JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B12, PAGES !9,811-19,824,NOVEMBER 10, 1991
Mantle Layering From ScSReverberations
4. The Lower Mantle andCore-MantleBoundary
JUSTIN
REVENAUGH
1ANDTHOMAS
H. JORDAN
Department
ofEarth,Atmospheric
andPlanetary
Sciences,
Massachusetts
Institute
ofTechnology,
Cambridge
Thisisthefinalinstallment
in a four-part
sequence
thatexamines
thenatureof mantlelayeringrequired
bythe
multiple-ScSphasesand internalreflectionsobserved
withinthe reverberative
intervalof SH-polarized
seismograms.
In this paper,long-period,
SH-polarized,multiple-ScSphasesreflectedoncefrom a mantle
discontinuity
(first-orderreverberations)
areusedto searchfor abruptshear-wave
impedance
contrasts
in the
lowermantle.Beneath
thegeographic
regions
sampled,
thedepthintervalof !000-2600km (Bullen'sregionD')
appears
freeof anydistinct,radiallayering,in agreement
withthemajorityof recentseismic
modelsandthenotion
of near-adiabatic
compression
of a compositionally
homogeneous
lower mantle. To passundetected,
discontinuities
in D' mustbe eithersmall(SH-polarized
reflection
coefficient
R0<1.0%),highlyintermittent
or
subject
tolateraldepthvariations
wellin excess
of 50 km.At greater
depths,
corresponding
totheD" regionof the
lowermost
mantle,wefindevidence
for a reflectorof long-period
seismic
energysituated
anaverage
of 325km
abovethecore-mantle
boundary(CMB), similarto the discontinuity
proposed
by T. Lay andcoworkers.
Our
resultsindicatea 4.4%increase
in shearwaveimpedance
anda 1.7%increase
in densityif the2.75%increase
in
shearvelocityproposed
by Lay andHelmberger
(1983)holdstruefor our studyarea.The discontinuity
is
observed
beneath
lessthanone-third
of the18 seismic
corridors
examined
anddoesnotappearto bea ubiquitous
featureof thelowermantle.This,plusanobserved
correlation
of discontinuity
depthandlower-mantle
velocity
heterogeneity,
andthelackof experimental
evidence
for deep-mantle
phasetransformations
stronglyfavora
compositional
origin.Mantle-side
layeringnearertheCMB, perhaps
associated
witha thinchemicalboundary
layer,wouldmimictheeffectsof the surfacecrust,producing
a complexwavetrainof first-andhigher-order
reverberations
superposed
uponall otherreverberative
intervalarrivals.If presentandunaccounted
for, CMB
structure
woulddriveestimates
of crustalstructure
andQs½s
obtained
by multiple-ScS
waveforminversion
away
fromtheirtruevalues.Comparedto independent
estimates,
ourresultsadmitlittle bias.MonteCarlotestsareused
to assess
the sensitivity
of thistechnique
to CMB layering,whichdecreases
with decreasing
boundary
layer
thickness.
Nonetheless,
theresultslimit CMB transition
zonesin excess
of 20 Ion to impedance
contrasts
lessthan
12% (8% for a 40-kintransition)beneathmuchof thewesternPacific,Australia,Melanesia,andIndonesia.
•NTRODU•ON
Oxburgh, 1967; Jeanloz and Richter, 1979; Doornbos et al.,
1986], mantle coupling of the core magnetic field [e.g.,
The lower mantle is easily the largestof Earth's layers,
Bloxham and Gubbins, 1987], the fate of subductedslabs[e.g.,
constitutingsome55% of its total volume and 45% of its mass.
Dickinson and Luth, 1971; Ringwood, 1975], and the source
As depictedin the vast majority of seismicmodels,the lower
region of mantle plumes [e.g., Yuen and Pettier, 1980; Holman
mantle is remarkably homogeneous, with near-adiabatic
and White, 1982; Staceyand Loper, 1983].
increases
of densityandelasticwavespeedsthroughout
mostof
D" supports heterogeneities of many length scales [e.g.,
its upper 2000 km [e.g., Jackson, 1983]. In the lowermost
Haddon, 1982; Dziewonski, 1984; Lavely et al., 1986],
150-300 km, identified with Bullen's D" region [Bulten,
making it difficult to isolate its radial, or global, structure,
1950], the picture changesdramatically;seismicvelocities
from local variations.Coupled with geographicallysparsedata
increaseonly slightlywith depth,in someprovincesmay even
setsand low signal-to-noiseratios (SNRs) and frustratedby the
decrease [e.g., Doornbos and Mondt, 1979], and Bullen's
high velocities of the lower mantle and the necessity of
"index of inhomogeneity"r/ [Bullen, 1963], measuring
"looking" through a heterogeneouscrust, upper mantle and
departurefrom ideal adiabaticcompression
of a homogeneous
transition zone, the mapping of D" structuresurely ranks as
medium,soarsfromvaluesclustered
nearunityto greaterthan
one of seismology's most daunting tasks. Nonetheless, a
4.5 for someseismicmodels[e.g.,Cleary, 1974],andas low as
numberof model D" structureshave beenforwarded,perhapsthe
0.75 in others.In the larger frrameworkof mantle and core
most striking of which are thoseof T. Lay and associates[e.g.,
dynamics,the importanceof D" clearlyrivals thatof its surface
Lay and Helmberger, 1983; Young and Lay, 1987a], which
counterpart,the tectosphere,reflecting the influence of, and
featurea 2.5-3% increaseof shearvelocity -275 km abovethe
directlyinfluencing,a numberof geophysicallyconsequential
processes,including evolution and compositionof the core core-mantleboundary (hereafter referred to as the CMB). The
[e.g., Ringwood, 1979; Ruff and Anderson,1979; Knittle and notion of a velocity increasein D" is difficult to reconcilewith
Jeanloz, 1986], heat flux into the mantle [e.g., Turcotte and straightforwardinterpretationof D" as a thermalboundarylayer
(TBL) and, if correct,has resoundingimplicationsfor mantle
composition and evolution. (For excellent reviews of the
relevant seismic, geodynamic,and geomagneticliterature, see
•Now at the CrustalImagingLaboratory,Instituteof Tectonics, Youngand Lay [1987b] andLay [1989].)
Universityof California, SantaCruz.
Copyright1991 by the AmericanGeophysicalUnion.
Papernumber91JB02163.
0148-0227/91/91JB-02163505.00
At the heart of all these matters
is structure
of the CMB
itself, the site of the largest, localized density contrastin the
Earth, and in many ways a mirror image of the free surface.
Tomographic inversions of large numbers of PKP and PcP
travel times have revealed the CMB to be a site of grosslateral
19,811
19,812
REVENAUGH
ANDJORDAN:
MANTLELAYERING
FROMScS,
4
inhomogeneity, with evidence of boundary topography in
excessof 6 km [Morelli and Dziewonski,1987] andchemically
distinctboundarylayers (CBLs) [Creager and Jordan, 1986],
perhapsmanifestedas "continents"rafting on the liquid outer
core [Jordan, 1979]. Severalpopularviews of what might be
conspiringin this complexregion of the lowermostmantle are
offered in Figure 1.
This report summarizesour attemptsto constrainlayering
of D', D", and the CMB with members of the class of body
wavesreferredto asScSreverberations
[Revenaughand Jordan,
1987; hereafterreferred to as RJ87], certainlythe most familiar
of which are the multiple-ScSphases(denotedScSn wheren is
the number of core bounces).ScS reverberations repeatedly
traversethe mantle along steeply dipping ray paths,producing
near-normal incidence reflections where they encounter
discontinuities. ScSn arrives at the receiver without being
reflected within the mantle, making it an exampleof a zerothorder reverberation, where order refers not to the number of core
bounces n, but rather the number of internal discontinuity
reflections suffered by the wave. On an SH-polarized
seismogram,ScS• and its depth-phaseequivalentsScS•are the
only zeroth-orderreverberations.Together they dominatethe
portion of time seriesfollowing the passageof the minor arc
surfacewave train and precedingthe first major arc, body wave
arrivals (the "reverberativeinterval"), an interval spanning2 <
n < 5 and roughly 45 min of record for epicentraldistancesof
the order of a radian or less. The only other deterministic
denizensof the reverberativeinterval are the first- and higherorder
reverberations
discontinuities.
First-order
excited
by
reverberations
crustal
and mantle
have been identified
Fig. 1. (a) Model of a well-stirredlower mantle free of large-scale
chemicalheterogeneity.
