GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 21, PAGES 4003-4006, NOVEMBER 1, 1998

Crustal deformation in the Baikal from GPS measurements

Eric Calais,Olivia Lesne,Jacques D6verch•re G6osciencesAzur UMR 6526, CNRSFUPMC, Valbonne and Villefranche/Mer, France

Vladimir San'kov, Andrei Lukhnev, Andrei Miroshnitchenko, Vladimir Buddo, Kirill Levi Instituteof the Earth'sCrust, Siberian Branch of the RussianAcademy of Sciences,Irkutsk, Russia

VjacheslavZalutzky VS-NIIFTRI, Time andFrequency Service Division, Irkutsk, Russia

Yuri Bashkuev BuryatScientific Center, Siberian Branch of the RussianAcademy of Sciences,Ulan Ude, Russia

Abstract. Three yearsand four campaignsof Global Positioning We processedpseudorange and phase data in single-day System(GPS) measurements(1994-1997) in the Baikal rift zone, solutionsusing the GAMIT software (version 9.6, King and largest active continentalrift systemin Eurasia, show crustal Bock, 1997). We solvedfor regionalstation coordinates, satellite extensionat a rate of 4.5+1.2 mm/yr in a WNW-ESE direction. 'statevectors, 13 troposphericzenith delay parameters per site A comparison with moment release of large historical and day, and phaseambiguities using doubly-differencedGPS earthquakessuggests that elasticstrain is currentlyaccumulating phase measurements.We used IGS final orbits, IERS earth in the Baikal rift zone along activefaults that currentlyhave the orientation parameters, and applied azimuth and elevation potentialfor a M=7.5 earthquake.The GPS-derivedextension dependantantenna phase center models, following the tables rate in the Baikal rift zone is at least two times greaterthan the recommendedby the IGS. Four primaryand eight secondaryIGS predictionof mostdeformation models of Asia.This resultcould stations (ONSA, GRAZ, TIDB, FAIR, HART, KOKB, TSKB, reflect the dynamic contribution of the Pacific-Eurasia USUD, TAIW, KITA, $HAO, IRKT) were included in the to intracontinental deformation in Asia, in addition to processingto serve as ties with the International Terrestrial the effect of the India-Eurasia collision. ReferenceFrame 1994 (ITRF94, Boucheret al., 1996). The least squaresadjustment vector and its corresponding Introduction variance-covariancematrix for station positionsand orbital elementsestimated for eachindependent daily solutionwere then The Baikal riff zone, located at the northernedge of the passedto a Kalmanfilter (GLOBK, Herringet al., 1990) in order centralAsia intracontinentaldeformation zone (Figure 1), is the largestactive continental rift systemin Eurasia(Tapponnier and Molnar, 1979). It extends over a distance of about 2000 km 60' 90' 120' 150' alongthe S-shaped suture that separates the Siberian 60.

