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6-1998 GPS Measurements Athe Northern Boundary Zone: Impact of Postseismic Relaxation Following Historic Earthquakes Fred F. Pollitz University of California

Timothy H. Dixon University of Miami, [email protected]

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Scholar Commons Citation Pollitz, Fred F. and Dixon, Timothy H., "GPS Measurements Athe Northern Caribbean Plate Boundary Zone: Impact of Postseismic Relaxation Following Historic Earthquakes" (1998). School of Geosciences Faculty and Staff Publications. 488. https://scholarcommons.usf.edu/geo_facpub/488

This Article is brought to you for free and open access by the School of Geosciences at Scholar Commons. It has been accepted for inclusion in School of Geosciences Faculty and Staff ubP lications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. GEOPHYSICALRESEARCH LETTERS, VOL. 25, NO.12,PAGES 2233-2236, JUNE 15, 1998

GPS measurements across the northern Caribbean plate boundary zone: Impact of postseismicrelaxation following historic earthquakes

Fred F. Pollitz Departmentof , Universityof California, Davis

Timothy H. Dixon RosenstielSchool of Marine and AtmosphericSciences, University of Miami, Miami, Florida

Abstract. GPS measurements in the northern Caribbean they are often explainedby postseismicviscoelastic relaxation suggestthat the rate of Caribbeanplate motionrelative to of the lower crustand uppermantle years to decadesafter the North America is about 10 mm/yr faster than predicted event. Postseismic relaxation from historic earthquakesin by global plate motion model NUVEL-1A. Severalof Hispaniola may explain part of the excess velocity because the key sites used in the GPS study are locatedin the large historic earthquakesin 1751, 1842, and 1946 are docu- Dominican Republic, near the rupture zones of large mented [Kelleher et al., 1973; Sykeset al., 1982] and near-field earthquakesin 1946 and in the previoustwo centuries. relaxation effects in other tectonicallyactive areasare known to Postseismicrelaxation of the crust and upper mantle is a persistfor decades[Thatcher et al., 1980; Tabei, 1989;Rydelek possibleexplanation for the plate velocity discrepancy. and Sacks, 1990; Pollitz and Sacks, 1992; Pollitz and Sacks, We explorea rangeof mechanismsand crustaland 1994] and suspectedto continue up to 150 years or longer mantle theology to place an upper bound on postseismic [Rydelekand Pollitz, 1994]. We shall systematicallyinvestigate relaxationeffects. The upper bound velocity contribution postseismicrelaxation effects following the historicearthquakes, in the southernDominican Republic is 5-6 mm/yr, and considering a range of crust and upper mantle rheologies. the most plausiblecontribution is 1-2 mm/yr, suggesting Specifically, we seek to place an upper bound on the possible that postseismiceffects cannot account for the discrepan- contribution of postseismicrelaxation to the excess velocity cy. This implies that the NUVEL-1A model underesti- field in the northeast Caribbean. mates the rate of motion of the Caribbeanplate.

