Seismic heterogeneity in the upper mantle - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B12, PAGES 23,753-23,766,DECEMBER 10, 1994

Seismic heterogeneity in the upper mantle G. Nolet Department of Geologicaland GeopkysicalSciences,Prince.*,on University,Princeton, New Jersey

1919-1994

S. P. Grand Department of Geological Sciences,University of Texas at Ar,stin B. L. N. Kenneff Research School of Earth Science, Australian National University, Canberra

Abstract. This paper gives an overview of recent seismologicalevidence on heterogeneityin the upper mantle. While there is a growing consensusamong seismologists on the low-order sphericalharmonic expansionof upper mantle structure, short- and intermediate-wavelengthfeaturesremain the subjectof intense debate. Regional studiesof Earth structure yield evidencethat the upper mantle is heterogeousat all resolvablescalesand that gl•)l•al models underestimatethe amplitudes of velocity anomalies. Some importanz problems still outstanding

concern:(1) the fate of subductingslabs,(2) the depthextent of low-velocity zonesunderspreadingcentersand (3) the depthat whichhotspotsoriginate.The grossradial layering of the upper mantle has beccnmmuch more evident to us in recent years, mainly as a result of the analysisof waveformsobservedby the modern generationoœdigital, broadbandstationsand by denselyspaced,narrow band arrays of seisinomelets.Nevertheless,important questionson the averageradial structure

still persist.The low shearvelocityzonehasbeen'mappedin detailbeneaththe continents,and Jordan's tectospherehypothesisis largely confirmedby regional

one-dimensional (l-D) and globaltomographic studies.However,it is lessclearto what extent these low shear velocitiesare matched by low compressionalvelocities. Anisotropy,for which compellh•gevidenceexistsin the observationsof split shear waves, may play a role in shaping our images of low velocity zones. Below the asthenosphere,the existenceoœthe Lehmann discc.ntinuityat about 220 km depth as a worldwide feature is unlikely, but early indications for a phase transition at 520 km have found some confixmationin stackedwave sections. Anomalouslylow velocitiesat greater depth have been observednear current,subductionzonesand even beneath more ancient suture zones. Recent progressin analysis of seismic

waveshasfollowedmajor advances in seismicinstrumentation of two kinds- (1) the installationoœvery broadbandstationsaroundthe world,and (2) the increased density oœstations in seismicnetworks. Future progresswill most likely depend on the expansionoœdensenetworks,including temporary and portable stations;on our ability to increasethe station coverageoœoceanicregions;and on advancingthe forward problemof computingwaveformsfor heterogeneous E•rth modelsto a level of efficiencythat allowsfor nonlinear optimization. Introduction The existence of lateral

notably the precisionof timing and the calibration of the instrument response. Early surface wave observavariations

in Earth's

seismic

tions(seeKnopoff[19721 andKovach[1979]forreviews)

velocitieswas knownevento seismologists suchas Jef- establishedthe presenceof a low shear velocity zone, or freys in the 1930s. I-Iowever•the study of lateral vari- LVZ, and showedconsiderablelateral variationsin the ations could not begin in earnest until the quality of thicknessof the high-velocitylid above the LVZ and in seismicobservationsimproved sufficientlyin the 1960s the minimum shear velocity found within it. Attempts were made to regionalize Earth into a small number Copyright1994 by the AmericanGeophysicalUnion.

of geophysical provinces(ancientshields,Phanerozoic

Papernumber94JB01892.

nentsand a subdivision accordingto agefor the oceans),

0148-0227/94/94JB-01892505.00

but efforts to define a common depth-dependentveloc-

platforms, and tectonically active areas for the conti-

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ity profile for equivalent provincesmet with little suc-