Platform from the Sayan-Baikalmobile belt (Logatchevand ! Zorin, 1992). We presentthe resultsof four campaignsof Global no. PositioningSystem (GPS) measurementsin the southernand westernparts of the Baikal rift zone.This data set providesthe 20- first directmeasurements of the currentdisplacement rates at the O' . boundarybetween stable Eurasia and East China. .;•,?:...,.•..•: ?,.:.:•.,.. $ ß.•>x".:ß ,.•.•.,/.-_....::,.•.?,•_•":::"••, .... GPS Measurementsand Data Processing ß . ". ; ,d ..:.... '.?,,.•.'Y k ' GPS measurements in the Baikal rift zone were initiated in •."' "':'.':' :•'"• i 52' July 1994 with the installationand first observationof a network 2-.';:....'•'•"•"-'"" "'- ' ' ß of 11 sitescovering its southernand westernparts (Figure 1). ,..:..:...... : .. The sitesIRKU and ULAN were reoccupiedin August 1995. 51' The whole network was remeasuredin August 1996 and August :. : .. •. • . .• . 1997. We used AshtechP12 and Z12 GPS receiverssampling at ...... -.:,ß ...... ' ß'. 'n.vs. sta: .'...':;):.;:.•,..o..•GO'•" L IA .....""- •\.• ...... '.. re'".' '...... •..:.'...... : --•..' "': '' 30 seconds.. Each site was surveyed 22 hours a day for an ":..' ":", ""•'"7'" '•:' ' ß ' ."v..'.' average of four consecutivedays during each campaign.We 100' 102' 104' 106' 108' benefit, since 1996, from the data from the Irkutsk permanent Figure1. Tectonicsetting of thesouthern Baikal riff zone:major IGS station(IRKT), located40 m away from the IRKU mark. active faults (SaF: Sayan , TuF: Tunka fault, Mo F: Morskoyfault, Ob F: Obruchevskyfault, O1F: Olkhonskyfault, Copyright1998 by theAmerican Geophysical Union. Pr F: Primorskyfault), instrumentalseismicity (dots, 1962-1980, M>2.8), historicalearthquakes (stars, see Table 2),-and focal PapernUmber GRL-1998900067. mechanismsof major earthquakes.The trianglesindicate the 0094-8276/98/GRL- 1998900067505.00 location of the GPS sites.

4003 4004 CALAIS ET AL.: CRUSTAL DEFORMATION IN THE BAIKAL RIFF ZONE

Table 1. Velocities (V, mm/yr) with respect to IRKU and 20 ' I ' I I I ' t ' I ' I I I I I ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' NS component- slope = 3.4 +- 0.2 mrn/yr,wrms = 2.4 mm associatedone standard deviation formal errors (o, mm/yr) 10 computedby scalingthe 1-o uncertaintiesof the final adjustment by the overallchi-squared per degreeof freedom. -10o E•'-20 ß,... ,..., ... , ...,... ,... ,. •. ,..., . ß Lon. Lat. Vlon Vlat cron cflat correlation Site code

, 108.24 52.97 3.• 1.1 1.4 1.0 -0.0323 TURK EW component- slope= -3.8 +- 0.2 mm/yr,wrms = 3.6 mm 107.62 51.81 2.8 -4.0 1.0 0.6 -0.0693 ULAN •_• 20•o '''' ...... • '''''...... '''''i 106.58 52.79 -0.7 2.2 1.2 1.0 -0.0430 ANGA o -10 106.49 50.74 3.5 0.2 1.2 1.0 -0.0549 KIAT • -20 ß, , , • ,... ,.. 106.01 51.17 4.6 -0.8 1.2 1.0 -0.0596 UDUN 105.50 53.06 0.5 0.5 1.2 1.0 -0.0629 BAYA .C_o UPcomponent - slope = 1.3+- 1.4mm/yr, wrms = 11.9mm 104.89 51.85 2.2 -0.5 1.4 1.2 -0.0975 LIST • 4020 '' ' lii'l liill''l''' I'''1''' I'''1'''1'''1• 104.32 52.22 0.2 0.4 1.4 1.2 -0.0589 IRKT 104.32 52.22 0.0 0.0 1.0 0.6 -0.0777 IRKU

103.74 51.77 2.3 0.2 1.2 1.0 -0.0517 KULT -4• ß i ß i i ß ß ' i ß ß 103.70 51.65 1.6 0.0 1.4 1.0 -0.0376 SLYU 49600 49700 49800 49900 50000 50100 50200 50300 50400 50500 50600

102.21 51.76 0.1 -2.3 1.4 1.0 -0.0460 BADA julianday number[07/25/94 to 08/12/97]

