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Title Two-layered structure in the lowermost mantle beneath the southwestern Pacific( Dissertation_全文 ) Author(s) Yamada, Akira Citation Kyoto University (京都大学) Issue Date 1997-03-24 URL https://doi.org/10.11501/3123273 Right Type Thesis or Dissertation Textversion author Kyoto University

Title Two-layered structure in the lowermost mantle …repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/...Two-layered structure in the lowern1ost n1antle beneath t he southwestern

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  • Title Two-layered structure in the lowermost mantle beneath thesouthwestern Pacific( Dissertation_全文 )

    Author(s) Yamada, Akira

    Citation Kyoto University (京都大学)

    Issue Date 1997-03-24

    URL https://doi.org/10.11501/3123273

    Right

    Type Thesis or Dissertation

    Textversion author

    Kyoto University

  • 学位申請論文

    印何回山

  • Two-layered structure in t h e lowern1ost n1ant le b eneath t h e southwester n P acific

    r\kira Yamada

    Department of Geophysics. I\yoto Uni\·ersity. T-.:yoto, .Japan

    Abstract . The heterogeneities in the D" layer beneath the southwestern Pacific are investigated by using the

    combination of earthquakes around the Fiji Islands and receivers in the .Japanese Islands. The data of short-

    period P wave are processed by the double-array stacking, a method to stack simultaneously the records from

    different events. The estimated depth of the D'' reflector varies from 270 km to 170 km in the southwestern

    Pacific. The numerical experiments using various P-velocity models are performed and the rapid velocity

    increase well explains the waveform of PdP. This increase may be at the top surface of the high-velocity layer

    (or high-velocity lamella).

    The data of broad-band transverse component are also examined. The D'' reflection (SdS) is clearly seen

    in some traces. The polarities of SdS are almost the same as those of the direct S wa\·es, which supports the

    D" t·cflcctor \\'ith the rapid S-wa.\'e velocity increase with depth, as well as for P wave. \\'hile the residuals of

    difl'erentia l travel time of ScS-S show fairly large variations, they show systematic large delays of ScS travel

    times. These delays may be caused by the existence of an S-wa\·e low velocity layer right abo\·e the (;f.!B.

    A probable cause of both P- and S-waves high-velocit.y anomaly is considered to be subducted slab material.

    The low S-velocity layer underlying the high-velocity anomaly might form a shape like a dome, which could be

    a seed of mantle plume.

    I nt r o duct io n

    The lowennost manti

  • rogo•iwitiP~ in D" hy 1 lw

    chemical reaction could he rdated with the tPmperatun• varia! ion in D".

    The ,·ariations in the wa\'elcngt h of the heterogeneit it::> have a range from a few thousands kilometers. inferred

    from the global tomographic technique [e.g., Sengupta and Toksoz, 1976: Inoue et al., 1990: Suet al., 199-1]. to

    a few hundreds kilometers [Haddon and Cleary, 197•1; \\'eher and Da.\'iS, 1990; \'idalc and Benz , 1993; 1\:riiger

    et al., 1995; Tono and Yomogida, 1996]. The great s ize of ·•avalanches" (subclucted slab metlerial) [e.g .. Tackley

    et al., 1993] and the rather small size of '' inclusions" by th

  • '''rlrral-conJporlt'lll rt'rord:-; fro111 1 hn••• arrays \\·ir h dianJt'l•·r sil.t's of a ff"w hundn·d kilolllo'tPrs. (:.!) short-pnic>d

    \'t>rliral-conrpont'IH n•cord..; from an array with of It'll kilon~t'l••rs and (:3) broad-hand horiwnral-comporwur

    H'Cords by STS I >'eismonwter:>. Tht' station location,; ar•' indicatt>cl in Figun• :l.

    Three large arrays in Figure ;j are to observe the micro earthquakt>s under the national project of earthquakt>

    prediction. which are opt'ratt•d hy 1\yoto University {Disaster Prevention Research In,;titute). the (.;niver,;ity of

    Tokyo (Earthquake H.e,;earch Institute) and Hokkaido University. respectively (hereaft.cr L,\1, Lr\2 and L,\:3).

    ThC' smal l array located in central .Japan is operated hy the .Japan ~lt'teorological Agency {Matsushiro Sei,-rnic

    Array System, \ISAS her••after). The data analysis by u::oing L:\1, 2 and :) is recently pre:>cnll'U Ill Yamada and

    :\akanishi (1996a and b] and by u:.ing \ISAS in Yamada ct al. (1996]. The results of tlwsc data analyses arc

    used in this paper to obtain dt•arcr image of the D .. layer. In addition to the above four arrnys for the P-wnw

    analy:.es, the broad-band records (1313 in Figure :3) a rc analpwd to st.ud_v S-wave structure of the IY layer.

    \Vt• usc earthquakes with det'p focus(> :300 km) and l aq~t' magnitude (111b > 5.5) which occurred around the

    Fiji Islands. The details about tht' event selection are found in Ynmada and Xakani~hi [I99Ga and b]. The event

    parameters used in this ~tudy are summarized in Tahl" I. Table 2 shows the range of the epicentral distance

    and the number of records used in this study.

    Figure ·1 shows the close-up of tlw bounce region {bcrwat h southwestern Pacific). \\'e project the distribution

    of 1 hcse bou nee points and the Fn•snel I' ll i pses on the ••a rt. h '$ su rfacc. The lengt h sea lc at l he depth of t.lw C.\ 113

    is about half compared with that on the earth's surfacP.