this way needsto have a high degreeof continuityover the full distancerange. The structure in the uppermost The excellent seismic data that became available with mantle is evident in observationsout to 1000 km, but the installation of the World Wide Standard Seismoresolutionof strugturein the transition zonedown to graph Network (WWSSN) and the growingpowerof 660 km typically requiresobservationsto 3000 km and computerstriggered the developmentof powerfulalgo- beyond. rithms for the computation of synthetic seismograms. Refractedwavesare sensitiveto velocitygradientsas Detailed velocity models became available for many re- well as to velocity jumps at discontinuities,so there can gionsin Earth through the careful matchingof observed be a significanttrade-off between different aspectsof and computedseismograms,generallyby trial and er- the model during the interpretation process. The use ror. On the other hand, the growing volume of simple of travel time information alone leads to a wide class travel time observationsfor seismic body waves from of modelscompatible with the observations,which can local and global networks inspired the new technique be refined by the application of synthetic seismogram of seismictomography, pioneeredin the 1970sby Aki methods to match the amplitude behavior. Becauseof and Lee [1976],Aki et al. [1977],and Dziewonskiet the long profile range requiredto examinevelocitywith depth, interpretations have commonly been based on Small-scalelength heterogeneitiesin the near-surface single-endedprofiles. Where constraintsare available have always received attention from seismologists, from multiple sourcesalonga singleprofilethere is clear though often as a nuisancewhose effectsmust be re- evidenceof lateral variationsin structure in the upper moved. The useof array observations,particularly with mantle [Mechieet al., 1993]. Other evidencefor the the larger LASA and NORSAR arrays, led to a recog- presenceof significanthorizontalvariability in structure nition that small-scale velocity perturbations are not comesfrom the correlation properties of arrivals from confinedto the upper crust but extend at least into the singlesourcesrecordedat multiple receivers[Kennett lower lithosphere[e.g., Kennett, 1987; W• and and Bodyman,1990]. 1990, Table 1]. The resultsof seismictomographynow There have been two main styles of approach to the indicate that velocity anomalies exist throughout the use of refracted wave information. The first has been to upper mantle, and these observationscontribute to our useshort-periodwaveswhichofferthe possibilityof high understandingof chemicaland thermal heterogeneity. resolution but are also influenced by lateral variations In keepingwith the spirit of other papersin this J(7R in structure. Generally, such studies have been based 75th anniversary series,we shall review the evidencefor on a high density of recordinglocations, either by a lateral heterogeneityin the upper mantle at a tectonic large array for a single sourceor, more commonly,the scale, discussits geodynamicsignificance,and attempt synthesisof a large aperture array by the superposition

cess[Okal,1977;Jordan,1981].

to look ahead

into the near future.

of observations from manysources.In somecases[e.g.,

As is common for a sciencethat is in rapid transition, Bowmanand Kennett,1990;De•l et al., 1993],stacking there is disagreementon the interpretation of various of different observationshas been used to emphasise seismologicalobservations,even among the authors of thosefeatures of the wavefield commonto many events. this paper. Rather than hide such problems,we have A representativeselectionof upper mantle P wavevechosento give more emphasisto unansweredquestions locity modelsderivedfrom short-periodobservationson than to acceptedsolutions. Unlike many other scien- different continentsis shownin Figure 1. Construction

tists, seismologists have little control over their experiments. Consequently,a mixture of techniquesis often used on a patchwork of data from various instruments. Many methods attempt to interpret the data in terms

of each of these models has benefited

from the assem-

bly of a dense record section of observationsto constrain travel time branches,and in most cases,theoretical seismogramcomparisonshave been usedto conof a one-dimensional (l-D, i.e. depth-dependent) model strain the amplitude behaviorwith epicentraldistance. which is valid for a particular region, rather than in The complexfine structure in somemodelsmay result terms of a three-dimensional structure. While the latter from the mapping of three-dimensionalstructure into approachis more logical,the first one givesresultsthat

are often morerobustin the usualcasethat not enough data are availableto allowfor an unambiguous solution in three dimensions.We shall first discusssomeimportant differencesbetween the two philosophies,before directing our attention more specificallyto the results obtained.

Inference

From

1-D

Studies

Most studies of upper mantle structure have been based on the use of refracted waves from natural or man-made sources with receivers at substantial distance

from t'he source. Any 1-D structureto be resolvedin

a one-dimensionai velocitydepth profile [Kenneffand Bowman,1990].The modelsspana widerangeof geographicenvironmentsand interpretation techniquesbut consistentlyshowthe presenceof the two major discontinuities near 410 and 660 km which mark the upper mantle

transition

zone.

The second approach has been to use long-period body waves which are not severely affected by smallscalelateral heterogeneitybut which have limited vertical resolution; in such work, detailed analysis of a limited number of seismogramshas been the usualprocedure. This approach advancedstrongly during the 1970s with the developmentof efficient algorithmsto computesyntheticseismograms.Rapid changesin ave-

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locity gradient, discontinuities, and low-velocity zones within the upper mantle will produce multiple arrivals at upper mantle distances. Synthetic seismogramscan accurately predict the relative amplitudes and timing

resulting in vastly more useful images of Earth's interior. These early tomographic methods used damped least squarestechniquesto solve the resulting system of linear equations, but computer speed and, most imof sucharrivalswhichcan then be directlycompared portantly, the large memory requirementsof orthodox to seismic data and reduce the nonuniquenessof seis- matrix solversposedlimitations to the size of the probmic modeling. Important early studies matched travel lem that could be handled. Iterative algorithms soon times as well as waveforms of P wavesat upper mantle brought the necessaryimprovementsin both computadistances[Helmbergerand Wiggins,1971; Wigginsand tion speed and problem size. The two techniquesused Helmberger,1973]. Upper mantle shearvelocity(SH) most at present were introduced 10 years ago in seismic profileswereobtainedby HelmbergerandEngen[1974]. tomography: the simultaneousiterative reconstruction These studies using perfectly elastic models generally technique(SIRT [Claytonand Comer,1983])and the resulted in simpler, i.e., smoother, models than earlier conjugategradientalgorithmLSQR [Nolet,1983].The work, although they did confirm the existenceof ma- matrix solver usedin tomographic inversionsdoesinflujor discontinuites near 400 and 660 km depth. They encethe result, but sucheffectsare now well understood also found evidence for a small jump in velocity near [Van der Slutsand Van der Vorst,1987;Nolet, 1993a]. 500 km depth and a low-velocity zone near 150 km Numerous tomography studies have been published depth. Since that time, there have been many studies that usehigh-frequencybody waves,rangingfrom global of the upper mantle beneath various different tectonic studiesthat includethe whole mantle [Inoue et al., provinces using synthetic modeling of upper mantle P 1990; Vascoet al., 1993]throughregionalscales(genand S waves including allowance for attenuation. In erally in subductionareas [e.g., Epakman,1991; Zhou North America, examples include those by Grand and and Cla•tton, 1990; Van der Hilst et al., 1991; Zhao

