Seismic evidence for distinct anisotropy in the innermost inner core

Transcription

1 Seismic evidence for distinct nisotropy in the innermost inner core FENGLIN NIU * AND QI-FU CHEN 2 Deprtment of Erth Science, Rice University, 6 Min Street, Houston, Texs 775, USA 2 Institute of Erthquke Science, Chin Erthquke Administrtion, No. 6 Fuxing Avenue, Beijing, 6, Chin *e-mil: niu@rice.edu Pulished online: 28 Septemer 28; doi:.8/ngeo4 Knowledge of the seismic structure of the innermost inner core is importnt for understnding the formtion of the inner core. It hs een suggested recently tht this region my exhiit distinct seismic nisotropy 4,5. Here, we use the difference in trvel times etween seismic wves reflected t the underside of the inner core oundry nd those trversing the inner core to constrin the seismic nisotropy. We clculted trveltime residuls for wves generted y two deep erthqukes tht occurred in Indonesi nd Argentin respectively, recorded y seismic rrys in Venezuel nd Chin. The trvel-time residuls re systemticlly lrger, y out.8 s, for wves tht trvel roughly long the equtoril plne of the inner core (Indonesi Venezuel) thn for those trvelling in direction t n ngle of 28 to the equtoril plne (from Argentin to Chin). The difference in trvel times is rguly most sensitive to the structure ner the centre of the Erth, nd thus provides evidence for deep lyering within the inner core. Our results re consistent with models invoking seismic nisotropy of the innermost inner core with the slowest direction tilted t n ngle of 45 to the equtoril plne 4. Seismic studies of the inner core using ody wves, such s, re lwys hindered y contmintion from mntle heterogeneities. An effective wy to eliminte mntle contmintion is to use differentil trvel time or reltive mplitude etween nd reference phse tht trvels long very close ry pth to in the mntle. Wves reflected t or refrcted ove the inner-core oundry (ICB), PKiKP nd PKPc, hve frequently een used to study the top 4 km of the inner core 6 8. On the other hnd, no pproprite reference phse hs een identified for modelling the deeper prt of the inner core 9. As result, the seismic structure of the inner 8 km of inner core is less constrined s compred with the outer 4 km. We found tht PKIIKP is n idel reference phse to for deciphering seismic structure t the centre of the Erth. PKIIKP trverses the inner core from either the minor-rc or the mjor-rc direction. Following the nming convention of surfce wves, herefter we refer to them s nd for the minor- nd mjor-rc phse, respectively. Ry pths of PKIIKPs re very close to those of in the mntle (Fig. ). The mximum seprtion etween the two phses is out 5 km in the upper mntle. In ddition, oth PKIIKPs nd propgte lmost verticlly cross the D lyer, nd susequently experience the minimum influence from seismic structure within the D lyer (Fig. ). In generl, it is difficult to oserve PKIIKP ecuse of the wek reflection t the ICB (ref. ). For deep erthqukes, however, Epicentrl distnce ( ) ICB CMB PKP Time reltive to (s) Figure Ry pths nd synthetic seismogrms of the core phses., Ry pths of the core phses: (lue), PKP (lck), (green) nd (red) t n epicentrl distnce of 78. ICB: inner-core oundry; CMB: core-mntle oundry., Synthetic seismogrms clculted y the direct-solution method. PREM nd the Hrvrd CMT solution were used in the clcultion. A single zimuth ws ssumed for ll of the sttions. Wveforms fter s indicted y the verticl dshed line re mplified y fctor of. The PREM rrivl time is indicted y green nd red squres for nd, respectively. nture geoscience ADVANCE ONLINE PUBLICATION