    S hOI·t-p

  • \'

    /).-1(/.rl) = .~· t ct·;.r;(t + ('!;''- '!'() + (~7i(d)- l"TJ(d))). •=I

    wlu•r•· I indicates the Ira1·.-1 tinH'. rl th·· distance of the n•fkctor from tlu> ('~lB. i iud••x lltlllllwr forth~ t'l't•nt-

    rPceiiW pairs (i = 1, .\'. th~ nu111her of.\' for each region i" li:-.ted in Tablt> 2). i =ltht• pi1·ot pair. !Ci amplitudt•

    normalization factor . .t•i{/) th~ ohs,•rvt>d seismogram. 1t onset time of P wave of the i-th pair, and l'J;(d)

    theoretical differential traveltime between P and PdP of the i-th pair for t.he reflector at d km from the C~ t n.

    In this study, the range of the d-\'aluc for the detection oft lw D" reflection is from 0 km to 400 km with an

    increnlt'nl of 10 km . The amplitudes of all traces arc normnlized by the maximum peak-to-peak amplitude.

    1;" is read from the original ::.cismogram. The correction for the polarity re\'ersal due to source mechanism is

    included in u,. ~lore detd here. ~loreo\'er radial-polarizl•d SKS is expected to arri\'e in the time window bctwePn the direct S wwC'

    and the C\IB reflection (ScS) and may contaminal

  • Shod-pt'r iod P wavt!. Tht> dt'lnilt>d Pxplanntion" for tlw r••=-u lt:; ;ue• descrihPd in our ll.'rt'lll papo•rs [Yamada

    ami :\akanishi, 1996a and h for thro•c largf' arrays ( L \ t. :.!

  • r

    :->hown in Ft:;ur•· 7 •n·· th t• s~nth••tic:-> calcuhtt>d J.y tlw rdl••rtivity nwtlwd anin·r ;;•'Oillt'tri.·s or tilt' array L.\:1 Th·· C(Jil\"

  • 1\lodl•l,.. of vc•locity cltau gl'

    Tho• P w locity modo·(~ ltl r:1lrnlat•• tit~· ,.:yn thdic,; are gronpo•d into fl\·e typo•,.:_ Tlwy an• ( I) a lanwlla with

    hig,h P wlocity. (:n a lamella wtth low P \·elocity. (:q a transitional increa~P in P wlocity. (I) a tran,-itional

    decrt>

  • (pllcctor

    suggest that the resolution of the determination of the D" reAl'ctor depth is a ft•w lens kilometer,;.

    Lam ella wit h hig h velocity. Among three results from thret• high-velocity lamell a models whose differences

    arc in t lw thickrwss of the lamella, Figure 9b shows the result frorn t.he lamella with the t.hickne:::s of :30 km.

    The wawform of PdP (th ird) is distorted wi t.h respect lo t.hat of thr direct P wave (top), due to the interference

    of rrllection,.; from the top surface (normal polarity) and the bottom surface (n.•vNsl'd polarity) of the lamella.

    Tht• second upward :;wing in PdP waveform corresponds to bottom-side reflt>ction, which is not seen in PdP

    from the P\ \' D I~ modt>l (Figure 9a). Comparing three n•sults oht airwd from thn.•e thick nes..;es of the lamella. it

    is found that the increase of the thickness increases the amplit udc of PdP. This is because a long wavelength

    wan• is not scnsrti\C to thin lamella. The amplitudP. of PdP for tht• :30 km-thick model is comparable to that

    for tht> P\\'DK model. The thicknt>:;s of 30 km is an approximat" \\·;wekngth of '2 ,.;t>c P \\·a,·e in the lowermo"t

    manti••.

    Lamclln with low velocity. TIH' interference of two n•flo•ctions from both surfaces of lamella is also found

    for t.his model. Figure 9c shows the result for Lhe low-veloc ity lanwlla wit h t.hP thickness of :30 km. In contrast

    wi th l lw previous high veloc i Ly lanwlla. the \\'a\'d rai n of Pd P consists of t lu' fi rst-nrri ,·al rPversed reflect ion (top

    surface) and so•condnr) arrival nor111al reflection (bottom surface). Tl 11~ st•pamt ion bt'I\\'Pen two reflect ions is

    olH'ious for tho• lanwlla \\ith thirkno•:n in tlw contour nwp of

    l·'ignn· !Jr.

    Tlw rl'llt-rr ion frt>lll Ill>' lop ,.;nrf;Ko' rnay 1 .. ~ qualit :11 iVt·ly "'nail. ,..i11ro• I h·· :-••i:-rni•· PTit'r~~ I har arriVt•,. at I It•:

    hountl:~r~ wath llf'.!!,ilti\·o· P-\'o•locity ronrra.;t tran:

  • incn•a,;t• in p VPiocir: llllh IIJt• rhickrw,; .... or :3o kill. In lhi,; '""''·tilt' CIIIIJ>Iitudt• and lilt' 11comes widt•. \\'t•

    also find a cont.inuous trrnd of delay of the tra\'1'1 Lime of PdP, correlating with thr increase of the thickne,;s.

    However the time delay of PdP accompanied with the changt' of the thickness frorn 10 km to 50 km is "rnall.

    about 1 sec.

    Transitional decrease in P velocity. The result for the model of tlw transit tOnal decrea..o;r in P velocity. with

    t ht• thick ness of 10 k m. is shown in Figure 9e. The signal of PJ P is characterized by (I) rat her small ampli t nd··

    due to the small reflection coeffirienL, (2) reversed polarity with respect to that of the direct P wave and (:3)

    rapid diminishing of amplitude with the increase of tlw thickness of the transition zone. The energy of PdP is

    beyond the level of a cut-off for plotting the contour map for the model with thf' thickne,;s greater than :30 km.

