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The 3 November 2002 moment magnitude 7.9 Denali faultearthquake generated large permanent surfacedisplacements in Alaska and large amplitude surfacewaves throughout western North America. We find goodagreement between strong ground-motion recordsintegrated to displacement and 1-hertz Global PositioningSystem (GPS) position estimates collected ~140 kilometersfrom the earthquake epicenter. One-hertz GPS receiversalso detected seismic surface waves 750 to 3800 kilometersfrom the epicenter, whereas these waves saturated manyof the seismic instruments in the same region. High-frequency GPS increases the dynamic range andfrequency bandwidth of ground-motion observations,providing a new tool for studying earthquake processes.

Our understanding of how faults rupture during majorearthquakes is limited by our observational capabilities; theyalso limit our understanding of the resulting grounddeformation. Within a distance range of about the samedimension as the fault rupture (the near-field) the surfacedeformation pattern is comprised of a highly spatiallyvariable permanent strain field generated by permanentdisplacements across the fault surface (coseismic offsets).These form within tens of seconds and are superimposed ontransient deformations generated by oscillating seismic wavesthat may be observed globally for large earthquakes.Traditionally, accelerometers and seismometers record rapidmovements of Earth that occur on time scales of seconds.Accelerometers record the strongest near-field groundmotions (measuring accelerations), and seismometers recordmore distant seismic waves (measuring velocities). GPSreceivers, on the other hand, record much slowerdisplacements of Earth that develop over days to years. Forexample, the slow persistent uplift of a volcanic edifice due tomagma influx or motion of tectonic plates can be measuredwith high-resolution using GPS receivers (1).

A GPS receiver determines position instantaneously—butimprecisely—by measuring the distance between the receiverand four or more satellites. By tracking the GPS constellationover a 24-hour period, horizontal precisions of 2 to 4 mm canbe achieved (2, 3). While many GPS receivers can operate at1 to 10 Hz, most geophysicists record measurements at .033Hz. In the past year, a growing number of continuousoperating GPS receivers have increased their recording ratesto 1 Hz to support missions in low Earth orbit (e.g., GRACEand CHAMP). Surveyors wanting solutions in real time alsooperate their receivers at 1 Hz. Here we examine whether 1-

Hz GPS receivers can be used to measure near-fielddeformation and seismic waves radiated by the Denali faultearthquake.

The moment magnitude (M) 7.9 Denali fault earthquakeruptured the Susitna Glacier-Denali-Totschunda fault systemon 3 November 2002 (4). This earthquake was complex, witha M7.0 thrust subevent followed by an M7.9 strike-slip eventthat ruptured to the east. Based on distant seismic recordings,the estimates of fault displacements at depth reach ~12 m.One-Hz GPS receiver data were collected from stations inFairbanks (FAIR), Anchorage (TSEA) and Skagway (TEM1)in Alaska and from stations in Washington (BREW),Montana (MSOL), Colorado (AMC2), California (GOLD),and Maryland (GODE) (5) (Fig. 1). One-Hz GPS data aresensitive to the satellite constellation, both its geometry in thesky and the number of satellites in view. If the geometry ofthe constellation in the sky is poor, one component may bebetter determined than the other. This is the case with FAIR,the site closest to the epicenter. At the time of the earthquake,the satellites were lined up in a way that leads to preciselongitude determinations but poor latitude determinations(Fig. 1, inset). For comparison, the visible satellites at BREWare also shown. In general, height is more poorly determinedby GPS than the horizontal position.

We analyzed and compared the near-field deformationrecorded at FAIR with some near-field seismic data. Thecomparison we make is complicated somewhat by the factthat FAIR is ~20 km from the nearest seismic station andalmost 140 km from the epicenter. Accelerograms fromFairbanks (station 8022 at 64.8735 N, 147.8614 W,Kinemetrics K2 instrumentation) were integrated twice toproduce displacement time histories. Accelerographs haveflat responses to acceleration and are operated primarily torecord large-amplitude, high-frequency (1 Hz) accelerationsfor engineering analyses. Integration to determine adisplacement time history, which constrains the coseismicoffset at depth, is inherently an under-determined problemand magnifies low frequency noise. Thus, we high-passfiltered the acceleration measurements to remove long-termdrift (cutoff period of 40 s) and then constrained theintegration to agree with surface coseismic displacements (4)(Fig. 2).