2 Tle Source prmeters nd the estimted differentil trvel times. Origin time Epicentre Depth Mw Arry Distnce Dte Time Lt. ( ) Lon. ( ) (km) (deg) C(s) O(s) O C(s) C(s) O(s) O C(s) 6/6/24 9:8: BOLIVAR //25 2:2: CDSN Distnce is mesured from the epicentre to the centre of the rry. C(s): clculted differentil trvel time in seconds sed on PREM; O(s): mesured differentil trvel time in seconds; O-C(s): differentil trvel-time residul % % c W 65 W 6 W Figure 2 Mp of the erthqukes nd the rrys., Mp showing inner-core ry segment of /2. S-wve velocity vritions of the D lyer from Grnd re shown in colour. Crosses nd open circles indicte the entrnce nd exit points t the CMB of /2 nd. The inset shows the exit points of (lue) nd PKIIKP (green) of the Indonesi Venezuel ry pths. The strs indicte the epicentres of the two erthqukes.,c, Mp of the CDSN () nd BOLIVAR (c) sttions. The insets show their geogrphic loctions. Strs indicte the ntipodes of the erthqukes. Red tringles show sttions with visile. Sttions mrked y open tringles were not used in stcking owing to low SNR. PKIIKP wves rrive 8 55 s fter the first rrivl which lies in the quiet time window tht hs no primry rrivls (Fig. ). There re only three plces on continents tht re locted t the opposite sides of deep erthqukes: northern Afric, northern South Americ nd estern centrl Chin. Seismic rry dt re ville t the lst two loctions. We found cler PKIIKP rrivls from two deep-focus erthqukes tht occurred in Indonesi nd Argentin, respectively (Tle, Fig. 2). The Indonesi event ws recorded y 6 sttions of PASSCAL deployment in Venezuel known s the BOLIVAR rry (Fig. 2c), wheres the Argentin erthquke ws recorded y 4 short-period nd rodnd mixed sttions tht elong to the Chin Digitl Seismic Network (CDSN) (Fig. 2). The Indonesi-BOLIVAR nd the Argentin-CDSN ry pths inside the inner core re 8 nd 28 from the equtoril plne. Thus, comprison of the two directions cn offer decisive dignosis of deep inner-core nisotropy. Owing to fvourle source mechnism to ntipodl sttions, the BOLIVAR recordings of the Indonesi event show clerly identifile tht hs reversed polrity to (red rrows in Fig. ). is lso visile with polrity similr to (green rrow in Fig. ) t the frthest sttion. These oservtions re consistent with Preliminry Reference Erth Model 2 (PREM) synthetic seismogrms (Fig. ). The polrity reflects comintion of the effects of negtive reflection t the ICB (ref. ) nd π/2 phse shift produced y the internl custic surfce for underside reflection wves 4. Another π/2 phse 2 nture geoscience ADVANCE ONLINE PUBLICATION

3 LETTERS 6/6/ km 5.9 Mw 2// km 6.9 Mw I Epicentrl distnce Epicentrl distnce Time reltive to (s) Time reltive to (s) Figure Exmples of seismogrms recorded y the two rrys., BOLIVAR seismogrms, ligned ccording to the hnd-picked rrivl times., CDSN seismogrms, ligned sed on the PREM-predicted rrivl times. Oserved nd rrivls re indicted y green nd red rrows, respectively. PREM predictions of nd re shown y stright lines. All of the seismogrms re filtered with the WWSSN short-period instrument response. The dt re further filtered with 2 s low-pss filter for stcking. The rw short-period BOLIVAR dt re shown only for presenttion purposes. The CDSN dt shown here re the low-pss filtered ones. shift is further dded to the mjor-rc phse for crossing the ntipode5,6. The Argentin erthquke, on the other hnd, exhiits no ovious PKIIKP rrivls in the CDSN records except for the three sttions ner its ntipode where the convergence of the PKIIKP wves from ll ck zimuths leds to significnt increse of their mplitude (green rrow in Fig. ). To detect the PKIIKP phses nd to etter determine the PKIIKP time, we pplied second-order phse-weight slnt stcking technique7 to the dt (see Methods section). Both PKIIKP phses re clerly identifile in the vespgrms of the two events with positive nd negtive slowness reltive to. Except for of the Indonesi