    Stack of lamellae . The model of the stack of lamellae ha:; a shape like 'teeth'. The contour maps for tlw~··

    show complicated distribution of PdP energy. Tlw rt•flt•ctions at both surfaces of f'ach high-velocity lamella and

    re\'erbcrations inside each lam(•lla and between lamt.>llae l'or111 continuous arri\'als of sei:;mic enrrgy (PdP-coda).

    The difference~ in 1 he w;wel'orm of PdP wavetmin, cau~Pd by the change of iniN\'als from 10 km to 50 km,

    are as follows. (1) 10 km interval (Figure 9f). Tlw first arrival is isolated and its ~hapl' is almost the ,;aJIIt'

    a,; that. for tlw P\\'1)1\ n1od .. J. 'J'Iw PdP-coda rs nol lll!!, in tint•' aud not IM

  • t Ia i rd pant•l:::. \\"hi le t !at• wawform from ••a cit ·tooth· i~ almo:-t idlarizc•d \\':1\'

  • I r I ilL· ro•llt rl or Ita" a fi IIi I,. ,..i /.t' iII ill•r i%1011 I al d i n>c I i· >11. Til.·~ ,;hr '"" Ill tlwi r t'X I ..... i 111•'111,.. I hilt tin• r..tlo•rt i· •II \\'it \(·I· I

    with ro'\•'r:-;o•tl pohrit: "Jll"'ar:- followi11g tltt• Jtortnal polarizt•d rl'll,•nioll du sitlocity at. the discontinuity is also suggested by the normal polarities of the SdS

    waveforms. The second swing, which is an e\'idencr for the lamella but against the one-sided discontinuity

    (e.g., P\\'01(-like model) , is found in scvl'ral traces, e.g., the observations on the left sidt• in Figure 7. We haw

    confirnwd by using the S synthetics (not presented here) t!Htt the lamella produces the second swing for long-

    period S wa,·e, but that the am pi it ude of second swing is ra 1 h.

    E:i titer the high-velocity I amelia or the d isconl in u i ty with fi 11 i le-size is possible and the combination of the

    two is also

  • for tlw Pxistt•nce of the low vdocity layl'r in the lowt•rrnost IIHIIltk henf'ilth the ,;ontllll'•'siPI"Il l'ncific [.\Iori a11d

    lldmh .. rgPr, 199:) and C:anwro and lfdmlwrger. l9%J. Tlwy ill"o suggest thnt it migiH ht• not a global hut a

    regional feature. The inclusion:; into the lowermost mantle by the chemical reaction may be of great contf'nt,. of

    iron compared with that of mantle [.Jeanloz. 1990 and It o ct al., I995j. The great change of the geotherm at tlw

    C'i\If3 may lead to the exist.enct• of tht:>rmal boundary layt>r with a finite thickness [LopN, l991 and Gaherty and

    lay, 1992J. It could be possibll' that the accumulation of the products by the chemical reaction, whose sci:>mic

    velocity may be lower than that of ambient mantle due to the iron-rich products. acts a role as the thermal

    boundary layer and can he setsmologically found as a low veloctty layer.

    The two-layered structure inferred from the above description is schematically illustratt•d tn Figure J I. \\'l•

    indicate in this sketch the values of the P-velocity increase within the high-velocity layer (or lamella), which arl'

    inferred from the preliminary estimates of the impedance contrast obtained in Yamada nnd Nakanishi [19D6bj.

    \\'t' assume when inferring the P-velocity increase that both the density and the P vdocity increase at the sanw

    rate. The stack of the low and the> lugh velocity structure, postulated here, changes its total thickness from 210

    km (LA I) to l 10 km ( Lr\:3). Assuming that the thickness of the high velocity layer is unchanged throughout

    the region sampled here, the low \'t•locity layer thickens in the region LAl. Lt. is one plausible, not sulllcicntly

    confident., speculation that. this thickened low vdocity layer in LA! is the plume ht•ad, which is now proc,'s:-:ing

    the upward motion as a "manllf1 plume" [.\Lori and lldmbcrgt'r, 1995). Unfortunately we can only search onl'

    direction among the rt•!!,ion :-:urrountlin~ LA I at JHt>.·-.•nl. The furtlwr examination of I)" structure in the n'giOn

    lwighhored on L:\ I \\'til JHO\ 1tl•· "" wtt h tltt: lllOrt' dt'

  • l'n•:-;tJt•l dlip-:,•. Such au auottwlou:< :-1 rttc!urctor hettt~nth L:\ l.!. and :3 may be up to tPn pt'I'Ct'llh. if \\'1' a:flector. Tht• d••tt•ction of the clear PdP :;ignab (t'.\CPpt for LA:?) indicat••;-; that the wa\dength

    of heterogeneities may be a fpw hundred kilometers. This difl'crences of scale lt>ngth of heterogeneities or tht'

    impedance contrast betwePn P and S waves might sugge,.;t the anomalous D" layer, where the rcspon;;e to tht>

    incidence is different betwet•n p and S waves.

    Conclusion

    The analyses of travel tim•' and w;weform are 11pplied to th•• data of both short-pNiod P wa\·e and long-p••riod

    S wave. \\'e can suggest the following image for the ()" layer hcneat h the southwestern Pacific sampled here.