Figure 3. Baseline time seriesbetween Irkutsk and Ulan Ude. Each point representsa daily solutionwith its 1-• error bar. The to estimatestation positionsand velocities.We imposedthe four setsof point representthe 94, 95, 96, and 97 measurements. reference frame by minimizing the position and velocity The dashedline is the best fit linear regressioncomputed from a deviationsof IGS core stationswith respectto the ITRF94 while weightedleast squares adjustment to the data. estimatingan orientation,translation and scaletransformation. The height coordinateand velocity were downweigthedby a factorof 10 relative to the horizontalcomponents. verticalcomponent (Figure 3). The GPS-derivedvelocity field (Figure 2) showsthat: Velocity Field 1. Thetwo sites located on thestable Siberian platfo .rm (IRKU, Our resultsover the threeyear period 1994-1997 are listedon BAYA) showno relativemotion at the 1 mrrdyrlevel. This result Table1 and displayedin Figure2. The formalerrors have been is a goodinternal test of theoverall quality of thevelocity field, computedby scaling the 1-• uncertaintiesof the final since both sites are located on the same aseismic and undeformed adjustmentby the overall chi-squaredper degreeof freedom. crustal block with no known active fault between them. The uncertaintiesin the horizontalcomponents of the velocity 2. The four sites located on the southeastern side of the rift estimatesrange between0.6 and 1.4 mm/yr (1-•). Long term (UDUN, KIAT, ULAN, TURK) show extensionacross the rift baselinerepeatabilities (weigthed RMS scatterabout the bestfit zone at an averagerate of 4.54-1.2mrrdyr (1-•). This resultis in linearregression to the positiontime series)are on the orderof goodagreement with geologicalestimates based on topographic 2 to 3 mm for the horizontalcomponents and 5 to 10 mm for the offsetsin holocenealluvial fans that indicate5 mrrdyrof total extensionin the northernpart of the Baikalrift zoneduring the last 10,000years (Houdry, 1994). The directionof extension varies between N80 and N140 (relative to IRKU). Given the 100' 102' 104' 106' 108' smallnumber of observationepochs available, it is not possible 54' 54- to determinewhether this variabilityof tectonicorigin or if it reflects GPS measurement errors. The direction of extension appearsto be obliqueto the directionof the majornormal faults, implyingleft-lateral wrench-extensional faulting in the southern 53' 53-part of theBaikal rift zone.This result is consistentwith earthquake focal mechanisms(Figure 1), stress tensor calculations, and microtectonic measurements in the southern 52' ....ß,,.., A T '"i. • ULAN4.• 52- Baikalrift zonethat predict slip vectors trending N105 on the majoractive faults (Petit et al., 1996;Delvaux et al., 1997). 3. The five siteslocated along the majoractive fault zonethat 51' 5l- bounds Baikal to the northwest and continues westward along the northernside of the Tunka basin (BADA, SLYU, MO N t30 L i A '"z7q•...... -"" '...... '-.\. KULT, LIST, ANGA) showvelocities on the orderof 2 mrrdyr 50' .. 5o- relativeto IRKU. All thesesites are located in theclose vicinity 100' 102' 104' 106' 108' of a major activefault and are thereforelikely to reflectelastic Figure 2. Velocities obtained from GPS measurementsin the strain accumulation. centraland southernparts of the Baikal rift and the Tunka basin. 4. We observeno significantrelative motion (at the 1 mrrdyr The arrow length is proportional to the displacementrate, level) betweenKULT and SLYU, located 10 km from each other indicatedin mm/yr next to the site codename. The velocitiesare on each side of the Sayan-southBaikal active fault. This expressed relative to IRKU. The ellipses represent 95% probablyindicates that the Sayan-southBaikal fault is currently confidence intervals. locked. CALAIS ET AL.: CRUSTAL DEFORMATION IN THE BAIKAL RIFT ZONE 4005