Helmberger[1984a,b],who studieddifferencesbetween andHasegawa,1993]to very local studiessuchasunder et al., 1983; Ellsworthand Ko•tshieldand tectonicstructure; Walck [1984],who stud- hotspots[Tr•tggvason ied upper mantle P velocity beneath the Gulf of Calianagi, 1977; I•ler et al., 1981]. Grand [1987,1994]has fornia;LeFevreandHelmberger[1989],whostudiedthe used a combination of refracted and vertical incidence upper mantle beneath the Canadian shield; and Zhao shearwavesto imagethe mantle. The shallowmantle in andHelmberger[1993],whostudiedthe P structurebe- these studies showsconsiderableheterogeneityon scale neath

the northwest

The structures

Atlantic.

for both

P and $ waves derived

from

lengths of 500 km or so, but the deeper upper mantle appearslargely uniform. For an extensivebibliography

longer-period observations (Figures2 and 3) showvari-

of tomographicstudies,seeI•ter and Hirahara [1993].

ability in the upper 300 km associatedwith the tectonic character of the surface regions. Shallow low-velocity zonesoccur in tectonically active regionsand beneath oceans, whereas high velocities axe sustained to some depth beneath shields. Comparisonof Figures2 and 3 shows that the low-velocity zones of P and S are different in character. S wave LVZs are generally broad

Low-order spherical harmonic inversionscan still be performed with noniterative methods and now include both waveformsand delay times from long-periodbody

wavesand their surfacemultiples[e.g., Woodhouse and Dziewonski, 1984; Woodwardand Masters, 1991; Suet

al., 1994]. The long-wavelength featuresof Earth show remarkable agreementamong these low-order models, (exceptfor shr14, which was actually derivedfrom P although the question has been raised that the limdata) and muchmoreconsistent from profileto profile. ited samplingfrom availablepaths might lead to spatial Continental models, such as k8 for Eurasia or s8 for aliasingeffectswhichbiasthe interpretation[Sniederet the northeasternUnited States, havenegativegradients al., 1991]. from 150 to 200 km depth but no well-developedlowAs a model that is representative for the most revelocity zone for P. From a petrological point of view cent sphericalharmonic inversioneffortswe have chosen this is to be expected if the low velocities are caused SH12/WM13 [Suet al., 1994]for occasionalreference by partial melting or subsolidusweakeningof the shear and comparison. This model expands the lateral hetmodulus. However, surface wave dispersion measure- erogeneity in Earth's whole mantle into sphericalharments, instrumental in mapping out the low-velocity monics to degree 12 for the horizontal coordinatesand zonesfor S waves, are insensitiveto P wave velocities; 13 Chebychev polynomials for the depth dependence. P wave LVZs are based on body wave analysesonly, The total of 2548 model parameters was estimated from and a bias causedby different analysis methods cannot 27,000 long-period seismogramsand 14,000 travel time completely be ruled out at this point. observations. Three-Dimensional

Studies

It has been arguedthat heterogeneitiesin the mantle occurpredominantlyat length scalesgreater than about

6000 km [Su and Dziewonski,199l]. The strongestargumentsfor this point of view are basedon the analyAbout 20 yearsago, Aki and Lee [1976],Aki et sis of long-period waves traveling over large distances, [1977],andDziewonski et al., [1977],usingtheincreased but it should be kept in mind that these waves themcomputing power available to them, made a dramatic selveshave a filtering effect becauseof the large size improvementin the interpretation of seismicdelaytimes of the Fresnelzonesinvolved(see,for example,Figure

NOLET ET AL.- UPPER MANTLE HETEROGENEITY

4

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1000 4,

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km/s

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Figure 2. Upper mantle modelsfor P velocityderivedfrom long-periodor broadbandobservations. Regionalcoverage:s25, North American Shield; k8, Eurasia; s8, northeasternUnited States;njpb, northernAustralia; ipremc,Isotropicversionof PREM modelwith continentalcrust.