    Comparing the observation;; with the numerical expcrinwnts, the high-\·elocity lamella is the preferred modt•l

    rather than the first-order discontinuity to exp lai n the wavcforn1 of PdP. However the discontinuity model cou ld

    st ill b!' a possible candidate if wP take the finite size of the 1) .. n•flector into account. The low-velocity layer.

    coated with this high-velocity layer, is inferred from the large delay of travel times of ScS. The total thickneN'

    of the:se two layers changes from :270 km to 110 km. Thi,.; he1crogcneous structure might be interpreted

  • of t.lw nwntle lwtH' . Let .. ..!.J. ~J77-9~0. 1!:!%.

    Haddon, 1\. A. \\· .. and .J. H. Cl••ary, Evidence for ~cattcring of seisntic PJ\P 11':1\'c>s Mar the corc-mantlt'

    boundary. Phys. Ear·th Planet. Inter .. 8. 2 11 -:2:3-l. 1974 .

    Hofmann, A. \\'., and \\'. 1\l. \\'hite, i\Iantle plumes from ancient oceanic crust, Ear·th Plauet :)ci. Letl., .57.

    421-436, 1982.

    Inoue, H., Y. Fukao, K. Tanabe, and Y. Ogata, Whole mantle P-wave travel time tomography, Phys. Earth

    Planet. Inter. 59, 294-328, 1990.

    Ito, E., I\. ~forooka, 0. Uj ike, and T. Katsura, Reactions between molten iron and s ilicat.e melts at. high

    pressure: Impli cations for the chemical evolution of earth's core, J. Geophys. Res., 100, 5901-5910, 1995.

    J ean loz, R., The nature of the Earth's core, Annu. Re11. Earth Planet. Sci., 18, :357-386, 1990.

    Kendall , .J.-1\L, and P. t\1. Shearer, On the structu re of the lowermost mantle beneath the south west PaciRc,

    southeast Asia and Australasia, Phys. Earth Planet. Inter., 9:2, 85-98, 1995.

    Kennett, B. L. N., eel., 1ASPE1 1991 Seismological Tables, Pub/. Australian J\.af. Univ., 1991.

    Kruger, F., 1\L \\'eber, F. Scherbaum, and J. Schlittenhardt, Ev idence for normal and inhomogeneous lowermost

    mantle and core-mantle boundary sl ru ctu re under the Arctic and northern Canada, Geophys. J. !Ill., 1 :'!2,

    637-657, 1995.

    Lay, T., The fate of descending slabs, Aunu. Rev. Earth Planet. Sci .. '!!2, :3:3-()1, 199·1.

    0. Loper, 0 . E., 1\lantle plumes, Tecto nophysics, 187, 373-384, 1991.

    Loper, 0 . E., and T . Lay, The corc-nmnt lc bonndary region, ./. C:cophJ;s. Rc.~ .. 100, 6:391-6-120. 199'>.

    !\Iori, .J ., and D. V. ll elmhP rgl'l'. Locali;r,Pd hout1dary layer lwlow 1111' tnid-PaciliU-10:H5!), J!J\J.").

    Ri chard,;, :0.1. ,\ .. and 1). c.:. Engd)rt•t,;otl. Larg

  • IIIIIIIIIIY Ill[) .. lay. •r ll'llh

    .J-;uray Ht'rord"': Pn·liuuna r~ IT"'ult"' . ./. C:comag. (:wcl

  • Fi~un· c;1pr ron-.

    Figure 1. Hayp;~lh" ]). il) result for n•gion [,,\I , h) L,\2 and c) Li\:3 . In r J>

  • low-pa:

  • Table 1. Event. paramdl'l'=' wwd in lhi" s tudy. Tlit' nol;llion,.; o f the columu ·· [)ala Se·t'· an' tlw ,.;anw

  • Tahl1· 1.

    Orit;tll ltnu•( (;\(1) L10 5.8 .\ ISASI 1987 Fe h. 10 00 59 28.51 19.4895 111.456\V :H):) ().2 !IISAS2 1981 Feb. ltl 13 :38 22 .11 11.9265 178.632\V 566 5.1 !11SAS1 1987 Apr. 29 1•1 27 :35.7 19.015 177.73\\ ' :385 5.~) LA3 1981 Aug. 26 6 56 46.2 20.745 118.46\V 56!) !>.7 LA:3 1988 .) an. 21 16 00 01.55 17.7635 178.737\ \' 566 5.1 l\ ISA51 1988 l\ov. 16 05 53 20.29 21.7685 179.426\V 582 5.8 !11SAS2 1989 Apr. 18 11 33 52.16 2:3.83·15 179.9·14£ 521 5.8 ~ I S:\S3 1989 :\lay 21 21 56 48.6:3 11.9525 178.593\\' 581 5.7 i\ ISAS1 1989 ::\o\'. 29 0:) 48 59.84 25.3745 119.6298 187 5.1 i\IS:\5:3 1990 :\lay 17 15 59 56.53 25.:3985 178.101 E 611 5.8 ,\JS:\53 1990 i\la~ 28 11 28 41.69 20.8145 171.981\\ ' •186 5.9 .\ ISAS2 1990 .Jun 26 12 08 29.39 22.0155 119.-1 t:3 \\' .')$1 6.0 .\15.-\52 1990 .Jun. 29 o:3 53 28.16 21.5525 179.3:32\\' 616 57 .\ISA52 1990 Aug 10 II II 36.78 J 9.8055 177.385\\' :!1:3 60 .\15A52 1990 Oct. 10 0:) 5-I 5:3.54 2:3.4915 179.0298 519 GO .\!S,\5:3 1991 :\ pr. 18 9 ll 20.1 22.925 179.:31\\' •Ill 5.1 L:\2 1991 Aug. 28 21 :32 35.4 22.2:35 179.6·1E .')99 55 LA1 1991 Srp. :30 0 21 46.4 20.8185 118.59\\' 566 6.3 013 1992 .Jan. I :3 9 :n 4:3.1 20.9:35 118. 72 \\' 515 5.7 L:\1 1992 Apr. ·I 1 11 12.:3 11.955 118.:31\\' 511 5.6 L:\ 1 1992 Aug. :30 20 9 5.8 17.925 118.71 \\' 565 5.8 LA1.2,1313 1992 Nov. 12 22 ::!8 51.5 22.4015 l/8.10 \ \' :360 5.9 BB 199:3 .\I a r. 21 5 'I 59.2 18.0..t5 I 78 .5:3\V 58$1 6.1 LA I .~ I SAS 1 ,f313 199:3 ,\ pr. 16 1·1 8 :38 .9 11.178$ 178.86 \V !)()!) G.O Bl3 19!'):3 r\ p r. 20 I (j 26 19.5 20.88S 118.70\\' !)!)::! 5.G LA 1.2 199:3 .) ul. !) 1:) :n 5:3.1 19.785 171.49\\' :398 6.0 LA1 199:3 r\ug. i 11 53 2·1. 2 23.8665 119.858 !)2:3 6.0 13B 199:3 Ort. 11 I :3 I 29.6 17.855 178.1:) \\' ::·· ·)').) 58 L,\ 2,~ 1 S,\ Sl.l313 19!1 I .\Jar. H :.!:3 28 6.1 18.0:395 118. 11 \\' ;j():l () (j 1313 I!)!) I ~Jar. :31 '2'2 10 :):!.1 :-, 12.051S 11!).:):!:3 \\ ' 51:'0 6. 1 ~I SAS2,BB I !l!) I Ort .,-_, :?-2 :.W ~----> :l5.118S li!Ul l: ;"',( q .-,,!) 1313 I !l!):) .Jan . II lfi 51 1:?.0 :?U.I:'IOS 119.~1\\' fi:ll fi () BB