Heat flux throughthe CMB may supporta
thermal boundary layer (TBL), producing a D"-like region. (b)
CoexistingCBL and mantleflow systems.Instabilitieswithin D" could
leadto mantleplumes.(c) D" CBL fed by remnants
of subducted
oceanic
lithosphere,a possibleend-memberof "penetrative"convection,in
which subducted
materialremainsdistinctfrom the surrounding
mantle
until retumingto the uppermantle,resultingin liule net flux between
upper and lower mantles.(d) Model of a stable, mantle-sideCBL,
perhapscreatedby reactionsbetweenmantleand core,swepttoward
regionsof upwelling.A core-sideCBL is shownby the stippledregion
beneaththe CMB andis a potentialadditionto any CMB model.
as distinct, isolated arrivals on long-period digital
seismograms[RJ87; Revenaughand Jordan, 1989; hereafter
referred to as RJ89], and used in previousinstallmentsof this
series to map long-wavelength variations in the depth and
strengthof the transitionzone discontinuities[Revenaughand normal-incidence,shear wave reflection coefficientsRo(z)-Jordan, 199lb; hereafterreferredto as ML2] and to decipherthe 1/261n/s(z)
assmallas 1-2%, whereIs(z)= vs(z)p(z)is nominal
complex assemblagesof shear wave reflectors in the upper shear impedance. Unfortunately, migration breaks down near
mantle [Revenaughand Jordan, 1991c; hereafterreferred to as the free surface and CMB. Discontinuities in these regions
ML3].
(e.g., the Mohorovi•i6, or M, discontinuity)producefirst- and
First-orderreverberationsare an ideal tool for probing the higher-orderreverberationsarriving within the wave trains of
lower mantle. Their near-vertical ray paths uniformly sample the zeroth-orderScSnand sScS, phases.Complicatedboundary
the lower mantle over a broad range of horizontalpath lengths interactions produce complex composite waveforms making
and renders them less sensitive to the deteriorative effects of
good fits to theoretical calculationsharder to achieve. This
lateral velocity heterogeneity.The accumulationof dynamic takes on added importance in migration; if zeroth-order
analogs(rays which arrive at the receiverby differentpathsbut reverberations are not completely removed from the
which have the same travel time, amplitude, and phase) with reverberativeinterval prior to migration, their left-over energy
increasingn raises and maintains their amplitudesat useable maps into both shallow and very deep discontinuitystructure.
levels, and because they reflect at subcritical angles, their Given the vast disparity in amplitudes of ScS, and sScS,
amplitudes are sensitive to density and velocity contrasts relative to first- and higher-orderreverberations,their spurious
alike, an invaluable asset in any description of mantle contributionto the reflectivity estimatescould be significant,
layering. Building upon the results of the previouspapersin prompting us not to seek estimatesof reflectivity within 30
this series[Revenaughand Jordan, 1991a; hereafterreferredto km of the upper and lower boundsof the mantle.However,it is
as ML1; ML2, ML3], we employ first-orderreverberationsto still possibleto use ScS reverberationsto map structurewithin
scan the lower mantle for reflectors of long-period shear these depth ranges by extending the techniquedevelopedfor
energy. To delimit permissible levels of CMB structural crustalmodelingin RJ89 to decomposethe sourcesof highercomplexity,we adaptour methodsof modelingScS, andsScS, order contamination.
waveformsto searchfor evidenceof contaminationby higherMotivation
and Methods
order reverberationsborn at transitionallayersnear the CMB.
BOUNDARY INTERACTION MODELING
Because the CMB resembles the free surface, both in terms
of intrinsic density contrast and the variation of physical
The methodof first-orderreverberation"migration"[RJ89; processesand characteristictime scales acrossthe interface,
ML2] is capable of resolving discontinuitiescharacterizedby there is every reasonto believe that it too supportsone or more
REVENAUGHAND JORDAN:MANTLE LAYERING FROM ScS,4
CBLs, be it buoyant slag floating on the molten core and/or
•ense lees settlingout of the manfie.
To remain hydrodynamically stable on geologic time
scales, intrinsic density contrasts associated with finely
stratifiedCMB layering have to be large [e.g., Sleep, 1988],
especiallyas the vertical extentof the compositionalboundary
layer is decreased. Assuming changes in velocity are
19,813
parameter in inversion) will be driven away from their true
values.To assess
themagnltude
of theseeffect,we generated
three synthetic data sets, mimicking data collected along
seismic corridor C5 connecting events in the Tonga-Fiji
seismiczone with AbbreviatedSeismicResearchObservatory
(ASRO) station CTAO in northeasternAustralia. For all three,
the mean crustalmodel employedis that providingthe best fit
commensurate
(i.e., ,•v•/•p > 0), CMB layeringcouldproduce to data (zu = 24 km, Ro(Zu) = 0.11; ML1), with random
first- and higher-order reverberationsof large enough variationsof up to 15 km in thickness,and 30% in reflection
amplitudesto detectthroughtheir contaminationof ScS, and coefficient,superposed
at eachsurfacebouncepoint of ScS, and
sScS,waveforms.Unlike spectralratiosof ScS,-ScS,.•pairs, sScS,. For two of the three synthetic data sets, additional
sensitivity of composite-ScS,waveformsto CMB structure layering at the CMB was imposedemployinga single, superincreaseslinearly with core reflection number n. The use of CMB, layer with R0 = 12% situated10 km and20 km abovethe
individual waveforms rather than ratios, however, requires CMB, respectively. Nonstationary noise (i.e., progressively
accuratemodeling of source,propagationand receivereffects. attenuated
noise),modeleduponthe actualdatanoisespectrum,
Although estimates of these quantities are made during was added to all synthetic traces. Examples of the resulting
waveform
inversion,
the estimates cannot be made
syntheticseismogramsare presentedin Figure 2. As can be
independentlyof CMB structure;it is by incorporatinga priori seen in Figure 2, the effect of the 10-km-thick CMB layer is
knowledge of crustal structure and attenuation that we can difficult to detectby eye, whereasthe 20-km-thicklayer has an
gainfully exploit boundary interactions to search for obviouseffect on the compositewaveformsof ScS, andsScS,.
impedancelayeringof the CMB.
Assume for the moment that the CMB
can be characterized
as an abrupt discontinuity with no extended transitionsor
manfie-side layering. In this simple scenario, only crustal
reverberationscontaminatethe waveformsof ScS, and sScS,.
Modelingof the crustduringwaveforminversionwill account
for this contaminationand in turn provide estimatesof M
discontinuitydepth and strength(zstandR0(zs•),respectively).
If we addstructureto theCMB, a simple,one-layermodelof the
crustwill no longerbe able to adequatelydescribethe full suite
of boundary interactions, now the result of complex
interferences of both boundaries. If the layering is strong
enough,estimatesof z•t, R0(z•t) andQscs(alsoincludedas a free
ScS2
sScS2
I
Quantifying Resolution
Waveform inversion of data and synthetic seismogram
suiteswasusedto producethe four panelspresentedin Figure3a
and 3b-3d, respectively, depicting contours of percent data
variance explained and Qscs as functionsof zst and Ro(zst).
Figure 3b, correspondingto inversion of syntheticdata with
no CMB layering, clearly offers an excellentmatch to data,
whereasthe two models incorporatingCMB layering (Figures
3c and 3d) just as clearly do not. In the latter, neitherthe shape
of the variance
reduction
surface nor the associated estimates of
Qs•sresemblethat of data, with some Qs•s estimatesbecoming
ScS•
ScS4
I
20 Km Loyer
10 Km Loyer
Data
I
40
50
I
60
I
70
Time (min)
Fig. 2. Comparisonof the reverberativeintervalsof data and three synthetictime series.Syntheticdata setshave a mean
crustalthickness
of 25 km withrandomlydrawnvariations
up to 15 km at eachScS,andsScS,bouncepointandmeanRo(zat
)
= 0.12 with 30% RMS variation.Noise adaptedfrom the datanoisespectrumhasbeenaddedto eachtrace.Bouomtraceis
data,aboveare syntheticwith no CMB layering,10-kmCMB layer,and20-km CMB layer,respectively.
The CMB layerhas
a 24% impedanceincrease(R0 = 12%) with depthacrossthe discontinuity.
