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A composite rupture model for the great 1950 Assam
earthquake across the cusp of the East Himalayan
Syntaxis
A. Coudurier-Curveur, P. Tapponnier, E. Okal, J. van der Woerd, E. Kali, S.
Choudhury, S. Baruah, M. Etchebes, Ç. Karakaş
To cite this version:
A. Coudurier-Curveur, P. Tapponnier, E. Okal, J. van der Woerd, E. Kali, et al.. A composite
rupture model for the great 1950 Assam earthquake across the cusp of the East Himalayan Syntaxis.
Earth and Planetary Science Letters, Elsevier, 2020, 531, pp.115928. �10.1016/j.epsl.2019.115928�.
�hal-02406768�
Earth and Planetary Science Letters 531 (2020) 115928
Contents lists available atScienceDirect
Earth
and
Planetary
Science
Letters
www.elsevier.com/locate/epsl
A
composite
rupture
model
for
the
great
1950
Assam
earthquake
across
the
cusp
of
the
East
Himalayan
Syntaxis
A. Coudurier-Curveur
a,
∗
,
P. Tapponnier
b,
E. Okal
c,
J. Van der Woerd
d,
E. Kali
d,
S. Choudhury
e,
S. Baruah
f, M. Etchebes
g,
Ç. Karaka ¸s
aaEarthObservatoryofSingapore,NanyangTechnologicalUniversity,50NanyangAvenue,Singapore639798,Singapore bInstituteofCrustalDynamics,ChinaEarthquakeAdministration,Haidian,Beijing,100085,China
cDepartmentofEarthandPlanetarySciences,NorthwesternUniversity,Evanston,IL60208,UnitedStates
dInstitutdePhysiqueduGlobedeStrasbourg,UMR7516CNRS,UniversitédeStrasbourg,5rueDescartes,67084Strasbourgcedex,France eWadiaInstituteofHimalayanGeology,Dehradun-248001,Uttarakhand,India
fCSIR-North-EastInstituteofScienceandTechnology,Jorhat-785006,Assam,India gSchlumberger-DollResearchCenter,Cambridge,MA02139-1578,USA
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Articlehistory:
Received18June2019
Receivedinrevisedform23October2019 Accepted26October2019
Availableonline15November2019 Editor: J.P.Avouac Keywords: Assam Himalayanearthquakes tectonicgeomorphology relocatedaftershocks landslidedistribution surfacerupture
Although the Mw=8.7, 1950 Assam earthquake endures as the largest continental earthquake ever
recorded, its exact source and mechanism remain contentious. In thispaper, we jointly analyze the spatial distributions ofreappraised aftershocksand landslides, and providenew fieldevidence for its hithertounknownsurfaceruptureextentalongtheMishmiandAborHills.Withinbothmountainfronts, relocatedaftershocksandfreshlandslidescarsspreadoveranareaof≈330 kmby100 km.Theformer are more abundantin theAbor Hills whilethe latermostlyaffectthe front ofthe MishmiHills.We foundsteepseismicscarpscuttingacrossfluvialdepositsandboundingrecentlyupliftedterraces,someof whichlessthantwothousandyearsorevenacouplecenturiesold,atseveralsitesalongbothmountain fronts.Theylikelyattesttoaminimum200 km-long1950surfaceruptureonboththeMishmiandMain HimalayanFrontalThrusts(MTandMFT,respectively),crossingtheEastHimalayanSyntaxis.Attwokey sites(WakroandPasighat),co-seismicsurfacethrowappearstohavebeenovertwiceaslargeontheMT asontheMFT(7.6±0.2mvs.>2.6±0.1 m),inkeepingwiththerelative,averagemountainheights (3200 mvs. 1400m), mappedlandslide scarnumbers (182vs. 96),and averagethrust dips(25–28◦ vs.13–15◦)consistentwithrelocatedaftershocksdepths.Correspondingaverageslipamountsatdepth wouldhavebeen≈17and≈11montheMTandMFT,respectively,whilesurfaceslipatWakromight havereached≈34m.Notethatthisamountofsuperficialslipwouldbeoutofreachusingclassic paleo-seismologicaltrenchingtoreconstructpaleo-earthquakehistory.Mostofthe1950firstarrivalsfitwith acompositefocalmechanismco-involvingthetwoshallow-dippingthrustplanes.Theirintersectionlies roughlybeneaththeDibangValley,implyingforcedslipparalleltoGPSvectorsacrosstheEastHimalayan Syntaxis. Successive,near-identical,terraceupliftsatWakrosuggest near-characteristicslipduring the lasttwosurfacerupturingearthquakes,whileterraceboulderagesmaybetakentoimplybi-millennial return timefor1950-size events.As inNepal,East-Himalayanmega-quakesare not blindand release mostoftheelastic,interseismicshorteningthataccumulatesacrosstherange.
©2019TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Great(Mw
>
8)19th/20thcenturyHimalayanearthquakeswerelongconsideredblind,whichmaderesearchontheirexactsources andreturn times difficult (e.g., Yeats et al., 1992; Nakata, 1989; Aderetal.,
2012
).The Assamearthquakeof15August1950 (also*
Correspondingauthorat:InstitutdePhysiqueduGlobedeParis,75238Paris, France.E-mailaddress:acoudurierc@ntu.edu.sg(A. Coudurier-Curveur).
known as Chayu or Medog earthquake), the largest continental eventeverrecordedinstrumentally(Mw
≈
8.
6–8.7)(Ben-Menahemet al., 1974; Chen and Molnar, 1977), has been no exception to that belief. Characterization of its source and focal mechanism, and therefore identification of the fault plane(s) involved in the rupture,are stilla matterofcontroversy. Whileits epicenter was locatedinremotemountainterrain,northeastoftheMishmiHills (Kingdon-Ward, 1951, 1953a, 1953b), above the Mishmi Thrust (MT) and close to the Po-Qu-Lohit strike-slip fault (Fig. 1 and Table 1), most of the aftershocks spread over several hundred
https://doi.org/10.1016/j.epsl.2019.115928
Fig. 1. ActivefaultingaroundEastHimalayanSyntaxis(EHS)(updatedfromTapponnierandMolnar,1977; Armijoetal., 1989)superimposedoncoloredSRTM3DEM (large-scalelocation:redrectangle,intop-leftinset).Surfacetracesofthegreat,15/8/1950,AssamearthquakeruptureonMainHimalayanFrontalThrust(MFT)andMishmi Thrust(MT)areoutlinedinred(dashedwhereinferredalongManabhumanticlinefront(MbA;Fig.2).Older,MainCentralandLohitThruststracesarealsoshown(dashed black).Coloredstarsandbeach-ballsare1950mainshockepicenterlocationsandfocalmechanismsfromdifferentsources(Green:(a)Tandon,1955;Yellow:(b)Ben-Menahem etal.,1974,Herrinetal.,1962;Orange:(c)ChenandMolnar,1977;Red:(d)thisstudy)(seecoordinatesinTable1).Reddotswitherrorellipsesareourrelocationsof aftershocksinfirstfourmonthsafter1950mainshock.Orangeparallelogramsareprojectionsofinferredco-seismicfaultplanescontainingthemainshockepicenters,the majorityofaftershocks,andmostmappedlandslidescars(Fig.2).Isoseismallines(dashedwhite)arefromPoddar (1950) (Mercalliscale,modifiedfromRossi-Forel).Brown triangleshighlightHimalayansummitswithelevationsabove6000m.Thin,purple,3500mcontourlineseparatesTibetplateau(lightyellow)andAborHimalayasand Mishmiranges(green)(e.g.,Avouac,2003; Bollingeretal.,2004).Brahmaputradrainagesystemisinblue.CompositeA–Alinereferstocombinedcross-sectionsinFig.3. (Forinterpretationofthecolorsinthefigure(s),thereaderisreferredtothewebversionofthisarticle.)