    Tab I

  • )

    Surface

    Solid Mantle hypocenter

    P.S PdP,SdS

    Liquid Core

    1· ~~5~,\~ I

  • 120" 140" 160" 180"

    40" 40"

    20" 20"

    o· o·

    -20" -20"

    120" 140" 160" 180"

  • 40

    30

    120

    • • • • •

    130

    • •

    ·~~~ • LAl

    • LA2

    • LA3 ~~~-

    • MSAS

    :~'% BB

    140 150

  • 15510~0

    ------~~==~~-----=====~~----.c======~------~ 15° 15° ~ 155° 160° ) 165°

    {*-~~ LAl

    ~8 LA2

    ~~) LA3

    MSASl

    1 oo ~ f1 MSAS2

    50 ~

    e MSAS3 eBB

    ~-----

    oo ~ ~~__/

    150° 155°

    ()

    • • •

    )

    -1 oo

    ~

    • • • ~ 50 • .. -• ·' •

    • ~ oo

    160° 165° F·a.,~ ~ +

  • 400

    300

    200

    100

    400

    300

    200

    100

    ..-... E 4oo

    .:::£ ..._... 0 300

    +oJ

    ~ 200 -"+-Q) 100 a:

    .... .... 0

    0 10 20

    0 10 20

    0 10 20

    Differential Travel Time(sec)

  • MSASl original

    • z

    z

    I

    -1 0 0 1 0 20 30 40

    Differential Travel Time (sec)

  • Obs. / Syn. 66:~f42° _________ ___ " ,"' ..... ~.,-·-------- ..

    'I

    -6--6-------------- / "v -- ----- , ' v o 64 2 0 - I ,'•·'' ',,,.. ,'\ -\ ,' __ , \ /,., __ , _________ _ I I •

    1)

    tl I I

    I I 66:24_6_0

    _____________ , \ ,' I I ,,

    6 6: 3-~ii 0----------- --~'

    66:;tti6_ __________ ,

    s SdS ScS

    )

    19920830 mb5.8 ________ _____ ,. d=565km 58.619° -

    19930321 mb 6.1 ___________ ,_,_, d=589km 58.825°

    19930416 mb 6.0 _____________ , d=565km 58.420° -

    19931011 " '

    ,' ... _,, ..... _ --- ... --------- ... __ .,,,-\, I I ,,,

    '~ -~~· .... _________________ ,, \ , ...... _,- ... _____ ___ . I I

    v

    ,, / I , ... , __ ....... _____________ , \ ,'

    1,,

    , ......... _____ _

    m 58 ..,, ., b • I IV " ,-, d=555km ss:s·s-io'-' \ ('•' .. ________________ _, \, ,- ... ___ _____ _ ,

    19940309 mb 6.6 ______________ _, d=563km 58.898°

    s

    I I 1,1

    ,-, ... ,,.......... ......... ,-... ,' \ I - .,..., ........................... \ , ... .,

    I I 1

    l I \ ,' \ I \ J ,I

    SdS ScS

    100 sec

    ~ - ... ... ,----

    F·~v.~~ ?