event, the oserved slowness is in good greement with the PREM prediction (Fig. 4,). The mesured differentil trvel times of nd nd their residuls with respect to PREM re listed in Tle. For ech event, estimtes of nd residul time gree very well with ech other. The Indonesi-BOLIVAR pth exhiits.8 s positive trvel-time residul, wheres the Argentin-CDSN pth shows no nomly with respect to PREM (Tle, Fig. 4). Bem-forming nlysis (see Methods section) indictes tht the two PKIIKP phses were received y the two rrys in the gret-circle ry-pth direction (Fig. 4c f). It is unlikely tht the differentil trvel-time residuls cn e explined with heterogeneities ner the core mntle oundry (CMB). In Fig. 2, inset we plot the exit points of nd PKIIKP t the CMB of the Indonesi erthquke using different colours, with lue () nd green (PKIIKP) circles centred t the exit points. The size of the circles is similr to the Fresnel zone of nd PKIIKP t the CMB, which is pproximtely 2 km for.5 Hz verticlly propgting P wve. The seprtion etween the PKIIKP nd is round 4 km t the CMB, resulting in n overlp of the Fresnel zone of the two phses. Lrge-scle heterogeneities ( few hundreds of kilometres) ner the CMB would thus ffect oth phses similrly nd re expected to hve little effect on the differentil trvel time. Smll-scle ( few tens of kilometres) velocity perturtions could ffect the differentil trvel times differently, resulting in positive nd negtive mixed residuls mong sttions within the rrys, which were not oserved. There is little difference in times picked from individul seismogrms (Fig. ). Mixed residuls could lso cuse destructive stcking, resulting in wek PKIIKPs tht re contrdictory to the lrge mplitudes oserved in the vespgrms. nd PKIIKP wves propgte nerly verticlly cross the D lyer; the totl trvel times of the two phses within the lowermost 2 km of the mntle re 28 nd s, respectively. If the oserved.8 s differentil trvel-time residul is ccumulted solely in the D lyer long either the or PKIIKP ry pth, then D model with 6% P-wve velocity nomly cross the entire 2 km depth is required. Otherwise, models tht hve % nomly cross the entire D lyer ner oth the entrnce nd exit points of nd nother % nomly with n opposite sign ner those of the PKIIKP ry pths re needed. The entrnce (or exit) regions of the two wves ctully overlp with ech other when their Fresnel zone size is tken into ccount. Thus, models requiring mixed nomlies cn in principle e ruled out. Current three-dimensionl P-wve velocity models8 predict tht the mntle contriution to the differentil trvel time is less thn.2 s, much smller thn the oserved.8 s. Ultrlow velocity zones re oserved to hve P-wve velocity reduction of % with mximum thickness of 5 km (ref. 9). The mximum trvel-time nomly they cn introduce is.4 s, not sufficient to explin the oserved nomly. The PKIIKP differentil trvel time is lso insensitive to Erth s ellipticity2 (see Methods section) nd chnges in inner-core nture geoscience ADVANCE ONLINE PUBLICATION

4 Slowness reltive to (s/ ) /6/ km 5.9 Mw Δ = 76.5 c 6/6/ km 5.9 Mw Δ = ?? Time reltive to (s) d Slowness reltive to (s/ ) Azimuth reltive ry pth ( ) Normlized power (db) 5 Normlized power (%) Slowness reltive to (s/ ) // km 6.9 Mw Δ = // km 6.9 Mw Δ = e Time reltive to (s)? f Slowness reltive to (s/ ) Azimuth reltive ry pth ( ) Figure 4 Results of stcking nd em-forming nlyses.,, Vespgrms of the BOLIVAR () nd the CDSN () dt. Hotter colour clusters represent greter energy. nd /2 re identified y rrows with crosses denoting the theoreticl rrivl time nd slowness clculted from the PREM model. Time nd slowness re reltive to. Note the erlier rrivl of in. The oserved slowness of in is slightly lower thn the PREM prediction, which could e cused y ICB