Table 1
Listoffocalmechanisms,momentestimates,magnitudes,andepicentrallocationsof1950Assamearthquake,fromdifferentpublishedsources.
Reference Focal mechanism Seismic moment Conventional magnitudes Coordinates (WGS84, Dec. Deg.)
(◦) δ (◦) λ (◦) Nature 1021 (N·m)
Mw Value Scale Lat. Long.
Gutenberg and Richter (1954) 8.6 PAS(Ms)
Abe (1981) 8.6 Ms
Tandon (1955) 270 75 285 Normal faulting 28.46N 96.67E Ben-Menahem et al. (1974) 334 60 175 Strike-slip 21 8.8 28.38N 96.68E Chen and Molnar (1977) 260 12 90 Thrust 9.5 8.6 28.38N 96.76E
Okal (1992) 14 8.7
This study 293 16 107 Composite 13 8.7 28.38N 96.72E
kilometers within the Abor Hills, highlighting a possible source on the Main Himalayan Frontal Thrust (MFT). Likewise, as de-rivedby differentauthorsfromfirst motiondatasetsandspectral amplitudes,the mainshockfocal mechanism remained controver-sial, implying either normal, thrust, or oblique strike-slip fault-ing, or possibly a combination of the latter two (Tandon, 1955; Ben-Menahem et al., 1974; Chen and Molnar, 1977; Molnar and Deng,
1984
)(Fig. 1andTable1).Also,whilewidespread liquefac-tion(e.g.,Reddyetal.,2009
; RajendranandRajendran,2011
),large landslides,andcatastrophicdebrisflowswerewidelydescribedin theAssamplain(Tandon,1950
;Kingdon-Ward,1953a
,1953b; Ra-machandraRao,1953
),anddespitededicatedresearch(e.g.,Kumar etal., 2010; Jayangondaperumalet al., 2011), no unequivocalev-idence of a primary surface rupture was found for a long time. Recently, however,inPasighat,
≈
400mnortheastofoneroadside sitepreviouslyidentifiedtobearcleartraceof1950surface defor-mation (Figs.2and6
b;Kalietal.,2013
;CoudurierCurveuretal.,2014a,
2014b
),shallow(≈
2 m)trenchinghaslocallyconfirmedthe existence of near-surface faulting inthe mid-20th century,hence likelyin1950(Priyankaetal.,2017
).Following the discovery ofa clear rupture forthe great 1934 (M
≈
8.
4) earthquakein Nepal(Sapkotaet al., 2013; Bollinger et al.,2014
) aswellasevidenceformedievalsurfacerupturesoflarge magnitudeeventsintheWestern Himalaya(e.g.,1300–1400 A.D., Kumar et al., 2001; Malik et al., 2008), we systematically revis-ited thewholeextentoftheArunachalPradeshmountainfronttoA. Coudurier-Curveur et al. / Earth and Planetary Science Letters 531 (2020) 115928 3
Fig. 2. 1950landslidesanddebrisflows.1950inferredearthquakesurfacerupture(Fig.1)isoutlinedinred.Mainshockepicentrallocations,asinFig.1.Whitecircles correspondto451picksoflandslidesscars,fromdetailedinterpretationofGoogleEarthimages(Largecirclesforlandslidescarsareasbetween0.4 km2and4 km2;Table S1).AreasofArunachalPradeshplaindevastatedbydebrisflowdeposits(southofMFTandMT)areoutlinedinbeige.Palered,orange,andyellowzonesaremountain areaswithdifferentdegreesofslopedevastation,frompost-eventaerialsurvey(RamachandraRao,1953;Ben-Menahemetal.,1974).YellowcontourdelimitstheSiang window.IsoseismallinesVIIIandIX(dashedred)arefromPoddar (1950) (Mercalliscale,modifiedfromRossi-Forel).500mtopographiccontoursarefromCARTOSATDEM. SF:SagaingFault.YellowdotsareplacenamesinFigs.3and4.
search for evidence ofa recentemergent break, assess the exis-tenceofprevious ones,anddeterminetheir geometryandlength. Becausethe 1950mesoseismal area is denselyforested, thefield identificationofactive/youngfaultscarpsandofsteepterrace ris-ersincisedbypermanentriversischallenging,whichmayaccount forthe scarcity ofprevious neotectonic studies. Acrossa few ac-cessible scarps, we used topographic leveling measurements to separaterecent,potentially1950co-seismicoffsetsfromolder cu-mulativeoffsets.Ourfield observationsandmeasurements of sur-facedeformationalongboththeMishmiandAborHillsmountain frontswerethencombinedwithourreappraisaloftheaftershocks andtriggeredlandslidescarsdistributionstodiscussanearthquake source model consistent with first-order, large-scale topographic andgeodeticevidence.
2. Mesoseismalsurfaceeffectsandaftershockdistributionofthe
1950Assamearthquake
The most spectacular effect of the 1950 Assam earthquake was the devastation of the Mishmi and Abor hills slopes and foreland by catastrophic landslides and associated debris flows (Kingdon-Ward,
1951
;RamachandraRao,1953
)(NotesS1a,S2, Ap-pendix A0,Fig. 2). An aerial survey carried out shortlyafter the earthquakerevealedthe extentof damageinboth hillslopes (Ra-machandraRao,1953
;Ben-Menahemetal.,1974
),summarizedin the qualitative map of Fig. 2 (red, orange, and yellow regions). Two distinct, “very severely” affected zones are apparent on ei-therside ofthe East Himalayan Syntaxis cusp (East Siang-UpperSiang-Dibang Valleys and Lower Lohit Valley, Fig. 2). Since large landslidescanpersistlongafteracausativeearthquake(NoteS1b, Nilsen and Brabb, 1975; Keefer, 1994; Meunier et al., 2007), as emphasized by the lack of significant changes in landslide scars locationoverthelast35 yr (AppendixA0andFig.S0),wemapped the present-day distribution andsize of freshlandslide scars us-ing recent satellite images (2006–2012) to further constrain the source parameters. Although such first-order mapping is not ex-haustive, especially forthe smallest landslides andin areas with mistorclouds,thelandslidedistributionshowninFig.2(cf.Table S1) nevertheless fits well with the “very severely” affected areas mappedshortlyafterthe earthquake.Notethat,while mostlarge landslide scars are located within the areas of maximum slopes (
≈
30–50◦,Fig. S1a)andpresent-dayrainfall(≈
4–7 m/yr,Fig. S1b), they mostlyandcloselyfollowtheAborandMishmirange-fronts (up to≈
80%, over a length of≈
400 km), while the maximum slope and rainfall areasextend much farther andelsewhere into themountains.If indicative of the extent of seismic rupture, the fresh scars mapped continuously along the Mishmi front all the wayto the Myanmar border (
≈
96.5◦E, Figs. 1 and 2) suggest that faulting in 1950 may have extended along the MT to the northernmost Sagaing Fault’s main branch. The gap between these fresh scars andothersfarthersouthalongthatnorth-trendingfaultmay sug-gest that faulting in 1950stopped at the 50◦ bendbetween the two faults.The southernmostlandslides mightthen berelatedto an earlier sequence of three M 7 to 7.6 events on the Sagaing strike-slipfaultbetween1906and1931.ThiswouldbeconsistentFig. 3. TopographyandtectonicsofEastHimalayanSyntaxis.(Top)North-lookingviewofsharptopographicdropbetweenMishmiHills(east)andAborHills(west)atDibang riveroutlet.Noteremarkablyflat,averagerangetops.(Bottom)CrustalsectionsofAbor/MishmisyntaxisalongcombinedAandAprofiles(Fig.1).Projecteddepthsof25 relocatedaftershocksbeneathAborandMishmiHillsareshownbyredandgreendots,respectively.ThedipsofMFTandMTthrusts(redandgreenlines,respectively)are consistentwiththesurfacebreakslocationsanddips(Fig.4),theaverageelevationsofthecorrespondingfrontalranges(≈1400 m,red,and≈3200m,green,respectively), andtheaftershockdepths.Coloredstars(asinFig.1)areprojectedlocationsof1950mainshockhypocenters.Notethatthethrustgeometriesarelikelymorecomplexthan simplyplanar.MainCentralThrust(MCT)“plane”locationanddiparealsoindicated(dashedgreyline).Verticalexaggeration(VE)oftopographyis5.