13 km/s 3

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shr14

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tna

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•,400

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13 km/s Figure 3. Upper mantlemodelsfor S velocityderivedfrom long-periodor broadbandobservations. Regionalcoverage:shr14, westernUnited States;tna, tectonicNorth America;sna, shield North America; njpb, northern Australia; ipremc, isotropicversionof PREM model with continental

crust.

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Mantle S wave at 25 deg

200

4oo

600

800

lO

20

3O

distance (deg)

O.Os

>5.5s

Figure 4. Fresnelzonesfor mantle S wavescan be very large and complicated. This plot shows the travel time delay for a wave that is scattered once but otherwise follows Snell's law. The travel time delay is plotted at the hypothetical scatteringpoint. The darkest regionsin the plot have a delay •1 s. Because of Fermat's principle, low-velocity zones with a delay •1 s can be evadedby wavestraveling in the darkest zones. The fact that the darkest regionis split into two zonesis a consequence of the existenceof velocity gradientsin the mantle and the triplication of the travel

time

curve.

4), and the conclusion that smallerscaleheterogene- anomaliesin SH12/WM13 and those determinedwith ity is unimportant seemsincompatiblewith more local

waveformstudies[e.g.,Rial at al., 1984;Paulssen,1987; ßDey et al., 1993;gielhuisandNolet, 1994]and with the many resultsfrom delay time tomographyusingshortperiodbody waves.However,the spotty coverageof cal studiesstandsin the way of computingglobalpower

1-D methods. Local studies generally seem to produce larger velocity anomalies than are found in the low-order global inversions,indicating that the filtering action of such models cannot be neglected. Figure 5 gives a histogram for the $ velocity variations at var-

ious depth levelsin model SH12/WM13. At a depth of 136 km, the average$ velocityis 4.452 km/s with a ambiguously.We note, however,that Gudmuhdsson et standarddeviationof 0.068 km/s and extremesof 4.305 al. [1990]use global International Seismological Cen- and 4.660; at 310 km, the averageis 4.707 4- 0.049 km/s tre (ISC) delaytimesto arguefor a whiteheterogeneity with extremesof 4.609 and 4.851 km/s. The range of spectrumfor upper mantle heterogeneitiesat least out velocityvaluesdecreases with depth (Figure 6). Thus, to the resolvingpowerof high-frequency P waves(100- for SH12/WM13, the maximum variation in Vs at 136 200 km), implyingthat the upper mantle is heteroge- km is 4-4% over the whole Earth, with about 95% of all neous at all resolvablescales,none of which is clearly valuesin a narrow range of 4-3%, within 4-2% at 310 dominant. km, and within 4-1% at 660 km. Suchvaluesare easily Additional support for the view that the global exceededin regional and local studies, even outside of models underestimate lateral heterogeneityin the up- the major subduction zones. The models presented by per mantle comes from a comparisonbetween velocity Grand and Helmberger[1984a]haveshearvelocitiesof at high angular order and resolving the question un-

NOLET ET AL.' UPPER MANTLE

Vs (km/s) at 80 km '

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5.5

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Figure 5, Histogramsfor S wavevelocityvariationsin the globalmodelSH12/WM13 [S• et 1994]at variousdepth levels.

Model SH 1 2

100

200

300

400

500

600

700

Depth (km) Figure 6. A sequenceof 1-D modelsgeneratedby e•aiuatingthe depth dependence of model SH12/WM13 at 411pointson Earth, spacedabout 1100km apart. The velocitydiscontinuities are imposedby the background model(PREM), the depthdependence of t;helaterallyheterogeneity is smooth with a resolvingpower in the parametrization of about 230 km.

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4.29 km/s and 4.775 km/s at 125 km depthbeneath ters[1991].Suchrootsforman essentialelementin Jorthe eastern Pacific and Canadian shields respectively. dan's[1975]tectosphere hypothesis.Structuraldetails A modelderivedby Humphre,Iset al. [1984]for south- within the tectosphereare likely. Grand and Helmberger ern Californiashowsa thin (about 50 km wide) tabu- [1984a]founda negativevelocitygradientnear 175km lar structurewith a 2-3% fast anomalyextendingfrom

depth beneaththe Canadian shield. œeFe•reand Helm-

near the surface to 250 km depth beneath the Trans-

berger[1989]founda similarstructurefor P waves,also

verse Ranges. A 2-4% slow anomaly is also present in this area to 125 km depth giving a total range in mantle heterogeneityof 4-7% within the upper 150 km of southernCalifornia. Humphreysa•d Duecker[1994] have extended their studies to arrays throughout the western United States, finding peak to peak variations of mantle velocity as large as 8%. Over the whole of

beneath

the western United States there appears to be a periodicity in the structural variations with a wavelength of about 500 km. Sipkin and Jordan [1980] have found a similar scale length variation in the western Pacific

using multiple bounce $c$ waves. It is also interesting to note that for the western U.S. arrays, Humphre,Is and Duecker[1994]only reducethe RMS of the travel time residualsby about 50%, even though they allow variations

to occur over scales as short as 20 km.