19,814
REVENAUGH
ANDJORDAN:
MANTLE
LAYERING
FROMScS,4
ridiculousfor the 20-km-thickCMB layer (Qscsestimates strength
of thereflector.Reflections
generated
by near-CMB
exceeding-3000 for reasonable combinations of crustal
reflectors
bracket
theparentScS,pulsewitha change
in sign
parameters,signalinga break down in the inversion).Best betweentopsidereflectionsprecedingScS, and bottomside
fitting crustal thicknessesand reflection coefficientsare also
reflections
whichfollowit, conspiring
to biastheapp.arent
biased.Througha seriesof suchexperiments
in whichboththe widthandhenceapparent
Qscsof the composite
waveform.
severityandCMB separation
of thebasalboundary
layerwere Variations
in theexacttimingand amplitude
of thesereflected
varied over wide limits, we have estimated the resolution arrivals
will affectthedegree
of biasintroduced,
butaslongas
attainablethroughthis boundaryinteraction
modeling(BIM) thesignof R0 remains
unchanged
so will the biasof Qs•s
obtainedby waveforminversion.
BIM remainsdie
technique,
whichis charted
in Figure4 andfoundto decay estimates
rapidly as the boundarylayer thicknessdecreases,with a methodof choicefor detecting
CMB layeringuntil the mean
minimumresolvableseparation
of' 10 km (corresponding
to boundary
layerthickness
reaches
60-80 km, at whichpoint
onlya ~5-km-thick
layer(crust)at thefreesurfaceowingto the separationof the crustal reverberationsexceeds the effective
migrationbecomesthe more
muchreduced
velocities
foundat shallow
depths)
at whichthe pulsedurationandreverberation
transitionappearsabruptthroughoutthe wavelengthband powerfulof the two approaches.
characteristic
of thisstudy(centered
upon200 km at theCMB).
Although
thislimitcanbe extended
by addinghigher-frequencyResults and Discussion
data,the rapidlydecayingsignalqualityof ScS reverberations
beyond 60 mHz will decide the ultimate level of resolution.
As in Figure2, noisy syntheticdata setsfor all 18 seismic
Outsidethis 10-km blind zone, the resolutionof BIM is not
corridors
werecomputed
usingthebestfittingcrustalmodels
obtained
fromdataandinverted.
In everycasethesynthetic
significantly
reducedby lateralvariabilityof the depthor
b
a
40
35
.............
,• 3O
E
v25
N 20
15
10
0.06
0.12
0.18
0.24
0.06
0.12
R(z•)
R(ZM)
35
35
3O
•,, 3O
E
E
v25
•-•25
N 20
N 20
15
10
0.06
0.24
d
4O
•
0.18
15
0.18
10
0.06
0.24
R(zM)
0.12
0.18
0.24
R(z)
10o
30oo
Qsos
..
Fig.3. Comparison
of fourgrid-search/waveform-inversion
results
for l0 seismograms
sampling
seismic
comdor
C5
connecting
events
in theTonga-Fiji
seismic
zonewithASROstation
CTAOin northeastern
Australia.
(a) Result
of data
inversion;
contours
correspond
tothepercentage
ofvariance
explained,
shaded
contours
toQscs.
Thedata
arebest
fitbyzst=
24km,Ro(zst
) = 0.11,andQs•s
= 210.Remaining
panels
areSimilar
inversions
of noisysynthetic
datasets:
(b)NoCMB
structure,
(c)10-km-thick
layerontheCMB,and(at)20-kin-thick
layerontheCMB.Thegood
agreement
ofFigures
3aand
3b suggests
nostrong
mantle-side
CMBlayering
for thispath.
REVENAUGH AND JORDAN:MANTLE LAYERING FROM ScS,4
19,815
synthetic
panelsis quitegood.Datafor thiscorridor
requireno
mantle-side layering of the CMB within the limits of
2O
resolution.
BIM
15
Taken as a whole, estimatesof zst obtainedfrom waveform
inversion and predictions garnered from the available seismic
refraction,reflection,surfacewave, and gravity literatureare in
substantialagreement(Figure 8 of ML1). The same is true of
-
o
RESOLVED
x.• lO
-
Qscs[e.g., Nakanishi, 1980; Sipkin and Jordan, 1980; Chan
and De), 1988], which also exhibits strongtectoniccontrol
and correlates with the depths of certain upper mantle
a:o
5
-
discontinuities (ML3),
100
biases .originating in the lowermost mantle. We conclude,
therefore, that there appears to be no strong, mantle-side
layering'ofthe CMB alorigthe 18 seismiccorridorssampledin
this study. There are mechanismsby which layering of the
CMB of the ilk associated
with a thin chemicalboundarylayer
(CBL) could remain hidden from BIM analysis.By limiting
UNRESOLVED
0
0
•
i
•
20
40
60
80
h (km)
Fig. 4. Resolution of mantle layering obtainable from first-order
reverberationmigration (FORM) and boundaryinteractionmodeling
(BIM; modelingof first- and higher-orderreverberationcontamination
providing further argument against
observations
to theSH wavefield,theanalysis
is madeblindto
fluid boundary layers and core-side CBLs. For mantle-side
of multiple-ScS
andsScSphases).
Layersthinnerthanh = 10 km (5 km
at the free surface)mimic abrupttransitionsat the wavelengths CBLs, chemical heterogeneity whose velocity-density
employedin this study(-200 km at the CMB). Darkershadingdenotes systematics do not produce significant shear impedance
higher resolution.
contrasts(e.g., •Jv,/•p < 0) is anotherplausible veiling
mechanism. To cite specific examples, Ruff and Anderson
[1979] proposea heterogeneous
layerof CaO- andA1203-rich
grid searchpanel provided a good fit to data. A data-synthetic
comparisonfor the seismiccorridor C11, connectingshocksin
the Philippines subductionzone with SRO station NWAO in
southwestern
Australia,is providedin Figure 5. Cll featuresa
strong bimodality of lithospheric sampling between two
tectonicallycomplex regions(back arc, marginal basin versus
continentaland continentalmargin regimes). zst estimatedby
waveforminversionis reasonablefor an averageof oceanicand
continentalcrusts(25 + 7), as is Qscs(194 + 35; ML1), and
even though bimodality is not explicitly included in the
synthetic data set, the agreementobtained between data and
refractory material residing on the CMB following expulsion
from the core, whiqh though slightly denser than overlying
lower mantle has wavespeedssome 5-7% slower. Similarly,
theadmixture
of Fein mantle
silicates
[e.g.,Adler,1966]acts
to increase
densitywhilereducing
•eismicwavespeed
[e.g.,
Birch,1961'Bonczar
andGraham,1982].Eltherscenario
could
render a CBL transparentto ScS reverberations while still
producinglarge delays in the travel times of transmittedwaves,
providing a potential avenue of reconciliationbetween our
observationof a structurallysimple CMB and the results of
CreagerandJordan[1986].
....... '"''"':'"';":'?•;•::•½%•::•:::':•!ii•
35
..............
'
.....
•:.f•.:•{•:•:•.`.:•..`.•:.;.::.•.;:.•::•..•t•..=============================================================:
ß•.';•t•*:;•;...•j::•;,•..
:....
'•::::::::i•::i::•ii::::•i::i::::11½•::ii...-.:½:;i::½½i::•:;i::i
::
......
•.•:-•,:',-.--:•,;•.•¾•-•ø'•...
½;,'-_-•:-t.•.-.-•-.:,•
:.:-:
....................................
.....
0.06
.'""'"'•••••......:••.••:•,;.•!i•!::::•::::::•i•:::::::::::'•
.......
*•'½'"'•
............
:::.:.:::::.;;;•:
O. 12
O. 18
0.24
0.06
O. 12
R(z)
O. 18
0.24
R(z)
90
300
Qs,:s
Fig.5.Comparison
of gridsearch/waveform
inversion
results
for14seismograms
sampling
seismic
corridor
•ll
connecting
events
in thePhilippine
seismic
zonewithSROstation
NWAO.(Left)Resultof datainversion;
datais bestfii by
zu = 25 km,Ro(zat
) = 0.22,andQs•s= 194. (Right)Similarinversionof a noisysynthetic
datasetwith no CMB structure.
Althoughdataare distinguished
by strongbimodalityof lithosphericsampling(backare, marginalbasinversuscontinental
and continental
marginregimes)not explicitlyincludedin the synthetics,
the fit is still quitegoodand consistent
with no
resolvableCMB layering.Note changein Qs•sscalefrom Figure3.
19,816
REVENAUGHAND JORDAN:MANTLE LAYERINGFROMScS,4
Ro(z) (x100)
Ro(z) (x100)
FS-8;0-4.0 0.0 4.0 8.0 -8.0
BIM allows us to probe D" in close proximity to the CMB,
but at greater separationsits resolution suffers relative to
migration of first-order reverberations. In the following
section we document the search for layering throughoutthe
lower mantle as revealedby profilesof shearwave reflectivity.
