analyzed data for the period ies are only feasible if the rates of plate motions are known to the same level of precisionand if the rates of between January 1991 and March 1996. The sites we motion overthe last few years(or few hundredyears) have chosenfor this study are listed in Table I and are describedadequatelyby a plate motion model aver- shownin Figure 1. Of these 38 sites, 16 were first obagedoverthe last fewmillionyears.Early spacegeode- servedduring the 3 week GIG campaign. There is then tic studieshave 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. BeNUVEL-1 model[Smithet al., 1990].In this studywe ginningin March 1992, we haveanalyzeddata from the will compareangularvelocitiesfor eight platesand var- IGS networkon a weekly basis. An analysisof IGS data ious plate pairs derived from our GPS data with the from January 1991 through November 1993 was previNUVEL-1A model. This study differsfrom the similar ouslypresented by Larsonand Freymueller[1995].In
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
9963
Table 1. Station Description
Site
Namea
IGS
Head
Plate
Lonl•itude, Latitude, aeg
deg
First
Last
Epoch
Epoch
Total
Epochs
I
Albert
ALBH
NOAM
236.513
48.201
1992.37
1996.25
161
2
Algonquin
ALGO
NOAM
281.929
45.765
1991.06
1996.25
200
3
Baltra Islandb
BALT
NAZC
269.741
-0.461
1991.08
1996.25
4 5 6 7 8 9 10 11 12 13
Bermuda Canberra Chatham Island Easter Island Fairbanks Fortaleza Hartebeestoek Hobart c Hercmonceaux Kourou
BRMU CANB CHAT EISL FAIR FORT HART HOBA HERS KOUR
NOAM AUST PCFC NAZC NOAM SOAM AFRC AUST EURA SOAM
295.304 148.980 183.434 250.617 212.501 321.574 27.708 147.440 0.336 307.194
32.198 -35.220 -43.766 -26.994 64.832 -3.852 -25.738 -42.614 50.681 5.218
1993.26 1991.06 1992.91 1994.07 1991.06 1993.46 1991.06 1993.20 1992.17 1992.88
1996.25 1996.25 1996.25 1996.25 1996.25 1996.25 1995.99 1995.96 1996.25 1996.25
125 181 16 48 199 123 188 92 143 130
51.994
1991.06
1996.25
192
21.994 27.607
1991.06 1992.51
1996.25 1995.99
173 157
1992.27 1991.06 1991.06 1993.20 1991.08
1995.99 1996.25 1996.25 1996.25 1996.25
131 199 164 94 63
5.810
9
14
Kootwijk
KOSG
EURA
15 16
Kokee Park Mas Palomas
KOKB MASP
PCFC AFRC
200.335 344.367
17 18 19 20 21
Matera, Italy Madrid, Spain McMurdo d North Liberty Ny Alesunde
MATE MADR MCM3 NLIB NYAL
EURA EURA ANTA NOAM EURA
16.704 355.750 166.675 268.425 11.865
40.461 40.241 -77.770 41.582 78.858
22
O'Higgins J•
OHIG
ANTA
302.100
-63.168
1991.06
1995.88
23 24 25 26 27 28
Onsala Pamatai Pie Town Penticton Perth Richmond
ONSA PAMA PIE1 PENT PERT RCM5
EURA PCFC NOAM NOAM AUST NOAM
11.926 210.425 251.881 240.375 115.885 279.616
57.222 -17.457 34.124 49.134 -31.632 25.466
1992.17 1992.17 1993.00 1992.17 1993.77 1992.57
1996.25 1996.24 1996.25 1996.25 1996.25 1996.25
180 106 71 179 78 134
37
29
Santiago
SANT
SOAM
289.331
-32.976
1991.06
1996.25
198
30 31 32 33 34 35 36
St John's Taiwan Tromso Townsville Tsukuba Westford Wettzell
STJO TAIW TROM TOWN TSKB WES2 WETT
NOAM EURA EURA AUST EURA NOAM EURA
307.322 121.537 18.938 146.814 140.088 288.507 12.879
47.405 24.876 69.538 -19.141 36.106 42.424 48.956
1992.41 1992.37 1992.17 1991.06 1993.96 1993.21 1991.06
1996.25 1996.25 1996.25 1992.88 1996.25 1996.25 1996.25
157 162 111 45 59 82 191
37 38
Wellington ¾aragadee
WELL ¾AR1
AUST AUST
174.783 115.347
-41.086 -28.885
1991.06 1991.06
1995.13 1996.25
61 204
aUnless stated otherwise, the local surveys between different monuments at the same site were taken from ITRF94
[Boucheret al., 1996]or the IGS CentralBureau(http://igscb.jpl.nasa.gov). bThe1996observations weremadeat Isla SantaCruz (GALA). Tie calculated for This paper:GALA minusBALT -4,973.763, 404.838,-31,181.929 m. CTie calculated for this paper: HOB2 minus HOBA 112.719, 50.737, -50.183 m.
•Thereareobservations at fourMcMurdomonuments, definedasfollows.McMurdoGIG: MCM1; McMurdo1992-1993: MCM2; McMurdo 1994: MCM3; McMurdo 1995-1996: MCM4. Ties calculated for This paper: MCM1 minus MCM3 -234.600, 17.358, 52.521 m; MCM2 minus MCM3 -73.423, 54.691, 36.920 m; MCM4 minus MCM3 -1,081.537, 400.997, 145.126
m.
eData from 1991-1992 excluded "The antenna had been hit by a stone... and was knocked over lying on its side," IGS Mail
135. No tie available
between
1991-1992
location
and 1993-1996
location.
fTie from GIG site and permanentsite, personnal communication, AndreasReinhold:OHIG-GIG (K4) minusOHIG (K5): 4.715, -0.277, 0.849 m.
that paper, a global set of stations was analyzed, but only the velocitiesof sites on the Pacific, Australian, and Antarctic plates were discussedin detail. In this study, we have expandedour analysisand interpretation to include the Eurasian, North American, African, Nazca and South American plates. Even though IGS data are availableon a daily basis, we have chosento analyze only one day of GPS data per week, except for special periods of interest such
as the GIG campaign. Our decisionis based on two characteristicsof GPS data. First, it has been shown that GPS estimates are highly correlated over periods
of severaldays[King et al., 1995].Thus,if solutionsare computed each day, they will not be independent. Second, station velocity uncertainties are more sensitiveto the time spanned by the data set than additional data points spacedcloselytogether in time. In order to analyze as uniform as possiblea data set and in an effort to
9964
LARSON ET AL.: GPS GLOBAL PLATE MOTIONS 180'
0'
180'
180 ø
0ø
180 ø
Figure 1. GPS stationsusedin this study.The sizeof the symbolscorrespond to the lengthof the time series,from more than 5 years,between4 and 5 years,between3 and 4 years,and less than 3 years. avoid correlatedestimates,we analyzed global tracking data only onceper week. By restrictingourselvesto a data set of manageablesize,we havemadeit possibleto reanalyzeas muchdata as neededto ensureconsistency of our solutionsover time as modelingtechniqueshave
The pseudorangedata are acquired by correlation of a code in the data signal, called the P code. During peri-
odswhenthe Pcodeis encrypted(antispoofing, or AS),
all modern receiverscan extract equivalentpseudorange data from the signalsby usingsignal crosscorrelationor other methodswhich tend to result in somedegradation •mproved. Although we have analyzedmore than the 38 sitesin Table 1, we computedvelocitiesonly for the in signal to noiseratio and data quality. This varies by sites for which we feel an accurate and reliable velocity receiver type and local environment and has lessened can be estimated. Therefore we excluded all sites with over time as GPS receivers have been improved. We lessthan 2 years of data. Unfortunately,this means choosestandard data weights of I cm for the carrier that we do not discuss some of the newest IGS sites phase data and, with a few exceptions, 100 cm for the in interestingtectonicareassuchas centralAsia. We pseudorangedata if the receiver records it. Pseudoralso excluded data from sites that have been displaced ange data from Rogue SNR-8 and Mini-Rogue SNRby earthquakes.In additionto uncertaintyassociated 800 receiversare biasedunder AS, so we excludedthese data duringperiodsof AS (AS hasbeenon with the coseismicdeformation,thesesitesmay also be pseudorange
affectedby postseismic deformation.As a result,we do almost continuouslysinceJanuary 30, 1994). Pseudonot discussseveralof the long-termIGS sitesin south- range data are also excludedfrom certain Turborogue ern California.
SNR-8000 receiversthat showpseudorangebiasesunder AS.
All raw data were passedthrough an automatic editing stage,duringwhichcycleslips(discontinuities in the All data presentedin this paper were analyzedusing phasedata) werefoundand correctedand outlierswere the GIPSY/OASISII software (release 3 andrelease 4) removed. For Rogueand Turboroguereceivers,the Turdevelopedat the Jet PropulsionLaboratory,California boEditalgorithm[Blewitt,1990]wasused.Forall other Instituteof Technology.The currentversionof the soft- receivers,the PhasEdit algorithm (J. Freymueller,unware is an evolution of the softwaredescribedby Lichten publishedalgorithm, 1996) was used. Both algorithms searchfor discontinuitiesin undifferencedgeometry-free andBorder[1987]. GIPSY uses undifferenced carrier phase and pseu- (and clock-free)linear combinations of the observables. dorangeobservables.For a generaldescriptionof the After data editing, the data from both GPS frequencies GPS observablesand data analysis, see, for example, are combined to form the ionosphere-freelinear combiHolmann-Wellenhof et al. [1993]and Leick[1995].The nation [see,e.g., Leick, 1995]and are decimatedto a carrierphaseis more precisethan the pseudorange but standard 6 min interval. Additional editing of the data is ambiguousby an integernumberof wavelengths so a is done manually based on inspection of postfit data carrierphaseambiguitymustbe estimatedfor eachcon- residuals, and data can be deleted or new phase ambitinuousphase-connected arc of GPS carrier phasedata. guity parameters inserted when outliers or cycle slips The pseudorangeis unambiguousbut has a noiselevel are found to have passedundetected through the autoapproximately100 times higherthan the carrier phase. mated editing. The number of suchediting changesre-
Data Analysis
LARSON
ET AL.'