Fault Models Introduction Representativefault planesof the 1751/1770, 1842, and 1946 Space-basedgeodetic techniques have foundwidespread ap- earthquakesequences are shown in Figure 2. For simplicity we plicationin recentyears to directlymeasure long term ground regard the three separateshocks which occurredfrom 1751 and motionin a varietyof tectonicsettings. Comparisons of predict- 1770 as part of a continuousrupture over a 300 km long part of ed horizontal ground velocity from global plate model the Enriquillo fault zone (EFZ) in southern Hispaniola. The NUVEL-1A [DeMets et al., 1994] and from VLBI, SLR, or 1842 fault plane correspondsto a single event which ruptureda GPS measurementshave yieldedexcellent agreement in nearly - 300 km long segmentof the westernSeptentrional fault (SFZ) all areaswhere comparisonshave been made [e.g., Robaudo in northernHispaniola [Mann et al., 1984]. Based on geologi- and Harrison, 1993; Robins et al., 1993]. A decade of GPS cal observations we assume that the 1751/1770 and 1842 events measurementsspanning the North America- Caribbeanplate involve purely left lateral strike-slipmotion on vertical faults. boundaryzone, however, indicate that the Caribbeanplate has a The 1946 sequenceis dominatedby the Ms = 7.8 August 4, velocity that is about 10 mrn/yr faster than the NUVEL-1A 1946 event, followed by a few smaller events with magnitude modelprediction [Dixon et al., 1998]. For example,the veloci- up to Ms =7.3 [Russo and Villase•or, 1995; hereafter ty of ROJO, southernDominican Republic, moves at 21+1 R&V95]. The distribution of aftershocksobtained by R&V95 mm/yr to the eastrelative to TURK in the Turksand CaicosIs- suggeststhat the total area involved in significantrupture has a lands on the stable North American plate (Figure 1). This length of about 175 km and width of about 20 km. The fault differs at 95% confidencefrom the predictedNUVEL-1A value geometry obtained by them (strike=N303ø E, rake=74ø on a of 11+3 mm/yr [DeMetset al., 1994]. The purposeof this pa- northward steeply dipping fault) has a significantcomponent of per is to testone possibleexplanation for thisdisagreement. reverse slip. A different fault geometryfor the August 4, 1946 The relative velocity between geodeticstations can change event has been advocatedby Dolan and Wald [1997; hereafter from one time periodto the next. Apart from the obviousex- D&W97] and involves a combination of thrust and minor left ampleof coseismicoffsets, these temporal changes are often as- lateral strike-slipmotion on a similar trendingfault with a shal- sociatedwith majorearthquakes preceding the observations,and low S/SW dip. D&W97 supporttheir model by interpretingthe focal mechanismsof shallow 1946 aftershocksand other nearby historic events as representingslip both updip and adjacentto Copyright1998 by the AmericanGeophysical Union. the 1946 main shock. As discussedby these authors the Papernumber 98GL00645. difference between the two models is in the choice of nodal 0094-8534/98/98GL-00645505.00 plane, the focal mechanismby itself yielding two equally plau-

2233 2234 POLLITZ AND DIXON: NORTHERN CARIBBEAN POSTSEISMIC RELAXATION

22'

20'

18'

I 21 14' 286' 288' 290' 292' 294' 296' Figure 1. Velocities and 95% confidenceellipses of severalGPS sitesin the northernCaribbean relative to TURK, locatedin the Turks and CaicosIslands on the stableNorth Americanplate, for the period 1986-1995 from Dixon et al. [ 1998]. OF is OrienteFault, SDB is SantiagoDeformed Belt, SFZ is SeptentrionalFault Zone, EFZ is Enriquillo Fault Zone, OBC is Old BahamaChannel, SSBF is SouthSamana Bay Fault.