,
2000[
In RJ89 we develop a techniquefor "migration"of first-
orderreverberations,
a process
which.
mapscomplex
ensembles
arrivals
ß
1500
Migration and Mantle Reflectivity
reverberation
' ' •t':•3
' '
i
1ooo
I
REFLECTIVITY MAPPING
of first-order
.....','::
_
5OO
Ro(z) (x100)
0 -8.0 -4.0 0.0 40 8.0
'into mantle
2500
[
CMB'
•
'
Data
Parsimonious
Fle$idual
shear wave
reflectivity, producing a low-pass-filteredestimateof Ro(z).
The algorithmis two-stage,first stripping(subtractingoff) a
model
ofzeroth-order
reverberation
waveforms
obtained
bythe
Fig. 6. Data (left), parsimonious
synthetic(center),andresidual(right)
reflectivityprofilesobtainedfrom first-orderreverberations
sampling
seismiccorridorC1 connecting
shocksin the Sumba,Philippinesand
NewBritainseismic
zoneswithSROstation
CHTOin Thailand.
Ro(z
) is
waveforminversionschemediscussed
in the previoussection, the low-passfiltered (comerat 20 mHz, maximumof passbandat 30
which includes estimatesof the path-averaged,Whole mantle
traveltime •rsc
s, qualityfactorQs,s,crustalstructure,andsimple
source corrections needed to describe the long-period wave
train of reverberativeinterval arrivals.Thesequantitiesare used
to synthesize Green's, or discontinuity response, functions
(DRFs), comprising the seismogram of first-order
reverberations expected for a unit discontinuity at some
specified depth z* in the mantle. In the second stage,
migration, expressedas the normalized cross correlation of
stripped data and DRFs, produces a maximum-likelihood
estimate of Ro(z*). By sweepingz* through the mantle and
stacking over suites of individual seismograms,an estimate
mHz) normal-incidence,
SH-polarizedreflectioncoefficientof the
mantle. The syntheticprofile is computedfor a mantle containing
reflectors
at onlythefourlabeleduppermantledepths.Large-amplitude
featuresin the lower mantle are false images of upper mantle
discontinuities,
artifactsowingto poor ray:parameter
coveragewhich
resultsin a nearmirrorimageprofile. Stipplediegiondenotesthe 5%
falsealarmprobabilitylevel. Gap in midmantleis dueto the destructive
interferenceof topside and bottomsidereflectionswhich leads to
instabilityduringmigration.While signaturesof the X (near 310 km),
"520-km" and "710-km" discontinuities
are evident(ML2; ML3), this
pathdisplaysno evidencefor reflectorsdeeperthani000 km.
J•0(z)of thetruemantlereflectivity
Ro(z
) is created
(Figure6). lower mantle discontinuityat zk simply by choosingzk such
Ideally,peaksin /•0(z) only occurat depthsof mantle that
discontinuities; in reality, the exigencies of experimental
geometry and inherently band-limited data introducestructure
ß,(p= 0)l, =
=0)l,
(3)
/=1
into Ro(z) that is not presentin the mantle.By and large these
artifacts can be detectedand accuratelyrecreatedby synthetic
This ambiguity of target depths manifest itself in
forward modeling and seldom pose a problem (ML3). One reflectivity profiles that are near mirror images about the
artifact, however, has proven more difficult to surmountand is midpointof the mantle with respectto travel time (-1250 km),
of directimportance
to paper.
Depth Ambiguity
A satisfactoryapproximationto reverberationtravel times
can be obtained from
•: _-•'• •(p=0) + pA, l,
(1)
i=l
whichbreaksthe mantleinto N discretelayersof thickness&i
andvertical, two-way travel time •.(p = 0), eachtraverseda total
of li times alongthe ray. Ai is the angulardistancecoveredby a
ray segmentin ith layer; in a higher-orderapproximation,Ai
would exhibit complex zi andp dependenciesalong individual
elementsof the ray; here it is adequateto assume
•= _'7"-•[
2z,+&,]
2nzc
(2)
with slight elongationin the lower mantle owing to greater
velocity. Near the travel time midpoint, topside and
bottomside reflections destructively interfere, leading to
instability in migration. As a result, no attempt is made to
retrieveRo(z)in a 100-km-depth
intervalcenteredaboutzx/2=
1250 km, explainingthe gap in the midmantleof Figure 6.
Ambiguity also explainsthe large-amplitudefeaturesobserved
in the lower mantle of the reflectivity profiles, which are
simply "false images" of the 410-km and 660-km
discontinuities.Mathematically, Ro(z)is derived from the true
functionRo(Z)via
½'•(z,))
= Ca•(*)*
JR0('0
- Ro(*s,s-,r)] (4)
where Cad(X) is the signal autocorrelation,and z, the depth
corresponding
to two-way,vertical travel time • measuredfrom
the free surface(notethe implicit mappingfrom time to depth).
For the 410-km and 660-km discontinuities,a priori
with A the epicentral distance,n the number of core bounces, knowledge of depth is good, and ambiguity presentsno
andzc= z•t-- 2885 km the depthof the CMB. Ray parameters
of
first-order reverberationsvary little with reflector depth or
increasing core bounce numbeg n, such that p A i is nearly
constantamongthe rays traversingthe layer. This produce
s an
interpretiveproblem apart from obscuringthe depth region
populatedby false images.In mostcases(ML2), the depthsof
the majortransition
zonediscontinuities
canbe estimated
to
better than +10 km. For the lowermost mantle, as well as the
unhappysituationwheretraveltimesof topside(bottomside)shallowestportions of the upper mantle where discontinuity
reverberations
froman uppermantlediscontinuity
at depthzj structuresare not nearly as well known, ambiguityprovesmore
can be matchedby bottomside(topside)reverberationsfrom a
noisome.For instance,a reflectormarkingthe lid-low-velocity
REVENAUGHAND JORDAN:MANTLE LAYERING FROM $cS, 4
zone interface(LVZ) at a depthof, say, 100 km, would also
registerasan impedance
increase170km abovetheCMB and
havea potentialreal-worldanaloguein the reflectorproposed
by Wrightet al. [1985]for theportionof D" beneaththesouth
China Sea. Reverberations alone cannot resolve depth
Ro(z) (x100)
FS-8;0-40
0.0 4.0
19,817
Ro(z) (x100)
8.0
-8.0
-4.0
0.0
4.0
Ro(z) (x100)
8 0
-8.0
-4.0
0.0
4.0
8.0
___2•ii½iF*:':'*'"'"':f.!
!__
•i•½:;•."?'
ambiguity;luckily, we have numerousother sourcesof
informationat our disposalwhich can.
In ML3 it is shownthat discontinuitystructurein the upper
mantleis tectonicallycorrelated;that is, corridorswith similar
2000
[
tectonic sampling,as quantifiedby the Jordan [1981]
•i•.......
regionalization,display essentiallysimilar assemblages
of
Data
Parsimonious
Residual
uppermantlereflectors.
The transitionzonediscontinuities
do
not appearto be tectonicallycorrelated(ML2), and it seems Fig. 7. Data (left), parsimonioussynthetic(center),and residual(fight)
logicalto extendthisobservation
to deeperdiscontinuities
as reflectivity profiles obtainedfrom first-orderreverberationssampling
.........
305 km
well. Given these assumptions,
it is possibleto empirically seismiccorridor C9 connectingshocksin the Sumba and Philippine
resolveaspects
of reflectivityprofile ambiguityby appealing
to tectonic correlationsin assigningreflectors to either the
upperor lowermostmantle.A secondand moreobviousavenue
is comparison with previous seismic studies, which agree
remarkablywell with our upper mantle interpretations(ML3)
and otherwise offer little a priori encouragement for the
widespreadoccurrenceof impedancedecreases
beneatha depth
of 150 km in the upper mantle. On the other hand, impedance
increases are widely spread throughout the upper mantle,
giving us confidencein assigninga lower mantle depthto any
feature that would otherwiserequire an impedancedecreasein
the upper mantle below a depth of 150 km. Conversely,any
featurewhich wouldrequirean impedancedecreasein the lower
mantle is interpretedas the false image of an upper mantle
reflector, a choice motivated not only by tectonic correlation
and abundanta priori observationsbut also by the certain
hydrodynamic instability of density decreasesin the lower
mantle (althoughwe must be careful in associatingimpedance
decreaseswith densitydecreases,viz avis the effect of Fe on
mantle silicates). While the analysisis subjective, we believe
the rewards of imaging lower mantle discontinuity structure
easily outweigh the dangers.