Discussion parallelfaults make suchprediction more questionable, but may indicatea smallerrecurrence interval (150 years)for a M=7.5 Comparisonwith SeismicMoment Release event. Accordingto the historicalseismicity catalogs of Solonenko (1977) and Kondorskayaand Shebalin(1982), six earthquakes Comparisonwith deformationmodels of Asia exceedingM=6 TM have occurred in the southBaikal rift since Severalkinematic models have been proposedrecently to 1700 (Table 2). Three events most probably occuredon the explain the present-daydeformation in centralAsia, basedon Obruchevskyfault, that boundsthe SouthernBaikal basinto the Quaternaryfault rates (Avouacand Tapponnier,1993; Peltzer north, whereas the other three occured in the Central Baikal and Saucier, 1996) and sparsegeodetic results (Molnar and basin along the Morskoy fault (Figure 1). Following the Gipson,1996; England and Molnar, 1997) or momenttensors of empiricalmagnitude-moment law givenby Hanksand Kanamori largeearthquakes (Holt et al., 1995). Whetherthese models use a (1979), thesetwo subsetsreleased minimum seismicmoments of continuum or a rigid block approach,they all use the 4.8x102øNmand 2.2x102øNm,respectively. In spiteof large India/Eurasiaconvergence as a kinematicboundary condition to uncertainties due to crude estimates of seismic moments for the the south.They predictan extensionrate that variesbetween 0- period 1700-1900, thesevalues are comparableto the 0.9 to 1 mm/yr(Peltzer and Saucier, 1996) and a maximumof 2 mm/yr 2.6x10•øNm seismicmoment release over the last 85 years (England and Molnar, 1997) in the Baikal rift zone, which is proposedfrom long-termand short-termkinematic deformation significantlysmaller than our GPSresults. An alternatemodeling models of Asia (Holt et al., 1995). approachhas recentlybeen proposedby Kong and Bird (1996) In order to convert this cumulated seismic moment release who useda dynamicthin-shell finite-element model taking into into total slip on each fault, and since active faults in the accountboth the India-Eurasiaand Pacific-Eurasiaconvergence southern and central Baikal basins are well delineated, we use as boundaryconditions. Their bestfit modelpredicts 19 mm/yr the approachproposed by Brune (1968). Using a seismogenic of extensionin the Baikal rift zone,which is four timeshigher thicknessof 25 km (D6vercht:reet al., 1993), the fault dip angles than our GPS results. The high heat flow values (and givenin Table2, anda rigidityof 3.3x10• dyn/cm•, we find a consequentlylow lithosphericthickness) used by theseauthors cumulatedslip of 3.3 m alongthe SouthBaikal fault and 1.0 m forthe Baikal rift (110to 130mW/m •) comparedto theaverage on the centralBaikal fault, correspondingto averagehorizontal valueof 70to 90mW/m • given by Lysak(1992) could partly slip ratesof 5.6 mm/yrand 2.1 mm/yrrespectively. These rates, explainsuch a discrepancy. that reflect seismic moment released over 3 centuries, are The fact that the GPS-derived extension rate in the Baikal rift comparableto the 4.5 mm/yr far-fieldextension rate acrossthe zone is fasterthan the predictionsof kinematicmodels based on rift obtainedfrom GPS measurementsover 3 years.This result, the hypothesisthat present-daydeformation in central Asia is togetherwith GPS resultsshowing that the Sayan-southBaikal enterely driven by the India/Eurasiacollision suggeststhe faultis locked,suggests that little or no aseismicslip is occurring existenceof an additionalprocess participating in the present- alongactive faults in thesouthern and western parts of theBaikal day openingof the Baikal rift zone.It haslong beensuggested rift zone, which are therefore currently accumulatingelastic that the Baikalrift