with the outer limit of the area of “suspected” 1950 landslides (Fig. 2), if it was not just drown to follow the Myanmar bor-der.Hence,weinferthatfaultingin1950stoppednearVijaynagar (27.19◦N,96.99◦E)(Figs.1and
2
).SimilarlyintheAborHills, clus-teredlandslidesalongtheSubansirivalley, neartheouterlimit of the“areaaffected bylandslides” (Fig. 2),likely markthewestern extent of faulting, close to the isoseismal line IX, implying that faulting ontheMFTextended to≈
94◦Ein1950. Furthermore,as observedforotherlargecontinentalthrustearthquakes(e.g.,2008, Mw=
7.
9, Wenchuan event),we interpret the dense distributionofthe largest landslidescars along the Mishmi front to attestto particularlylarge peak accelerations,in turnpossibly linked with particularlylargeco-seismicdisplacementsontheMT(Fig.2)(e.g., Meunieretal.,
2007
; Yuanetal.,2013
).Giventhe lackof observed surface faulting atthe time, infer-ences on faulting and initial source length estimates relied only onthe initial aftershock distribution(Kanamori andAllen,
1986
). A250 km-longsourcewasfirstinferredusingthespatial distribu-tionof54relocatedaftershocksrecordedinthefirst8months af-terthemainshock(e.g.,Tandon,1954
; Ben-Menahemetal.,1974
; Chen and Molnar, 1977). Since the difference between source length and aftershock zone extent is commonly small for large earthquakes(e.g.,dePoloandSlemmons,1990
; Wellsand Copper-smith, 1994), we re-evaluated the distribution of theaftershocks usingall94eventslistedby theInternationalSeismological Sum-mary (ISS) for the 4 months following the earthquake. We relo-catedthemusingtheiterativeinteractivemethodofWysessionet al. (1991) (Fig. 1, Table S2, Note S3). The depths of 46 of thesehypocenterscouldberetrievedbythefloatingdepthrelocation ap-proachdevelopedby ReesandOkal (1987) (TableS3).Weverified thatthedistancebetweenfixedandfloatedepicentersdidnot ex-ceedafewkilometers.
At a regionalscale, aside froma few distant (likely triggered) events along the Cona/Yarlung-Zangbo graben, across the Naga Hills,northofthePo-Qufault,andalongthenorthwesternSagaing fault (Fig. 1),mostofthe aftershocks,within their confidence el-lipses,liebeneaththeAborandMishmiranges.Theyaremostly lo-catedwestoftherelocatedmainshockepicenters,allfourofwhich areconsistentwithadeeprupturenucleationjustwestofthe Po-Qu-Lohit fault (Fig. 1). They extend over a length of
≈
350 km between the northernmost extremity of the Sagaing Fault main branchnearVijaynagartotheeast,andtheSubansiriRiver tothe west(Fig.1).Theprincipalaftershockzoneisboundedsouthwards by theMain Himalayan Frontal Thrust(MFT) andMishmi Thrust (MT) faults, andtothe northwestandnortheast by theHigh Hi-malayanrangeandPo-Qu–Lohitstrike-slipfault,respectively.Both theepicenter locations andtheaftershock distributionimply cou-pled co-seismic slipon both the MT and MFT,possibly down to thesegeological boundaries(Figs. 1and3
). Notethat thesmaller number of aftershocksin the Mishmi Hillsmay be suggestive of more complete stress release on the MT during the mainshock. Amongthe46hypocenterdepthsweretrieved,weusedthe25foci locatedbeneaththeAborandMishmiHillstoinferplausible, pla-nargeometriesfortheMFTandMTincross-section.Suchselected datasuggestthattheMTmaybeabouttwiceassteepastheMFT, withdip anglesof≈
28◦ and≈
14◦,respectively, inkeepingwithA. Coudurier-Curveur et al. / Earth and Planetary Science Letters 531 (2020) 115928 5
Fig. 4. MorphotectonicmarkersofactivethrustingalongtheAborandMishmiHillsmountainfronts(locationsofsitesinFig.2).(a)Tectonicscarp(about7m-high)marking thesurfaceemergenceoftheMFTalongtheAborHillsfront,northofNiglok.(b)SectionalongeastbankriserofSikuriver,westofMebo,showinguplifted,flat,modern strathterrace,cappingfault-bendfolded,Siwaliksiltstones/sandstonesoverthrustedatopflat,modernalluviumby35◦NNEdippingemergentMFT.(c)MFTscarpacrossyoung, upliftedSikuterracejusteastofrivercutsectioninFig.4b,westofMebo.(d)MishmiThrustscarp(upto10m-highand350m-long)aboverivercutexposing15◦E-dipping MTplane,northofRoing.(e)Rivercutsectionacross20◦NE-dippingMishmiThrustalongNWbankofTebangriversouthwestofLohitpur.(f)Close-upofTibangrivercut (Fig.4e)showingtheemergentMTplaneemplacing25◦NE-dipping,fine-grained,lightgreygravelbeds(fault-bendfolded)atopsub-horizontal,coarser-grained,buff-colored modernalluvium.That≈27 mverticallyoffsetyoungalluviumyieldsaminimumcumulativethrustslipof≈79 m(seeequationinsection5.2formoredetails).(g)Uplifted hanging-wallterraces,NNEof17m-highscarp,attestingforcumulativedeformation,onDemmaiRiverwestbank,eastofTezu.(h)Treetrunksbeheadedby1950debris flowinDemmairiverbed(Lohittributary),eastofTezu(cf.Fig. S3).(i)Cumulative,14to15m-high,MTscarp,southeastofWakro(Fig.5C3).
theelevationdifferenceresultingfromthelong-termgrowthofthe tworanges(Fig.3).