The 20-km separation between FAIR and seismicinstruments and the dispersive nature of the large amplitudeseismic surface waves means the two types of records will beout of phase. Nevertheless, the agreement between therecords is striking. The difference in noise characteristics ofthe east and north GPS records, most apparent before the

Using 1-Hz GPS Data to Measure Deformations Caused by the Denali FaultEarthquakeKristine M. Larson,1* Paul Bodin,2 Joan Gomberg3

1Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309–0429, USA. 2Center forEarthquake Research and Information, University of Memphis, Memphis, TN 38152, USA. 3U.S. Geological Survey, Memphis,TN 38152–3050, USA.

*To whom correspondence should be addressed. E-mail: kristine.larson@colorado.edu

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arrival of the seismic signal, reflects the satellite geometry(Fig. 1). Although there is still a long-period discrepancybetween the GPS and seismic data, it is clear from thisexample that 1-Hz GPS instruments can capture the completeground-motion time history during large earthquakes, thusproviding insights into fundamental fault propagationmechanisms. Moreover they provide a means of measuringstatic displacement offsets that is less sensitive to noisecontamination than seismic data. Accelerometer noise levelmay be related to the amplitude of the signal. For example,close to the rupture tilting of the ground surface locallybeneath the accelerometer may become significant, and theaccelerometer cannot distinguish between rotations and linearaccelerations. However, close to the rupture the GPS noiselevel should be the same as at more remote sites, while thesignal would increase to about half the fault slip (as much as4 m for the Denali fault rupture).

Although signals from large earthquakes are now routinelyrecorded on high dynamic range, broad bandwidthseismometers, the Denali fault earthquake signal amplitudesstill exceeded the recording capabilities out to distances ofthousands of km (6). The largest amplitude waves beyond thenear-field are dispersive surface waves, with frequenciesbetween about 1 and 0.01 Hz and group velocities of ~1 km/sto 4 km/s. Our comparison of seismic and geodetic recordsindicates that GPS can be used to measure these seismicwaves without the saturation problems of the seismometers.The east and north component records from TEM1, BREW,MSOL, AMC2, and GOLD show displacements beyond thedistance of any detectable permanent coseismic deformation(Fig. 3).

The GPS data record the arrival of surface waves atdistances as far as 3800 km (AMC2). Notably, the GPSrecords at TEM1 are on scale, while the more distantbroadband seismic recordings at a seismometer (stationDLBC at 58.437 N, 130.027 W) almost 300 km farther fromthe earthquake epicenter are clipped. Such observations arekey for studying focusing due to rupture directivity anddynamic earthquake source processes. Our results, forexample, show larger ground motions in the direction ofrupture propagation and expected strongest directivity (alonga great-circle path roughly through MSOL and AMC2),relative to motions at GOLD that are the same magnitude asthe measurement noise. Such observations also are critical forinterpreting the pattern of remotely triggered seismicity (7).

Our results demonstrate that GPS provides anotherimportant tool for studying earthquakes. Specifically, weshow that GPS can resolve displacement changes that vary ontime scales of seconds. For several types of measurementskey to earthquake studies, GPS offers several advantages overseismic instrumentation. Estimation of earthquake generatedcoseismic offsets and strain fields require measurement ofground displacements. The processing required to estimatethese from seismic data inherently enhances noise. In GPS,displacements are the basic measurement and thus, estimatesdo not suffer from this noise source. In addition, seismicwave amplitudes vary over many orders of magnitude andalthough the dynamic ranges of the best seismometers cancapture most of these, many saturate for the largest—andmost interesting—earthquakes. The accuracy of GPS dataactually improves as the magnitude increases and there is nosaturation. Ultimately quantitative assessment of the accuracyof both seismic and GPS measurements will require datarecorded at the same location. Of the sites shown in Fig. 3,only GOLD has a nearby seismometer (station GSC, at

35.302 N, 116.806 W, ~15 km from GOLD). Even though thesignal is smallest at GOLD, there is still striking agreementbetween the seismic and GPS data (fig. S2). At largerdistances it becomes difficult to separate differences inwaveforms associated with local geologic structure andradiation pattern from those due to measurement imprecision.

These results also suggest an interesting, although not yetrealized, possibility for measuring temporally and spatiallyrapidly varying strains associated with seismic waves, whichcan cause ground failure and certain types of structuraldamage. Differenced displacement data recorded by high-sample-rate GPS receivers deployed as arrays with aperturesof the order of hundreds of meters or less (fractions ofseismic wavelengths) would provide direct strainmeasurements. These could be deployed both in the field forstudies of earthquake processes and within built structures forengineering studies. Attempts to make such measurementsusing integrated seismic array data suffered from noiseenhancement associated with integration and from the limitedbandwidth of most seismic instrumentation (8).