zonewas the restlitof "activerifting", with strainto be releasedin futureearthquakes. Considering the date crustalextension primarily caused by thediapiric upwelling of a of thelast major event (1742 event, M=7 TM, which released about mantleasthenolith or plume (e.g. Zorin, 1981). However,recent 50% of the seismic moment of the South Baikal region in the results from gravity models (van der Beck, 1997; Petit et al., past 300years) and a constantfar-field extensionrate of 1997), finite-elementdeformation models (Lesne et al., 1998), P- 4.5 mm/yr, we can infer that the South Baikal fault has wave delay-time tomography(Petit et al., in press), and accumulated2.7 m of potentialslip (averagedisplacement) since geochemistryof mantlexenoliths (Ionov et al., 1995) showvery that time. This would correspondto a M=7.5 earthquakeif it little mantleuplift underthe Baikalrift zoneand suggest that the were to be releasedin a singleevent (Wells and Coppersmith, present-dayextension is mostly driven by horizontal forces 1994).If magnitude7.5-7.7 earthquakes(about 3 m of potential related to the far-field kinematics.If local processesdo not slip accumulation)are characteristicof the SouthBaikal fault significantly contribute to the force balance in the Baikal rift zone (an assumptionthat is compatiblewith our historical zone,the additionaldynamic contribution suggested by our GPS knowledgeand with its 150km longcontinuous rectilinear trace resultsmust be soughtin boundaryforces not accountedfor in asunderlined by the instrumentalseismicity), a constantfar-field classical deformation models, in particular at the Pacific extensionrate of 4.5 mm/yr would imply a 350-yr recurrence subductionzone, whose dynamic effects on lithosphericstresses interval for such events. In the central lake, the numerous sub- and strain in continental Asia are poorly known. Possible processesthat could significantly influence the force balance several thousands of kilometers inland from the Pacific subduction include the effect of shear traction in the subduction Table 2. List of large historical earthquakesand main border zone (Kong and Bird, 1996) and the influenceof the sinking faults in the South Baikal rift. Estimated magnitudes(M) are oceanic on the neighboring subcontinental taken from Solonenko (1977). Seismic moments (Mo) are expressedin 1020Nm. SB= SouthBaikal, CB=Central Baikal. convectionby a lateral coolingeffect (Nataf et al., 1981). See Figure 1 for fault and eventlocations. Conclusions Area Eventdate M Mo Associatedfault LengthDip SB 06/27,1742 7TM 3.7 Obruchevsky150 km 60ø Three years of GPS measurementsshow that the Baikal rift SB 10/24,1769 7TM 0.9 Obruchevsky150 km 60ø zoneis currentlyopening at a rateof 4.5+1.2mrn/yr in a NW-SE SB 04/11, 1902 6.9 0.2 Obruchevsky150 km 60ø direction.Since the entirenetwork has only be measuredthree CB 01/12,1862 71/2 1.8 Morskoy 200km 50ø times (only IRKU and ULAN have been measuredfour times), CB 11/26, 1903 6.8 0.2 Morskoy 200 km 50ø theseresults are still preliminary.They are howeverin good CB 08/29,1959 6.8 0.2 Morskoy 200km 50ø agreementwith recentgeological estimates of fault slip ratesand with the strain and stresspattern deduced from instrumental Holt, W.E., M. Li and A.J. Haines, Earthquakestrain rates and seismicity. instantaneousrelative motions within central and eastern Asia, VLBI resultsat Shanghai(Heki, 1995; Molnar and Gipson, Geophys.J. Int., 122, 569-593, 1995. 