3. Geomorphicandstratigraphicevidenceforsurfacethrusting
alongtheAborandMishmirange-fronts
Wedescribe here, for the firsttime along more than 150 km acrosstheSyntaxis,geomorphicandtectonicfeatures attestingto recent,modern,surfacethrustingalongboththeAborandMishmi mountainfronts(NoteS4,Figs.2and
4
).North of Niglok (27.87◦N,95.23◦E,Fig. 2), a clear, 6 to 7 m-highand600m-long,cumulativetectonicscarp(Fig.4a), orthogo-nal to the Sille river cutsacross rice paddies.The height of this scarp increases westwards for
≈
2 km along the edges of older terraces,anddecreasesstepwise eastwards to≈
3 mand≈
1.5m before disappearing towards the river (Fig. S2). As it marks the northern edgeof the Assamplain, thisscarp clearly corresponds to the frontal emergence of the MFT. Shortly to the east, this thruststepsleftnorthwardsby≈
5 km,northofMikung(27.92◦N,95.24◦E,Fig.2),toboundtheupliftedMiremterraces(Fig. S4).Yet farther northeast, past another
≈
2 km northward step, the MFT cutsacross Pasighat(28.06◦N,95.32◦E,Fig.2), sharplyseparating flights ofupliftedhanging-wallterraces,initiallydepositedbythe nearlyorthogonalSiang-BrahmaputraRiver,fromfootwallchannels inthatriver’sfloodplain(Kalietal.,2012
,2013)(Fig.6A).NorthofPasighat,pastanotherleft-stepacross the Brahmapu-tra,theMFTcontinuesnortheastwards,boundingupliftedflat sur-facesinanareadenselycoveredbyforest.IntheSikurivervalley, westofMebo(28.17◦N,95.42◦E,Fig.2),a30◦north-dippingthrust faultemplacessimilarlydipping bedsofmudstonesandfractured pebbles atop horizontal layers of coarser cobbles (Fig. 4b). The young-looking,
≈
5 m-high surface scarp corresponding to that thrust may be followed along the mountain front for≈
4 km eastwards (Fig. 4bandc).Farther east,theMFT continues north-eastwards for about 30 km to the Dibang river outlet (28.29◦N, 95.71◦E)whereitmeetsalmostorthogonallywiththeMT(Figs.1BetweentheDibangandtheDeopanirivers(28.16◦N,95.85◦E, Fig.2),theMTrunsmostlyparalleltothelargerisersofthesetwo rivers.JustnorthofRoing,attheDeopanioutlet,the thrustveers by90◦ tocuttheterraceriseralongtheriver’ssouthbankbefore continuingfarthersouthwards across youngterracesof that river for
≈
400m.Thenorth-facing, verticallyincisedDeopaniriser ex-poses the≈
15◦ east-dipping MT thrust in section (Fig. 4d). The MTsurfacetrace thencontinuesalongthe baseofan escarpment whose height increases southwards from≈
6 to≈
10 m, bound-ing the edge of a flat, uplifted alluvial-fan surface. Between Ro-ingandLohitpur(28.01◦N,96.21◦E,Fig.2), particularlythick, far-reachingdebrisflowsmostlikelytriggeredbythe1950mainshock (NoteS5),facingthemostprominentlandslidescarsintheMishmi Hills,coverthe forelandSWof theMTtrace. WestofLohitpur, a south-facingrivercutalong theTebangrivervalley exposesa par-ticularlyclearsection acrosstheemergent20◦NE-dippingMishmi Thrust that emplaces sheared, 25◦NE dipping Quaternary fluvial gravels on top of younger, horizontally-bedded alluvium (Fig. 4e andf). Farthersouth,betweenTezu (27.92◦N,96.16◦E)and Brah-makund (27.87◦N, 96.36◦E, Fig. 2), the≈
17 m-high MT surface scarpboundsmultipleabandonedterracesoftheDemmairiver,a Lohittributary (Fig.4g).Beheadedtreetrunks,whose≈
4m-high topsremainspectacularlyincrustedwithfluvialpebbles,stillstand erectabovethepresent-dayDemmaifloodplain,attestingtothe ef-fectofparticularlycatastrophic1950debrisflows,asdescribedby Kingdon-Ward(1953b) (Fig.4h,S3,andNoteS5).At the Lohit River outlet, the MT bends againsouthwards, to bound the
≈
30 km-longnorth-southtrending mountainfront all the way towards the piggy-back side of the northeast dipping limb ofthe curved, actively growing, Plio-Quaternary Manabhum Anticline (Dasgupta, 2011; Borthakur et al., 2013). Near Wakro (27.78◦N,96.34◦E, Fig. 2), both north and south of the Kamlang River,weidentifiedparticularlysteepandyoungescarpmentsand recentlyuplifted terraces (Figs.4i and5). We followed the trace ofthe MTsouthwards, northeastofthe Manabhumanticline and identifiedsets ofuplifted, abandonedterraces suggestingthat re-curring thrustingextends alongthe UpperDihing Valleypossibly asfarasVijaynagar, nearthe junctionoftheMT withthe north-ernmostbranchoftheSagaingFault(Figs.1and2
,NoteS4).4. Quantitativemeasurementsof1950andpenultimatethrust
offsets
We leveled topographic profiles perpendicular to most ofthe tectonicscarpsweidentifiedtoestimatetheirheightandseparate recent,potentially1950co-seismic throwsfromolder,cumulative ones.Wecollectedquartz-richsamplesbothatopandinside(depth profiles)theuplifted terracesboundedbysuch scarpsfor cosmo-genic 10Be dating (technical details in Appendix A4). The results
obtained at two key sites on the MT and MFT, as well as the constraintstheyprovideonco-seismicsurfacedeformation,are de-scribedindetailbelow.
At and south of Wakro along the MT, we found three steep scarps, roughly perpendicular to the Kamlang River (Fig. 5). The Mishmi Thrust truncated and uplifted the lowest, most recent, strath terrace (KL, Fig.5B)by 7.6
±
0.2m on the northbank of thatriver.The strathdepositsofthisterrace standupon abraded, exhumed metamorphic bedrock whose top is also uplifted, by 7±
0.4 m, relative to the river level downstream from the thrust (Fig. 5C1). Alongthehanging wall river-cutedge, thesub-verticalterraceriserisstillaffecteddailybymultiplerockslides(Fig. S5A) attesting to ongoing incision. The maximum slope of the convex Kamlang-river thrust scarp is particularly steep (up to 60–70◦, Fig. 5C). Such convexity and steepness that are similar to those observed across the co-seismic scarps of contemporary thrust earthquakes (e.g., 2005, Mw
=
7.
6, Muzaffarabad, Pakistan, andof the1999, Mw
=
7.