GPS GLOBAL
PLATE
MOTIONS
9965
Each station position estimate is based on 24 hours of
quired has varied somewhatwith time. In general,prior to the introduction of AS almost all data problemswere detected and correctedby the automatic algorithms. Data quality worsenedconsiderablyafter the introduction of AS and has been improvingsteadily sincethen as the receiver hardware has been improved. Parameter estimationin GIPSY is carriedout usinga
GPS data. We followeda strategysimilar to that describedby Heftin et al. [1992]. The coordinates of six
globallydistributedsiteswereconstrainedto agreewith VLBI/SLR coordinateswith an a priori uncertaintyof 10 m. The remaining sites were constrainedwith an a priori uncertainty of I km. These loose constraints are SquareRootInformationFilter (SRIF) algorithm[Bier- sufficientto avoidsingularitiesin the GPS solutionsbut man, 1977]whichallowsus to estimateparametersas are not sufficientto specify the referenceframe. We have made a considerableeffort to analyze the enconstants or varying in time. We estimate both station and satellite clock errors relative to a user-specified tire time seriesof data consistently.The same models referenceclock as white noiseparameters,uncorrelated and strategiesare used throughout, which has meant from epochto epoch. We estimate the wet tropospheric reanalyzingthe earlier data as modelshave improved. path delay at zenith as a time-dependent parameter Through this effort we hopeto avoidaliasingsystematic
with a randomwalk noisemodel[Lichtenand Border, errors into our estimated station velocities and tectonic 1987; Tralli et al., 1988]. Orbits are modeledby inte- interpretations. There is one unavoidabledifferencebetween data collected in 1991-1993
grating the equationsof motion and estimating corrections to the initial conditions,as describedby Lichten
and data collected
in 1994-1996. On January 30, 1994, antispoofingwas and Border[1987]. We estimatesolarradiationpres- implemented on the GPS constellation. As described sure coefficients for the ROCK4 or ROCK42 model as above, this reducedthe quality of the carrier phase data appropriate[Fliegelet al., 1992]. We estimatea single and the amount of pseudorangedata which was availX-Z solarradiation pressurescaleparameter,plussmall able to us. However, we have not been able to identify independentX and Z corrections,and a Y-bias param- any significantchangesin velocitywhich correlatewith eter. For satellitesthat are eclipsing(passingthrough the introduction of AS. Earths shadowon eachrevolution),we estimatetimevarying solar radiation pressureparameters. The GPS Reference Frame
yaw attitude modelis described by Bar-Sever[1996]. A summaryof the modelsusedin GIPSY/OASIS and
As describedabove, we estimate GPS station coordinates for each day of data in loosely constrainedsolu-
our parameter estimationstrategy is given in Table 2. Table 2. Data Analysis Summary Models Data
interval
Value 6 min
Elevation angle cut-off Geopotential
15 ø
JGM3 degreeand order 12
Precession Nutation Earth Orientation
IAU 1976 precessiontheory IAU 1980 nutation theory
Difference phase center correction
Schuplerand Clark [1991] Bar-Sever[1996]
Yaw attitude Reference clock
Pseudorangeweight Carrier phase weight
IERS
bulletin
B
Algonquin 100 mm 10 mm
Estimation
Standard
force model force model white noise
0.00001km/s
Solar pressureYbias
constant
10-19' km s-•s -•
SolarpressureX/Z
constant
1% of Ybias
Eclipsing satellites Station position, reference Station position, nonreference
stochastic
1 hour updates
constant
0.01 km I km
Station
white
Parameter
Satellite position Satellite velocity Satellite
clock
clock
constant
Phaseambiguity(real-valued)
constant
Zenith tropospheredelay
random
noise
Deviation
I km ls
ls
0.1 km
walk
10 mm/sqrt(hour)
9966
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
We can transform
tions. That means that we tightly constrainedneither the coordinatesof the tracking sites nor the orbits of
our solutions into ITRF94
in dif-
ferentways.LarsonandFreymueller[1995]estimateda the GPS satellites(seeTable 2 for a descriptionof our seven parameter transformation for each GPS solution analysisstrategyand constraintdefinitions).In our so- and then simultaneouslyestimated linear fits to all sites lutions, the orbits of the GPS satellites are not in a well-determined
reference frame.
The entire GPS con-
stellation can be rotated in longitude without degrading the fit of the data to our models.Equivalently,the longitudesof our estimatedstation coordinatescan be rotated without degradingthe data fit, althoughthis is not true for either height or latitude. However, while the entire GPS network and GPS constellation
can be
transformedas a rigid unit, our looselyconstrainedsolutions still determine relative frame-invariant quantities
very precisely. Station geocentricheightsand baseline lengthsare determinedvery preciselyin the looselyconstrained solutions,subject to someuncertainty in scale. In order to use the coordinates
derived
from these
solutions, we need to transform all of the loosely constrained
using the entire time series. Unfortunately, this technique is sensitiveto data outagesat the referencesites. Owing to unavoidablerandom or systematicerrors in the referencesite coordinatesand velocities,a different set of referencesites will produce a different realization of the referenceframe. In this study we have applied the referenceframe constraintsdifferently. We first estimate the velocity and epochposition of each site from the unconstrainedsolutions,using the full covariance information from our GPS solutions. This velocity solution is, like the individual GPS solutions,looselyconstrained. We then apply reference frame constraints by using the published ITRF94 positions and velocities for selectedreferencesites as pseudo-observations, weightedby the ITRF94 covariancematrix.
solutions into a consistent reference frame so
that we can derive•ates of site motion (and plate motion) from the time seriesof coordinates.The reference frame definesthe scale, origin, and orientation of our geodeticcoordinates.The referenceframe is specified
The reference sites we have chosen are listed in Table
3, and their locationsare plotted in Figure 2. These
werechosenfor their (1) highaccuracy,(2) geographic distribution,and (3) inclusionin the time seriesfrom 1991 through 1996. For geometric reasons,we would
by meansof a prioriinformation aboutthecoordinateslike to add a site in east Asia, but there were no sites and/or velocitiesof sites, or other similar quantities. that met our criteria for the period 1991-1996. There Since all plates on the Earth are moving, we must use a kinematic referenceframe, that is, one which includes the time evolution of the reference frame parameters. A referenceframe is realized through the coordinates and covariancesof individual stations. A sevenparameter transformation
can be estimated to transform an un-
are many other sitesin Europe and North America that could have been chosen, but these sites are close together and provide no additional geometric strength. We do not want to constrainthe velocitiesof too many sites, becauseour objective is to study the tectonic implications of the GPS velocities.
constrainedGPS solutioninto a specificreferenceframe. The quality of the transformationwill dependon the ac- Site curacyof the coordinatesand velocitiesof the reference
Velocities
stations which are used to derive the transformation
Using the techniquesdescribedabove, we estimated velocitiesfor 38 GPS sites. Velocity estimates, transaddition to random errors in coordinatesand velocities, formed into horizontal and vertical components, are the accuracyof the referencestation coordinatescan be listed in Table 4 along with the NNR-A velocity precompromisedby errors in local survey ties. diction and ITRF94 velocity,if available. This velocity For this study we haveadoptedthe ITRF94 reference solution is based on our entire time series of data.
and on the geographicdistributionof thosestations. In
frame (InternationalTerrestrialReferenceFrame 1994 It is instructive to compare the velocity estimates [Boucheret al., 1996]).This is the bestfit modelof po- based on all of the data with an individual time sesitionsfor 240geodeticsitesusingthe VLBI, SLR, GPS, ries of coordinatesderived from the same solutionsby and DORIS techniques.Velocitiesfor someof the sites transforming each individual solution into the ITRF94 are alsoincorporatedinto ITRF94 if there are sufficient data to determine an accurate velocity estimate. In practice, ITRF94 velocitiesare availablefor siteswith Table 3. Reference Stations longhistoriesof VLBI and SLR measurements.ITRF94 Site Name Location closelyfollowsthe developmentof previousframes, in
particular,ITRF92 [Boucheret al., 1993]and ITRF93 [Boucheret al., 1994],with improvements both in data quality and estimationstrategy. ITRF94 is designedto agreeon averagewith the NNR-A absoluteplate motion model, so ITRF94 velocitiesof sites on plate interiors should be directly comparableto the predictions of NNR-A if the plates are rigid.