sible choices. Assumingidentical slip valuesand constraintson 1946 (Steeply Dipping Foult) 1946 (Steeply DippingFoult) the depth extentof faulting, the choiceof nodalplane is impor- +1842 +1842 +1751 tant for the predictedpostseismic relaxation patterns because the r/c=1.0 X 1020Pa-s r/c=1.0 X 1020Pa-s area of the shallowlydipping plane is largerthan the area of the r/re=Infinity r/re=Infinity steeplydipping plane. The predictednear-field relaxation pat- TURK terns also depend heavily on the choice of fault geometry. .-.2122 • . . . © Therefore we consider both fault geometries. For each 2o, geometry we parameterizethe total 1946 moment releasein c • 19 termsof slip on a singlefault planetrending parallel to the ma- '.• jor faultsin northernHispaniola (Figure 1). We fix the striketo ø,18 be N69 ø W (or S69ø E) and either: 1. fixed the dip at that value }•151 mm/•/r I I I , ,I , determinedby R&V95 (62ø towardsthe NE) and fixed the rake -74 -72 -70 -68 -74 -72 -70 -68 at 45 ø, rather than the 74ø determinedby R&V95 for the Au- r/c=1.0 X 102oPo-s r/c=1.0 X 1020Pa-s 7)m=1.0X 1020 Po-s r/m=1.0X 102o Pa-s gust 4, 1946 event, to allow roughly equal componentsof 22 strike-slipand thrustmotion; or 2. fixed the dip at 12ø towards the S/SW with rake of 55ø, yielding a motion vector of the ,,•. 21 ,.., . footwall of S55ø W, somewhatmore westerlythan the motion -• 20 vectorof S34ø W determinedby D&W97. •19 Since precise constraintson the total moment of any of the events are not available, we fix the slip associatedwith each 0,18 fault plane at 5 m. This is consistentwith the fault lengthsof [•151 m/f I I I ' the various earthquakeruptures and their large felt magnitudes. - •4 -72 -70 -68 -74 -72 -70 -68 Mann et al. [1984] suggesta 300 year recurrenceinterval for r/.=l.0 X 1020Po-s r/c=1.0X 1020Po-s great earthquakesin northernHispaniola, in which case a max- r/•n=l.0X 10TM Pa-s r/rn=l.0X 1019Pa-s imum characteristicslip of 5 m is implied by the maximum possiblelong term relative plate velocity (20-25 mm/yr) com- bined with the upper bound of 75% total plate motion that ei- -o 20 r, ..•-•t•"'•-.•...•,'-e-'e-'•.-'.-',"• • '1 20 [. • .••• • • • '1 ther the SFZ or EFZ are likely to sustain [Dixon et al., 1998]. The possibilities of aseismic interseismicslip and significant strain accumulation offshore north of the island also argue ,'; againstlarger slip valuesfor the great inland earthquakes. •71• s•/•*r-', ' ' 4,<< I ' '1 17 In order to determinethe time-dependentdeformation follow- -74 -72 -70 -68 -74 -72 -70 -68 Longitude(deg.) Longitude(deg.) ing an earthquake,we specify a three-layerelastic-viscoelastic coupling model as follows: 1. Purely elastic upper crust •igure 2. Left panels: postseismicvelocity fields over the (0- 16 km depth), bulk modulus r = 65 GPa, modulus period ]986-]995 calculated for the summed ]75 ], ]842, and g = 36 GPa; 2. Viscoelastic lower crust (16- 33 km), r = 95 ]946 events for three different viscosity combinations. Right GPa, g = 53 GPa, viscosity= tic; 3. Viscoelasticupper mantle p•els: corresponding postseis•c velocity fields for the (> 33 km), K = 150 GPa, g = 70 GPa, viscosity= rim. This summed]842 and ]946 evemso•ly. A steeplydipping fault for simple crustal structureappears appropriatefor several of the the ]946 eaAhqu•e is assumed. 1946 (ShallowlyDipping Fault) 1946 (Steeply Dipping Fault) Discussion and Conclusions +1842 +1842 We restrict attention to those Caribbean-North America r/c=3.0 X 10TM Pa-s r/c=3.0 X 10•9 Pa-s t)m=l.0 X 10TM Pa-s •]rn-----1.0X 10 TM Pa-s (CAR-NAM) GPS observations relative to TURK, rather than a 22 broader North American rigid plate framework, because the .•21 ,, former observations are associated with smaller uncertainties

