JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B8, PAGES 15,783-15,797,AUGUST 10, 1994
Shear wave splitting in refracted waves ceturned froin
the upper mantle transition zone beneath northern
Australia
C. Tong, O. Gudmundsson,and B. L. N. Kennett
ResearchSchoolof Earth Sciences,AustrMian National University, Canberra
Abstract. The broadbandrecordingsite at Warramunga(WRA) in the Northern
Territory of Australiaprovidesgoodcoverage
of seismicwavepropagationthrough
the upper mantle for sourcesin the earthquakebelt throughIndonesiaand New
Guinea. $ wavesrecordedon the radial ($V) and tangential(SH) components
are
of comparablequality becausethe hard-rockrecordingsite minimizesthe influence
of couplingto P on the radial component.Refracted$ wavesfrom the uppermantle transition zone show a clear advanceof $H wave arrivals comparedwith
$V. Eleven polarizationanalysesof wavesreturned from the transition zoneyield
an average time shift of 2.3 s with the fast direction scattered about the transverse
direction. Nine polarizationmeasurements
of wavesreturnedfrom the top of the
lowermantleyield an averagetime shift of 1.7 s, againwith the fast directionnear
the transverse. No appreciable time differencesare observedbetween the radial
and transversepolarizationsfor paths refractedwithin the lithosphericlid. Because
the observations
of shearwavesplittingin wavespassingthroughthe low-velocity
zone, '•he transition zone, and the top of the lower mantle are not coherentin their
absolutepolarization,the causecannotlie in azimuthalanisotropy
at shallowdepths
under the WRA station. The most plausibleexph•nationis transverseisotropyin
shear within the low-velocityzone under the unusuallythick mantle "lid" under
Australia. A possiblecontributionmay comefrom anisotrol•yin •-olivine at the
top of the upper mantle transition zone. Transverseisotropyin the 200-kin-thick
layer below the lithospheredown to the transition zone with a 1% faster shear wave
speedsfor horizontalpolarizationcomparedwith verticalpolarizationwill explain
the splittingdata. For this asthenospheric
regionthe levelof anisotropyis quite
reasonable
and the polarizationis consistent
with lateralflow. The geometryof the
availablepathsfor wavespropagating
withinthe mmltlelid i• not sufficient
to place
constraints
on the anisotropic
propertiesof this heterogeneous
a,ndlow-loss
region.
Introduction
ture using sourcesin the major earthquake belt run-
ning throughIndonesiaand New Guinea (Figure 1).
Most seismic body wave studies of upper mantle
structure have been based on the analysis of t' wave
arrivals on vertical componentinstruments or on the
use of $H waves recorded on a componenttrmisverse
to the path froxnthe source. Suchnatural polarization
is not easyto find and has limited the extent of $ body
wave studies of the upper mantle. Very little work has
been done with
$V
wave records from the radial com-
The granites of the Warramungu group provide excellent recording sites with high-velocity material at or
very near the surface. Drilling at the site indicates that
the weathered layer is less than 30 m thick or much
thinner than one wavelength of the waves we record
on the WRA broadband instrument. The presence of
high P wavevelocitiesat the surface(around6 km/s)
minimizes
the contamination
of the $V
wave field.
In
consequence,it is possibleto exploit $V recordi•gs on
ponent becausethe waveformsare usually complicated
both the radial and vertical componentsof broadband
due to couplingbetweenthe $V arrivals and P waves instruments.
in the shallow structure
The Proterozoic
near the receiver.
rocks of the Tennant Creek inlier in
A three-component set of broadband seisn•oxneters
(Guralp CMG-3) has beenoperatedsince1988at the
the Northern Territory of Australia lie at an appropriWarramunga
array (WRA) in the NorthernTerritoryof
ate distancerange to investigateupper mantle struc-
Paper number 94JB00460.
Australia, 35 km to the southeastof the small mining
town of Tennant Creek. The site is installed o•t granite
and has yielded very good data for both P and $ waves
propagating through the upper mantle transition zone.
0148-0227/94/94JB00460$05.00
Kennettet al. [1994]havepresented
recordsections
for
Copyright 1994 by the American GeophysicalUnion.
15,783
15,784
TONG ET AL.: SHEAR WAVE SPLITTING IN 'rHE UPPER MANTLE
A
-lO
-2o
1 O0
110
120
130
140
150
160
170
longitude(degrees E)
Figure 1. The locationsof the eventsusedin the analysisof shearwave splitting, and the
Warramungaarray (WRA) broadbandinstrumentin the NorthernTerritoryof Australia. A
distinctionis madebetweeneventsin the Indonesian
region(opencircles)andNew Guineaand
SolomonIslandsregion(solidcircles).