I 2
Yaragadee Santiago
Australia South America-Nazca Africa
3
Hartebeestoek
4
Madrid
Europe
5
Kokee
Pacific
6
Algonquin
North America
7
Fairbanks
North
Park
America
LARSON
ET AL.: GPS GLOBAL
180 ø
PLATE
0ø
MOTIONS
9967
180 ø
.30 ø
0ø
.60 ø
_60 ø -90
-90 ø
180 ø
0ø
180 ø
Figure 2. Stationsusedto definethe referenceframe (seeTable 1 for stationidentifications). Table
4. Station Velocities and Standard Deviations GPS
Station
North
East
ALGO
2.6+1.2
-16.1+1.2
ALBH BALT BRMU
-7.1+1.5 9.2+1.9 8.04-2.0
-6.0+1.6 41.7+5.0 -13.44-2.2
CANB CHAT EISL FAIR
55.6+1.3 38.4+2.0 -4.9+2.9 -21.6+1.2 10.14.2.1 16.9:t:1.3 15.14.1.5 55.44.2.2 33.44.1.2 15.74.1.2 11.84.1.8 15.74.1.2 17.74.1.7 18.94.1.6 -10.54-1.4 -2.54.1.9 14.0:t:2.1 11.84.1.7 14.44.1.5 32.84.1.6
17.3+1.5 -48.3+2.5 77.4+4.2 -9.8+1.2 -9.04.2.5 16.64.1.3 16.34.1.6 16.24.2.5 -61.44.1.2 19.74.1.3 -4.44.2.1 20.24.1.2 16.64.1.9 23.94.1.8 8.74.1.5 -16.24.2.1 15.0•:2.2 15.34.1.9 18.04.1.6 -74.8 :t: 2.6
-10.54.1.5 55.54.2.4 -8.24.1.8 2.04.1.6
-14.64.1.6 41.34.2.7 -14.64.2.1 -12.74.1.8
17.54.1.3 14.64.1.6
17.24.1.3 -!6.2:t:1.7
-12.84.1.6 56.54.3.3 16.04.1.3 -16.64.2.6 34.44.1.7 3.14.1.9 14.84.1.2 56.5 :t: 1.2
27.14.2.1 23.34.4.2 16.44.1.4 -10.84.3.2 -19.44.2.2 -15.64.2.1 22.14.1.3 38.44.1.2
FORT
HART HERS HOBA KOKB KOSG KOUR MADR MASP MATE MCM3 NLiB NYAL OHIG
ONSA PAMA PENT PERT PIE1 RCM5 SANT STJO TAIW TOWN TROM TSKB WELL WES2 WETT YAR1
In millimeters per year.
NNR-A
Up 4.5+1.2 -0.9+2.0 -21.2+8.6 -1.3+2.9
8.4+1.8 1.54.4.4 -4.7+6.6 -1.6+1.2 5.04.3.6
-0.24.1.3 -1.64.2.2 5.44.3.5 -0.34.1.2 -4.94.1.7 0.14.3.1 2.94.1.2 1.34.2.5 -4.84.2.3 -1.94.2.3 0.04.2.7 9.94.4.6 -7.44.3.1
0.74.2.0 -1.54.3.5 -2.44.1.9 7.64.3.9 6.64.3.0 -0.54.2.4 5.44.1.3 -2.44.2.0 -6.24.2.6 19.34.7.2 1.54.2.1 -1.94.4.2 8.34.3.2 -0.44.2.7 -2.04.1.8 6.24.1.2
North 3.2
ITRF94
East -17.0
-14.0 9.6 8.3
-14.2 61.7 -12.3
53.7 31.4 -8.9 -20.2 11.7 20.1 15.2 54.4 32.3 14.5 11.1 15.7 17.5 12.8 -11.7 -2.2 13.6 10.2 13.6 31.5
17.7 -40.5 79.4 -10.3 -5.5 20.7 17.6 12.8 -58.3 18.5 -5.9 18.6 17.1 22.0 7.5 -15.9 12.9 16.3 18.6 -62.9
12.7 59.2 -8.7 2.2 9.5 12.6 -13.3 54.7 12.4 15.7 37.1 5.7 13.5 59.1
-15.1 38.0 -12.8 -10.7
-0.9 -14.8 22.3 30.0 17.2 -19.2 -0.6 -15.7 20.3 39.0
North 1.0+1.4
Up
East
4.6+1.4
-16.5+1.4
53.6 + 2.2
21.4 + 2.2
-1.6
+ 2.2
-12.0 + 2.2 -22.9 + 1.5
75.1+2.3 -8.2 + 1.6
-1.0
+ 2.2
-2.5
4. 1.5
15.2 4. 2.1 15.9 :t: 1.3
16.2 4. 2.3 18.0 4. 1.5
-1.6
4. 2.4
33.4 4. 1.7 15.3 4. 1.5
-60.0 4. 1.6 18.6 4. 1.9
-1.0
16.14.1.4
18.74.1.5
18.0 4. 1.7
23.44.1.5
1.9 4. 1.3
4- 1.5
- 1.1 4. 1.5
1.94.1.4 -2.44.1.6
4. 1.8
-13.5
4. 2.8
13.04.1.5
17.94.1.6
-1.1
4. 1.5
3.3 4. 1.4 20.24.2.8
-8.94.1.4 18.44.3.2
15.64.1.8
16.44-1.9
14.2 4. 1.2 58.44.1.4
19.7 4. 1.4 38.54.1.4
-2.5 4. 2.7
-14.6
1.74.1.2 3.0 + 2.8
-0.3+2.0
-3.3 4. 1.3 4.24-1.3
9968
LARSON
ET AL.'
GPS GLOBAL
PLATE
MOTIONS
0.4
0.2
(o) (b) -0.2
04 1990
1992
1994
1996
years
Figure 3. Individual epochsolutionsin latitude, longitude,and height for Yaragadee,Australia. At each epoch, a seven-parametertransformation has been estimated between the unconstrained
solutionsandInternationalTerrestrialReference Frame1994[Boucheret al., 1996].Formalerrors are onestandarddeviation. The linesshownare the fits to the globalGPS solutions,as described in the text.
reference frame independently. The most frequently the northernhemisphere,Kootwijk (KOSG), locatedin observedsite in this study is Yaragadee(YAR1), lo- the Netherlands. For Kootwijk, the weightedRMS decated in western Australia. The latitude, longitude, viation about the best fit line is 3.7, 4.9, and 9.4 mm and height estimatesof this site as a function of time for latitude, longitude, and height, respectively. The are shown in Figure 3 along with the linear fit of the improvement in position standard deviation for both global solution to the individual epoch solutions. The Kootwijk and Yaragadeefrom 1991 to the presentis weightedRMS deviation about the best fit line is 4.5, due to the increase in the number of satellites in the 7.6, and 12.2 mm for latitude, longitude, and height, GPS constellation, from 15 satellites in 1991 to 24 torespectively. In Figure 4, we show a typical site from day. The contrast in precisionbetween Yaragadee and
0.4
0.2
_(o)
0.0
_(b)
-0.2
-0.4
1990
1992
1994
1996
years
Figure 4. Individual epoch solutionsin latitude, longitude, and height for Kootwijk, Netherlands. At each epoch, a seven-parametertransformation has been estimated between the un-
constrained solutionsand InternationalTerrestrialReference Frame1994[Boucheret al., 1996]. Formal errorsare onestandarddeviation. The linesshownare the fits to the globalGPS solutions, as describedin the text. Note that the Kootwijk coordinatesare more precisewhen comparedto Yaragadeein Figure 3. This is due to the strengthof the IGS network in the northern hemisphere relative to the southern hemisphere.
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
9969
e = 0.5 mm/yr, At is in years,andC = 5.5 Kootwijk reflectsthe greater number of tracking sites whereO'fram
(and better realizationof the ITRF) in the northern ram, correspondingto the upper bound additive error suggested for VLBI data by Argusand Gordon[1996].
hemisphererelative to the southernhemisphere. It is crucial that we properly estimate the uncertainties in our velocity estimates. It has long been known that the formal errorsderivedby GIPSY usingthe analysisstrategydescribedin Table 2 underpredictthe true scatter, or repeatability, of individual estimates. We have therefore scaledthe position variancesso that the reduced chi squared statistic of the velocity solution is approximately1; this resultsin a variancescalingfactor of 9. This scaledoesnot compensatefor systematic referenceframe biases,possiblenon-Gaussianerrors, or possiblecorrelationsbetween solutions. The assumption of uncorrelateddata may be optimistic, sincethere is growingevidencefor temporal correlationsin GPS
The
2 afo,,m,,t istheGIPSYvariance multiplied by9, as
discussedearlier. We considerthis to be a safe, conservative estimate of the uncertainties. In effect, for sites present throughout the entire time series, the two ad-
ditiveerrorsadd1.44mm•'/yr•' to the variance of each velocity component, so none of our velocities will have
an uncertaintylowerthan about 1.2 mm/yr. Note that for sites present throughout the entire time series,the additive factors are larger than the scaleduncertainties based on random
errors.
errors are uncorrelated
We assume that
the additive
from site to site.
The velocity estimates and their adjusted covariance are then used to estimate angular velocities for eight solutions.King et al. [1995]determinedautocorrela- tectonic plates: Africa, Antarctica, Australia, Eurations for a 10 km GPS baseline using a 384 day time sia, Nazca, North America, Pacific, and South America. series of data and found nonzero correlations for time The three-dimensionalvelocity v of a geodetic site on lagsup to 20 days,althoughthe autocorrelations for all any plate can be written as
componentswere 0.1 or lessfor a time lag greaterthan 10 days. Long-term geodeticmonumentinstability is anotherpotential sourceof correlationsbetweenour so-
v=xr
(2)
lutions. Langbeinand Johnson[1997]haveanalyzeda where • is the angular velocity of the plate and r is the longtime seriesof data from two-colorlaserline length positionof the site (all Cartesianvectors).In the plate
tectonic model, all station velocitiesare explicitly horifor long-termcorrelationsin line length measurements zontal. The vertical componentof the site velocity thus that can be describedby a random walk process.Based contributesnothing to the estimation of the angular veon a similar length time seriesfor a regionalnetwork in locity, so there are in reality only two data per station. southernCalifornia,Bock[1995]suggests that a reason- With three parameters per angular velocity, velocities ablerandomwalkvariance wouldbeoforderi mm2/yr, from two sitesare required to determine all components althoughthere can be considerablevariation from site of the angular velocity of a plate. A priori information to site dependingon the local conditionsand the way can be applied to estimate an under-determinedanguthe GPS antenna is attached to the ground. However, lar velocity,but in this paper we only estimatedangular velocitiesfor plates with at least two siteson them. The Herring[1996]hassuggested that theseGPStime series relative angular velocity for a plate pair is simply the are too short to determine whether a random walk error model is required. The significanceof these results differenceof the absoluteangular velocitiesfor the two measurements
in California
and found clear evidence
independentplates. Angular velocitiesare frequently expressedin terms of their pole of rotation and angular been answered and is still an area of active debate. Choosinga conservativeapproach,we increasedour speed,and we follow that conventionin this paper. scaled uncertainties by additive factors to compensate for the possibleeffects of referenceframe biases and
for the interpretation of geodetictime serieshas not yet
correlations in the data.