The

unexplained delays appear coherent and are unlikely

to be due to randomerrorsin time measurements (E. D. Humphreys,personalcommunication, 1993),and it may be that there are large-amplitudeheterogeneities with very short wavelengthsthroughoutthe upper few hundred

kilometers

of the mantle.

Evidence for pervasive smaller-,•;cale heterogeneityis alsofound in observationsof short-period arrivals across

the Canadian

Low-Velocity

shield.

Zones

The existence of a low-velocity region beneath the Phanerozoic platforms has been establishedin many early surface wave studies. A preciseinterpretation of the dispersion data is hampered by,recognition that the upper mantle under many continental regions ex-

hibits seismicanisotropy[Babuskaet al., 1984; Regan and Anderson,1984] which, by itself, also may show largelateral changes,as observedfrom shearwavesplitting measurements [Silver and Chan, 1988]. For example, recent broadband observationsbeneath northern Australia [Kennett et al., 1994; 1994]haverevealeda verylargecontrastbetweena 210km-thick lithospherewith very low intrinsicattenuation and the astenosphere beneathwith a very high lossand slightlyreducedshearvelocities. Shearwavesplitting observations for the refractedS waves[Tonget al., 1994] are compatiblewith about 1% higher horizontalshear wavespeedsin the asthenospheric zone,whichmay well representthe locationof greater mantle flow. SH12/WM13 showsa pattern of low shearwavevelocitiesextendingto at least 250 km beneath ocean

portable arrays with a range of aperturesand station ridges,confirming earlysurfacewavestudies[Wielandt

spacings[Ke•mettand Bodyman, •990]. To matchthe

andKnopoff,1982]. 2hanga•d Tanimoto[1992]have

relatively rapid changesin amplitude behaviorfor refracted wavesfrom singleeventsacrosssucharrays,heterogeneityon scalelengthsof 200-300km with at least 1% variability is required. Kenneff a•d Nolet [1990] have demonstrated that such small-wavelengthheterogeneitydoesnot have a significantinfluenceon longerperiod surfacewavesand so would not be detectedin typical waveform tomography.

recentlychallengedthe view that the low velocitiesunder the ridgesextendto a depthwellin excessof 100km. However,the poor fit of the predictionsfor the Zhang

Major Upper Mantle Features In this section we review various results on the seismic

expressionof tectonicfeatures,and will focusattention to someimportant problemsstill outstanding. Deep Continental

Roots

and Tanimoto

model to observed $$ wave travel times

putstheir conclusion in doubt[$uet al., 1992].This debate over the depth extent of mid-oceanicridgesseems to mirror that over the power in small-scale heterogeneities:in both cases,data setswith differentresolution lead to incompatibleresults. The imagingof smallscaledetail of upwellingof magmabeneathoceanridges is made very difficult by the combinationof very few seismicstations located in oceanic areas, the poor reso-

lution at depth usingfundamentalmode surfacewaves, and the fact that imagingof low velocitiesis hampered by diffractionof wavesaroundthe low-velocityobstacles [Wielandt,1987]. The same is even more true for hotspots. Imaging ex-

ModelSH12/WM13 [$uet al., 1994]has400-km-deep perimentsbeneath the Hawaii hogspot[Ellsworthand keelsbeneath Canada, Baltica, much of Eurasia, Africa, Ko•anagi, 1977],Yellowstone [I•er et al., 1981],and et al., 1983]all indicatea sublithoBrazil, and the western part of Australia. The vertical Iceland[Tr•lggvason sphericorigin for the mantle plume, but resolutionis quicklylost at depth, denyingus an answerto the important questionof upper or lower mantle origin for hotspots.Surprisingly, Nataf and VanDecar[1993]reand Helmberger[1984a,b]and œerner-œam and Jordan port a very small but clear delay time signalthat may [1987],who showhigh 5H velocitiesextendingto such be causedby a plume beneaththe Bowiehotspot,indidepth, and fits the correlation of large SS-S residuals catinga lowermantle originfor the upwellingmaterial. resolution is that of the radial parametrization used, or about 230 km, making a depth extent for roots beneath shield areas to at least 300 km very probable. This is in agreement with regional models of Grand

with tectonic province shown by Wood•vardand Mas-

This observation could be made thanks to the fortunate

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location of a denseseismicarray, the WashingtonRegional SeismicNetwork, with digital recording,making very accurate delay time readingspossible. Low velocities, together with low Q, have also been observedin or just above the transition zone in active subductionzones.Sacksand Okada[1974]report very low Qp valuesabovethe deepparts of the slabsin Japan and Fiji and similarly low Qp under Peru. Zhou and

it, t

orro-

orated these observationsby imaging low velocitiesin the west Pacificcorrespondingto the low Qj> zonesand alsofoundvelocityanomaliesoftenextendingawayfrom the slab. This is alsoobservedby Revenauõhand Sipkin