REFLECTIVITY OF THE LOWER MANTLE
Data Processingand SyntheticForward Modeling
To
alleviate
the
deteriorative
effects
of
seismic zones with ASRO station MAJO on Honshu. The synthetic
profile was computedfor a mantle containingreflectorsat only the
threelabeleduppermantledepths.The otherlabeledpeak, at 305 km
above the CMB, may be evidence of a reflector atop region D".
Filtering and otherconventionsare the sameas Figure 6.
significant featuresof the data profile. To model the shallow
structureof this path, we haveplacedimpedanceincreasesat 73
km and 217 km depth, a structure broadly consistent with
nearby corridors and upper-mantle models derived through
independent means [ML3; Hales et al., 1980]. Similar
comparisonsof the reflectivity for corridorsC9 and C6 are
shownin Figures7 and 8. C9 utilized 14 eventsin the Sumba
and Philippinesseismiczonesrecordedat ASRO stationMAJO
in Japanand requiredonly a small impedancedecreaseat 86 km
in the upper 300 km of the mantle, a refl•tor we associatewith
the low-pressurelimb of the low-velocityzone (LVZ) and refer
to as horizon G in ML3. C6, connectingSRO stationGUMO on
Guam with 20 events in the Sumba and Philippine seismic
zones, is recreated with remarkable fidelity by a mantle array
featuringonly a singlediscontinuityin the upper 300 km, in
thiscasean impedanceincreaseat 61 km depth(the H horizon).
The residual of data and syntheticprofiles for thesecorridors
reveals unmodeled reflectivity peaks and troughs at several
depthsin the lower mantle,with conspicuous
Ro(z) crestsatop
D" (305 km and 340 km above the CMB, respectively). The
reverberation
Ro(z) (x100)
Ro(z) (x100)
Ro(z) (x100)
splitting (destructive interference and dephasing due to
-8 0
-4 0
0.0
4 0
8 0
FS8o -4
8
,o _ ,
velocity heterogeneity and discontinuity topography), the
reflectivity profiles consideredin this report are computedfrom
data subjectedto a more restrictivelow-passfilter (comer at 20
mHz, tapering smoothly to zero admittanceby 30 mHz) than
1ooo
i•:.•:•,:•
the conventionsdetailed in ML1. Besidesreducingthe impact
of reverberationsplitting, filtering also improves the level of
fit obtained between data and synthetic profiles, producing
simpler waveforms and attenuating apparent structure in
residual profiles arising from inevitable modeling and noiserelated mismatchesof data and s,vntheticprofiles.
,.
,
, ••
,
LI
CM
BL-•---• Data
Parsimonious
Residual
Figure 6 compares data and parsimonious synthetic
reflectivity profiles for C1, the heterogeneous corridor
Fig. 8. Data (left), parsimonious
synthetic(center),and residual(fight)
connectingshocksin the Sumba,Philippines,and New Britain reflectivityprofiles obtainedfrom first-orderreverberationssampling
,
,
,
,
5oo•'
•- :'_i_'---'-•iii
•ooo :•!ii!i
ii:
seismic
zones
with
$RO
station
CHTO
in
Thailand.
A
discussionof synthetic forward modeling is included in ML3,
which carefully details the assumptionsand criteria adopted
during profile modeling, and we will not go into the procedure
here other than to say that "parsimonious" refers to the
simplest reflector array adequatelyrecreating the statistically
seismiccorridor C6 connectingshocksin the Sumba and Philippine
seismiczoneswith SRO stationGUMO on Guam.The syntheticprofile
was computedfor a mantle containing reflectors at only the three
labeleduppermantle depths.The large negativepeak near 2100 km
depthis the false image of the "520-km" discontinuity(not includedin
the parsimonioussyntheticprofile). Filtering and other conventions
are the sameas Figure6.
19,818
REVENAUGH
ANDJORDAN:
MANTLELAYERING
FROMScS,4
latter features,if we were to opt for the uppermanfie alternative
interpretation, would correspond to impedance decreases
(presumablyvelocity decreases)in the range of 210-260 km
depth,resultingin a picutreof the uppermanfiethat would be
difficult to reconcile with the tectonic setting of these paths,
especiallyC9 which showsclear evidenceof a much shallower
low-velocity zone. Either way, given the limits of resolution
attainable
with
small numbers of events, none of these
reflectivity loops is highly significant. Nonetheless, it is
intriguingthat similar peaksatop D" are notedbeneathroughly
one-third of the 18 seismic corridors and that the sightings
display some, albeit tentative, geographical coherence,
clusteringin the lowermostmantle beneathwesternAustralia
and the PhilippineSea (Figure9 andTable 1).
In fact, this feature is the only lower mantle R o(Z) loop to
display any consistencyacrossthe data set, suggestingthat the
other peaks and troughsare largely the result of randomnoise
processes.As discussedin ML3, our primary dictum at this
level of forward modeling was to be as parsimonious as
possible in explaining the data; that is, we have proposed
additional discontinuities in the lower mantle only where
clearly necessitatedby data. If we have erred in this analysis,
our errors should favor simpler lower mantle structures.In the
future, the data volumes generatedby a mature global network
composed of Incorporated Research Institutions for
Seismology (IRIS), National Net, Geoscope,Poseidon, and
other instrumentsaugmentedby carefully designedpassive
sourceProgramfor Array Studies(PASSCAL)experiments,
will
quickly outstrip the modest data base now within our
possessionand provide a more definitive statementon lower
mantle reflectivity.
A Discontinuityin D"
In the meantime,to improvethe upon the resolutionof the
single-corridor
estimateswe have stackedthe entire 18-corridor
suite of data and parsimonioussyntheticreflectivity profiles.
Assuminguncorrelated,Gaussiannoise, this operationought
to producea factor 4 improvementover the SNR of a typical
seismiccorridor(Figure 10). We find that apartfrom two small
impedanceincreasesat 710 km and 900 km discussed
in ML2
(the false imagesof which appearnear 1830 km and 1640 km,
respectively),
theonlylowermantleJ•0(z)peakexceeding
the
5% false alarm level occurs 325 km above the CMB.
Due to
depth ambiguity, this crest can also be attributed to an
impedancedecrease200 km beneath the Earth's surface.
Fig. 9, Mercatorprojections
of surveyed
regionsdepictinglocationsof sources
(triangles)andreceivers(squares).
Shading
denotesthe portionof the lowermantlesampledby the 18 seismiccorridors.Mean apparentseparation
of horizonB from
theCMB relativeto thePREM[Dziewonski
andAnderson,
1981]. Corridor
numbers
referto Table1.
REVENAUGHAND JORDAN:MANTLE LAYERINGFROM$c$, 4
TABLE 1. Ro(z) Estimates
for HorizonB
Reflecti_o•
Source
No. Region
3
Sumba
Station
CTAO
hs *(km)
_t
Coefficient
Ro(Zs)
_t
6 Sumba-Philippines
GUMO
340+
25
0.018
9 Sumba-Philippines
MAJO
305 + 25
0.022
325 + 25
0.015
330+25
0.012
11 Philippines
13
Sumba
17
New Britain
NWAO
TATO
_t
270+
18 Japan-Izu
_t
25
0.018
B Not Observed
Sumba-Philippines
CHTO
2
New Britain
CTAO
4 Philippines
5 Tonga-Fiji
7
Izu-Bonin
8 Tonga-Fiji
GUMO
KIP
10 Japan-Izu
MAJO
12
NWAO
New Britain
14 Tonga-Fiji
15 Tonga-Fiji
SNZO
16 Philippines
TATO
et al. [1988] found no evidenceof a B discontinuity,whereas
their data for the neighboringcorridor connectingdeep South
American eventswith the U.S. and Canadastronglysupported
the Lay and Helmberger [1983] model, implying a spatially
intermittent or oscillatory B with expressionsat moderately
short length scales (-2000 km). This, and the comings and
goings of sightings within our data set, suggestwe should
repeat the stacking experimentsafter differentiating between
corridors that do and corridors that do not evidence B.
A stack of the 13 seismic corridors along which B is not
clearly observed is shown alongsidethe 5 for which it is in
Figure 11. Any putativesignatureof B in the formeris strongly
attenuatedrelative to the latter, confirming that it is either
much weaker (by nearly a factor of 2), not present,or varying
so greatly in thicknessalong a majority of the paths sampled
as to precludedetection.