1996) show8_+0.5 mm/yr of eastwardmotion of SouthChina Houdry, F., M6canismesde l'extensioncontinentale dans le rift nord- relative to Eurasia. Zhang et al. (1998) determinea Plio- Baikal,Sib6rie: contraintes des donn6es d'imagerie SPOT, de terrain, de sismologieet de gravim6trie,PhD Thesis,Universitd Pierre et Quaternaryextension rate of 54-2mm/yr in a NW-SEdirection Marie Curie, Paris VI, 356 pp., 1994. acrossthe Ordos system, that probably accounts for part Ionov, D.A., S.Y. O'Reilly, and I.V. Ashchepkov,Felspar-bearing of the South China-Eurasia relative motion. Our GPS results lherzolite xenoliths in alkalibasalts from Hamar-Daban, southern show4.5-+1.2 mm/yr of NW-SE extensionacross the Baikal rift Baikal region,Russia, Contrib. Mineral. Petrol., 122, 174-190, 1995. zone,a valuethat is consistentwith the resultsmentioned above. King, R.W., and Y. Bock, Documentationfor the GAMIT GPS software The strainin the Baikalregion is howeverlocalized on a narrow analysis,release 9.4, unpublished,1997. (50 to 100km wide) rift structurewhich appearsto be Kondorskaya,N.V., and N.V. Shebalin, New catalog of strong concentratingthe present-daydeformation between Eurasia and earthquakesin the USSRfrom ancient times through 1977, 608 pp., East China. WorldData Cent.A Solid-EarthGeophys., Boulder, Colorado, 1982. Kong, X., and P. Bird, Neotectonicsof Asia: thin-shell finite-element A comparisonbetween the GPS resultsand the seismic modelswith faults, in "The Tectonic Evolution of Asia", An Yin and momentreleased by historicalearthquakes suggests that elastic T. Mark HarrisonEds, CambridgeUniversity Press, 19-34, 1996. strainis currentlyaccumulating in the Baikalrift zonealong Lesne,O., E. Calais, and J. D6verch•re,Finite elementmodeling of activefaults that havethe potentialfor a M=7.5 earthquakeif crustaldeformation in the Baikal rift zone:new insightsinto the that elasticstrain were to be releasedin a singleevent today. If active-passivedebate, Tectonophysics, 289, 327-430, 1998. aseismicslip is neglected,we hypothesize recurrence intervals of Logatchev, N.A. and Y.A. Zorin, Baikal rift zone: structure and 150-350years for large (M~7.5-7.7)events along the lake geodynamics,Tectonophysics, 208, 273-286, 1992. shoreline,which could imply the occurrence of a strongshock in Lysak,S.V., Heat flow variationsin continentalrifts, Tectonophysics, 208, 309-323, 1992. the near future. Molnar, P., and J.M. Gipson,A boundon the rheologyof continental The ratherhigh extensionrate found here for the Baikal rift lithosphereusing very long baselineinterferometry: The velocityof zone comparedto the predictionof mostdeformation models of SouthChina with respectto Eurasia,J. Geophys.Res., 101,545-553, Asia could reflect the dynamic effect of the Pacific-Eurasia 1996. subduction in addition to the well-kown effect of the the India- Nataf, H.C., C. Froidevaux,J.L. Levrat, and M. Rabinowicz,Laboratory Eurasia collision. This hypothesisneeds to be confirmedby convectionexperiments: Effects of lateralcooling and generationof additional GPS measurementsand further testedusing dynamic instabilitiesin the horizontalboundary layers, J. Geophys.Res., 86, deformation models. 643-6154, 1981. Peltzer,G. and F. Saucier,Present-day kinematics of Asia derivedfrom Acknowledgments.We are particularlygrateful to AcademicianN.A. geologicfault rates, J. Geophys.Res., 101, 27,943-27,956, 1996. Logatchev,former director of the Instituteof theEarth's , Siberian Petit, C., E.B. Burov, and J. D6verch•re, On the structure and Branchof the RussianAcademy of Sciences,for his strongsupport of mechanicalbehavior of the extendinglithosphere in the Baikal rift this work. We thank all the field operatorswho participatedin the fromgravity modeling, Earth Planet. Sci. Lett., 149, 29-42, 1997. acquisitionof the GPS data.Insightful comments from G. Peltzerand Petit,C., J. D6verchb•re,F. Houdry,V.A. Sankov,V.I. Melnikova,and D. S. Wdowinskyhelped