'• 20 [Dixon et al., 1998]. The precedingresults suggest that we can explain at most 5-6 mrn/yr of the excess 10 mrn/yr CAR-NAM :19 ._ velocity with a model of postseismicrelaxation from historic earthquakes. Figure 4 shows results of of horizontal GPS velocitiesfor CAR-NAM velocity, assumingthat all boun- 17 •4 -72 -70 -68 -74 -72 -70 -68 dary deformationis concentratedon the SFZ, with a locking depth of 20 km. This approachappears justified by the fact that Figure 3. Postseismicvelocity field averagedover 1986-1995 the SFZ is more active than the EFZ to the south [Mann and for the summed 1842 and 1946 events on both the shallowly dipping [D&W97] and steeply dipping [R&V95] fault geometriesfor the 1946 sequence. Observed Velocity D=20 kin; vpar =20 mm/yr 25 _ v----Vpor/2-- (1//x)(Vpor) tan-l(distonce/D) Caribbean islands, where the crust-mantle transition is much 20 [ ROd••:••.:•••]•i•...... 1.?..:•!•.•.•.•.:..`•...•••••i•,...... :;..J•..:..•'"•'"""'"' deeperthan in the Caribbeanoceanic basin [Edgar et al., 1971]. :;...:•i-:-----,•,•-.-...:•F'---'".---'•:•i :'---.:".•....:• •.'--':..-;•½--"::.---"• • 15 x;i...... '"' PostseismicVelocity ---e10 - 5 We calculatedthe total postseismichorizontal velocity fields averagedover the period 1986-1994 using the methodof Pollitz 0 TURK -200 - 1 O0 0 1 O0 200 [1992] for several different combinationsof viscositiesqc and Corrected Velocity rl,,, summingover the historicearthquakes described in the pre- 3O vious section. We first searchedfor those viscosity combina- D=20 km' Vpor =15 mm/yr tions which lead to the largestpostseismic velocity contribution 25- V=Vpor/2-- (1/?T)(Vpor)tan-l(distance/D) from the 1751/1770 sequencealone, finding that a lower crustal 20- :":•!i":::'"''""''' ' viscosityof about10 TM Pa s wasoptimal for producingthe larg- 15•'•••••i ...... ""'•"'"•'"'""•"•••••••ii'"'"'""•:•'•i est postseismiceffects, regardlessof the mantle viscosity. We ••::------•...... ';.':.::-:.'"•""'"'"'"":•::•:--.------•;iiii'"'"'" '"":"•:"•;.'iiii then addedthe 1842 and 1946 sequencesto the calculation. In i ':':":':':':':':'"':':':':':':':':':'::::::::::::::::::::::;'""::.::•::'&::.' .:::'::':•::'8•:B? these calculations we assumedthe R&V95 fault geometry for r the 1946 sequence. Figure 2 showsthe resultingvelocity fields 0 for three viscositycombinations when 1751+1842+1946events -200 - 100 0 100 200 are summed (left hand panels) and when only 1842+1946 Distance from Septentrional Fault (kin) events are summed (right hand panels). Comparing the left Dominican Republic Velocities hand and right hand panels it is clear that the 1751 event is responsiblefor about one-half of the total 3-4 mrn/yr predicted at ROJO. At TURK the largest contributionscome from the 22[(•Observed •GRAND TURK(fixed) 1842 and 1946 sequences.It is apparentthat the highestpost- •• 20 • _ CABOFRANCES VIEJO seismiceffects result from relatively low mantle viscosity. In order to increasecalculated post-1946 relaxationeffects, it :19 was found sufficient to fix the mantle viscosity at rl,,,= 1.0x1019Pa s and choosethe crustalviscosity in the rangefrom 1.0 to 10.0x10•9 Pa s. We comparedthe post-1946 18,, , , 'R•. velocity patterns obtained with the shallowly dipping fault --74 -72 -70 -68 model [D&W97] and the steeplydipping fault model [R&V95] Longitude (deg.) for severalviscosity combinations and found that the maximum Figure 4. GPS velocitiesresolved parallel to the SFZ are in- velocity achievedat ROJO did not exceed2 mrn/yr with the vertedfor CAR-NAM relativeplate motion with fault-parallel steep fault geometry or 3 mrn/yr using the shallow fault rate v•,. assuminga locking depthof 20 km, both with and geometry.Figure 3 showsthe postseismicvelocity patternsfor withoutcorrection for maximumpossible postseismic relaxation the summed 1842+1946 events for the maximal viscositycom- effects.An analyticformula appropriate for strainaccumulation bir•ation using both the shallow and steep fault geometriesfor in the strike-slipsense on an infinitelylong vertical fault [Sa- the 1946 event. It shows that the summed velocity reaches vage and Burford, 