AssumingB is the local manifestationof the discontinuity
seenelsewhereby Lay and Helmberger[1983], we can usetheir
estimateof the percentincreasein shearvelocity (Svs/vs =
2.75%) to constrainthe associatedcontrastin density. While
this is far from an ideal procedure,in lieu of any othermeansof
separating the contributions of velocity and density to
impedance,it is the best we can do. Somejustificationcan be
amassedfrom the consistency
of 8vdv• estimatesobtainedfor a
number of geographically diverse study areas [Lay and
Helmberger, 1983' Young and Lay, 1987a], which vary by
only 10-15%. In the ensuing analysis, we have adopted the
value of R0(2560) = 0.022 appropriate for the estimate in
Figure 11 scaledby a empiricalmultiplicativecorrectionof 1.3
to accountfor splitting of first-orderreverberations.Unlike the
the 660-km discontinuity, it is not possible to directly
estimate
'Height above CMB relative to the PREM of Dziewonski and
Anderson [ 1981].
tAlthougha reflectoris indicatedin data,interferencewith upper
manfie structure
precludes
accurate
estimation
of depthorR0.
Although it is not possible to distinguish between these
alternativeswith the presentdata set, the agreementof the D"
interpretation with the discontinuity proposed by Lay and
Helmberger [1983] is compelling,whereasthere is little prior
evidenceof an upper mantle impedancedecreasenear 200 km
depth with the possible exception of the LVZ, which was
included in individual syntheticprofiles when necessitatedby
data (ML3) and needing to be uncharacteristically deep
regardlessof our modeling.
Though admittedly subjective,our preferred interpretation
is an impedanceincreaseatop D". Following the conventions
establishedin ML3 for naming intermittentand heterogeneous
mantle discontinuties, we refer to this feature as the B horizon
in honor of Bullen's seminal contributionsto study of deepEarth structure. It must be noted that even after stacking,
horizon B is not strongly required by data, with R0(2560) =
0.85% only slightly bestingthe 5% false alarm level. We must
be cautiousin applyingthis significanceestimate,however, as
there are several questionsneeding to be addressed:(1) Lateral
velocity heterogeneityand topographicrelief on the reflecting
horizon will lead to destructiveinterference(splitting) between
individual first-order reverberations in families of dynamic
analogs,reducingcorrelationwith DRFs, and biasingRo(z) to
lower values. (2) By stackingover the entire data set, we have
includedseismiccorridorsalong which B may not exist. In a
studyof Tonga-Fiji eventsrecordedin North America,Garnero
19,819
the standard
deviation
of travel
times
for first-order
reverberationsexcited by the B reflector from the amplitude
spectrum of R0(2560). Rather, to compute the splitting
correction, we assume B supports short-wavelength
topography of the magnitude observed on a corridor-bycorridor basis. This approach seems reasonable given that
minor-arc paths are as long as the apparent scale length of
topography(Figure 9). The correction allows for roughly 50
km (two standard deviations) of peak-to-peak discontinuity
topographyplus 2 s of uncorrelatedmanfie heterogeneity.(See
ML2 for a discussion of splitting and derivation of the
amplitude correction.) R0(2560) = 0.022 implies a density
increaseof 1.7% (Sp = 0.09 g/cm3) acrossthe discontinuity,
and a value for 81nv•/81np= 1.62 close to the Birch's law
scaling of 1.4 appropriate for many upper mantle silicates
[Andersonet al., 1968]. Conversely, an uncorrectedestimate
taken directly from the global stack of Figure 10 predicts a
0.5% decreasein density.The certain hydrodynamicinstability
of densityinversionsin D" precludesthis possibility.
Horizon B is also apparentin full-bandwidthglobal stacks
(Figure 10 of ML2). Given that reverberation splitting will
have a more pronouncedeffect in the higher-frequencystack,
the discontinuity must appear abrupt through most of the
wavelengthband of interestto remain visible. In other words,
it cannot
be both broad
and mobile
and still be detected
at
higher frequencies. The full-bandwidth estimate of ML2 is
centeredon 25 mHz, correspondingto a dominatewavelength
of ~200 km near the CMB. Referencedto Figure 4 of ML2, this
observation
limits
the transition
width
of B to 40 km. Other
observations are even more stringent [cf. Young and Lay,
1987a], though they are geographically removed from our
study region.
19,820
REVENAUGH
ANDJORDAN:
MANTLELAYERING
FROMScS,4
-8.0
,
-4.0
I
0.0
4.0
,,,I._
I
Ro(z) (x100)
Ro(z) (x100)
Ro(z) (x100)
FS
8.0
-8.0
-4.0
0.0
4.0
8.0
-8.0
i
-4.0
I
0.0
i
500 -
4.0
8.0
i
i
_
:::.•:!:!
:::.•::::
1000 -
_
I
I
1500 -
2000
-
2500
CMB
325 km
I
•
,
I
Residual
Parsimonious
Data
Fig. 10. Comparisonof data (left) and parsimonious
synthetic(center)reflectivityprofilesobtainedby stackingindividual
profilesfor 18 seismiccorridors.Individualsyntheticprofilescontainmodeldiscontinuities
near 410 km and 660 km depth,
plusadditionaldiscontinuities
in the upper300 km of themantleasrequiredby data(ML3). ResidualRo(z) (right)exceedsthe
5% false alarm level only twice beneatha depthof 1000 kin. The impedanceincreaseat 1810 km is the false imageof an
impedanceincrease70 km below the 660-km discontinuity
(ML2). A secondpeak 325 km abovethe CMB is evidenceof
horizon B. Filtering and other conventionsare the sameas Figure 6.
Ro(z) (x100)
-4.0
FS
,
-2.0
0.0
i
2.0
Ro(z) (x100)
4.0
i
i
-4.0
-2.0
0.0
2.0
i
4.0
i
5OO -
1000
_
•:::t:::',
i
I
:::.:.:.:•.
:.:.:.:
500
2000
_
2500
--:!•iiii!!i•i•}ii:•i:i!:•
........
325km
I
CMB
Corridors Without D"
I
I
Corridors With D"
Fig. 11. Comparisonof stackedresidualreflectivity profiles for 13 corridorsalong which B was not identified on an
individualprofile basis(left) and five pathswhereit was (fight). Only a small impedanceincreasenear 300 km abovethe
CMB can be seenin the left-hand profile, suggestingthe discontinuityseenc!eady on the right is either absent,severely
attenuated,or varyinggrosslyin depthalongthe 13 corridorsincluded.Filteringandconventions
are the sameas Figure6.
REVENAUGH
ANDJORDAN:
MANTLELAYERING
FROMScS,4
DISCUSSION
19,821
360
PossibleOrigins of a D" Discontinuity
340
Becausethe two major componentsof the lower mantle,
perovskiteand magnesiowtlstite,
are predictedto remain stable
at pressuresand temperaturesexceedingconditionsat the CMB
,_• 320
[e.g., VassiliouandAhrens,1982;KnittleandJeanloz,1987],
'" 300
E
it is unlikely that the B marks a phase transformationin a
...c280
compositionally homogeneous mantle. Changes in the
symmetrysystemof perovskitehave been observedin analog
260
componentsand may occur in silicateperovskites[e.g., Wolf
and Bukowinski, 1985] but are predictedto occur at fairly
240
shallowdepthsin thelowermantleif at all andmustbe regarded
as unlikely candidatesfor a discontinuityatop or in D". The
-0.8
-0.4
0.0
0.4
recentlynoted high-pressuretransformationof stishoviteto a
csr0,,(sec)
CaC12structure[Tsuchidaand Yagi, 1989] is hamperedby the
low abundance
of stishovite,and may be of the wrongsensein Fig. 12. Scatterplotof horizonB's separation
fromtheCMB h, versus
6xo., thetwo-way,shearwavetraveltimedelaythrough
thelowermost
• [Jeanloz,1989],to providean adequate
explanation
of B.
by modelL02.56[Dziewonski,1984]
This leads us to favor structural interpretationsof D" 400 km of themanfiepredicted
a scaling
of P toS wavevelocity
perturbations
of 6vJ6v
r=
heterogeneity and layering, a number of which have been assuming
I,
forwarded. For instance, flow of softened lower mantle material
1.1. The strong correlation (coefficient of correlation r = 0.93)
suggeststhat B is farthest removedfrom the CMB in locally slow
regions,consistentwith sweepingof intrinsicallydensemantledregs
toward regionsof locally hot and seismicallyslow upwelling. Data
point labelled SYL1 (triangle) is from Youngand Lay [1987a] and
samplesD"beneathIndia andthe IndianOcean.
toward "catchments,"or sites of plumelike upwelling, may
locally resemblechemical inhomogeneitywith vertical length
scalescomparableto D" thickness[e.g., Stacey and Loper,
1983]. Hot mantle material would be both buoyant and
seismicallyslow relative to the surroundingmantle, however,
in direct oppositionto our results(R0(2560) > 0). And while
mantle structureand low SNRs, experimentaluncertaintiesin
the model does predict 1000-km scale lengths for
the apparentdepthsof B are large (+25 km). Nonetheless,its
heterogeneity
radiatingoutwardfrom the catchments,
lengths
scalesperpendicularto these"spokes"are only of order 10 km,
and it seemsunlikely that our samplingof the lower mantle
wouldbe so frequentlyalignedwith the along-graindirectionof
flows. While in substantialagreementwith observationsof
directionallydependentscatteringin D" [e.g., Haddon, 1982],
this sort of structure is an unlikely explanation of our
observations. As an alternative, Knittle and Jeanloz [1986]
suggestthat the lower 300 km of the mantle may be a residuum
layer created by oxygen dissolutioninto the core, with an
increasein lower mantleelasticwavespeeds
owing to attendant
increasein stishoviteabundance[Knittle and Jeanloz, 1989]. A
third possibility is accumulation of the crustal and/or
lithosphericcomponentof subductedslabs upon the CMB
inferredseparationfrom the CMB, h, is positivelycorrelated
with the two-way,verticaltraveltimedelaythroughregionD",
8x'o.,at greaterthanthe 95% confidence
level (Figure12).