6,Chi-Chi, Taiwan,Figs. S6 andS7) suggestthat theKamlang scarpformedduringarecent,singleevent.Two well embedded quartz-rich gneiss boulders sampled on the top of the KLterrace, several metersaway fromthe unstableterrace riser edge, yield 10Be exposure ages of 173
±
27 yr and 1188±
180yr(AS13-51 andAS13-50,respectively;Fig.5,AppendixE, and Table S5). These two ages imply rather young terrace aban-donment, either a little more than a thousand years ago, or as recently asduring the past two centuries. Consideringthe steep, freshmorphologyoftheKamlangscarp,weinterprettheyoungest cosmogenicagetoconstraintheonsetoffloodplainabandonment and therefore the maximum ageof co-seismic uplift.In all like-lihood, such very recentabandonment should be correlatedwith the1950Assamearthquake,theonlyknownregionaleventinthe entireregion largeenough toproduce theparticularlyhighuplift oftheKamlang terrace.Giventhelimitednumberofsample ages discussed here, additional dating is required to corroborate this interpretation.Theothertwosteepscarps,thatboundupliftedalluvialsurfaces northandsouthoftheKamlangRiver(WKandSK,Fig.5A),are al-most exactly twice as high(
≈
14.5and≈
14 m) asthe Kamlang terracescarp,anddisplaycompositeformsattestingtocumulative uplift (Fig.5C). Thenorthernscarp showstwoslopebreaks sepa-rating a steep (40◦) middlepart froman upper bevel andlower colluvium withgentler≈
16◦ and≈
13◦ slopes, respectively. The offset ofthe scarp’s steepest central part, mostlikely dueto the last surfacerupturingearthquake,is7.7±
0.1m.Thisvalue is es-sentiallyidenticaltothe7.6±
0.2moffsetoftheyoungestKL ter-race.The southern scarpalsoshowsdistinct slopesupwards(21◦ and29◦)anddownwards(40◦)butnosymmetry,possiblybecause of secondary faulting upslope and quarryexcavation at thebase. Measurementsofinsitu-producedcosmogenic10Beconcentrations in 7exposed quartz-richboulders atop the SKand WK hanging-wallandfootwallalluviumare showninFigs.5,S8,andTableS5. Twoof thethree samplesfromthe SKterrace provide consistent exposure ages of 1636±
486 yr and 1710±
513 yr,yielding a young, mean exposure age of 1670±
500 yr. The third sample is≈
1000 yr older(2734±
405 yr) andthus may be less repre-sentative oftheexposure ageoftheSK terrace (seePDF Fig. S8). Onthe WKfan surface,thefoursamplesyield exposureages be-tween 2133±
395and3292±
384 yr,withameanageof≈
2.8±
0.4 kyr.However,a2m-deeproadcutdepthprofileacrossthe footwallsectionofthatfansurfaceyields10Beconcentrationsthatarebestmodeled witha2.3
±
0.3kyrageprofile(Fig. S8).Within uncertainties, andinview ofthedifferentsamplinglocations, the youngest surface abandonmentage (2.1±
0.4kyr) ofthe Wakro alluvial fanisinfairagreementwiththatdeducedfromits depth profile(2.3±
0.3kyr).Given the aforementioned descriptions, we consider that the youngest ages on both surfaces best constrain exposure ages of
≈
2.2±
0.5kyrfortheWakroalluvialfan,and≈
1.7±
0.5kyrfor the South Kamlang terrace (WK, SK, respectively on Fig. 5) (Ap-pendixE).TheyoungerageofSKislikelyrelatedtoitsnatureand simplerexposurehistory(emergedriverterraceratherthan aban-donedalluvialfan)andmightbestconstraintheageofsurface up-liftofboth,ifcontemporary.Thesimplestscenario accountingfor thesequantitativeobservationsisacumulative,≈
14mco-seismic upliftofthetwoterracesasaresultoftwolargeevents,including the 1950Assamearthquake,withnearly identicalverticalthrows of≈
7m. Aquantitative analysisof theshapesofthesescarpsin termsofdegradation bydiffusion(e.g.,Hanks andWallace,1985
; Tapponnieret al.,1990
; Avouac, 1993; Avouac andPeltzer, 1993; Carretieretal.,2002
)mightbeusedtocorroboratethisscenario.In Pasighat, topographic profiles leveled perpendicular to the MFT scarpacross threedistinct, uplifted andabandoned Brahma-putraterraces(profile4onFig.6)constrainthethrowsdueto
suc-A. Coudurier-Curveur et al. / Earth and Planetary Science Letters 531 (2020) 115928 7
Fig. 5. 1950surfacebreakalongtheMishmiThrust(MT)nearWakro.A.MapofemergentMTtrace(red)andsectionsacrossyoungscarpsboundingupliftedalluvialsurfaces (yellowandorange).BackgroundimagefromGoogleEarth.Numberedwhitelinescorrespondtoscarpprofiles1,2,and3inFig.5C.Whitepoint-of-viewsymbolsreferto photographsinFigs.5BandS5B.Small,numberedblack/whitecirclesindicatesamplelocations(TableS5andFig.5C).R:Rapids.WK:Wakroalluvialfansurface.KL:Kamlang terrace.SK:SouthKamlangterrace.B.Viewof1950co-seismicscarpboundingupliftedKLterraceonNEbankofriver.C.Topographicprofiles1,2,and3acrossMTscarps inFig.5A.
cessiveevents. The corresponding seismic scarp heights increase southwestwards from 2.6
±
0.1 m, to 7.3±
0.1 m, and 11.5±
0.1m,acrossterracesT1,T2,andT3,respectively(Fig.6).Theflat, topandbottomsurfacesofthesmallestescarpmentare offsetby a steep,convex upward, up to56◦ steep slope that topsa small colluvial wedge (Fig. 6C1). The two highest scarps have gentlermaximumslopes (23◦ and29◦),withprofilespartlysmoothedby humanaction,andlargercolluvialwedges(Fig.6C2,3).Eventhough
alterationofthethreescarpsbyroadandhouseconstructionis ev-ident,weconsidertheheightofthesmallest(2.6
±
0.1 m), steep-est,henceyoungest,scarpasaminimumboundforthelastevent throw (Kaliet al., 2013; Coudurier-Curveur et al., 2015). A simi-lar,≈
3.1m-highscarp,studiedandtrenched≈
400mnortheastof ourlocation, hasalso been inferred to result fromsurface fault-ing in 1950 (Priyanka et al., 2017). The other two scarp heights wemeasuredmaybeinterpretedtoyieldevidenceforcumulative offsetsdue to one andtwo comparableearthquakes. In-situ cos-mogenic10Beconcentrationsderived from4samplescollectedontopofthehangingwall onprofile2(TerraceT2, Fig.6C2,4)range
inagefrom2722
±
290 yrto4393±
432 yrwithamean expo-sureage of3.7±
1.2 kyr. As in Wakro,the age ofthe youngest ofthesesamples might be takento constrain a maximum aban-donmentageoftheT2terrace andthereforeamaximumagefor upliftassociatedwiththepenultimateevent,whichwouldthenbe youngerthan2722±
290 yr(TableS5).Herealso,however, addi-tionalsamplinganddepthprofilingareneededtobetterconstrain theT2andT3exposureagesandthereforetheearthquakehistory ontheMFTnearPasighat.5. Solvingthe1950Assamearthquakedilemma?
5.1.Sourcegeometry
5.1.1. Atwo-faultsourcemodel
Thecombinationofrelocatedaftershocksandlandslidescar dis-tributions(Figs.1–3) stronglysuggeststhatbothfaultplanes
rup-turedduringthe1950earthquakeoveratotallengthof
≈
330 km, asstronglysupportedbyfieldobservationsoffresh-lookingscarps alongtheMishmi(MT)andMainHimalayanFrontal(MFT)thrusts (Figs. 4–6). We propose a two-fault source model schematically representedbytwoplanes(orangeparallelogramsinFig.1),whose intersectionprojectsjustwestoftheDibangvalley.Theyhave dif-ferent widths and dips, consistent with1/ the lower average el-evation ofthe Abor relative to the Mishmi hills (a factorof≈
2, Figs.1–3andS9),2/oursurfacedipmeasurements(Figs. S10and S11), and3/ thedepthsofthe largest1950,relocated aftershocks (Fig.3,TableS3).TheMTsourcecoversaprojectedsurfaceareaof 180 kmby 80 km,fromtheMyanmarbordertotheDibangRiver (Fig. 1).The MFTsource extendsover aprojected surface area of 150kmby100 km,fromtheDibangtotheSubansiririvers.The similar orientations of (1) the intersection between both planes, (2) the slip directions in the focal mechanisms, and (3) the average trends of hanging wall GPS vectors relative to India (e.g., Kreemer etal., 2014; Vernant etal., 2014) (Figs. 1, 8, and S12A) suggest similar slipdirections on both thrust planes. This wouldbeconsistent withoblique slipon eachofthethrusts, im-plying a left-lateralcomponent alongthe MFTanda right-lateral one along the MT (Fig. S13). Although lateral offsets were gen-erally hard to detect in the field, the respectively left and right steppinggeometriesoftheMFTandMTareinkeepingwith oppo-siteobliquitiesofthrustingalongboth.Alternatively,assuggested by newlocalGPS data,smallscale, clockwisedrag-rotation ofan Assammicro-block(Guptaetal.,
2015
),amechanicallyreasonable inferenceattheeasterntipofimpingingIndia,wouldalleviatethe need fora right-lateral slipcomponent along the Mishmi Thrust (Fig. S12B). This, however would still require significant left slip alongtheMFT.Clearly,morefieldexplorationforkinematic indica-tors,andhigherresolutiongeodesyarebothneededtodetermine whichofthetwo processes(rotationoroblique slip)ismore im-portant.Fig. 6. 1950surfacebreakalongtheMainHimalayanFrontalThrust(MFT)atPasighat.A.MapofemergentMFTthrusttraceandsectionsacrossyoungscarpsbounding upliftedBrahmaputraterracesT1,T2,andT3(green,yellow,andorangesurfacesrespectively,Fig.6C).BackgroundimagefromGoogleEarth.Numberedwhitelinescorrespond toscarpprofiles1,2,and3andterracesurfaceprofile4inFig.6C.Whitepoint-of-viewsymbolreferstophotographinFig.6B.Small,numberedblack/whitecircleindicates sampleslocation(TableS5andFig.6C).Yellowdashedlinesindicatebeheadedformerchannelsinfloodplain.Orangelineindicatestrench(T)dugbyPriyankaetal. (2017). B.Viewofco-seismic1950,T1escarpmentmodifiedbyroadbuilding,northofairport(Profile1).C.Topographicprofiles1,2,and3across1950andcumulativethrustscarp. Profile4,paralleltoscarp,showselevationdifferencesbetweenterracesT1,T2,andT3.