Our reference frame realization
Geodetic
Results
is not unique,and the geometryof the chosenreference In this section we discuss the velocities of individstations is dictated by availability rather than optimal ual sites, and the discrepancieswith respectto NNR-A geographicdistribution. If we vary the set of reference sites, we can produce small changesin our estimated (Table 4). Differencesbetweenour estimatedvelocivelocities. We estimate that an additional site velocity ties for most sites in the plate interiors and the NNR-A uncertaintyof 0.5 mm/yr is sufficientto characterize the predictionsare quite small worldwide, indicating that possiblesystematicbiasescausedby a particular choice our referenceframe is aligned with NNR-A. Differences
of referencesites. To addresslong-term correlationsin at individual sites could be due to real differences in the data, we follow the approachof Argus and Gordon plate motions, local tectonic motions or site instability. [1996]and add a time-dependent velocityerror, which In this study we have ignoredeffectsdue to postglacial decreasesas the length of the time seriesincreases.We rebound, although there have been observationsof postglacial rebound from a longer time seriesof VLBI data modify the velocity varianceas follows:
[Argus,1996]The effectdue to postglacialreboundis 2
2
C •.
2
t7ne w--t7 formal -]-•-• -]-t7 fram e
(1)
primarily in the vertical component and we model the horizontal velocities exclusively.
9970
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
Table 5. Plate Angular Velocities Angular Velocity
Source
Pole Error Ellipse
Latitude,
Longitude,
a•,
O'max,
O'min ,
deg,
deg,
deg/m.y.
deg
deg
deg
•,
2.8
90
3.6
1
deg/m.y.
Africa (Hartebeestoek, Maspalomas) This paper
50.0
-86.8
0.26
NNR-A
50.8
-74.0
0.29
This paper
60.5
-125.7
0.24
NNR-A
63.1
-115.9
0.24
This paper
31.4
40.7
0.61
NNR-A
34.0
33.2
0.65
5.3
OOl
Antarctica(McMurdoand O'Higgins) 6.6
0 03
Australia(Perth, Yaragadee,Canberra,Hobart, Townsville) 3.1
1.0
-61
OOl
Europe(Hersmonceaux, Onsala,Tromso,Ny Alesund,Madrid, Kootwijk, Wetzell) This paper
56.3
- 102.8
0.26
NNR-A
50.8
-112.4
0.23
This paper
40.6
-100.7
0.70
NNR-A
48.0
-100.2
0.74
5.7
1.7
43
02
1.7
-5
05
Nazca (Baltra Island and Easter Island) 7.6
North America(Bermuda,NorthLiberty,Westford,Richmond,Algonquin,Fairbanks,St John's) This paper
-0.4
-84.5
0.22
NNR-A
-2.5
-86.0
0.21
4.3
2.0
0
Ol
Pacific (Pamatai, KokeePark, Chatham) This paper
-63.1
95.9
NNR-A
-63.2
0.70
This paper
-21.0
-183.5
0.16
NNR-A
-25.6
-124.0
0.12
107.4
2.3
0.9
-82
0.01
7.4
-71
0.06
0.64
SouthAmerica(Kourouand Fortaleza) 29.6
One sigmaerror ellipsesare specifiedby the angularlengthsof the principalaxesand by the azimuths(•b, givenin degreesclockwisefrom north) of the major axis. The rotation rate uncertaintyis determinedfrom a one-dimensional marginaldistribution[DeMets et al., 1990,Table 2a]. We also discussthe estimated angular velocity for
eachplate (seeTable 5). We first discussthe platesfor
procedurefor Madrid and the estimation of the Eurasia angular velocity and found a 30% increasein standard
which we have more than two sites with long time his- deviation when Madrid is removed. Only in the case tories, as theseare the best determined. Along with the of the Africanplateare our poleestimatesstronglydeangular velocitiesand their uncertainties,Table 5 lists pendenton the assumedvelocityof a referencesite. the sites used to define each plate. In some cases,stations that were used as reference sites were also used to
Eurasia
define the plate. It should be noted that while ITRF94 incorporatesinformation from NNR-A, ITRF94 velociAll of our siteson the stableEurasiaplate are located ties are in many casesdistinct from NNR-A predictions, in westernEurope. Thesesitesall havelongtime series, and one of our reference sites is not located in a stable as they were establishedas permanent sitesin 1992 and plate interior. For plates that include one of our refer- many were alsoobservedduring GIG. Site velocitiesand ence sites, we carefully examine the pole fits to ensure residualswith respectto NNR-A are shownin Figure5. that our results are not biased by the inclusion of ref- All sitesin the plate interiorexceptTromsoagreewith erencesites. For example,the North Americanangular NNR-A velocitieswithin 3 mm/yr and are well within velocity is basedon the velocitiesof sevensites,of which 95%confidence limits. The discrepancy at Tromsoaptwo, Algonquinand Fairbanks,are referencesites. If we pearsto be real, asour estimateagreeswith an indepenremoveAlgonquinand Fairbanks,the estimatedpole of dentanalysisof GPS data for that site [Boucheret al., rotation changesby 1.7ø in latitude and 0.3ø in longi- 1996]. Matera,Italy, is locatedin the plateboundary tude, and the maximum pole uncertainty increasesfrom zone between Eurasia and Africa, and thus we do not 4.3ø to 7.9ø. The change in the estimate of the angu- expectit to agreewith NNR-A. Our velocity(18.9•-1.6 lar velocity is much smaller than the uncertainty, so we mm/yr north and 23.9•-1.8 mm/yr east) agreeswell conclude
that
the inclusion
not bias our estimate.
The
of the reference increase
sites does
in standard
error
is caused by the geometry of the sites, meaning that Fairbanks is an important site for the estimation of the North America angular velocity. We followeda similar
with the SLR measurements (18.0•-1.7 mm/yr north and 23.4+1.5 mm/yr east) reportedin ITRF94. Our velocityfor Taiwan is surprisinglycloseto that predictedfor the stable Eurasianplate, even though it is locatedwithin a plate boundaryzone(Figure6).
LARSON
ET AL.:
GPS GLOBAL
PLATE
MOTIONS
9971
plate is 4.84-2.0mm/yr at N96øE, about 40% slower. The westward motion of Taiwan relative to Shanghai presumablyis due to elastic deformationcausedby the collisionof the Philippine Sea plate with Eurasia. Our estimate of the Eurasia angular velocity agrees with NNR-A within 95% confidence,but the uncertainty in our estimate is large. In order to reduce the uncertainty, we need a better distribution of siteswithin the plate rather than more precise velocities for sites in western Europe. For example, if each of the European sites usedfor our angular velocity estimate had a
TROM
standarddeviationof i mm/yr, the maximumpoleposition uncertaintywould be 4.8ø (with the actual data it is 6.3ø). With the additionof an equallyprecisesite
ONSA
in eastern Eurasia, the maximum pole position uncertainty would be reducedto 2.5ø. An accurate velocity from one of the new IGS sitesin Moscowwould provide a similar improvement.
KOSG
•$ø HERS •
/•toø
WETT
North
MADR
MATE
America
We have goodgeometriccoverageof the North American plate, with seven sites in the plate interior ranging from Alaska to Bermuda. We have also included
Albert Head (British Columbia), Penticton (British, Columbia),and Pie Town (New Mexico) in our analFigure 5. GPS station velocityestimatesand NNR- ysis of North America, although we have not assumed A residualsfor the Eurasia plate. The 95% confidence they are on the stable interior of the North American regionsare shownattachedto the residuals. plate. The site velocities and residual velocities rela-
oo
15ø
tive to NNR-A are shown in Figure 7. Fairbanks has Molnar and Gipson[1996]presented VLBI resultsfrom a marginally significantsouthward velocity relative to
with VLBI. Of the Shanghai,about 800 km to the north of Taiwan, which NNR-A (2.14-1.1mm/yr), consistent showthat south China is moving 8 + 0.5 mm/yr at three sites we removed from our angular velocity estiN116øE+4.1 ø with respect to the Eurasia plate. Our mate, only Albert Head showssignificantmotion relaestimated velocity for Taiwan relative to the Eurasian tive to North America,11.4+1.6 mm/yr at N56øW, in 1O0ø
120 ø
140 ø
160 ø
180 ø
200 ø
220 ø
40 ø
40 ø
20 ø
20 ø
0o
_20 ø
_20 ø ::
-40ø 1 O0 ø
120 ø
140 ø
160 ø
.......................... •i•:•:•::'"•:•:•::• ilii iiiii iiii I!-40ø 180 ø
200 ø
220 ø
Figure 6. GPS stationvelocityestimatesand NNR-A residualsfor the Australiaand Pacific plates. The 95% confidenceregionsare shownattachedto the residuals.