HETEROGENEITY

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ily visible in their data and do showvariationsin depth with peak to peak amplitude variationsof about 30 km,

possiblyanticorrelated[Revenaugh and Jordan,1991a] with the largest uncertainty in the depth contributed

by the upper mantle velocity [Stammleret al., 1992]. A negative correlation would be in agreement with a thermal origin for the variations since the sign of the Clapeyron slope is positive for the olivine to •3 spinel transition at 400 km but almost certainly negative at

660 km [Ito and Takahashi,1989]. Evidencefor longwavelength topography was also found from underside

reflections PiooP [NeelearidSnieder,1991]. Recent modelsshowa relatively sharp changein ve-

[1994],whoreporta zoneof low SH velocityjust above locity at the major discontinuities,althoughthe details the 410-kin discontinuityextendingfrom the Japanese

of the

subduction

lengths involved. The model shr14 for shear velocity

zones beneath

China.

Zielhuis

and Nolet

transition

cannot

be resolved

with

the wave-

[1994]observea low S wavevelocityanomalyat tran-

[Helmbergerand Enger•,1974]did not includeattenu-

sition zone depths beneath an ancient suture zone in

ation, but an allowance for attenuation was included

easternEurope,while Taylor and Toksoz[1979]found in later studies[e.g., Grand and Helmberger,1984b]. low P wavevelocitiesat the locationof the subducting Kennett [1975]pointed out that when attenuationwas proto-Atlantic in the northeastern United States. No- includedin upper mantle models,the pattern of amplilet and Zielhuis[1994]conjecturethat suchzonescan tudes associatedwith the presenceof the major velocbe explained by a mechanismof water injection which ity transitionscould be matchedwith a rapid change would permanentlyerodethe tectosphereat its edges. compatible with the constraints on transition thickness inferred by Richards[1972]from observationsof preUpper Mantle Discontinuities cursorsto P'P' (PKPPKP). A high gradient over a A commonfeature of both short- and long-periodre- depth interval of 10 km will produceessentiallythe same fraction studiesis a relatively high gradient within the synthetic seismogramat wide anglesof incidenceas a transitionzone(from410 km to 660 km depth). The sharpdiscontinuityevenat frequenciesas high as I Hr. overall increasein velocity from about 425 to 625 km is [Burdickand Orcutt, 1979]. At longerperiods,such well resolved;however,the detailsof the gradientwithin as those used to study shear structure, the situation is this depth interval are uncertain, and the existenceof a worse,and a sharpjump in velocityor a high gradient over a depth interval of about 40 km cannot be distinguished. The strongestevidencefor the sharpnessof The magnitude of the jump in P wave velocity across discontinuities comes therefore from vertically incident the 410-km discontinuityis highly variableamongdif- waves. Adams[1968, 1971],Engdahland Flinn [1969], ferent models;it variesfrom as little as 0.23 km/s for and Whircomband Anderson[1970]were the first to PREM [Dziewonski andAnderson,1981]to as muchas observeprecursorsto PKPPKP waves at frequenciesof 1.20 km/s in a modelfor the Fiji plateau [Frohlichet about 1 Hr. which are interpreted as near-vertical incial., 1977]. Sucha large disparitymost probablyarises dence reflections from discontinuities within the upper from differencesin the style of interpretation in combi- mantle. Nakanishi [1989]and Benz and }ridale[1993] nation with the limited resolvingpower of the available have performed more recent studies on the sharpness data, rather than from significanttectonicsignals.The of the transition. In general, the 660-kin discontinuvariability for the jump in $ wave velocity at the 410- ity appearsto be a good reflectorof near-vertical, 1-Hr. km discontinuity among different models is much less P waves indicating that the transition is completed in an interval of 4 km or less[Richards,1972]. Support[Nolet and Wortel, 1987]. The data modeled with synthetic seismogramsare ing evidencefor the sharp nature of the discontinuity generally wide-anglereflectionsfrom discontinuitiesor also comesfrom P to S convertedwaves[Vinnik, 1977; turning rays. The travel times of suchphasesare very Stareruleret al., 1991]. Local focusingand defocusing sensitive to velocities in a r.one about 100 km thick of these waves has been used to infer a topography on above the discontinuities,resulting in a trade-off be- the 660-kin discontinuity of more than 10 km amplitude tween velocity and discontinuity depth. The travel over wavelengthsas short as 200 km beneath western times of near-vertical reflections from discontinuities are Europe[Paulssen,1988; Van der Lee et al., 1994]. Such better suited to estimating depths of discontinuities, pronouncedtopography could be expected to disrupt but depth estimates are still susceptibleto the influ- the refracted wave field and so alter the inferred sharpence of the reference velocity used to convert time to ness of the discontinuity. The 400-kin discontinuity is generally a much poorer d•pth. ••• •d •o•d• [•OO•,b], S•,• [•], and ShearerandMasters[1992]haveusedstackedlong- reflector, indicating that it is probably spreadover more period data to searchfor near-verticalupper mantle re- than 5 km depth or that it exhibits topography that flections.The 410- and 660-km depth reflectorsare eas- influences the amplitudes of reflected waves. It should

sharp discontinuityat about 520 km depth is not well established.