5.1.2. Crustalandsuperficialfaultdipangles
Consideringan
≈
30 km-thickseismogeniccrustandadowndip slip limit located roughly beneath the high Himalayan range (≈
3500masl elevationcontour,Avouac,2003
; Dhital,2015
),the MTandMFTthrustdipangleswouldbeabout25and15◦, respec-tively,inkeepingwiththe≈
28and≈
14◦ dipsconsistentwiththe aftershocksdepthdistributions(Fig.3).The3Dgeometryofplanes thatbest fittherelocated aftershockhypocenters and the simpli-fied3Dsurfacerupturetracemaybeconstrainedfurtherusingthe PetrelE&Psoftwareplatform(AppendixA3).TheresultingMTand MFTplaneshavedipanglesof25and13◦,respectively(Figs.7and S14).Insummary,averagedipanglesof≈
25–28◦ fortheMT,and≈
13–15◦fortheMFT,accountbestfortheavailablegeologicaland geophysicalevidence.Inkeepingwitha25–28◦dipfortheMishmi Thrust,thehypocenter depthofthe1950Assamearthquakewould beabout37±
3 km(Figs.3and7
).At several sites in the field, our local, detailed mapping and georeferenced profiles also provide strong constraints on the thrusts’ shallow dip angles. At Wakro, co-referenced scarp pro-filescutting the thrust trace at three differentelevations (Fig. 5) constrain a planar, shallow MTdip of13
±
1◦ (3-point method, Appendix A2, Fig. S10 and Table S6). This is less than the 20◦ measured along the Lohitpur River section (Fig. 4g) but consis-tentwithHimalayanmegathrustfrontaldipselsewhere(e.g.,Malik et al., 2008; Sapkota et al., 2013). At Pasighat, the smalleleva-tion difference (
≤
1.5 m) between 3co-referenced profiles across the MFT scarps (Fig. S11) makes assessing a dip by using the 3-points method moreprecarious, butthepoorly constrained re-sult(1±
0.8◦,AppendixA2,andTableS6)maybetakentoimply a much shallower dip than those measured along the MT. Note that veryshallowMFTsurface dipsare alsovisibleintrenchesat there (Fig. 3a, b in Priyanka etal., 2017) aswell asnear Niglok (Coudurier-Curveuretal.,2015
).5.2. Shallowandaverageco-seismicslipamounts
Measuredco-seismicverticalthrow(u)andthrustdipangle(
α
) constrain theminimumco-seismic slip:d=
u/
sin(
α
)
considering a pure thrustingevent withnegligible slip obliquity (see section5.1.1).At Wakro,the7.6
±
0.2m co-seismicthrow oftheKL ter-raceandtheshallowMTdipof13±
1◦ wouldimply,iftheyoung KLexposureageisconfirmed, ashallow1950down-dipslipofas muchas34±
2.5m.AtPasighat,combiningtheminimumheight (2.6±
0.1m) ofthesmallest, youngestscarpwithashallowMFT dipof1±
0.8◦unfortunatelyyieldsanimplausiblylarge1950 sur-facedown-dipslip(≈
150m!;AppendixA2,Fig. S11,andTableS6), duetotheverypoor3-pointconstraint,clearly notrepresentative ofdeeperpartsofthethrust.Combining the large scale, constant, average dips compatible with seismic observations (26.5
±
1.5◦ and 14±
1◦) with theA. Coudurier-Curveur et al. / Earth and Planetary Science Letters 531 (2020) 115928 9
Fig. 7. 3DviewoftheEastHimalayanSyntaxis,withinferredsourcegeometryforthe1950Assamearthquake.TopographyfromSRTM3data.Blueandpurplesurfacesare reconstructedco-seismicMishmiThrust(MT)andMainHimalayanFrontalThrust(MFT)ruptureplanesdipping25–28◦and13–15◦,respectively.3Dimagewasbuiltusing PetrelE&Psoftware.
Wakro/Pasighat surface throws (u) of7.6
±
0.2m and2.6±
0.1 m(Figs.5C1 and6C1) wouldyield averageco-seismic slipvalues(d)of
≈
17±
1m and≈
11±
1mon theMTandMFT, respec-tively.Notethatsuch slipamounts arecompatiblewiththe16m average slipcalculated by Chen and Molnar (1977) on a unique, low-angleMFTfault.5.3.Dualfirstmotionfocalmechanism
Thekinematicallycomplexdualsourceweproposeforthe1950 Assamearthquakelikelyaccountsforthefactthatitsexact geome-tryhaslongbeenthesubjectofcontroversy.Tandon (1955) carried outanearlyinvestigationofthefocalgeometryofthe1950event, andproposed a normal faulting mechanism with parameters in-ferred to be
φ
=
270◦;δ
=
75◦;λ
=
285◦ (Fig. 8A andTable 1). AslaterdiscussedbyBen-Menahemetal. (1974),however,several ofhisreadingsmighthavebeenerroneous, andhis methodology, whichpredatedtheintroductionofthedouble-coupleconcept,also did not handle core phases, the latter being incompatible with normal faulting. His dataset remains valuable, however, notably for stations in Central and Southern India. Ben-Menahem et al. (1974) investigated the focal mechanismof the1950 earthquake basedontheinterpretationofbothfirstmotionsandspectral am-plitudesoflong-periodsurfacewaves.Theyproposedastrike-slip solutionona steeplydippingplane(φ
=
334◦,δ
=
60◦,λ
=
175◦; Fig.8B andTable 1) that would correspond roughly to the Jiali-Po-Qu-Lohitfault,lateridentifiedasalarge,active,strike-slipfault (e.g.,MolnarandTapponnier,1975
; TapponnierandMolnar,1977
; Ni and York,1978; Armijo et al., 1989). Subsequently, Chen and Molnar (1977) arguedthata shallowdippingthrust withastrike close to that of the MFT (φ
=
260◦,δ
=
12◦,λ
=
90◦) (Fig. 8C andTable 1) provided an equally acceptable fit to the first mo-tiondataset.Infact,neitherthesteepdipofthePo-Qu-Lohitfault, nor the length of an inferred rupture along it can account for thedistribution of aftershocks andmagnitudeof the 1950event (e.g.,Armijoetal.,1989
).Furthermore,whileBen-Menahemetal.’s (1974) solution skillfully runs fault planes through available first motiondatasets (assuminga crustal source velocity of 6.5 km/s, andignoringmostofTandon’s(1955) datafromstationsinIndia), itremainsunconvincinggivenimpulsivearrivalsinthevicinityof itsproposednullaxis.Ontheotherhand,whileChenandMolnar’s (1977) pure thrust solution fits well with the broad aftershock zonebeneaththeeasternHimalayasandwiththeMFT’s≈
NE-SW trendand dip consistent withour field observations, it accounts neitherfortheepicenterlocation intheMishmi Hillsnorforthe dominant1950thrustingandlandslides along thenearly orthog-onalMT.It seems clearthata compositesource, assuggestedbyourfield resultsandthebroaderaftershock/landslide distribution, withapossibletimelagofafewsecondsbetweenrupturesontwo thrustfaults,isrequired.