improve the first versionof this paper.This Delvaux,Present-day stress field changesalong the Baikal rift and researchwas supportedby the French Ministry for Educationand tectonicimplications, Tectonics, 1171-1191, 1996. Research ("DSPT3"), the French Ministry of Foreign Affairs Petit, C., I. Yu. Koulakov,and J. D6verchb•re,Velocity structurearound CEnveloppe Echanges Scientifiques 1994-1997"), CNRS-INSU the Baikal rift zonefrom teleseismicand local earthquaketraveltimes Programs("Tectoscope-Positionnement 94", "Int6rieurde la Terre97"), andgeodynamic implications, Tectonophysics, in press, 1998. NATO (grant LG#961302), and the SiberianBranch of the Russian Solonenko,V.P., (Ed.), Seismic zoning of Eastern Siberia and its Academyof Sciences.Contribution G6osciences Azur No. 215. geologicaland geophysical base, Nauka (in Russian),303 pp., 1977. Tapponnier,P. andP. Molnar,Active faulting and tectonics of the Tien Shan,Mongolia and Baikal regions,J. Geophys.Res., 84, References 3425-3459, 1979. Avouac,J.P. and P. Tapponnier,Kinematic model of deformationin vander Beek, P., Flankuplift andtopography at the CentralBaikal Rift centralAsia, Geophys.Res. Lett., 20, 895-898, 1993. (SE Siberia):A test of kinematicmodels for continentalextension, Boucher, C., Altamimi, Z., Feissel, M., and Sillard, P., Results and Tectonics, 16, 122-136, 1997. analysisof theITRF94, IERS Technicalnote 20, 191p., 1996. Wells,D.L., and K.J. Coppersmith,New empiricalrelationships among Brune, J.N., Seismicmoment, seismicity, and rate of slip along major magnitude,rupture length, rupture width, rupture area, and surface faultzones, J. Geophys.Res., 73, 777-784, 1968. displacement,Bull. SeismSoc. Am., 84, 974-1002,1994. Delvaux,D., R. Moeys,G. Stapel,C. Petit,K. Levi,A. Miroshnichenko, Zhang,Q.Z., J.L. Mercier,and P. Vergely,Extension in the graben V. Ruzhich and V. San'kov, Paleostress reconstructions and systemsaround the Ordos (China), and its contributionto the geodynamicsof the Baikal region,Central Asia. Part II: Cenozoic extrusiontectonics of southChina with respectto Gobi-Mongolia, rifting,Tectonophysics, 282, 1-38, 1997. Tectonophysics,285, 41-75, 1998. D6verch•re,J., F. Houdry,N.V. Solonenko,A.V. Solonenko,and V.A. Zorin, Y.A., The Baikal rift: an example of the intrusion of Sankov,Seismicity, active faults and stressfield of the NorthMuya asthenosphericmaterial into the lithosphere as the cause of disruption region, Baikal rift: new insightson the rheologyof extended of thelithosphere plates, Tectonophysics, 73, 91-104,1981. continentallithosphere, J. Geophys.Res., 98, 19,895-19,912,1993. England,P., and P. Molnar, The field of crustalvelocitiy in Asia calculatedfrom Quaternaryrates of slip on faults,J. Geophys.Res., E. Calais, O. Lesne, and J. D6verch•re, G6osciencesAzur, 250 rue 130, 551-582, 1997. Einstein,06560 Valbonne, France. (e-mail: [email protected]) Hanks,T.C., and H. Kanamori,A moment-magnitudescale, J. Geophys. V. Sankov, A. Lukhnev, A. Miroshnitchenko, and K. Levi, Institute Res., 84, 2348-2350, 1979. of the Earth's Crust, 128 Lermontova Street, 664033 Irkutsk, Russia. Heki, K., Movementof the Shangaistation: Implication for the tectonics V. Zalutsky,VS-NIIFFRI, Time and FrequencyService Division, 57 of easternAsia, J. Comm.Res. Lab., 42, 65-72, 1995. Borodina Street, 664056 Irkutsk, Russia. Herring, T.A., J.L. Davis, and I.I. Shapiro,Geodesy by Radio Y. Bashkuev,Buryat ScientificCenter, 6 SakhyanovaStreet, 670047 Interferometry:The Applicationof KalmanFiltering to the Analysis Ulan Ude, Russia of Very Long BaselineInterferometry Data. J. Geophys.Res., 95, 12561-12581, 1990. (ReceivedJune 8, 1998;revised July 31, 1998;accepted August 6, 1998)