  • 15" ~--=====--=======---=====---=+=1

    1 o·

    5"

    0

    ScS-S Residual (sec) 0 oof\

    2.0 4.0 6.0 Si

    0

    150" 155" 160"

    a)

    0

    0

    0

    165"

    15 ° ~-r::::::::====--=======---=====---==+=1

    D" thickness from SdS (km) b) 0 0 0

    100 200 300

    1 o·

    5 LAl

    0 LA2 ;·~·· ,· LA3

    MSASl o· MSAS2

    e MSAS3

    150" 155"

    0

    160" 165"

  • a) b)

    ll 0 10 20 0 10 20

    4()()

    300 300

    200 200

    100 100

    0 0 10 20 0 10 20

    15

    ]j ·10 ·15

    0 10 20 0 10 20

    Differential Travel Time

    2400 2500 2600 2700 2800 2900

    Differential Travel Time

    ~"S--- I ->~1Jt= I I~ 2300 2400 2500 2600 2700 2800 2900

    Depth(km) Depth(km)

    c) d)

    ~~ 0 10 20

    400

    300 300

    200 200 Pdl~""- -- -Z31J J{,ii

    100 100

    0 0 10 20 0 10 20

    15

    Jl ·15

    Figure g

  • e) D

    0 10 20 0 10 20 400~----~~---------------~--~

    300 ~JP 2300 2400

    Depth(km) Depth(km)

    g) h)

    0 10 20 0 10 20

    0 10 20 0 10 20

    .:1+1--:\f\V:f.::~l ·15 .l...---------------',.!!.!-----------'--

    0 10 20 0 10 20

    1:: ~-~::er~n~:~:;~~ > 2300 2400 2500 2600 2700 2800 2900

    Differential Travel Time

    ul·~---------~.----~------~r· t I -----.---~'ULC~lf > 2300 2400 2500 2600 2700 2800 2900

    Depth(km) Depth(km)

    Figure 9 (Continued)

  • i)

    0 10 20

    0 10 20

    Differential Travel Time u ,4 r' --~--~--~--~~--~ th : · : . --~ r > 2300 2400 2500 2600 2700 2800 2900

    Depth(km)

    Figure 9 (Continued)

  • 60 a) LAl Direct P 40

    20 0

    -20 -40 -98

    0

    -10

    0 10 20

    20 Direct P 10

    0 -10 -~§ P P 280 km

    0

    -10 0 10 20

    4 c) LA3 Direct P

    0

    1:~ 0.5 0.0 -

    -0.5 -1.0

    0 10 20

    L /0 fl ~~,I rQ

  • )

    LAl LA2 LA3

    270km

    ~ .· High Vp (1.4%) HighVp(2~

    High ':,~.~~ :. .. .. .. .... .. ......... High V s(?) ...___ir-: --H-ig-h-=-v=-. p:( 2=-. :9 ~~o) .............. .... : . .. 1111111

    ? : v· s(?) ,,,, . - . ,,, .

    ,,,,,,,,,,,,, ................ ir. ....•............................ .... , 170km

    LowVs LowVs Vs?

    CMB

    southwest 500km northeast

    F·~IA\I'Q. I I

  • (e.g., the Indian Ocean), but is constant beneath mid ocean

    ridge (e.g., the Mid Atlantic Ridge).

    Introduction

    The mid ocean ridges (spreading regions) have been considered

    to play a passive role in the force balance of plate tectonics5 and

    only shallow structure has been studied in detai 16 . Seismic tomogra-

    phy using surface waves7•8 has a limited resolution in both horizon-

    tal and vertical direction, especially poor resolution in the depths

    and contrasts of the upper mantle discontinuities.

    \Ve examine the results of previous studies1- 4 by analysing the

    data from recent large deep earthquakes in Fiji and Bolivia, whose

    bounce points of P'P' are located near the Mid Atlantic Ridge and

    far from the Mid Indian Ocean Ridge, respectively. We show the

    regional difference of the underside reflections of P'P' from the 420-

    and the 660-km discontinuities (referred to as P'420P' and P'66oP'

    here after) and attempt to interpret the differences in terms of

    chemical composition with an assumption of pyrolite mantle.

    D ata

    The Fiji earthquake of !\larch 9, 1994 (mb= 6.6) and the Boli-

    vian earthquake of June 9, 1994 (mb= 7.0) provide us with a rare

    chance for the analysis of P'420 P' and P'66oP'. We analyse the data

    of short-period vertical-component seismograms from the Fiji and

    Bolivian earthquakes recorded by the micro earthquake network

    2

  • of Tohoku University and Southern California USGS-Caltech net-

    work. \Ve use the hypocenter parameters li sted in mon thly PDE

    (Preliminary Determination of Epicenters) report to calculate the

    travel times and bounce points of P'P', P'42oP' and P'660P ' . Figure

    1 shows that the bounce points for the Fiji earthquake are located

    close to the Mid Atlant ic Ridge and that those for the Bolivian

    earthquake far from the Mid Indian Ocean R idge. This diffe rence

    of the bounce point locations encourages us to study structural dif-

    ference of mantle beneath the mid ocean ridge and normal ocean.

    Strict inspection of all data by display of original traces and fil-

    tered traces obtained from several pass bands leads to 20 traces for

    the Fiji earthquake and 82 traces for the Bolivian earthquake. \Ve

    use the short-period vertical-component seismograms filtered with

    a pass band of 0.2-0.5 Hz (Fig. 2). The epicentral distance ranges

    from 67.1 o to 70.2 o for the Fiji eart hquake and 64.4 o to 70.5 o fo r

    t he Bolivian earthquake.

    Analys is

    The slowness is one of important parameters to identify the un-

    derside reflections of P 'P' from the upper mantle discontinuities.

    The slant-stacking of the filtered seismograms in Fig. 2 is made

    with a slowness interval of 0.1 sec/deg to measure the travel times

    and slownesses of significant signals in the data. The comparison of

    the calculated t ravel t imes and slownesses with those of clear peaks

    3

  • in the contour maps indicates that P'660 P' from the Fiji earthquake

    (Fig. 3a), and P'ssoP' and P'42oP' from the Bolivian earthquake

    (Fig. 3b) can be identified without ambiguity. The fact that the

    signals of P'660P' and P'42oP' consist of three dominant peaks (Fig.