topogrphy, interference with other phses nd the constnt slowness ssumption in the slnt stcking. c f, Bem power showing the rrivl slowness nd ck zimuth of the two PKIIKP phses. rdius (Fig. ). Our ry trcing indicted tht km increse in the inner-core rdius results in only.4 s chnge in the differentil trvel time, nd consequently the oserved lrge residul time is unlikely to e cused y vritions in inner-core rdius 2. It is lso difficult to explin the oserved residul times with possile error in source loction. A km error in the hypocentre depth results in chnge of differentil trvel time y only. s, wheres km error in the epicentrl loction could led to chnge of differentil trvel time y s much s.7 s. Becuse of the wide distriution in sttion zimuth (Fig. 2), lrge error in the epicentrl loction will result in positive nd negtive mixed trvel-time residuls mong the sttions within the rrys, which is not oserved in the dt. Thus, the oserved differentil trvel-time residuls cn e ttriuted only to the heterogeneity nd/or nisotropy within the inner core. Here, we further rgue tht the origin of the differentil trvel-time residuls lies in the centre of the Erth. PKIIKPs trvel through only the top 8 km of the inner core nd hlf of their ry pths in the inner core flls roughly in the estern hemisphere nd the other hlf in the western hemisphere. The degree-one heterogeneity in the uppermost inner core thus hs little effect on the trvel time of PKIIKPs, which is estimted to e <. s sed on the E/W models 22. The inner-core ry segments of the PKIIKP phses in our dt set hve glol coverge of lmost 7% (Fig. 2), which mkes it hrd to rgue for ny smll-scle velocity nomlies ner the ICB (ref. 2) s plusile origins of the oserved residuls. In fct, there is lso significnt mount of crossing nd overlp of PKIIKP rys etween the two events; ny nomlies long the PKIIKP rys will show up in the dt of oth events. Thus, the oserved trvel-time residuls re est explined y erlier rrivls. Both the Indonesi Venezuel nd Argentin Chin pths elong to the so-clled equtoril pths in which the ry ngles re less thn from the equtoril plne, mking it hrd to explin the difference y existing nisotropic models with the fst direction prllel to the rottion xis 24,25. The uppermost 4 km of the inner core, which is well constrined from differentil trvel-time studies, is lso expected to produce no significnt trvel-time nomlies to ccording to current inner-core models 26. Thus, the oserved differentil trvel-time residuls must originte from the deeper portion of the inner core. Within this deeper region, rys of the two pths re seprted y <28 km, smller thn the Fresnel zone of the.5 Hz wve. Consequently, ny loclized heterogeneity centred long one ry pth will ffect the other pth s well. Thus, the 4 nture geoscience ADVANCE ONLINE PUBLICATION

5 oserved.8 s trvel-time residul long the Indonesi Venezuel ry pth cn e explined only y seismic nisotropy with the slowest propgtion direction eing tilted wy from the equtor. Our dt hve no resolution on the depth distriution of the distinct nisotropy nd nture of the trnsition etween the two nisotropic regimes. Our oservtion here is consistent with the innermost inner core (IMIC) model 4 derived from ctlogue trvel-time dt. The IMIC hs rdius of km with the slowest direction 45 from the equtoril plne. Trnsition etween the IMIC nd the rest of the inner core is oserved to e grdul 27. Thus, the tilted nisotropy could e distriuted smoothly over wide rnge of depth in the innermost 8 km of the inner core. Both hexgonl close-pcked nd ody-centred-cuic crystl ggregtes re found to e le to produce nisotropy tht mtches seismic oservtions 28,29. Thus, the two nisotropic regimes my e cused y different crystl structure or different proportions