We thus re-assessed the first motion dataset by personally readingrecordsat12historicalstations (allcompressionalexcept Jakarta), and complementing it by first motions reported by the ISSandbyTandon (1955) (Fig.8).Wefindthatthefaultgeometry derivedfromourfield investigationalongtheMFT(
φ
=
245◦,δ
=
15◦,
λ
=
70◦)iscompatiblewiththefirst-arrivaldatasetonFig.8D, while a source on the MT (φ
=
315◦,δ
=
20◦,λ
=
120◦) would violatethedilatationalarrivalsatBrisbaneandRiverview,in East-ernAustralia(Fig.8E).Suchobservationsmightbecompatiblewith an initial ruptureonthe MFT,anda time-lagging ruptureon the MTnotcontributingtofirstmotionpolarities.However,this inter-pretationwouldbeatoddswiththemutuallyconsistentlocations ofthe1950epicenter80 kmNEoftheMishmiThrust(Figs.1–2; Tandon,1955
;Ben-Menahemetal.,1974
;ChenandMolnar,1977
). Hence, the 1950rupturemust havenucleated on theMT, whose plane, with an average dip of 20◦ (Fig. 8E), would fit the entire dataset save for arrivals at Brisbane and Riverview, whose picks weunfortunatelycouldnotindependentlyconfirm.Additionally, 3 dilatational arrivals (at Cartuja,Owase, and Nagasaki), which are inside the densest, best-defined compressional cluster, are in all likelihooderroneous.Toaccommodatedeepaftershocks,structural geology, field observations,andpossible changes ofdipat depth, we consideranaverage dipof25◦ forthe sourceontheMTthat best fits the entire dataset. The two sub-mechanisms (φ
=
315◦,δ
=
25◦,λ
=
120◦, andφ
=
245◦,δ
=
15◦,λ
=
70◦, on the MT andMFT,respectively),withscalarratiosof1:1.6(seepotency ra-tios, below), combine into a nearly pure double-couple orientedφ
=
293◦,δ
=
16◦,λ
=
107◦ (Fig.8andTable1).Remarkably, thefull, combined,5-dimensionalmoment tensor derivedfromthedual(MT
+
MFT)sourcemodelwepropose(Fig.8) satisfies all clearly non-erroneous first motion arrivals, including BrisbaneandRiverview(Fig. S15).Thisexcellent fit,however,still runsintotheproblemthatthesourcecouldnothavebeen instan-taneous, given the necessary propagation time (on order oftens of seconds)over a distance ofat least100 km between thetwo rupturesegments.5.4. Independentseismicmomentestimates 5.4.1. Seismicmomentfromfieldobservations
Under the assumption of uniform slip on the faults, we can estimate the resulting seismic potencies Po of both fault planes corresponding to the product of the average slipd and the rup-tured area S. Using d
=
17 m and 11 m, and S=
16000 km2Fig. 8. (Top)First-motionfocalmechanismsforthe1950Assamearthquake.Thedatasetusedcombinespolaritiesreadaspartofthisstudy(largedots), andreported fromthe ISS(smalldots),or transcribedfromTandon (1955) (smalltriangles,allIndianstations). Solidand opensymbolsarecompressionalanddilatationalarrivals, respectively.A.NormalfaultingmechanismproposedbyTandon (1955):noteinconsistenciesforlargedistances.B.Strike-slipmechanismfromBen-Menahemetal. (1974):
noteinconsistenciesforseveralIndianstations.C.Shallow-dippingthrustfaultingsolutionfromChenandMolnar (1977):noteremaininginconsistenciesatafewofTandon’s (1955) Indianstations.D.Mechanismforshallow-dippingthrustingonMFT:notesuperiorfittoentirefirstmotiondataset.E.MechanismforobliquethrustingonMT:note onlysmallnearnodalinconsistenciesat2Australianstations.(Bottom)Focalmechanismscombinationconsistentwithfieldobservations.(Left)MechanismsonMFTandMT, respectively,withbeachballsscaledlinearlytoseismicmoments.Blackarrowsareslipvectors.(Right)Bestdouble-couplederivedfromcombinedMFT+MTsolutions.
and15 500 km2 (Fig.7),yields potenciesof
≈
270and≈
170 km3 fortheMTandMFTfault planes,respectively. Assumingacrustal rigidityμ
of 3×
1010 Pa (or N/m2), the corresponding seismic moments (M0=
μ
P0) wouldbe≈
8×
1021 and≈
5×
1021 N·
m,on the MT and MFT, respectively. This would imply a moment sumof1
.
3×
1022N·
mcorresponding toamomentmagnitudeof Mw=
8.
7 (Table1).Taking more gentle dips for the thrusts, consistent with our fieldmeasurements,wouldincreasetheresultingslipsontheMFT andMT to
≈
20 and≈
40 m,respectively, and the resulting mo-ment sum to 2.
8×
1022 N·
m (Mw
=
8.