    3b) is a strong evidence for the signals from the great Bolivian

    earthquake, because the waveforms of P waves recorded by broad-

    band instruments suggest that three dominant shocks occurred dur-

    ing Lhe rupture process of the earthquake. Another apparent signal

    with slowness of -4.3 sec/deg (Fiji) and -4.3 sec/deg (Bolivia) is not

    interpreted as P'dP' ( d < 420-km), but as SI

  • Ocean Ridge. The data sets analysed in the present study does not

    show such a clear regional variation for the 660-km discontinuity.

    However, one of the array studies of P'dP'2 suggests the possibility

    of a weak reflectivity of the 660-km discontinuity beneath the ~lid

    Atlantic Ridge.

    Implications on chemical composition

    Advances in high-pressure experiments have reached a depth of

    about 800 km in the Earth's mantle13- 15 . It may be possible for us

    to interpret the seismological results of the present study in terms

    of mineralogical phase relation. We assume Mg2Si04-Fe2Si04 sys-

    tem in the following discussion.

    420-km discontinuity

    Recent high pressure experiments16'17 of olivine (a) to modified

    spinel ( (3) transformation, which is considered to correspond to the

    420-km discontinuity, suggest the transformation width of 11 to 19

    km in temperature range of 1400 to 1750 ·c for the composition of

    (Mgo.sgFeo.11 )Si04 16, and the decrease of the width with tempera-

    ture increase16•18 . Our observational result from the Fiji earthquake

    is consistent with this experimental result. However, the clear re-

    flection from the Bolivian earthquake seems inconsistent with the

    width range. An easy way to overcome this inconsistency is to

    change the Mg-Fe ratio from (Mg0 .89 Fe0 . 11 )Si04 beneath the dis-

    continuity. They 16 infer that Mg content is grcaLcr than 95% or

    5

  • smaller than 80%. We can suggest that the increase of Fe content

    to 17% beneath the discontinuity is plausible. The effect of H2 0 19

    on the transformation should be investigated with careful experi-

    ment.

    660-km discontinuity

    Recent high pressure experiment 13 shows that the transforma-

    tion of spinel to perovskite + magnesiowi.istitc take places at a

    pressure of 23 GPa with a pressure interval less than 0.15 GPa at

    a temperature of 1600 °C. These pressure values are converted to a

    depth of 653 km and a depth interval less than 5 km. These values

    are consistent with the results of the present study for both the Fiji

    and Bolivian earthquakes.

    One of the recent array studies2 suggests the possibility of the

    reflectivity decrease with decreasing distance of the bounce points

    to the Mid Atlantic Ridge. This might require a change of chemical

    composition near the 660-km discontinuity across the Mid Atlantic

    Ridge.

    Conclusion

    An unambiguous regional variation of the reflectivity of 420-km

    discontinuity is observed between the bounce points close to the

    Mid Atlantic Ridge and those far away from the Mid Indian Ocean

    Ridge. The reflections from the 660-km discontinuity arc clearly

    observed from the both bounce points. Recent array stucly2 by one

    6

  • of the authors of the present study finds an evidence for the change

    of reflectivity of the 660-km discontinuity beneath the !\lid Atlantic

    Ridge.

    Interpretation of these results in terms of chemical composition

    is not unique. However, the change of chemistry near the disconti-

    nuities is a simple but most plausible interpretation. Interpretation

    by temperature variation seems difficult. The focussing and defo-

    cussing due to heterogeneities in the Earth cannot be ruled out.

    We must point out that the bounce points for the Fiji earthquake

    are close to a hot spot20 (Fig. l(a)). There found no hot spot near

    the bounce points for the Bolivian earthquake.

    Acknowledgmen ts.

    The data analysed in this study were provided by courtesy of To-

    hoku University and California Institute of Technology.

    7

  • References

    1. .:'-Iakanishi, I., Seismic reflections from the upper mantle disconti-

    nuities beneath the ~lid-Atlantic Ridge observed by a seismic array

    in Ilokkaido region, Japan, Geophys. Res. Lett., 13, 1458-1461,

    1986

    2. Nakanish i, I., Reflections of P'P' from upper mantle discontinu-

    ities beneath the Mid-Atlantic Ridge, Geophys. J. R. Astron. Soc.,

    93, 335-346, 1988

    3. Nakanishi, I., A search for topography of the mantle discontinu-

    ities from precursors to P'P', J. Phys. Earth, 37, 297-301, 1989

    4. Benz, H. M. and J. E. Vidale, Sharpness of upper-mantle discon-

    tinuities determined from high-frequency reflections, Nature, 365,

    147-150, 1993

    5. Forsyth, D. and S. Uyeda, On the relative importance of the

    driving forces of plate motion, Geophys. J. R. Astron. Soc., 43,

    163-200, 1975

    6. Detrick, R. S., P. Buhl, E. Vera, J. i\Iuttcr, J. Orcutt, J. Madsen,

    and T. Brecher, Multi-channel seismic imaging of a crustal magma

    chamber along the East Pacific Rise, ~ature, 326, 35-41, 1987

    7. Nakanishi, I. and D. L. Anderson, 1\[easurements of mantle wave

    velocities and inversion for lateral heterogeneity and anisotropy-

    fl. Analysis by the single-station method, Geophys. J. R. Astron.