1973] is employed. Shadedarea indicates about 3-4 mm/yr at ROJO. Allowing these absolutevelocities the extentof the localplate boundary deformation zone between to representCaribbean to stable North American relative plate the Enriquillo-PlantainGarden Fault Zone and the Old Bahama motion, the maximum amountof excessCaribbean plate veloci- Channel.Light solid/dashedlines in the lowerpanel indicate ty relative to stable North America which can be explained predictedvelocities relative to fixed TURK without/with correc- throughpostseismic relaxation effects is thus about 3-4 mrn/yr. tion for postseismicrelaxation effects. Heavy solid/dashed lines Since TURK exhibits a small relative velocity with respectto givethe corresponding CAR-NAM relative motion decomposed stable North America, the maximum amount of excess Caribbe- into its fault-paralleland fault normalcomponents; the fault- an plate velocity relative to TURK which can be accountedfor normalcomponent is takento be equalto the fault-normalcom- by the samemodel is slightlyhigher at 5-6 mrn/yr. ponent observedat ROJO. Burke, 1984; Prentice et al., 1993]. The corrected velocity reflection in the Caribbean Sea, Amer. Assoc. Petr. Geol. correspondsto subtractingout postseismicrelaxation effects for Bull., 55, 833-870, 1971. a particularviscosity model 01c = 3.0x1019Pa s, T]m=1.0x1019 Kelleher, J.A., L. Sykes, and J. Oliver, Possible criteria for predictingearthquake locations and their applicationto major Pa s) and assumingthe R&V95 fault geometryfor the 1946 plate boundariesof the Pacific and the Caribbean,J. Geophys event. Use of the D&W97 1946 fault geometrywould increase Res., 78, 2547-2585, 1973. the size of the correctionat ROJO by about 1 mm/yr and lead Mann, P., K. Burke, and T. Matumoto, Neotectonics of to a somewhatlarger correction for FRAN. The valueVpa r • 15 Hispaniola:Plate motion, sedimentation,and seismicityat a mrn/yr obtained after correction for postseismicrelaxation restrainingbend, Earth Planet. Sci. Lett., 70, 311-324, 1984. effects shows that the fault-parallel velocity is underestimated Mann, P. and K. Burke, Neotectonics of the Caribbean, Rev. by at least4 mrn/yr by Model NUVEL-1A. Geophys.Space Phys., 22, 309-362, 1984. Although the velocity observationsof neither FRAN nor Pollitz, F.F., Postseismicrelaxation theory on the spherical CAPO require a CAR-NAM relative velocity greater than 11 Earth, Bull. Seism. Soc. Am., 82, 422-453, 1992. Pollitz, F.F. and I.S. Sacks, Fault model of the 1891 Nobi earth- mrn/yr, the observationsat sites more distant from the plate quake from historic triangulation and levelling, J. Phys. boundary deformation zone appear to require a significantly Earth, 42, 1-43, 1994. greater rate of relative motion. In particular,site ISAB on Prentice, C., P. Mann, F.W. Taylor, G. Burr, and S. Valastro, PuertoRico (Figure 1) is not associatedwith any obviouspost- Paleoseismicityof the North American plate boundary(Sep- seismicrelaxation effects from historic earthquakes(Figures 2 tentrional fault), Dominican Republic, Geology, 21, 49-52, and 3). Its eastward velocity of 20 mm/yr suggeststhat 1993. l•par ---- 20 mrn/yris appropriatefor the region.A similarinter- Robaudo, S. and C.G.A. Harrison, Plate from SLR and seismicloading analysisapplied to site GTMO in easternCuba VLBI data, in D. Smith and D. Turcotte, eds., Contributions (Figure1) alsopoints towards l•pa r • 20 mm/yr. Takentogeth- of Space Geodesyand Geodynamics,Am. Geophys.U. Geo- er, these observationssuggest that the postseismicrelaxation dynamicsSeries, 23, 51-71, 1993. effects manifested at ROJO are much smaller than the max- Robins, J.W., D.E. Smith, and C. Ma, Horizontal crustal defor- mation and large scaleplate motionsinferred from spacegeo- imum possible, suggestinga lithosphericrheology producing detic techniques,in D. Smith and D. Turcotte,eds., Contribu- only locally significantrelaxation effects. An indeterminately tions of SpaceGeodesy and Geodynamics,Am. Geophys.U. high viscositymantle combinedwith a crustalviscosity of GeodynamicsSeries, 23, 21-36, 1993. - 102øPa s, as