YoungandLay's [1987a] estimatefor the regionbeneathIndia
and the Indian Ocean also agreesquite well, thoughother
estimates[Lay and Helmberger,1983] for regionsmorewidely
separated
fromoursdo not. •O" is computed
by pathaveraging
the L02.56-predicted[Dziewonski, 1984] travel time delay
through the lowermost 400 km of the mantle. In ML1,
tomographicmodelsof the mantle were shownto accurately
predictwholemantletraveltimedelaysobtainedfromScS•and
sScS,,andin ML2 to reducethe varianceof apparentdepthsof
the 410-km and 660-km discontinuities, providing
justification for their use in this application. Since model
[e.g., Ringwood, 1975], a model which also providesa longL02.56strictlyappliesto P wavespeeds
only it is necessary
to
term storagearea for oceaniccrust [Hofman and White, 1982].
assumea scalingbetweenP andS wave speedperturbations,
Any sortof chemicallydensermaterialat the baseof the mantle
basedon recentlowermantle
will be swept towards regions of upwelling, and thinned whichwe setat By,= 1.18vp
tomographyresults [e.g., Dziewonski and Woodhouse,1987;
elsewhere[Davies and Gurnis, 1986; Hansen and Yuen, 1988],
et al., 1988].We notethattheprecise
value6vd6v
p is
which may accountfor the geographicallyintermittentnature Giardini
relatively unimportantin the following analysis,controlling
of horizon B.
only the slope, not the degree, of correlation. A further
Fortunately, some constraints on the viability of these
multiplicative correction of rn = 1.1 was obtained from
rather diverse mechanismscan be gleaned from the more
calibrationof observedand model-predicted
ScS, travel time
detailed examination
of our observations
treated in the
residuals(ML1) and probably reflects over dampingin the
following sections.Becausethese analysesattempt to draw
tomographic
models.The rangeof computed8•O.valuesis ~1
additional meaning from admittedly noisy observationsand
s, or about 1% of total two-way travel time. We note that
extrapolations(e.g., assuminga velocity contrastfor B from
although the correlation is visually very strong, the error
the results of Lay and coworkers), they must be treated as
bounds
assignedto individual depth estimatesare appropriate
speculationawaitingmore and betterdata.
and we regard the extreme degree of correlation as somewhat
fortuitous. Nevertheless, this observation supports our
Depth Variation
interpretation of horizon B over the horizon G (LVZ)
The depthof B appearsto vary by about70 km for the five alternativeintroducedby depth ambiguity.
seismic corridors where it is most confidently observed;see
The sense of correlation in Figure 12 is such that the
Table 1. Due to interferencewith the false imagesof upper discontinuitymigratesfarther away from the CMB in locally
19,822
REVENAUGHAND JORDAN:MANTLE LAYERINGFROMScS,4
slowregionsof D" anddoesso by an amountmuchgreaterthan
could be accountedfor by lowered velocities.Since we would
expectthe increasein vs associated
with the highlymobileB
horizon to be a major contributor to D" heterogeneity,and
thereforeto &o-, the Signof the correlationis oppositewhat
we would predict if all other effects were held constant,
suggestingan additional, perhaps thermal, contribution to
heterogeneity.To pursue this hypothesis,we have assumed
regional temperature anomalies are coherent in sign and
magnitudeacrossD" (a reasonableassumptiongiven the 1000to-2000-km horizontal length scalesunder consideration)and
limit the source of compositional heterogeneity to the
discontinuity itself, resulting in a simple yet remarkably
successful
model of D" heterogeneity(Figure 13). Let h0be the
maximum thicknessof D" (taken to be 400 km, althoughthe
precisevalue is relatively unimportantin what follows), and h
the observedseparationof B from the CMB; for a prescribed
temperature
anomalyAT, we find
AT(•r&ro.)
=-AT•rv
•2h0
- AT•rh' '
Fig. 13. Schematic
of oneCMB-D" scenario
consistent
with the model
of equation
(5), relatingvariations
in i5•o. to heighth of B abovethe
CMB. A manfie-sideCBL, suchas Figure 1d, wouldbe swepttoward
regionsof manfieupwellingandbecomeheated(regionH). Although
intrinsicallydenserand fasterthan the mantle it displaces,in these
warm regionsit mightbe slowerthanadjacent,cooleri'egionsof D"
(regioas
C). BemuseevenheatedCBL materialremains
denser
thanthe
overlyingmanfie,R is marginallystableand capableOf producingup
to 6 kanof CMB topography
and explainingthe anomalous
correlation
of slow(fast)D" velocitiesandtopographic
lows (highs)on the CMB.
Magnitude
of ATi}r• is exaggerated
for clarity.
(5)
layer can be calculatedfrom simple force balancearguments
where• is the meanD" velocityand Or denotesthe partial
whichsuggests
boundarytopography
8h scalesas
,
derivative with respect to temperature.Written this way, the
first and last terms can be estimated from Figure 12,
&~.•/3ituL (6)
specifically:AT(•r6•:o.)-- 1 s, andAT(•vh) -- 70 km. Assuming
v-•=7.2km/s,•rff•=-4.4 ß10'4km/søC
(ML2),and(6%I vs)h =
2.75% [Lay and Helmberger,1983], we obtainAT = 230ø C, a
value not unreasonable for long-wavelength temperature
variationsnear the base of the mantle and almost certainly a
lower bound owing to the predicteddecreaseof •}r• with
where
L
is
the
characteristic
horizontal
half
width
of
topography,It the dynamicviscosityof the overlying fluid, d
thescaleheightof flow(i.e.,vertical
extent
of theconvective
boundarylayer), and u the horizontalfluid velocity away from
(above)the interface.The scalelengthL is inferredfromthe
increasingpressure[Anderson,1987]. A modelconsistingof a
lack of correlation between discontinuity depth and velocity
layer of intrinsically dense mantle dregs, swept towards
heterogeneitywith the other regions studied by Lay and
regions of locally hot and seismically slow upwelling is
Helmberger [1983], the spatialpatternof D" velocity
capableof explaining the senseof correlationbetweenh and
8•'D- (Figure13). The altemative,whichreckonstopography
is
producedby the thermalundulationsof a phasetransformation
or disassociation,requires a Clapeyron slope an order of
magnitude greater than experimentally determined for the
transitionzone discontinuities(yD.>_--16.4MPa/øC), and as
stated earlier, there is little experimental evidence for deepmantle phase transitions, prompting us to favor the CBL
interpretationof B.
variationsnoted by Wysessionand Okal [1988], and the
distributionof sightingsdocumentedin Figure 9, all three of
which are consistentwith-2000
km coagulationlengths for
individual dreg "pools." 6p is taken from the previ6us
analyses(0.09 g/cm3); adoptingreasonablevalues for the
remainingparameters
(It = 102aPa s, d = 300 kin, andu = 0.1
m/yr) suggests6h of order 200 km which is broadly
comparablewith that observed.