23,762

NOLET

ET AL.: UPPER

MANTLE

be noted, however,that in someregionsthe 400-kin dis-

continuitydoesreflectshort-periodwaves[Bowmanand Kennett, 1990; Vidale and Benz, 1992]. The variability of observationsof reflected or conv,;rtedphasesfrom the major discontinuitiesis a longstandingproblem in seis-

mology. The variations may reflect lateral changesin the character of the discontinuity, but a definite answer cannot yet be given. Someregionalmodels[e.g.,Mechieet al., 1993]have a high gradient zone or a discontinuity near 520 km depth with lower gradientsabove and below this depth. Shearer[1990]recentlyrevived the discussion on the existenceof a phasetranformation in the transition zone by his observationof stackedphasesoriginatingat this depth,whileRevenaugh andJordan[1991a]alsoobserve

HETEROGENEITY

is still considerabledebate on the fate of subducting slabsonce they are old and denseenoughto sink to the bottom of the upper mantle. In an effort to avoid the problemsposedby poor res-

olution, Creagerand Jordan [1984]averagethe effects of wave propagationoutside the mantle by projecting all delay times on a residualsphere. From their results and forward modeling,they concludethat slabpenetration into the lower mantle occurs, implying a convec-

tive regimethat involvesthe wholemantle rather than a stratified system. For portions of some subduction zonese.g., the northern Kurilesand the Marianas,penetration of slabs is confirmed by seismictomography. In other areas, such as Izu Bonin and central Japan, the slab appearsto deflect[Van der Hilst et al., 1991] and may remain stagnant on top of the 660-kin discona minor reflector at a mean depth of 520 km. Other modelshave a more uniform gradient which is greater tinuity [Fukaoet al, 1992]. The nature of the fate of than that predicted for self-compressionof the spinel subducted slabs appears to have a strong correlation phaseof olivine [Bassand Anderson,1984]. However, with the tectonichistoryof a region[Van der Hilst and Joneset al. [1992]and Cummins.et al. [1992]find no $eno, 1993]. compellingevidencefor a jump in velocitynear 520 km from analysisof waveformsreturned from the transition What Next? zone.

Secondarydiscontinuitiesare no• as well constrained.

A numberof profilesfrom shieldareas (8-13 in Figure 1) havestructurenear 210 km with a zoneof reduced velocity beneath. These observationsdirectly

contradictan earlierview by Anderson[1979]that the Lehmann discontinuity, with a positive jump in velocity, is a worldwide feature with a common cause and

Much of the progressin unraveling small-scaleseismic structure of the upper mantle was generated by a formidable increasein both the quantity and quality of seismicdata sincethe 1960s. The availability of almost 30 years of arrival times from a worldwide network of seismicstations reported to the International Seismo-

logical Centre made possiblemuch of the imaging of commoncharacteristics.tlevenaugh. andJordan[1991b] subductionzonesthrough delay time tomography.The have reported $H wave discontinuitiesnear 300 km be- installation of the WWSSN network, now largely reneath the shield region of westernAustralia, and near placed with the new digital, broadband stations, trig200 km in northernAustralia whereother studies[e.g., gered the analysisof long-periodwavesso important Leven, 1985; Bowmanand Kennett, 1990] (Figure 1) to the global imaging of structure in the upper mantle

have proposeda velocityjump usingrefracted P waves. and below. Increased access to observations is now beRecentinterpretationsof Russiandeepseismicsounding ing providedthroughthe Internet directly from stations data [Mechieet al., 1993]introducea complexfeature and through internationally operatingdata centerssuch in velocity near 210 km; see profiles 12 and 13 in Fig- as the IRIS Data Management Center in Seattle or the

ure 1. Vidale and Benz [1992]alsoreport observinga

Orfeus

reflector near 200 km depth within subduction zones. These discontinuities,however,have not been proposed as global features, and their interpretation is still an open question. Subduction

Despite all these gains in data a major problem in mapping heterogeneitycontinuesto be the limited resolving power of the available data sets. Local tomographic investigationsare hampered, often more than global studies, by the patchinessof data coverage.The

Subduction zones cause the most pronouncedvelocity anomalies in Earth's upper mantle. Gubbins and

The density of seismic•a,tions at the surfaceof Earth generally falls far short of what would be needed to

Data

Center

in the Nether]ands.

lackof resolutionis causedby a numberof factors:(1)

inferred,(2) $nieder [1991] report arrival times for compressional match the scalelengthsof heterogeneity wavesthat indicate path-averagedslab velocity anoma-

the sources,earthquakesor explosions,are not spread

lies in excessof 5% for the Tonga-Kermadecslab, although the layer with the highest velocity may be thin

uniformly,(3) the precisionof routinedelaytime readingsis oftenlow, and (4) thereare theoreticallimita-