consideredin the top panelsof Figure2, then Rosencranz,E., Openingof the CaymanTrough and the evolu- appearsappropriate for the Hispaniolaregion. The postseismic tion of the northern Caribbean plate boundary, Geol. Soc. relaxation predictedon such a model does not exceed 1-2 Am. Abs. w. Progs., 27, A-153, 1995. mrn/yrat any of the highest-velocityGPS sites. Rosencrantz,E. and P. Mann, P., SeaMARC II mapping of If postseismicrelaxation can be ruledout as a significantcon- transformfaults in the CaymanTrough, Caribbean Sea, Geol- tributor to the excessCAR-NAM velocity determinedby GPS, ogy, 19, 690-693, 1991. then an alternative explanationis that the CAR-NAM rate Russo, R.M. and A. Villasehor, The 1946 Hispaniola earth- quakesand the tectonicsof the North America-Caribbean predictedby the NUVEL-1A modelis too small. Recentrein- plate boundaryzone, northeasternHispaniola, J. Geophys. terpretationof marine magneticanomaly data in the Cayman Res., 100, 6265-6280, 1995. Trough[Rosencrantz, 1995] yields a rateof about19 mrn/yr. It Russo,R.M. and A. Villasehor,Reply, J. Geophys.Res., 102, is also recognizedthat the Cayman Trough representsnot 793-802, 1997. directly CAR-NAM motion but rather the motion between Rydelek, P.A. and I.S. Sacks,Asthenospheric viscosity and NAM and the Gonave microplatewedged between Cuba and stressdiffusion: a mechanismto explain correlatedearth- Hispaniola[Rosencrantz and Mann, 1991]. The possibilityof quakesand surface deformations in northeastJapan, Geophys. significantlong term Gonave-CARrelative motion along the J. Inter., 100, 39-58, 1990. southernboundary of the Gonaveplate (Rosencrantz,personal Rydelek,P. and F.F. Pollitz,Fossil strain from the 1811-1812 communication,1997), suggeststhat geologicestimates of the New Madrid earthquakes,Geophys. Res. Lett., 21, 2303- 2306, 1994. relative motionrate are equal to or may exceed20 mm/yr, in Savage,J.C. andR.O. Burford,R.O., Geodeticdetermination of agreementwith GPS estimates. relativeplate motionin centralCalifornia, J. Geophys.Res., 78, 832-845, 1973. Acknowledgements.We are gratefulto Eric Rosencrantzfor Stein, S., C. DeMets, R. Gordon, J. Brodholt, D. Argus, J. sharingunpublished results and his latestinterpretations of plate Engeln,P. Lundgren,C. Stein,D. Wiens,and D. Woods,D., kinematicsaround the CaymanTrough. Constmctivecomments A test of alternativeCaribbean plate models,J. Geophys. from James Dolan and two anonymousreviewers are ack- res., 93, 3041-3050, 1988. nowledged. Sykes,C.R., W.R. McCann,and A.L. Kafka,Motion of Carib- beanplate during last 7 millionyears and implicationsfor References earlier Cenozoicmovements, J. Geophys.Res., 87, 10656- 10676, 1982. DeMets, C., R.C. Gordon, D.F. Argus, and S. Stein, Effect of Tabei, T., Crustal movementsin the inner zone of southwest recentrevisions to the geomagnetictime-scale on estimatesof Japanassociated with stressrelaxation after major earth- current plate motions, Geophys.Res. Lett., 21, 2191-2194, quakes,J. Phys.Earth, 37, 101-131,1989. 1994. Thatcher, W., T. Matsuda, T. Kato, and J.B. Rundle, Lithos- Dixon, T.H., F. Farina, C. DeMets, P. Jansma, P. Mann, and E. phericloading by the 1896 Riku-u earthquake,northern Calais, Relative motion between the Caribbean and North Japan: Implicationsfor plate flexureand asthenospheric Americanplates and relatedboundary zone deformationfrom rheology,J. Geophys.Res., 85, 6429-6435,1980. a decade of GPS observations,J. Geophys.Res., in press, 1998. F. F. Pollitz, Departmentof Geology,University of Califor- nia, Davis, CA 95616 (email: [email protected]) Dolan, J.F. and D.J. Wald, Comment on "The 1946 Hispaniola T. H. Dixon, Rosenstiel School of Marine and Atmospheric earthquakeand the tectonicsof the North America - Caribbe- an plate boundary zone, northeasternHispaniola" by R.M. Sciences,University of Miami, 4600 RickenbackerCauseway Miami, FL 33149-1098 (email: [email protected]) Russo and A. Villase•or, J. Geophys.Res., 102, 785-792, 1997. (ReceivedMay 30, 1997; revisedJanuary 26, 1998; Edgar, N.T., J.I. Ewing, and J. Hennion,Seismic refraction and acceptedJanuary 30, 1998.)