of the hexgonl close-pcked nd ody-centred cuic crystls t different depths. It could lso reflect chnge of fric lignment with depth, suggesting tht different episodes my hve existed in the growth history of the inner core. METHODS PHASE-WEIGHT SLANT STACKING Let s n,m represent the mplitude t the mth sttion t the nth time for the cse with M sttions. For given slowness of p, the phse-weighted stck 7 n (p) is given y n (p) = M M N s n+τm,m exp{iϕ n+τm,m }, M M m= where ϕ n,m is the instntneous phse clculted from s n,m nd its Hilert trnsform H[s n,m ]: m= ϕ n,m = tn{h[s n,m ]/s n,m }. τ m is the time lg of the mth sttion nd is relted to slowness p y τ m = D m p, where D m is the epicentrl distnce for the mth sttion minus the epicentrl distnce of the centre of the rry. N is the power index of the phse-weight stcking nd we used N = 2. We pre-processed the rodnd dt first with deconvolution of instrument response, nd then convolution with the WorldWide Stndrdized Seismogrph Network (WWSSN) short-period instrument response. The WWSSN short-period dt seemed to possess the est signl-to-noise rtio (SNR) for the nd PKIIKP phses. We further filtered the dt with 2 s low-pss filter. A SNR > 2 ws used to select the dt for the finl processing. We then mnully picked the first pek of the wves nd ligned them t the picked times. The slowness ssumed for ech stck is vried with respect to tht of the rrivls (defined s zero) in increments of. s deg within the rnge of ± s deg. The resulting 6 stcked wveforms re susequently converted to mplitude envelopes using the Hilert trnsform. The mximum mplitude is chosen from ll 6 stcked trces nd is used to normlize the trces in units of deciels. BEAM-FORMING ANALYSIS The slnt stcking nlysis descried ove ssumes tht the lter phses rrive from the sme ck zimuth s tht of. To check this ssumption, we used nother rry nlysis technique, the so-clled em-forming method. In em-forming nlysis, ll of the seismogrms re linerly stcked fter time correction clculted from the ssumed slowness nd ck zimuth. The est slowness nd ck zimuth were determined when the summed mplitude reches mximum. More specificlly, we first ligned the seismogrms so tht rrivls coincide with the PREM rrivl time. We vried the slowness from to s deg for nd to s deg for in increments of. s deg. The zimuth ws devited from the gret circle within the rnge of ±9 t increments of. A 2 s time window centred on the pek PKIIKP rrivl times ws used to clculte the em power. ELLIPTICITY CORRECTION We used the method of Kennett & Gudmundsson 2 to clculte the ellipticity corrections. PKIIKP ws not included in the phse list of Kennett & Gudmundsson 2, nd ws computed y comintion of two legs with the sutrction of PKiKP segment: T PKIIKP (z s, ) = T (z s, ) + T (, ) T PKiKP (, c ), where z s is the source depth nd, nd c re the rc lengths of the two legs nd the PKiKP leg, respectively. They were clculted y mtching the ry prmeter of the PKIIKP phse. Received Novemer 27; ccepted 27 August 28; pulished 28 Septemer 28. References. Jcos, J. A. The Erth s Inner Core. Nture 72, (95). 2. Buffett, B. A. in Erth s Deep Interior: Minerl Physics nd Seismic Tomogrphy From the Atomic to Glol Scle Vol. 7 (eds Krto, S. et l.) 7 62 (Geophys. Monogr. Ser., AGU, Wshington, 2).. Yoshid, S., Sumit, I. & Kumzw, M. Growth model of the inner core coupled with the outer core dynmics nd the resulting elstic nisotropy. J. Geophys. Res., (996). 4. Ishii, M. & Dziewonski, A. M. The innermost inner core of the erth: Evidence for chnge in nisotropic ehviour t the rdius of out km. Proc. Ntl Acd. Sci. 99, (22). 5. Beghein, C. & Trmpert, J. Roust norml mode constrints on inner-core nisotropy from model spce serch. Science 299, (2). 