(lessthan 15 km) as evidencedby the dispersive char- tionsto the resolvingpowerevenif we had an ideal coverageof wavepathsthroughoutthe planet. Progressis

acter of the wave. Since the anomaly is positive, the fast rays arrive before any diffracted waves, making slabseasierto image than low-velocityzones. Moreover, there is abundant seismicity at depth in most conver-

gent areas, resulting in increasedresolvingpower, despite the common lack of seismic stations seaward of the zone. Despite these fortuitous circumstances,there

to be expected on all these points. The density of seismicstations can be improvedconsiderablywith moderncommunicationmethods. Cellular phone use is expectedto grow stronglyin the next decade,and the planned use of a multisatellitesystem for commercial

communication

will make the transfer

NOLET ET AL.: UPPER MANTLE

of digital data from a massivenetwork of simple seismometers with accurate time stamps possible. Digital analysisof wavesenablesvery accurate measurementof

delay timesusingcross-correlation techniques[VanDecar a•d Cro•on, 1990]. By improving measurement procedures [here is a good chance that the accuracy of travel time observations can be upgradedby an order of magnitude, with a consequentimprovement in resolvingpower, whereas to achievethe sameimprovementin resolutionthrough statistical averagingof more observationswould require an increase in the quantity of the data by 2 orders of magnitude. At sea,deploymentof large arrays of hydrophonesis a provenmilitary technology,and the use of data from arrays like the secret acousticsubmarinemonitoring net-

HETEROGENEITY

23,763

arrays would actually be quite modest and might give us the seismologicalequivalent of a Hubble telescopefor only a fraction of the cost. The recent rate of improvement in the quality of seismic data has outstripped the comparable development of interpretation tools. Investments in recording technologywill be of greatestbenefitif it can be matchedby improved methods of modeling the influenceof heterogenity on the seismicwavefield. Nonlinear optimization

hasbeensuccesfully appliedto waveformfitting [Nolet, 1990],extendingits applicabilityto frequencies as high as 50 mHz. For the interpretation of higher frequencies, it will be necessary to incorporate scattered wave energy into the inversion. Current techniquesfor the coupling of normal modes are too inefficient to be applied directly in nonlinear optimization schemes. Tarantola

works(knownby acronymssuchas SOSUS)in scientific [1987]and Geller andHara [1993]haverecentlydevelinvestigationsawaits a relaxing of security requirments, and a more agressivepolicy of data preservation until then. Since such arrays are commonly located at the seaward side of oceanic trenches,a large increasein resolving power would become available that might help decide the fate of subductingslabs. We have to wait passively for earthquakes to occur, and just now seismologyhas gone through a few

oped efficient techniquesto invert waveform data which have already found application in low-orderglobal stud-

very quiet decades.For example,the deep(600 km)

lies of surface waves contain information that is only

earthquake that occurredon June 9, 1994, in northern Bolivia was the first large deep earthquake since the Colombian event of July, 31, 1970. Since then the very broadband, digital networks have been installed worldwide, and it is expected that these will yield superior observationsof normal mode overtones. Among other results, such data could greatly improve our knowledge of lateral variations in the attenuation factor Q. Global variations in this important seismicparameter

ies [Hara et al., 1993]. Alternatively, Sniecler[1986] has shown how to use linear inversion techniques for scattered fundamental mode Rayleigh waves in a firstorder Born approximation, and this method is potentially very powerful when large, dense seismicarrays become

available.

Polarization

and azimuthal

anoma-

beginningto be modeled[Lerner-Lainand Park, 1989; Le•shin et al., 1994; Laske and Masters, 1993]. Attempts to regionali•.eseismicanisotropyare crude and

likely to improvein the nearfuture [e.g.,Nishimuraand Fors•lth,1989;Park and Yu, 1992].

are now only knownto the lowestangularorder [e.g., Romaaowicz,1990], and more local studiesstill suffer

In order to improve the quantification of small-scale heterogeneity in the upper mantle it will be necessary to be able to model the propagation of seismicwaves at higher frequencies. This will need the extension of interpretation methods to include higher order scattering, involving mode-mode conversions,for both body

from a variety of instrumental and interpretation prob-

and surface waves.

lems [Nakanishi,1993]. In the absenceof earthquakesourcesin many regions of Earth, there is a growing demand for safe but powerful artificial sources. The technologyfor large vibrators has recievedlittle attention so far, but efforts by Russian seismologistshave been successful,and such instruments could be deployed in inactive areas. Finally, the limits on resolvingpower in current theory can be improved considerablyif we find ways to efficiently incorporatescatteredenergyin the interpretation. This does not necessarilyinvolve only theoretical progress. The example of the oil exploration industry showsus that the availability of a massiveoverkill in in-

strumentationallowsusto applysimplified(firstorder)

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(ReceivedMarch 7, 1994;revisedJuly 15, 1994; acceptedJuly 19, 1994.)