P, $V and $H propagation and have illustrated the
value of the broadband recording by deriving both S
and P velocity models from the same set of events. The
$V and SH wave record sectionsare very consistentin
their general features, but when they are exami]•edin
detail, a perceptible time advance may be observedfor
many of the $H wave arrivals relative to their SV wave
counterparts.
The object of this paper is to quantify the extent
of shearwave splitting for these $ wavesrefracte(lback
from the upper mantle and to attempt to determinethe
location and orientation of the anisotropywhich induces
the splitting. The geometryof S wave propagationin
the presentstudy is very differentfrom that employedin
measurementsof shearwave splitting for phasessuchas
$KS [SilverandChan,1991;Vinniketa!. 1992]or ScS
[Fukao,1984]. In suchworkthe S wavesare traveling
closeto vertical, and the resulting splitting represents
the cumulative
effect of the structure between the core
mantle boundary and the surface. For the reft'acted
waves through the upper mantle transition zone, consideredhere, the propagation paths are near horizontal.
Onlyin thepassage
throughthe uppermost
mantle(the
mantle"lid") arethepropagation
pathscloseto vertical
and more comparableto the usual SKS configuration.
The geometryof the propagation
paths(seeFigure2)
enablesus to put direct constraintson the likely location of anisotropy.
Anisotropy
in the Earth's Mantle
would need to be describedby 21 elastic moduli, 1)utthe
available configurationsfor seismic observationshave
tended
to limit
the class of models
which
have been
consideredin the upper mantle. In particular, because
S body wave studies have focusedon waves traveling
closeto vertical, attention has been concentrated on the
variation of the S wave speedsfor different polarizations
in the horizontal plane.
Transverseisotropy is a situation where there is cylindrical symmetry in the wave velocity about an axis but
where the wave velocity along this symmetry axis differs from the wave velocity in the perpendicular plane.
In seismologicalusagethis term has often been applied
to a w•rtical symmetry axis, and then there will be a
difference between the elastic wave speedsof vertically
and horizontally polarized waves. The term azin•uthal
anisotropy is also often used in a restricted senseto refer to observationsof a directional dependenceof the
seismicwave speedson the angle of propagation in a
horizo]•tal plane.
Anisotropy can occur in a much more complicated
fashion than these simple configurationsand giw•srise
to a complexdependenceon orientation. However,since
these simplifiedmodelsform the basisof most previous
interpretation, we will use them to guide our discussion.
Two manifestations of anisotropy in seismic data
have been recognized for some time. First, Pn velocities from refraction profiles in the oceea•srevealed
in the 1960s a directional dependenceconsistent with
5% azimuthal anisotropyin compressional
velocityin
the subcrustaloceaniclithosphere[Hess,1964: Raitt
et al., 1969; Shearerand Orcutt, 1986]. Sindlarob-
Evidencefor anisotropyof variedgeometryin Earth's
mantle continuesto grow. In general,suchanisotropy servations have subsequently been made in continental
TONG ET AL.: SHEAR WAVE SPLITTING
IN THE UPPER MANTLE
Shear velocity(krn/s)
3
4
5
6
15,785
Distance(degree)
7
0
210
210
410
410
2
4
6
8
10
12
14
16
18
20
22
24
.......................................
o
660
660
Q = 1300
lower mantle
Figure 2. The shear velocity structure under northern Australia and a schematicdrawing of
ray paths in this study and their relation to the main structural units of the upper mantle.
settings[Fuchs,1983; Hearn,1984]. Second,the dis- wedgeof subductionzones[Andoet al., 1983;Fukao,
crepancy between the dispersioncharacteristicsof Love 1984; Bowman and Ando 1987] and under the conand Rayleigh waves along comparable paths was rec- tincnts [Vinnik et al., 1984; Silver and Chan, 1991;
ognize(tin the early 1960s[Anderson,1961]and was Vinnik et al., 1992]. The time separationof the two
split waves at continental sites is generally a.round1
s which would imply anisotropyof the order of 3% if
it were to be evenly distributed throughout a 150-kinicant volumenear the top of the mantle [e.g., Yu and thick lithosphere. The total set of stations for which
Mitchell,1979;Journetand Joberr,[1982].Dziewonski $K$-splitting measurementshavebeenmadegivescovof observaand Anderson[1981]estimatethe differencebetween erageof mostcontinentswith concentrations
the wave speeds for vertically and horizontally polar- tions in Europe and North America. The $KS-splitting
ized shear waves, needed to reconcile Love wave and resultsreveal someinterestinggeometricalpa•tcn•s and
Rayleighwave(lispersion,as 2-3%in the top 200 km of a generalcorrelationwith surfacetectonics[Vinnik et
al., 1992]. But it shouldbe recalledthat the splitting
the mantle in their global preliminary referenceEarth
observations
themselves include no direct inforn•ation
model (PREM). Reganand Anderson[1984]foundregionalized models for age provincesin the oceanswith on the localization of anisotropy in depth.