9972
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
210 ø
225 ø
240 ø
255 ø
270 ø
285 ø
300 ø
315 ø
210 ø
225 ø
240 ø
255 ø
270 ø
285 ø
300 ø
315 ø
Figure ?. GPS station velocityestimatesand NNR-A residualsfor the North Americanplate. The 95% confidenceregionsare shownattached to the residuals. For clarity, the velocity and residual for Alberthead, British Columbia, are not shown.
goodagreement with the previousanalysisof Argusand Townsville, and Canberra were observed as early as GIG. Perth and Hobart came on-line with Rogue reHeftin [1995]. Our data do not showevidenceof significantinternal ceivers in 1993. The Townsville site was abandoned for plate deformation, which agreeswith an independent continuous observationsin 1992, but we include it here analysisof VLBI data by Argusand Gordon[1996].We for completeness.The locations of these sites and their also see little evidence of vertical deformation from the velocitiesare shown in Figure 6. The size of the error GPS data. Algonquinrises4.5+1.2 mm/yr, a conse- ellipsesreflects the time span of the observations. We betweenNNR-A and the geodetic quenceof the ITRF94 frame constraint. North Lib- find no discrepancies
erty (0.0+2.6 mm/yr), Richmond(-0.5+2.4 mm/yr), velocities at the 95% confidence limit. The baselines Westford(-0.4+2.7 mm/yr), and Bermuda(-1.3+2.9 betweenthe differentAustralian sitesalsoshowno sigmm/yr) all showno vertical deformationwithin one nificant lengtheningor shortening,which is consistent standard
deviation.
Our vertical
estimate
for North
Liberty disagrees with the ITRF94 predictedsubsidence of 13.5+2.8 mm/yr, which is basedon VLBI observa-
with the NUVEL-1A assumption of no internal plate deformation.
The discrepancybetween the NNR-A pole and our tions. The resolution of this discrepancy will require geodeticpole is 2.6ø in latitude and 7.5ø in longitude, a careful comparisonby the VLBI and GPS analysis with a maximum uncertainty of 3.1ø. The Australia centers,althoughwe note that our result is more plau- angular speedis smallerthan predicted by NNR-A. Al-
though the NNR-A pole discrepancyis not significant at 95% confidence,we have conductedseveraltests to determine the sensitivityof the Australia angularvelocGiventhe goodagreementbetweenpredictedand observedvelocities in stable North America, it is not sur- ity to our data. For example, if we removeYaragadee prisingthat the poleof rotation and angularspeedalso as a referencesite and replace it by Canberra, the Ausagreewell with NNR-A. Our pole agreeswith NNR-A tralia pole is still shifted 7ø east of the NNR-A pole. to within 2ø in pole position, well within one standard If we removethe Yaragadeeor Canberra data from the deviation. angular velocity estimation, the pole moves less than 1ø and the angularspeedchangeslessthan 0.01ø/m.y. Australia Fortunately, the Australian plate is wel] instrumented Our analysisof Australian plate motion is based on with GPS receivers and more accurate velocities should the motionsof five sites: Yaragadee,Canberra, Perth, be availablein a few years. Currently, the discrepancy Townsville, and Hobart. Yaragadee and Perth are lo- betweenour angular velocity for Australia and NNR-A cated on the western coast, and Canberra and Ho- is not significantat the 95% confidencelimit. bart are located on the eastern coast and on the isAlso shownin Figure 6 is Wellington, New Zealand, land of Tasmania, respectively. Of these, Yaragadee, located in the Pacific-Australianplate boundary zone. sible than the VLBI
result and the difference could be
explainedby subsidenceof the VLBI antenna.
LARSON
ET AL.:
GPS GLOBAL
PLATE
MOTIONS
9973
A permanentGPS receiverwasoperatedtherethrough- ceiver was installed in early 1995. The differencesbeout 1991-1992 and then was abandoned. Fortunately we
tween NNR-A predictions and our velocities for Mc-
have been able to augment our Wellington time series with campaign measurementstaken in January 1994 and January 1995. Wellington's velocity is consistent with the plate boundarydisplacementfield derivedfrom
Murdo (< 1 mm/yr) and O'Higgins(< 2 mm/yr) are
We have analyzed data from two continuousGPS sites on the Pacific plate: Kokee Park, Hawaii, and Pamatai, French Polynesia(Figure 6). We have a 5 year time seriesat KokeePark and a 4 year time se-
Africa
remarkably small. The Antarctica pole agreesbetter with NNR-A in latitude than longitude, but the standard deviations are also larger in longitude than latterrestrialgeodetictechniques by Bibbyet al. [1986]. itude. The prospects for future Antarctica measureOur newlyestimatedvelocityfor Wellingtonagreeswith ments are good. Three additional sites on the Antarcthat of Larsonand Freymueller[1995]to better than 1 tica plate were added during 1994: Casey and Davis on the continent and Kerguelen Island. All of these mm/yr and 2ø in azimuth. sitesare in the IGS networkbut werenot installedearly Pacific Plate enoughto contribute to this analysis.
The African plate is sampledat Hartebeesthoek,South
Africa, and on the Canary Islands(Mas Palomas). The Mas Palomas velocity agreeswith NNR-A to within 1 ries at Pamatai. Kokee Park is a reference site and mm/yr. Hartebeestoekis one of our referencesites,so thus agreesbetter with ITRF94 than NNR-A. The reits velocity has been constrainedto agreewith ITRF94, sultingvelocityfor KokeePark is 3 + 1.5 mm/yr faster and its agreement with NNR-A is only within three than NNR-A. The NNR-A velocity for Pamatai is 70.3 standard deviations. Since we do not have enoughinmm/yr, but our GPS velocityis 81.2+ 2.9 mm/yr, dependent data from the African plate to evaluate the about 15% faster. Initial SLR results for the nearby site significanceof the discrepancyat Hartebeesthoek, we at Huahine, French Polynesia,were reported as 874-3 cannot be sure that our estimate of African plate momm/yr [Robbins et al., 1993]but havesincebeenrevised tion differssignificantlyfrom NNR-A. In any case,with downward to 714-3mm/yr [Boucheret al., 1996]. To only the two sitesthe angular velocity is not determined expandour set of siteson the Pacificplate, we havealso precisely,with an uncertainty of 7ø in pole position lonanalyzedtemporary and permanentdata spanning3.3 gitude. Additional data from siteson the stable African yearsfrom Chatham Island. The velocityof Chatham plate are neededto improvethe estimate of the angular Island is about 20% faster than predicted by NNR-A. velocity. At present, there is only one additional site Velocitiesof all three are fit well by a pole of rotation on the African continent, and it has a short time histhat lies11ø (4a) to the westof the NNR-A poleof ro- tory. This site (Malindi, Kenya) is located east of the tation and has a angularspeedgreaterby about 10% East African Rift System, so it is not on the African (6a). The Pacificpoleis the mostpreciselydetermined plate. We expect it will be severalyears before a better in our study becausethe GPS siteson the plate are so estimate of African plate motion can be obtained. widely spaced. No other plate in this study has an angular veloc- Nazca ity sodifferentfrom that predictedby NNR-A. To test Sites on the Nazca plate are necessarilylimited to our angularvelocity,we useit to predictthe velocities islands. SLR measurementswere made prior to the inof SLR and VLBI sites on the Pacific plate. Our prestallation of a permanent GPS site on Easter Island in dictedvelocitiesfor Kwajalein(VLBI), andMaui (SLR) 1994. Our GPS velocity, shownin Figure 8, agreesat andHuahine(SLR) all agreewith the ITRF94 velocities the two standard deviation level with both the NNRfor those sites within the 95% confidence limits of the
data. If we combine ITRF94 velocities for Kwajalein,
Maul, andHuahineand our velocitiesfromPamataiand Chatham, the resultingpole is -63.3 ø latitude, 96.6ø
A and the ITRF94 value. With only one site on the Nazca plate, we would be unable to estimate an angular velocity, so we have also included data from two temporary sitesin the GalpagosIslandsthat were occu-
longitude, andthe angularspeedis 0.68ø/m.y.We sug- pied as part of the Centraland SouthAmerica(CASA) gestthat the motionof the Pacificplate overthe last experiment[Fre•maelleret al., 1993].We includedata 5 yearsdoesnot agreewith its motion overthe last 3
from Isla Baltra from 1991 and 1994, and data from a site on Isla Santa Cruz, about 30 km to the south, which was observed in 1994 and which became a perAntarctica manent site in early 1996. The two sites are 30 km There are two sites on the Antarctic plate that meet apart and were assumedto have the same velocity. The our criteria of a 2 year time span: McMurdo and data are consistentwith this assumption,and the 5 year O'Higgins. Both McMurdo and O'Higgins were ob- time seriesyields a velocity that is significantly slower servedduringthe GIG campaign.A permanentreceiver than NNR-A predictions. The differencebetween our wasplacedat McMurdo in February 1992 but has been velocity and the NNR-A prediction for that site is 20+5 movedtwice sincethen. The permanent O'Higgins re- mm/yr (Figure 8). Our estimatedpole of rotation for m.y.