Another advantage to using GPS instrumentation toaugment seismic arrays is cost. The cost of a geodetic qualityGPS receiver does not depend on the sample rate. Allcontinuously operating GPS receivers in the United Statescould be run at 1 Hz; the only additional cost is fortransmitting and archiving the data. The NSF Earthscopeinitiative (9) proposes to add about 800 GPS receivers to thewestern United States. This study strongly suggests that theybe operated at 1 Hz. Finally, the resolution of high-frequencyGPS has not been fully examined. It would be extremelyuseful to study GPS capabilities in the 1- to 10-Hz frequencyband.

References and Notes1. P. Segall, J. L. Davis, Annu. Rev. Earth Planet. Sci. 25, 301

(1997).2. J. Zhang et al., J. Geophys. Res. 102, 18035 (1997).3. A. Mao, C. Harrison, T. Dixon, J. Geophys. Res. 104, 2797

(1999).4. D. Eberhart-Phillips et al., Science, in press.5. Information on materials and methods is available on

Science Online.6. A variety of stations across the western United States

clipped (amplitudes exceeded the recording capabilities)with the passage of surface waves from the Denali faultearthquake. All of the Yellowstone and Utah short periodseismometers (L4C and S13s) and some broadbandseismometers (CMT-40) clipped in the Yellowstone array(B. Smith, University of Utah, personal communication).

7. D. P. Hill et al., Science 260, 1617 (1993).8. J. Gomberg, G. Pavlis, P. Bodin, Bull. Seismol. Soc. Am.

89, 1428 (2000).9. Earthscope is a multi-disciplinary NSF funded project, and

includes seismology (USArray), geodesy (Plate BoundaryObservatory) and borehole instrumentation (San Andreasfault at Depth). More information is available atwww.earthscope.org.

10. The Harvard centroid moment tensor (CMT) for theDenali fault earthquake was taken from their onlinesearchable database at www.seismology.harvard.edu/CMTsearch.html.

11. K.L. acknowledges NSF Grant EAR-0003943. G.Rosborough derived the coseismic constraints for theintegration of seismic accelerations and produced the finalform of Fig. 1 (Generic Mapping Tools). The IGS, CORS,

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JPL (section 335), CDDIS, SOPAC, and UNAVCO Inc.provided significant infrastructure support for this study.C. Clarke of AeroMap U.S. provided the data from TEM1.K.L. thanks J. Freymueller, J. Ray, P. Shearer, Y. Bock,and W. Prescott for helpful discussions. Two anonymousreviewers provided helpful feedback that significantlyimproved the manuscript.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/1084531/DC1Materials and MethodsFigs. S1 and S2Table S1References

14 March 2003; accepted 1 May 2003Published online 15 May 2003; 10.1126/science.1084531Include this information when citing this paper.

Fig. 1. Location of Denali fault earthquake epicenter (star)along with 1-Hz GPS sites used in this study. The surfacefault rupture, locations of FAIR and 8022, and the Harvardcentroid moment tensor (CMT) focal mechanism for theearthquake are given on the right (10). The inset on the leftshows the satellite constellation geometry at the time of theearthquake for FAIR and BREW. The azimuth directions arenorth, east, south, and west; elevation angles of the satellitesare given by the concentric circles, spaced evenly between15° and 60°. The center of the plot corresponds to when thesatellite is directly overhead (elevation angle of 90°).

Fig. 2. Integrated seismic accelerations for station 8022compared with 1-Hz GPS positions for FAIR. Time is relativeto 3 November 2002 22:12:41 UTC. The integration wasconstrained to agree with the GPS measured surfacecoseismic displacement. The coseismic displacements arederived by averaging the coordinates of FAIR 4 days beforeand after the earthquake (4). The 8022 observations wereshifted by 3 s.

Fig. 3. East and north component GPS surface waveobservations at TEM1, BREW, MSOL, GOLD, and AMC2(body waves are not apparent). Time is relative to 3November 2002 22:12:41 UTC. The dominantly strike-slipDenali fault earthquake mechanism favors the generation ofLove waves (horizontally polarized transverse shear motion).This is consistent with the generally larger amplitudes on thecomponents oriented closest to perpendicular to the rupturestrike (e.g., the east component at MSOL). Vertical positionestimates are also available (fig. S1).

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