The last decade has also seen surfacewave ton•ogravaried degrees of anisotropy which could be confined
to the asthcnosphcrcin the oceans. L•v•que and Cara phy attempt to map the lateral variation of anisotropy
modeled in terms of a transversely isotropic shear velocity structure with the vertical polarization a few percent slowerthan the horizontal polarizationsin a signif-
[1985]useddispersion
data for both fundamentaland [seee.g.,Montagnerand Tanimoto,1991].Natal et al.,
higher modesfrom eventsalong the Tonga-Kcrmadcc [1986]founda correlationof regionswith a fast vertitrench recorded in North America to obtain transversely
isotropicmodelsfor the Pacific Ocean on one hand and
North America on the other. Their models require horizontally polarized shearwavesto remain faster than ver-
ticallypolarizedshearwavesby about1% to depthsexceeding400 km under North America. In their oceanic
model, anisotropypersiststhrough the lithosphereand
the asthcnosphcreand into the transition zone at a
higher level than beneath the North American conti-
cal polarization with the locations of mid-ocean ridges.
Tanimotoand Anderson[1984]founda high degreeof
correlation between their map of the orientation of azimuthal anisotropy with the lateral flow pattern predicted at the relevant depth by Hager and O'Connell
[1979]. Both of theseresultshave been interpretedin
terms of crystal alignment in the mantle, particularly
of fcrromagncsiumsilicatessuchas olivine, induced by
flow at high temperature.
It is now clear that at least parts of the upper mannent. Nishimuraand Forsyth[1989]reach a similar
conclusionfrom a more extensive path coverageof the tle are significantlyanisotropicand that this anisotropy
Pacific Ocean. However, it should be noted that the can have a significanteffect on wave propagatimt in didepth resolution attainable in such surface wave stud- verse ways. For simple models of anisotropy the efits is limited, particularly at greater depths.
fects on travel times (Pn), phasevelocities(surface
Measurementsof shearwavebirefringenceor splitting waves),and shearwavesplitting are well understood.
of near verticallytravelingbody waves(S, ScS,SKS) Recent developmentsin wave theory in anisotropic mehave revealed azimuthal anisotropy in the back arc dia demonstrate that gradients in anisotropic proper-
15,786
TONG ET AL.: SHEAR WAVE SPLITTING IN THE UPPER MANTLE
wavesreties can causesignificantdistortionof seismicwaveforms larger distancesthe onsetof S, representing
throughcouplingof the splitshearwaves[Thomson
et fracted back from beneath the 410- or 660-km disconcontent(0.25-0.3
al., 1992]. Further, ray bendingthroughanisotropic tinuities,has a muchlower-frequency
arrivalsalsocharacterize
regionscan resultin complexmultipathing.Elaborate Hz). Suchintermediate-period
analysisinvolvingforward waveformmodelingwill be the S waves returned from the 410- and 660-kin disconnecessaryto handle the full rangeof complexityinduced tinuities at shorter distances. The frequencydifferences
havebeeninterpretedby Gudmundsson
et al. [1994]in
by anisotropy.
Most experimentssensitiveto anisotropyhave limited directional
resolution.
Observations
of shear wave
terms of a zone of enhanced attenuation
and above the 410-km
below 210 km
transition.
splitting for $K$ wavesrepresentthe integrate([effect
of the differencesin the S wave speedsthrough the region of anisotropy,for the two orthogonalpolarizations
in a near-horizontal plane, and provide no depth resolution. Love and Rayleigh waves taken together do
potentially provide three-dimensionaldirectionalresolution, but strong trade-offs exist with lateral hetero-
In all we havelookedat about 120seismograms
which
satisfythe aboveselectioncriteria, recordedat WRA
betweenDecember1989throughMay 1992. In many
cases,no shear wavesare discerniblefrom the P wave
geneity[Andersonand Dziewonski,1982].
which we obtain 34 measurementsof splitting associated with different phase branches. The geographical
Measurements of shear wave splitting in more com-
coda. In other casesthe signal-to-noiseratio is not sufficient to obtain
a reliable
measurement
of shear wave
splitting. In the end we have 29 useful eventsfrom
configurationof the WRA recordingstation and the
epicentersof the earthquakeswhichprovideusefulshear
dimensionaldirectional coverageneededto resolvethe wavesplitting measurementsare displayedin Figure 1;
full anisotropy of the mantle, although such measure- the hypocentersof the eventsare also presentedin Taments are admittedly difficult. This is what we attem- ble 1. The turning points in the mantle correspondpt to do in the present study. We are motivated by ing to these propagation paths lie mostly beneath the
observationsof seismogramsfor shear wavesrefracted northern margin of the Australian continent. Studiesof
through the upper mantle which displaya •imc separa- both shortperiodarrivals[Dey et al., 1993]and broad-
plicatedgeometriesthan the SKS geometrysho•fidbe
explored, since they can provide some of the three-
tion between the onsets of SV
bandrecords[Kennettet al., 1994]haveindicatedthe
and $H waves.