9974
LARSON
240 ø
260 ø
280 ø
300 ø
ET AL.: GPS GLOBAL
320 ø
340 ø
20 ø
20 ø
0o
0o
_20 ø
_20 ø
_40 ø
_40 ø
PLATE
MOTIONS
The large uncertainty in the pole position is controlled by the relatively large uncertainty in the velocity of Baltra. When the velocitiesof the Galapagos and Easter Island sites are determinedwith a precision
of I mm/yr, thesetwo siteswill be su•cient to determine a precisepole of rotation (maximumpole uncertainty 2.5ø), althoughdata from additionalsiteswould be required to determine whether the Nazca plate is deforming internally. Data from a regional campaign have been taken at a site in the Juan Fernandez
in the southeast part of the Nazca plate, which may eventually help resolvethis issue. South
_60 ø
240 ø
_60 ø
260 ø
280 ø
300 ø
320 ø
340 ø
Figure $. GPS station velocity estimatesand NNRA residuals for the South America and Nazca plates.
The 95% confidenceregionsare shownattached to the residuals.
islands
America
We have analyzed data from three permanent GPS siteson the South American plate. Santiagois located in the SouthAmerican/Nazcaplateboundaryzone. On the stable portion of the plate, we have observations from Fortaleza, Brazil, and Kourou, French Guyana. Their horizontalvelocitiesand NNR-A discrepancyvectors are shown in Figure 8. Their velocitiesagree to better than 1 mm/yr with NNR-A in the north component and within two standard deviations in the east
the Nazcaplate differsby 8ø from the NNR-A pole, but the uncertaintyis almostas large (7ø). A small shift in the pole position and angular speedcan accountfor a component. The angular velocity for South America is the most large differencein velocitybecausethe pole is located poorly determined of the eight plates estimated in this fairly closeto the plate.
Previouslypublishedresultsfor Baltra [Freymueller paper. This is simply becauseKourou and Fortaleza are et al., 1993]gavethe motionof Baltra relativeto Jeru- lessthan 2000 km apart, yieldingpoor sensitivityto the salen in Ecuador based on data from 1988, 1990, and longitudeof the pole (maximumstandarddeviationof 31ø). The addition of another site in southernSouth 1991. The 1990 and 1991 results for Baltra are consisAmerica wouldsubstantiallyimprovethe geometryfor tent with the low rate obtained in this study, although
the 1988 data are not.
The
1988 CASA
results also
determiningthe pole of rotation. The uncertaintyin the angular speedwill be reducedby about 50% when the
show an unexpectedeast-westmovementof Baltra relative to Isla del Cocoon the Cocosplate, which couldbe site in La Plata (near BuenosAires) has a sufficiently explainedif the coordinatesobtainedfor Baltra in 1988 precisevelocity. The longitude of the pole will remain poorly constraineduntil a site in western South Amerwere biased to the west. We conclude that the 1988 solutions for Baltra were probably biasedand that the re- ica, but east of the deformingAndes, is included. No maining data are consistentwith a rate of motion much permanent sites meeting that criterion have yet been established.
lowerthan predictedby NNR-A. Resultsfrom 1991 and 1994 for Isla Malpelo, about 800 km to the northeast of Baltra and also on the Nazca plate, are also con- Relative Angular Velocity Vectors sistent with a lower velocity than would be predicted by NNR-A. The motion of the Nazca plate is well conRelative angular velocitiesdescribethe relative mostrained in the NUVEL-1A model since it is surrounded tions of a plate pair and can be derivedby differencing on three sidesby spreadingcenters,so we would not ex- the absoluteangular velocitiesfor the two plates. Anpect NUVEL-1A to havean incorrectestimateof its mo- gular velocitiesderived from GPS data are generally tion. Active volcanismin the GalapagosIslands occurs correlated, due to the correlations between sites in the about 75 km to the west, on Isabella and Fernandina GPS velocity field. Just as GPS relative velocitiesare islands[Simkinand$iebert,1994].Westwardmotionof more precisethan absolute velocities,the uncertainties both GalapagosIslands GPS sites could be causedby of relative plate angular velocitiesare smallerthan those ongoingflexureof the lithospheredue to the load of the of absoluteplate motions. Relative angular velocities active volcanic islands if these islands were still subsidare also lesssensitivethan absoluteangular velocities ing today. However, we have no explanation that can to referenceframe errors in the GPS velocities,or the definitively account for the entire discrepancy.It may no-net-torque assumption used to derive the NNR-A be that the plate is deforminginternally. Data from the model from NUVEL-1A. We can compareour relative Galapagosand Malpelo will be examined more fully in angular velocitiesdirectly with the NUVEL-1A relative a future paper with the other CASA regionalcampaign plate motion model, and unlike NNR-A, standard dedata.
viations are available for NUVEL-1A.
This allows us to
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
9975
better assessthe significanceof discrepanciesbetween estimate agrees well with both NUVEL-1A and Godthe plate model predictionsand our geodeticanalysis. dard but has a relatively large uncertaintyin the pole In Table 6 we compare our relative angular velocity positionestimatedue to the poor geometryof the GPS estimatesto NUVEL-1A and other publishedgeodetic siteson the Eurasianplate. The GPS-only solutionwill studies. We have listed all plate pairs which share a be improvedwhen sitesoutsideof westernEurope conboundary. For comparisonwith an independent GPS tribute. The JPL-GPS and VLBI pole positionsare lo-
analysis,we list Argusand Heftin [1995]valueswhen catedmorenortherlyof NUVEL-1A. Our angularspeed available(hereafterJet PropulsionLaboratory(JPL)- agreeswith NUVEL-1A, as do all of the other geodetic GPS). Our study usesa longertime seriesthan JPL- solutionswith the exception of the VLBI solution. GPS, and includes more sites. We have also made a greater effort to augmentour velocitiesby using data from temporary occupationsof sites. The JPL-GPS paper also showedangular velocitiesderived from VLBI data, which they have made available(D. Argus and R. Gordon, manuscriptin preparation,1997) (hereafter VLBI). For comparisonwith a recent multipletechnique analysis,welist Smithet al. [1996](hereafter Goddard). This groupcombinedseparateanalysesof VLBI, SLR, GPS, and DORIS data to'estimateangular velocities for many of the plates we discuss. The Goddardstudy has the advantageof havingmore data and more sites becausethey use severaltechniques,although inconsistenciesbetween the velocity solutions used could potentially causebiasesin the results. For severalplates, they rely only on GPS data, and we expect goodagreementof resultsfor theseplates. We note two trends in Table 6. First of all, there is good agreementbetweennearly all our GPS derived relative angular velocitiesand NUVEL-1A, with the exceptionof someof those involvingthe Pacific plate. In general, there is also good agreementbetween the independentgeodeticanalyses,This is encouraginggiven that VLBI, SLR, DORIS, and GPS are quite distinct geodetictechniquesand the data wereanalyzedand referenceframe constraintsapplied in very different ways. The one exception to this good agreement is for the North America-Africa pole position. Upon closer in-
Pacific-North
America
Pacific-NorthAmerica relative plate motion has critical implicationsfor deformationin the plate boundary zonesof Californiaand Alaska. Our estimatedangular velocity (Figure 10) is significantlydifferentthan NUVEL-1A, both in pole locationand angularspeed. Our angularvelocitydisagreeswith the other geodetic studiesin longitudebut agreesin latitude and rate. All of the geodetictechniques estimatea fasterangular speedthan NUVEL-1A, but only our rate and the VLBI rate exclude the NUVEL-1A
rate from the 95% confi-
denceregion. The VLBI and Goddardangularvelocities are basedon differentsetsof stations. For VLBI, the sites are in the northern hemisphere,specifically Marcus Island, Hawaii, and Kwajalein. The GPS estimates are based on Hawaii and sites from the southern
Pacific. The Goddard solutionwill averageboth northern and southernhemisphereas VLBI, SLR, and GPS data contribute to the angular velocity estimate. We are the only analysis listed in Table 6 which usesmeasurementsfrom Chatham Island. OngoingGPS measurementsfrom sites such as Kwajalein and Chatham Island shouldresolveissuesregardingthe Pacificplate. Despite the significantdifferencebetweenour pole and the NUVEL-1A pole, both predict the samerelative motion along almost the entire Pacific-North America plate boundary(Table 7). For a point in southernCalspection, it becomes clear that pole uncertainties are ifornia near VandenbergAir Force Base, we predict a poorly definedat extremelyhigh latitudes(the pole is relative platemotionvectorof 46.4+ 2.8 mm/yr toward locatedat a latitude of 79ø). In this case,we havealso N40.3øW+1.8, 2.7ø westerly of NUVEL-1A but with inspected the Cartesian uncertainties, which indicate the same rate to within 0.4 mm/yr. The azimuthdifferagreementwith NUVEL-1A at better than two stanenceis not significantat the two sigmalevel. In the Gulf dard deviations. In Table 7 we show the predicted relative motion at of California, our model predicts relative motions 1.3 severallocationsalong plate boundaries. Two angular mm/yr fasterthan NUVEL-1A towarda direction5.6ø velocitiesfor a givenplate pair may be significantlydif- more westerly. The rate differenceis insignificant,but ferent and yet predict motionsalongthe plate boundary the azimuth differencewith NUVEL-1A is possiblysigthat are not significantlydifferent. This is the casefor nificant. Our predicted rate and azimuth all well within our Pacific-North America angular velocity, for exam- the onesigmauncertaintyrangefor spreadingratesand ple. Where our predicted relative motions on the plate transformfault azimuthsin the Gulf of California,however[DeMetset al., 1990].DeMets[1995]showedthat boundary differ from those predicted by NUVEL-1A, the 3.16 m.y. averagespreadingrate in the Gulf of Caliwe can compare our relative motions to the raw data forniais slowerthan both the 0.78 m.y. averagespreadfrom which NUVEL-1A is derived.