While theselast few analysesare of a qualitativenature,we
For a coefficient
of thermalexpansion,
a = 2.5 x 10'5/øC concludethat the inferreddensitycontrast,topographyon both
[Jeanloz and Thompson, 1983], almost certainly a gross
the discontinuity and (perhaps) the CMB, and thermal
overestimate [Chopelas and Boehler, 1989], AT = 230ø C
anomaliesassociated
with the proposedCBL, all appearto be
decreasesdensities within the dreg layer by less than 0.03
internally consistentand reasonablefrom the standpointof
g/cm3. Eventhisoverstated
reduction
is smallin comparison
to mantledynamics,andfavor a modelin whichB marksthe upper
the inferreddensityincreaseacrossthe discontinuity(tSp=
boundaryof a compositionally
distinctbasallayer occupyinga
0.09 g/cm3),implyinga net massexcessin seismically
slow fractionof the Earth'slowermantle,perhapsresemblingFigure
regions of the lower mantle, reconciling the seemingly
anomalous correlation of topographic lows (highs) on the
CMB with seismicallyslow (fast) regionsof D" [Davies and
Gurnis, 1986; Morelli and Dziewonski, 1987]. A 300-km
accumulation
of dregswoulddepressthe CMB in excessof 5
kan, consistentwith the estimates of Morelli and Dziewonski
l d. As a final comment, we stress the need for accurate
amplitudeinformationin the studyof mantlelayering,as none
of the precedingdiscussion
wouldhavebeenpossiblein lieu of
go(2560).
Other Lower Mantle
Discontinuities?
[1987], though we do not consider this to be convincing
Apart from horizonB and impedanceincreasesconfinedto
evidence of topographyof that magnitude in light of the
numerousgeodynamicalargumentsagainst it [e.g., Bowin, the upper 1000 km of the mantle (ML2, ML3), no Other
1986; Gwinn et al., 1986; Wahr and de Vries, 1989].
The amountof accumulationwe might actuallyexpectfrom
viscous drag associatedwith convectiveflow over the dreg
discontinuitiesare required in the lower mantle. The
seismological
literature
contains
numerous
references
to lower
mantle discontinuitiesin velocity or its first derivative, with
REVENAUGH
ANDJORDAN:
MANTLELAYERING
FROMScS,
4
someclusteringof observations
neardepthsof 1200 km, 1550
km, and 2300 km [e.g., Johnson, 1969; Corbishley, 1970;
Everndenand Clark, 1970]. Unfortunately,the roles of mantle
heterogeneity
and sourceandreceiverstaticswere oftenignored
in theseearly studies,drawing into questionthe identification
of 1% velocity variationsin geographically
diversedata sets,
althoughthe clusteringof depthsis intriguing.To be present
in the regionsof the lower mantle sampledin this study and
pass undetected,these discontinuitieswould have to be small
19,823
Birch, F., Compositionof the Earth'smantle,Geophys.J. R. Astron.
Soc., 4, 295-311, 1961.
Bloxham, J., and D. Gubbins, Thermal core-mantle interactions,
Nature, 325, 511-513, 1987.
Bonczar, L.J., and E.K. Graham, The pressureand temperature
dependence of the elastic properties of polycrystal
magnesiowfistite,J. Geophys.Res., 87, 1061-1078, 1982.
Bowin,C., Topographyat the eore-manfieboundary,Geophys.Res.
Lett.,13, 1513-1516, 1986.
Bullen, K.E., An Earth modelbasedon a compressibility-pressure
hypothesis,Mon. Not. R. Astron.Soc.Geophys.Suppl.,6, 50-
(lessthan the nominalRo(z) resolutionof 1%), geographically
59, 1950.
intermittent,or subjectto lateral depthvariationsin excessof Bullen,K.E., Introductionto the Theoryof Seisrnology,381 pp.,
CambridgeUniversityPress,New York, 1963.
50 km, or coincidentwith the falseimageof an uppermantle
Chan,W.W., andZ.A. Der,Auenuation
of multipleScSin various
parts
discontinuity. As it stands our results are in excellent
of the world, Geophys.J., 92, 303-314, 1988.
agreementwith the majority of modem seismicmodelsof the Chopelas,A., and R. Boehler, Thermal expansionmeasurementsat
lower mantle,favoringthe notionof a smooth,homogeneous,
very high pressure,systematicsand a case for a chemically
homogeneousmantle, Geophys.Res. Lett.,I6, 1347-1350, 1989.
and nearly adiabatic increase of velocity and density
Cleary, J.R., The D" region, Phys, Earth Planet. Inter., 9, 13-27,
throughout
D' punctuated
by notabledepartures
within D".
1974.
CONCLUSIONS
A thoroughsearchfor layeringin the lower mantle tums up
evidenceof only one reflector. This feature (horizon B) resides
at a mean of 325 lcm above the CMB
beneath western Australia
and the Philippine Sea, and is similar to the discontinuity
proposedby Lay and Helmberger [ 1983]. Careful migration and
modeling permits an estimateof B's SH reflection coefficient
R0(2560) - 0.022, implying a 1.7% increasein density for the
Lay and Helmbergerestimateof shearvelocity contrast.It does
not appear to be a ubiquitous feature of D", showing up
strongly beneath less than one-third of the seismic corridors
examined.Where it is extant, we find strong correlationof the
apparentdepth of B with lower-mantle velocity heterogeneity,
an observationfavoring a model of a compositionallydistinct,
basal mantle layer, perhaps exceeding 300 km thickness in
someregionsof the lower mantle, and capableof producingup
to 5 lcmof CMB topography.
Strong mantle-side layering situated in close proximity to
the CMB, as associatedwith the type of CBL structureput forth
by Jordan [1979] and Jordan and Creager [1986], does not
appearto be required by the data, but could be presentif it is a
liquid (such as a core-side CBL), accompanied by small
impedance contrasts, or of limited vertical extent (<10-15
km). Extensionof the boundaryinteractionmodelingtechnique
used to reach this conclusion to higher frequencies has the
potential to place more stringentboundson CMB complexity
and offers a useful complimentto traditionalspectralratio and
diffractiondecayrate studiesof the CMB.
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of file P-wave travel
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Creager,K.C., andT.H. Jordan,Asphericalstructure
of thecore-mantle
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Davies, F.G., and M. Gumis, Interaction of mantle dregs with
convection, Lateral heterogeneityat the core-mantle boundary,
Geophys. Res. Lett., 13, 1517-1520, 1986.
Dickinson,W.R., and W.C. Luth, A modelfor platetectonicevolution
of mantle layers, Science, 174,400-404, 1971.
Doornbos, D.J., and J.C. Mondt, P and S waves diffracted around the
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J. R. Astron. Soc., 57, 381-395,
1979.
Doornbos, D.J., S. Spiliopoulos, and F.D. Stacey, Seismological
propertiesof D" and the structureof a thermal boundarylayer,
Phys. Earth Planet. Lett., 41,225-239,
1986.
Dziewonski,A.M., Mappingthe lower mantle:determination
of lateral
heterogeneityin P-wave velocity up to degreeand order 6, J.
Geophys. Res., 89, 5929-5952, 1984.
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model (PREM), Phys.Earth Planet. Inter., 25,297-356, 1981.
Dziewonski,A.M., and J.H. Woodhouse,Globalimagesof the Earth's
interior, Science, 236, 37-48, 1987.
Evemden,J.F., and D.M. Clark, Studyof teleseismicP, I, Travel-time
data, Phys. Earth Planet. Inter., 4, 1-23, 1970.
Garnero,E., D. Helmberger,and G. Engen,Lateralvariationsnear the
core-mantleboundary,Geophys.Res. Lett., 15, 609-612, 1988.
Giardini, D., X.-D. Li, and $.H. Woodhouse,Splitting functionsof
long-period normal modes of the Earth, J. Geophys.Res., 93,
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Gwinn, C.R., T.A. Herring, and I.I. Shapiro, Geodesy by radio
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Acknowledgments.
This work was supportedby NSF grantsEAR8708548 and EAR-9018589, and DARPA under AFGL contractF19628-
Hales, A.L., K.J. Muirhead, and $.M.W. Rynn, A compressional
velocity distributionfor the upper mantle, Tectonophysics,63,
309-348,
1980.
87-K-0040. J. Revenaugh was supportedby a Shell Doctoral
Fellowship.We wish to thankA.M. DziewonskiandJ. H. Woodhouse Hansen, U., and D.A. Yuen, Numerical simulationsof thermal-chemical
of HarvardUniversityfor accessto their GDSN data archivesand CMT
instabilitiesat the core-mantleboundary,Nature, 334, 237-240,
1988.
catalogs,S. A. Sipkin of the USGS for providingthe HGLP data,and
M. E. Wysessionfor a thoughtfulreview. Lunchtime Software Guild
Hofman,A.W., andW.M. White, Mantle plumesfrom ancientoceanic
contribution
7.
crust,Earth Planet. Sci. Lett., 57, 421-436, 1982.
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(ReceivedFebruary28, 1991;
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