    Soc., 78, 573-617, 1984

    8

  • 8. Woodhouse, J. H. and A. M. Dziewonski, Mapping the upper

    mantle: Three-dimensional modeling of Earth structure by im·er-

    sion of seismic waveforms, J. Geophys. Res., 89, 5953-5986, 1984

    9. Engdahl, E. R. and E. A. Flinn, Remarks on the paper "Early

    reflections of P'P' as an indication of upper mantle structure," by

    R. D. Adams, Bull. Seism. Soc. Am., 59, 1415-1417, 1969

    10. Engdahl , E. R. and E. A. Flinn , Seismic waves reflected from

    discontinuities within Earth's upper mantle, Science, 163, 177-179,

    1969

    11. Whitcomb, J. H. and D. L. Anderson, Reflection of P'P' seis-

    mic waves from discontinuities in the mantle, J. Geophys. Res., 75,

    5713-5728, 1970

    12. Adams, R. D., Reflections from discontinuities beneath Antarc-

    tica, Bull. Scism. Soc. Am., 61, 1441-1451, 1971

    13. Ito, E. and E. Takahashi, Postspinel transformat ions in the sys-

    tem Mg2 Si 0 4 -Fe2Si04 and some geophysical implications, J. Geo-

    phys. Res., 94, 10637-10646, 1989

    14. Anderson, D. L., Theory of the Earth, Blackwell Scientific Pub-

    lications, Boston, 1989

    15. l rifune, T., Phase transformations in Lhe earth's mantle and

    subducting slabs: Implications for their compositions, seismic ve-

    locity and density structures and dynamics, The Island Arc, 2, 55-

    71, 1993

    9

  • 16. Katsura, T. and E. Ito, The system I\1g2Si04-Fe2Si04 at high

    pressures and temperatures: Precise determination of stabilities of

    olivine, modified spinel, and spinel, J. Geophys. Res., 94, 15663-

    15670, 1989

    17. Akaogi, M., E. Ito, and A. Navrotsky, Olivine-modified spinel-

    spinel transitions in the system Mg2Si04-Fe2Si04 : Calorimetric

    measurements, thermochemical calculat ion, and geophysical appli-

    cation, J. Geophys. Res., 94, 15671-15685, 1989

    18. Ilelffrich, G. and C. R. Bina, Frequency dependence of the vis-

    ibility and depths of mantle seismic discontinuities, Geophys. Res.

    Lett., 21, 2613-2616, 1994

    19. Wood, B. J., The effect of H20 (water) on the 410-kilometer

    seismic discontinuity, Science, 268, 74-76, 1995

    20. Turcotte, D. L. and G. Schubert, Geodynamics: Applications

    of continuum physics to geological problems, John Wi ley & Sons,

    450pp, 1982

    10

  • r

    Figure Captions

    Figure 1. Surface bounce points (solid circle) of P'P'. The lo-

    cations of hot spots20 (solid stars) are also shown. (a)Fiji-Japan.

    (b )Bolivia-Southern Carifornia.

    Figure 2. Short-period vertical-component seismograms filtered

    with a pass band of 0.2-0.5Hz. (a)Fiji-Japan. (b)Bolivia-Southern

    Carifornia.

    Figure 3. Contour map showing the result of slant-stacking

    of the filtered seismograms in Fig. 2. Also shown are the locations

    of P'P', P'42oP', P'66oP', and SI

  • . 0

    ....... . .

    L_~ I ~

    'v->!v

    r

    . 0 -· 7' I \

    )

    -

    - ) .

  • 60. go· 120.

    -3o·

    g2·

    go· 120.

  • r

    0 Vl

    0 :::> ("") ('l)

    ><

    Time (sec) (VM OFF) X 10+2

    YMZ.uth

    'J.XPOOC~~NfYtA uiJIIII'o'.oii~SNR.ulh

    HOJ.uth HSK.ulh

    -TBS.uth .~~NIB.ulh

    ~~--w~-OGA.ulh fVV'IW~,.,.,.,._.1-TMR. uhs

    --t"-------~~.-vv-~----~~--.......,......

  • ><

    0 + ('...)

    X 1012 Time (sec) [VM OFF)

    f;J _ 2 ( hJ

  • 0 ....--.... 0) (1) -1

    "'0 --as -2 (/) ....._.... (/) -3 (/)

    ~ -4 $ 0 -5

    (f) -6

    I

    - ~ -

    -P' 66oP' ~

    I

    -

    -

    I

    Fiji-Japan I I I

    ~ ~J' I .. 4 ,, t I

    I , -

    I~~~~~~' I ~ 1-I P'P' . ~ I._ ~ ~~ l~ l/1

    ij -

    SKKKP I ~ 1

    ,v l' t ~ I VI~ ~~~ 1 R ~

    1-

    I ,~ ' I I I

    I I I I

    2100 2150 2200 2250 2300 2350 2400

    Travel Time(sec)

  • Bolivia-US o~~~~~~~~~~~~~~~~~+

    ....--.... (J) Q) -1

    ""0 --() -2 Q) (/) .....__. (/) -3 -(/)

    Q) -4-

    ~ . t ~ ~ 0 -5 .

    (J) .

    P' 66oP '

    r I

    -6+·~~~~~~~~~~~~~~~~~

    2100 2150 2200 2250 2300 2350 2400

    Travel Time(sec)

  • Fiji-Japan

    500

    400 P'P'

    300

    200

    100

    0 2100 2150 2200 2250 2300 2350 2400

    Travel Time(sec)

  • Bolivia-US

    40

    30 P'P'

    20

    10

    o~~~~~~~~~~~~~~~~~+

    2100 2150 2200 2250 2300 2350 2400

    Travel Time(sec)

    000200030004000500060007000800090010001100120013001400150016001700180019002000210022002300240025002600270028002900300031003200330034006800690070007100720073007400750076007700780079008000810082008300840085