9). Such larger values,however,should only be considered as upperlimitssince, as in-ferredforrecentmegathrustearthquakes(e.g.,2011Tohokuevent; Ammonetal.,
2011
; Leeetal.,2011
),onemightexpectspatialslip heterogeneitiesonthethrusts,suchasslipdecreaseand/ordip in-creasewithincreasingdepth.5.4.2. Seismicmomentfrommantlewaves
Based on ourbest double-couple geometry, we computedthe spectralamplitudesofmantleRayleighandLovewaves(Okal and Talandier, 1989) from long-period records at Uppsala, Göttingen, SanJuan,andHuancayo,complementedbyourpreviousdatasetat Pasadena(Okal,
1992
).Suchamplitudesrequireaseismicmoment of≈
1.0×
1022N·
matthelongestresolvableperiods(200–250 s; Fig.9).This valueiscompatiblewith, although23% smallerthan, that estimated fromour field observations. The discrepancy may beattributedtouncertainties onthedepthextentoffaulting and toprobableslipheterogeneitieson thefault planes.On theother hand, the inferred total potency would amount to≈
440 km3, a factorof1.6smallerthansuggestedbyBen-Menahemetal. (1974). Our moment is also compatible with Chen and Molnar’s (1977) estimate (9.5×
1021 N·
m) computed for a low angle thrust onthe MFT only (Table 1). To our knowledge, such moment values stillsingleoutthe1950Assamearthquakeastheonlycontinental earthquakeon record witha seismic momentvalue of 1022 N
·
m (Mw=
8.6/8.7).Finally,theweakdependenceofMc onfrequency(slopeofonly
−
0.06 logarithmic unitsper mHz,Fig. 9) indicatesthat,despiteitscompositemechanism,the1950Assamearthquake did not display anomalous source slowness (generally associated with slopes of
−
0.08 or more in absolute value) (e.g., Okal and Borrero, 2011; Okal, 2013). Rather, this slope suggests a rupture time ofno more than≈
65 s, a standard value for theproposed dimensionsofthecompositerupture.5.5. Returntime
The cosmogenic ages of the cumulative, 14 m offset terrace doublets near Wakro suspected to have recorded near-identical upliftamountsbytwosuccessiveevents,includingthe1950 earth-quake,maybeusedtoestimatethereturntimeofsuchevents.The olderages (2.3
±
0.3kyr, 2.1±
0.4 yr,and1.7±
0.5 yr; Fig. S8, Section4)onterracesWKandSK,northandsouthoftheKamlang River, respectively, imply nearly identical uplift by a comparably greateventatleast1200andpossiblyasmuchas2600 yr earlier (i.e.,1900±
700 yr).FollowingthesamelogicatPasighat,theless wellconstrainedyoungestabandonmentageofthesecondterrace (T2,Fig.6),whichlikelyrecordedupliftby2events,wouldsuggest a maximumreturntime of≈
2700 yr betweentheseevents.Such returninterval ontheMainHimalayan FrontalThrustwould nev-erthelessbecompatiblewiththeupperboundofthatestimatedat WakroalongtheMishmiThrust.Local, recently published interseismic GPS velocities imply a shortening rate of 17
±
0.5 mm/yr across the MT (e.g., Devachandra et al., 2014; Gupta et al., 2015). That shortening rate andthe minimum 34±
2.5m 1950 co-seismic slip we in-feratWakro,wouldbeconsistentwithareturntimeof≈
1980±
160 yrfor1950Assam-sizeeventsontheMT.Suchrecurrence in-terval would be compatiblewiththat (≈
1900±
700 yr)derived fromcosmogenicdating ofupliftedterracesatthesame location. It is clear however that more quantitative geomorphic and age constraintsalong boththrustsareneededtobetter assessthe re-currence interval of great,1950-type earthquakes across theEast HimalayanSyntaxis.A. Coudurier-Curveur et al. / Earth and Planetary Science Letters 531 (2020) 115928 11
Fig. 9. DeterminationofseismicmomentfrommantleRayleighandLovewaves.ColoredsymbolsshowcorrectedmantlemagnitudeMc(OkalandTalandier,1989),based onFig.8compositeMFT+MTfocalmechanism,atvariousstations(HUA,GTT,SJC,UPP,andPASareHuancayo,Göttingen,SanJuan,Uppsala,andPasadena,respectively;Ri andGiaresubsequentsurfacewavepassages).DashedvioletlineshowsbestlinearfittoMcvaluesasafunctionoffrequency(bottomleftequation).Blackdashedlineand yellowbandshowaveragedMcvalueandassociatedstandarddeviation,respectively,acrossentirefrequencyspectrum.Weakmomentincreasewithperiodsuggestsavalue of1×1022N·matthelongestavailableperiods(∼250 s).
6. Conclusions
Eventhough the exact geometryof the seismic source ofthe great1950Assamearthquakeremainstobebetterascertained,our wide-ranging,multi-methodobservationsandmeasurements sup-port theco-involvement oftwo distinct, nearly orthogonal thrust planes.Theearthquakeappearstohavebeenassociatedwith sub-parallel components of oblique slip both on the shallow-dipping partofHimalaya’sMainFrontalThrust,beneaththeSiangwindow, andonthesignificantly steeper MishmiThrust. Such adual fault geometryandthecorrelated amounts ofslipon both thrustsare compatiblewithacompositemainshockfocalmechanismandwith atotalseismicmomentof1
.
3×
1022 N·
m thatconfirms itasthelargest continental eventever quantified. Our landslide scar and relocatedaftershockdistributions constraintheextentofthe rup-turearea,
≈
330 kmby90 km,acrosstheEastHimalayanSyntaxis. Our field measurements likely attest to a minimum≈
200 km-long primary 1950 surface rupture along the Mishmi and Abor hills fronts. The fact that the ratios between the co-seismic sur-face throws on the MFT and MT, and between the two moun-tainfrontalelevationsarecomparable(0.34and0.44,respectively; Figs. 3 andS9) is in keeping with such a complex, dual source andhighlightstheirstronglinkwithlong-termtopographicgrowth (e.g.,Kinget al.,1988
). We interpretour3D thrust-dip measure-ment(≈
13◦)nearWakrotoindicate thatthe1950shallow seis-mic slip on the Mishmi Thrust there reached at least 34±
2.5 m, given the 7.6±
0.2 m seismic uplift of the young Kamlang River terrace. In the same area, older, twice larger, cumulative terrace uplifts of 14 and 14.5 m may be taken to imply near-characteristicslip behavior during two similar great earthquakes. TheGPSshorteningrateandourcosmogenic10Beterraceagesare consistent witha first-order,nearly bi-millennial return time for suchmega-quakes.Thevery largeamountsofco-seismic slipand uplifton theMishmi Thrust(>30 and>7 m,respectively), where the thrust cuts the youngest deposits at the level of a perma-nent riverchannel, rule out the useof classic paleoseismological techniquesto investigate the long-termhistory and returntimes ofThrustMega-quakes (M≥
8),exceptatatypical, possibly unre-liable sites.Our findings around the Assam cusp provide further evidence that large Himalayan megathrust earthquakes are not “blind”, supporting mechanical models in which the bulk of the GPS-measured,elasticshorteningacrossthelarge mountainrange isultimatelyandprimarily released by slipon thefrontal, emer-gentthrusts.Acknowledgements
This research is partly supported by the National Research Foundation Singapore and the Singapore Ministry of Education under theResearch Centresof Excellenceinitiative in the frame-workofanMoUwiththeCSIR-NorthEastInstituteofScienceand Technology (Jorhat,India) co-signedbyP. Banerjee,P. Tapponnier, andS.Baruah,withcontributionfromIPGStrasbourg(France).We thankS. R. Ildefonso(Aero 360Solutions, Philippines),S. Sharma and S. Baruah (NEIST, India), and A. Ahsan (Geological Survey of Bangladesh) fortheir essential contribution inthe field (topo-graphicsurveys,rocksampling).WethankLaurentBollinger(CEA, France) for the tri-stereo Pleiades image correlation at Pasighat. Pleiades DEMs were processed using CNES data (2014), Astrium Services Distribution/Spot ImageS.A.,France (allrightsreserved, commercial use forbidden). We acknowledge help fromOta Kul-hánek andDon Helmberger to access the Uppsala andPasadena seismological archives.We thanktheASTER AMSnational facility (CEREGE, Aix-en-Provence, France, supported by INSU-CNRS, IRD, andCEA)andDr.StevenBinnie (AMSCologne,Germany)for cos-mogenicdating.PT alsothanks Prof.A.Kausar (Geologicalsurvey of Pakistan) for his contribution in the field near Muzaffarabad, Azad-Kashmir, in January 2006. P. Tapponnier is grateful to the Asian School oftheEnvironment (ASE)forallowing himto com-pletethisworkbyextendinghiscontractatNTUforoneyearand ahalf.WearegratefultotheEditorandthreeanonymous review-ersforfastidiousandconstructivecommentsthathelpedimprove theoriginalmanuscript.
Thiswork comprisesEarthObservatory ofSingapore contribu-tionno.269.
Appendix. Supplementarymaterial
Supplementary material related to this article can be found onlineat
https://doi
.org/10.1016/j.epsl.2019.115928.Thesedata in-clude the Google map of the mostimportant areas described in thisarticle.References
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