Eurasia-North
America
In Figure 9, we showthe pole positionof the EurasiaNorth Americanangularvelocity. In eachcase,we have plotted the position and its 95% confidenceellipse. Our
ing rate and the NUVEL-1A closure-fittingrate (the Pacific-NorthAmericarelativemotionpredictedby the NUVEL-1A data excludingdata from that plate boundary), probablybecause the Gulf of Californiaspreading centers did not accommodate
the entire Pacific-North
Americarelativemotion until about 2 m.y. ago. Our
9976
LARSON
ET AL.:
GPS GLOBAL
PLATE
MOTIONS
Table 6. RelativeAngular Velocitiesfor Plates Sharinga Boundary Angular Velocity
Source
Pole Error Ellipse
Latitude,
Longitude,
c•,
deg
deg
deg/m.y.
This paper
68.1
NUVEL-1A VLBI Goddard JPL-GPS
62.4 74.0 66.7 78.5
This paper
-49.6
NUVEL-1A VLBI Goddard JPL-GPS
-48.7 -50.5 -49.8 -49.1
This paper
76.3
NUVEL-1A Goddard JPL-GPS
78.9 78.8 80.9
Ëmax, O'min ,
•
deg
deg
Europe-North America 126.6 0.24 6.5
3.9
-30
0.02
1.3 2.4 1.2 4.9
-11 -48 -39 -8
0.01
135.8 111.3 126.8 122.0
0.21 0.26 0.22 0.23
4.1 5.4 3.0 8.2
Pacific-North America 95.7 0.83 2.0
deg
deg/m.y.
0.02 0.01 0.03
1.0
-86
0.02
1.3 2.0 2.6 4.1
1.2 0.8 1.1 2.2
61 -84 -86 -83
0.01
Africa-North America 103.5 0.21 7.1
5.7
76
0.01
1.0 3.4 11.1
77 36 15
0.01
101.8 104.1 103.1 107.0
0.75 0.78 0.77 0.79
38.3 0.24 3.8 39.2 0.24 6.3 16.7 0.22 14.5 South America-North America
0.01 0.02 0.03
0.02
0.04
This paper
-11.1
126.7
0.29
6.6
3.9
27
NUVEL-1A JPL-GPS
-16.4 -6.5
121.9 124.4
0.15 0.28
6.2 8.3
3.9 7.4
9
0.08 0.01 0.12
2.1
0.9
-68
0.02
1.3 2.4 3.3
1.2 0.8 2.2
-90 -76 -88
0.02
0.02 0.01
This paper
-61.5
Pacific-Europe 90.0 0.97
NUVEL-1A Goddard JPL-GPS
-61.2 -61.9 -60.2
94.2 98.4 95.6
0.86 0.90 0.95
-55
0.02 0.05
Australia-Europe This paper NUVEL-1A Goddard JPL-GPS
8.6
48.5
0.65
3.7
1.4
-46
15.2 12.4 9.9
40.5 44.6 47.4
0.69 0.66 0.72
2.2 1.7 4.9
1.2 0.6 4.0
-45 -51 -53
0.02 0.05
This paper
-23.5
Africa-Europe -29.8 0.05
35.0
21.4
25
0.02
NUVEL-1A Goddard JPL-GPS
21.2 18.4 -11.7
-20.6 -24.6 -27.3
6.2 13.3 41.7
0.8 10.0 36.1
-4 -71 36
0.02 0.03 0.03
0.12 0.10 0.07
This paper
65.7
Australia-Pacific 2.9 1.04
1.7
1.5
2
0.02
NUVEL-1A Goddard JPL-GPS
60.2 60.8 57.2
1.7 3.9 6.5
1.1 1.9 2.6
1.0 1.0 2.4
58 29 43
0.02
2.0
1.5
44
0.03
1.2 2.7
1.1 1.7
82 84
0.06
1.07 1.07 1.13
This paper
63.6
Antarctica-Pacific -95.0 0.93
NUVEL-1A Goddard
64.5 64.8
-84.0 -82.3
0.87 0.90
0.02 0.04
0.01
This paper
-10.6
Africa-A ustralia -127.3 0.65
3.3
1.9
58
0.02
NUVEL-1A Goddard JPL-GPS
-12.5 -10.1 -11.2
-130.2 -127.0 -127.4
1.3 2.5 6.1
1.0 1.8 4.3
39 -62 -22
0.01
-9.8
-136.8
0.65
4.4
2.6
20
NUVEL-1A Goddard LF
-13.2 -11.1 -12.8
-141.7 -138.6 -143.1
0.65 0.64 0.65
1.3 5.2 6.7
1.0 2.5 3.5
63 30 15
This paper
-47.4
South America-Africa 131.4 0.35 14.0
4.6
3
0.06
NUVEL-1A JPL-GPS
-62.6 -39.9
140.6 131.7
0.8 7.4
11 -7
0.01
0.63 0.63 0.71
0.03 0.04
Antarctica-Australia
This paper
0.01 0.01 0.03 0.03
0.31 0.38
2.7 16.2
4.2
1.4
2
0.04
1.9 6.3
0.9 1.5
-1 26
0.02
This paper
52.2
Nazca-Pacific -94.5 1.37
NUVEL-1A Goddard
55.8 67.3
-90.1 -81.1
1.36 1.27
0.01
0.03
LARSON ET AL.: GPS GLOBAL PLATE MOTIONS
Table 6.
9977
(continued) Angular Velocity
Pole Error Ellipse
Latitude,
Longitude,
•o,
er,•ax,
er,•i•,
•p
er•,
deg
deg
deg/m.y.
deg
deg
deg
deg/m.y.
Source
South America-Antarctica
This paper
-63.75
126.5
0.29
20.2
4.4
6
NUVEL-1A
-86.44
139.3
0.26
3.1
1.2
24
This paper
-4.49
Africa-Antarctica -42.5 0.11
24.2
13.8
3
5.64 5.6
-39.2 0.13 -39.1 0.13 Nazca-Antarctica
4.6 26.1
1.4 14.1
-41 19
This paper
30.0
-94.0
10.7
2.9
NUVEL-1A Goddard
40.7 73.3
-95.9 0.52 -77.0 0.37 South America-Nazca
4.7 21.4
2.0 4.1
NUVEL-1A Goddard
0.49
0.05 0.01
0.02 0.01 0.04
2
0.06
-9 -30
0.02 0.06
This paper
-43.8
95.2
0.74
9.1
5.5
18
0.07
NUVEL-1A
-56.1
86.0
0.72
3.7
1.5
10
0.02
NUVEL-1A from DeMets et al. [1990]and DeMets et al. [1994];VLBI from D. Argus and R. Gordon(manuscriptin preparation,1997); Goddardfrom Smith et al. [1996]; JPL-GPS from Argusand Herin [1995];LF from Larsonand Freymueller[1995]. Pole error ellipse conventiondefined as in Table 5
predicted spreadingrate agreesalmost exactly with the NUVEL-1A closure-fittingrate, and the 0.78 m.y. av-
tion at both the Pacific-Australia
and Pacific-Eurasia
plate boundariesthan NUVEL-1A. In thesecases,rela-
eragespreadingrate of DeMets[1995]lies within our tive plate motion on the boundaryis significantlyfaster onesigmauncertainty.At Kodiak Islandin Alaska,our in our model but with the same azimuth as NUVELpolepredictsrelativemotion1.6mm/yr morerapid and 1A. Because both of these boundaries are subduction oriented 3.1ø more northerly than NUVEL-1A; again, thesedifferencesare not significant.Only in the western Aleutians is our predicted Pacific-North America relative motion significantlydifferentthan NUVEL-1A,
boundaries where the plate boundary data are sensitive to the azimuth of relative plate motions, our model is just as consistentwith the data from those plate boundaries as is NUVEL-1A. No data from the and there it is different in rate rather than azimuth. Pacific-Australiaplate boundary were usedin determinSince the only plate boundary data from the western ing NUVEL-1A. On the basisof the faster convergence Aleutians comefrom earthquake slip vectors,which are rates predictedby our model, about 22% faster for the sensitive to the azimuth of relative plate motion, our Pacific-Australiaboundaryand about 12% fasterfor the faster rate of subduction here remains consistent with Pacific-Eurasiaboundary,our data are consistentwith the available plate boundary data. a correspondinglyhigher rate of seismicmoment release Pacific-Australia and Pacific-Eurasia at these boundaries. Similar implicationshold for the Unlike at the Pacific-North America plate boundary, otherplate boundaries in the westernPacific,including our model predicts significantlydifferent relative mo- the Pacific-PhilippineSeaplate boundary.
Table
7. Relative
Plate Motions
at Selected Locations
on Plate Boundaries
This study
Location
Latitude,
Longitude,
deg
deg
Rate ,
NUVEL1-A
Azimuth
Rate
,
Vandenberg
34.6
-120.6
46.44-2.8
-40.3 4- 1.8
46.8-4-1.3
Gulf of California
23.5
-108.5
48.74-2.8
-59.8
47.44-1.2
Kodiak, Alaska
57.6
-152.2
58.34-1.7
West Aleutians
51.0
Alpine fault, New Zealand Tokyo, Japan
-43.5 36.1
East Pacific Rise East Pacific Rise East Pacific Rise
-10.0 -19.0 -30.0
4- 2.0
-18.34-2.5
173.1
79.44-1.7
-42.84-1.9
170.0 140.1
45.44-1.7 103.44-2.6
-103.64-2.6 -67.64-1.2
-110.0 -113.0 -112.0
136.34-3.6 145.54-3.6 151.34-4.0
Azimuth
100.6 4- 1.4 101.84-1.3 100.84-1.2
56.7 4- 1.4 74.34-1.4
37.04-1.4 92.74-1.7 139.9 4- 1.6 147.44-1.5 151.14-1.6
-37.64-1.5 -54.24-1.5
-21.4 4- 1.2 -45.64-0.9
-109.14-2.1 -69.04-0.9 102.04-0.8 103.04-0.8 102.24-0.7
All rates are givenin millimetersper year, and azimuthsin degreesclockwisefrom north. Uncertaintiesare one standard deviation.