6. Niu, F. & Wen, L. Hemisphericl vritions in seismic velocity t the top of the Erth s inner-core. Nture 4, 8 84 (2). 7. Creger, K. C. Anisotropy of the inner core from differentil trvel times of the phses PKP nd. Nture 56, 9 4 (992). 8. Tnk, S. & Hmguchi, H. Degree one heterogeneity nd hemisphericl vrition of nisotropy in the inner core from PKP(BC) PKP(DF) times. J. Geophys. Res. 2, (997). 9. Breger, L., Tklcic, H. & Romnowicz, B. The effects of D on PKP (AB-DF) trvel time residuls nd implictions for inner core structure. Erth Plnet. Sci. Lett. 75, 4 (2).. Aki, K. & Richrds, P. G. Quntittive Seismology (W. H. Freemn, New York, 98).. Dziewonski, A. M., Chou, T.-A. & Woodhouse, J. H. Determintion of erthquke source prmeters from wveform dt for studies of glol nd regionl seismicity. J. Geophys. Res. 86, (98). 2. Dziewonski, A. M. & Anderson, D. L. Preliminry reference Erth model. Phys. Erth Plnet. Inter. 25, (98).. Tkeuchi, N., Geller, R. J. & Cummins, P. R. Highly ccurte P-SV complete synthetic seismogrms using modified DSM opertors. Geophys. Res. Lett. 2, (99). 4. Choy, G. L. & Richrds, P. G. Pulse distortion nd Hilert trnsformtion in multiply reflected nd refrcted ody wves. Bull. Seismol. Soc. Am. 65, 55 7 (975). 5. Brune, J. N., Nfe, J. E. & Alsop, L. E. The polr phse shift of surfce wves on sphere. Bull. Seismol. Soc. Am 5, (96). 6. Ril, J. & Cormier, V. Seismic wves t the epicentre s ntipode. J. Geophys. Res. 85, (98). 7. Schimmel, M. & Pulssen, H. Noise reduction nd detection of wek, coherent signls through phse-weighted stcks. Geophys. J. Int., (997). 8. Fuko, Y., Widiyntoro, S. & Oyshi, M. Stgnnt sls in the upper nd lower mntle trnsition zone. Geophys. Rev. 9, 29 2 (2). 9. Grnero, E. J. & Thorne, M. in Erth s ULVZ: Ultr-Low Velocity Zone, in Encyclopedi of Geomgnetism nd Pleomgnetism (eds Guins, D. & Herrero-Berver, E.) (Springer, Netherlnds, 28). 2. Kennett, B. L. N. & Gudmundsson, O. Ellipticity corrections for seismic phses. Geophys. J. Int. 27, 4 48 (996). 2. Souriu, A. & Souriu, M. Ellipticity nd density t the inner core oundry from sucriticl PKiKP nd PcP dt. Geophys. J. Int. 98, 9 54 (989). 22. Wen, L. & Niu, F. Seismic velocity nd ttenution structures in the top of the Erth s inner core. J. Geophys. Res. 7, doi:.29/2jb7 (22). 2. Vidle, J. E. & Erle, P. S. Fine-scle heterogeneity in the Erth s inner core. Nture 44, (2). 24. Morelli, A., Dziewonski, A. M. & Woodhouse, J. H. Anisotropy of the inner core inferred from trvel times. Geophys. Res. Lett., (986). 25. Sherer, P. M., Toy, K. M. & Orcutt, J. A. Axi-symmetric Erth models nd inner-core nisotropy. Nture, (988). 26. Creger, K. C. in Erth s Deep Interior: Minerl Physics nd Seismic Tomogrphy From the Atomic to Glol Scle Vol. 7 (eds Krto, S. et l.) 89 4 (Geophys. Monogr. Ser., AGU, Wshington, 2). 27. Cormier, V. F. & Stroujkov, A. Wveform serch for the innermost inner core. Erth Plnet. Sci. Lett. 26, 96 5 (25). 28. Steinle-Neumnn, G., Stixrude, L., Cohen, R. E. & Gulseren, O. Elsticity of iron t the temperture of the Erth s inner core. Nture 4, 57 6 (2). 29. Belonoshko, A., Skorodumov, V., Rosengren, A. & Johnsson, B. Elstic nisotropy of Erth s inner core. Science 9, (28).. Grnd, S. P. Mntle sher-wve tomogrphy nd the fte of suducted sls. Phil. Trns. R. Soc. A 6, (22). Acknowledgements We thnk the BOLIVAR tem, FUNVISIS (Venezueln Foundtion for Seismologicl Reserch) nd CEA (Chin Erthquke Administrtion) for providing the dt, N. Tkeuchi for providing the DSM code nd H. Kwktsu nd S. Tnk for helpful discussions. This work is supported y Rice University nd the NSF. Author informtion Reprints nd permission informtion is ville online t Correspondence nd requests for mterils should e ddressed to F.N. nture geoscience ADVANCE ONLINE PUBLICATION 5