9978
LARSON ET AL.: GPS GLOBAL
PLATE MOTIONS
10 mm/yr for NUVEL-1A. The increasedfault-normal contraction predicted by our model should be observable geodetically,and ongoingGPS observationsin this area should be capable of testing our model prediction in the future.
GPS results from the southwest
Pacific
[Beyiset al., 1995; Taylor et al., 1995]are consistent with our proposed Pacific-Australia convergencerate but are too impreciseto test the differencebetween our model prediction and NUVEL-1A. The rate of underthrusting of the Pacific plate beneath the Japaneseislandsdependson both the PacificEurasia relative plate motion and the motion of the Japaneseislands relative to Eurasia, which we will not attempt to addresshere. At a location near Tokyo at the southern end of the main Japaneseislands boundary with the Pacific plate, our model predicts 103.4+2.6
6•0o
mm/yr of relative motion of the Pacific and Eurasian plates,11.7mm/yr fasterthan predictedby NUVEL-1A and oriented
110 ø
120 ø
130ø
.•40 o
at the same azimuth
within
uncertainties.
Pacific-Nazca
Figure 9. Eurasia-NorthAmericanpole position,with 95% confidence regionfor DeMets et al. [1994](trian-
gle),Smithet al. [1996](circle),VLBI (D. ArgusandR. Gordon,manuscriptin preparation,1997) (square),ArgusandHeftin[1995](diamond),and this paper(star). On the Alpine fault in New Zealand, our model predictssignificantlyfaster relative plate motion, 8.4 mm/yr fasterthan NUVEL-1A and directedmoreobliquely to the trend of the Alpine fault. Projectedonto the N55E trend of the Alpine fault, our velocity givesa fault-parallelrate of 42 mm/yr and a fault normalcontractionrate of 16 mm/yr, comparedto 36 mm/yr and
Although our velocity for Baltra Island was significantly different than that predicted by NNR-A, the angular velocity for the Nazca plate agreed with NNRA within
its stated
uncertainties.
We have tested
our
Nazca-Pacific angular velocity by looking at predicted velocitiesat severallocationsalong this plate boundary. As shown in Table 7, the agreementbetween NUVEL1A and our model is very good. At -19.0 ø latitude,
our modelpredicts145.5+3.6 mm/yr at an azimuth of 101.8ø + 1.3ø, whereas NUVEL-1A predicts 147.4+1.5
mm/yr. For the samelocation, Wilson[1993]finds a spreadingrate of 153.7mm/yr, slightlyfasterthan our rate but within 95% confidence limits. Ant arct ica- A ust r alia
100 ø
110 ø
9O'
1•0o
In Figure 11 we showthe pole of rotation for Antarctica-Australia. There are no VLBI or SLR angularvelocity estimatesfor the Antarctica plate. Goddard combined DORIS and GPS. For completeness,we compare
ourestimatewith the LarsonandFreymueller[1995]estimate for data that spanned 1991-1993. In that paper, the z componentof the angular velocity was constrained to agreewith NNR-A becausethere wasonly one site on the Antarctica plate. Our new estimate is basedon the velocitiesof two sites on Antarctica, so there is no need to constrainthe angular velocity. Again, the pole of ro-
ø60ø
9o' øøø øø
'60'
o
Figure 10. Pacific-NorthAmericanpole position,with
tation latitude agreeswell between NUVEL-1A, Goddard, and our estimate. The angular speedsalso agree within two standard deviations. The pole longitudes, as with Pacific-North America, agree lesswell. Africa-Australia
The Africa Euler pole is not well determinedby any 95% confidence regionfor DeMetset al. [1994](trianof the geodetic techniquesdiscussedin this paper, but gle), Smithet al. [1996](circle),VLBI (D. ArgusandR. the Africa-Australia relative angular velocity pole is relGordon,manuscriptin preparation,1997) (square),ArgusandHeftin[1995](diamond),andthispaper(star). atively well determined. The NUVEL-1A standard de-
LARSON ET AL.: GPS GLOBAL PLATE MOTIONS
220 ø
9979
signals. We are thus able to compare GPS velocities
230 ø
•0 ø
o with plate models, specificallythe NUVEL-1A absolute plate motion model NNR-A. For all but a few sites, the
agreementwith NNR-A is better than 95% confidence. "
0"
Specifically,sites in North America, Antarctica, South America, Eurasia, Africa, and Australia with long time
seriesagreewith NNR-A to better than 3 mm/yr. The
,Z•O ø 220' 230'
eo o
Figure 11. Antarctica-Australia and Africa-Australia
pole positions,with 95% confidenceregionfor DeMets et al. [1994](triangle), Argusand Heftin [1995](diamond),Smith et al. [1996](circle),Larsonand Freymueller[1995](invertedtriangle),andthis paper(star).
viation for this plate pair is also quite small. Figure 11 showsthat all the geodeticestimatesagree within a few degrees,and all are offset from NUVEL-1A by 3ø. The agreementbetweenthe different geodeticanalyses is likely because all three are controlled by the GPS data from Mas Palomas and Hartebeesthoek.
The dis-
crepancybetween NUVEL-1A and the geodetic analyses is most likely controlled by the GPS data from Hartebeesthoek, as discussedearlier. North
America-South
discrepanciesthat do exist on the Pacific and Nazca plates are intriguing. GPS sites from the Pacific are faster than plate models would predict. In addition, sites in the south Pacific have larger discrepanciesthan sites in the north Pacific. On the Nazca plate, Baltra Island is almost 50% slowerthan NNR-A predictions. A nearby permanent GPS installation on the GalapagosIslands will be able to confirm this result within the next few years. For the most part, significantvertical deformation is limited to reference sites that required it or sites where we mixed permanent installations and campaigns(e.g., Wellingtonand Baltra). In theselatter cases,antenna height recording errors can produce significantvertical error. The data used in this analysis were available as the result of a cooperative international effort to install and operate G PS receiversthroughout the world. With just 5 years of data, we were able to estimate angular velocities for eight tectonic plates. Continued expansion of the IGS network should allow for angular velocity estimation for most of the remaining tectonic plates by the end of the century. We currently assumethat all site velocities vary linearly in time. With extension of thesetime series,we will be able to addressthe validity of that assumption,as well as investigatingthe significance of vertical
America
P90ø300 ø 310 ø $20ø
Finally, we show a plate pair, North America-South
America (Figure 12), for which there are no conventional plate motion data (i.e., seafloorspreadingrates, transformfault azimuths,earthquakeslip vectors).The NUVEL-1A pole uncertainty for this plate pair is as large as the geodetic standard deviation, as shown in Figure 12. The differencesbetween our estimate, NUVEL-1A, and JPL-GPS are not significantat the 95% confidence
deformation.
limit.
Conclusions
0o
0
In this paper, we have summarized the results for
the analysisof a 5 year time span of global GPS data. We have concentratedon siteswith long time histories, and for the most part, we have avoided sites in plate boundary zones. In several cases,we have been able to supplement continuousGPS measurementswith earlier campaignstyle measurements,thus extending the time seriesby many years. We have also avoided sites contaminated with coseismicand postseismicdeformation
P90o
$•0ø 300 ø
310'
Figure 12. North America-SouthAmerica angular velocitypole position,with 95% confidence regionfor
DeMetset al. [1994](triangle),ArgusandHeftin[1995] (diamond),andthis paper(star).
9980
LARSON
ET AL.: GPS GLOBAL
PLATE
MOTIONS
Acknowledgments. This research was funded by Boucher, C., Z. Altamimi, and L. Duhem, ITRF 92 and its associatedvelocityfield, IERS Tech. Note 15, IERS Cent. NASA NAG5-1908. We are grateful to many organizations Bur., Obs. de Paris, 1993. and individuals for providing data and software support, includingZuheir Altamimi, John Beavan, Geoff Blewitt, Boucher,C., Z. Altamimi, and L. Duhem, Results and analysis of the ITRF 93, IERS Tech. Note 18, IERS Cent. Yehuda Bock, Claude Boucher, James Campbell, Chuck Bur., Obs. de Paris, 1994. DeMets, Carey Noll, AndreasReinhold,WolfgangSchlueter, TeresaVan Hove, and JPL section335. We thank Don Ar- Boucher,C., Z. Altamimi, M. Feissel,and P. Sillard, Results and analysisof the ITRF 94 , IERS Tech. Note 20, IERS gus,RichardGordon,MichaelHeftin,Jim Ray, JohnRobCent. Bur., Obs. de Paris, 1996. bins, and GeorgeRosboroughfor helpful discussions and providingus with their current results. Chuck DeMets Clark, T.A., D. Gordon, W.E. Himwich, C. Ma, A. Mallama, and Richard Gordon made many helpful suggestionsfor and J.W. Ryan, Determination of relative site motions in improvementof the manuscript. This study would not the western United States using Mark III very long basehave been possiblewithout the developmentof the IGS. line interferometry, J. Geophys. Res. 92, 12,741-12,750, 1987. Plate models described in this paper may be viewed at http:/ / spot.colorado. edu/kristine/jgr.plates.html. Chase, C.G., The n-plate problem of plate tectonics, Geophys. J. R. Astron. Soc., 29, 117-122, 1972. Chase, C.G., Plate kinematics: The Americas, East Africa, and the rest of the world, Earth Planet. Sci. Left., 37, References 355-368, 1978.
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