We have to acknowledgethe difficulty posedfor measurementsof shear wave splitting arising from possible
contamination by phase conversions,which can give rise
to precursorsto the true shearwave. Suchconversions
are much more likely to occur on the radial component
given l.hat to zeroth order Earth is a sphericallysymmetric body. Thus, when shear waves on the radial
componentappear to be advancedin relation to shear
waves on the transverse component, scattering must be
consideredas a potential cause. In this study virtually
all the observations
are of earlier
shear waves on the
transw•.rsecomponent or a fast polarization near the
transw.•rse
direction.
Data Selection and Analysis Procedure
The eventsusedfor shearwave splitting analysiswere
extract;ed from the broadband
records at the WRA
site
presenceof regional variations in mantle structure under Northern Australia, and so we have divided the
sourceregioninto two parts: (1) alongthe FloresArc,
Indonesia, with propagationunder northwesternAus-
tralia, and (2) in New Guinea,with pathsto the NNE
of the array. The events in each classare identified by
open or solid symbolsin Figure 1.
In the course of the work on the P and $ wave ve-
locity structure in the upper mantle beneath northern
Australiarecordsectionsfor both the radial ($V) and
transw.•rse
($H) components
of $ wereconstructed
[see
Kennettet al., 1994]andit becameclearthat therewas
a systematic advance of SH wave onsetscomparedwith
the SV onsets, for arrivals from the transition zone at
distam:esbeyond 2100 km. In Figure 2, we illustrate
the nature of the $ velocity profile which has been pro-
posedfor the northernAustralianregion [Kennettet
al., 1994];a fast and ratherthickmantlelid extendsto
210 kin.
The impressionof SH advancegainedfrom the record
sectionswas confirmedby carefulhand picking carried
tralian region[Kennettet al., 1994]. The eventsspan out independentlyby at least two peoplefor eachevethe distancerange from 12ø to 30ø from the Indonesian nt. However,for the high-frequencyonsetswhich have
and New Guinea earthquake belt. Shallow eventswith propagatedthrough the mantle lid there was no disbody wave magnitudesbetween4.9 and 5.5 have been cerniblepattern of earlier $H waves,but tile correlaselectedfor which there is good signal strength for S tion of arrivals on the different components was much
compared to the P wave coda. With this type of event poorerthan for the arrivalsfrom greaterdepth.
we try to secureas simple a waveform as possibleand
In orderto improvethe quantitativemeasuresof shealso minimize the influence of subduction zone strucar wave splitting, we have undertakena correlation
tures near the source.
analysisfor arrivals associatedwith each of the major
For epicentral distancesout to 2100 km, the onsets classesof propagationpath through tile mantle. For
of the $V and SH waveformsshow high frequencies each event, a set of time windows is selectedto span
of
(greaterthan I Hz) associated
with arrivalspropagat- the expectedtime for the arrivalsfor differentclasses
ing within a thick lid extendingto 210 km depth. At propagationpath, derivedfrom interpretationof cornas part of a study designedto determinethe P and $ velocity structureswith depth beneath the northern Aus-
TONG
ET AL.: SHEAR WAVE SPLITTING
IN THE UPPER. MANTLE
15,787
Table 1. Hypocenters
andMagnitudes
of Eventsfor WhichShearWaveSplittingWasSuccessfully
Measured
Event
Date
OriginTime, UT
I
2
3
4
5
6
7
Jan. 5 1990
Feb. 2 1990
Feb. 9 1990
Feb. 10 1990
Feb. 10 1990
Feb. 26 1990
March 3 1990
10
07
21
13
13
18
17
8
9
10
11
12
13
April 4 1990
April 13 1990
April 25 1990
May 7 1990
May 15 1990
May 21 1990
14
15
June I 1990
June 6 1990
16
17
Latitude,deg Longitude,deg Depth, km Magnitude
-8.800
-10.233
-9.886
-5.263
-10.506
-9.577
-5.561
106.442
110.290
119.050
151.271
120.304
149.798
129.633
29.0
45.8
41.7
10.0
33.0
33.0
33.7
5.3
5.8
4.6
5.3
4.6
5.2
5.1
19 51:48.95
22 46:55.32
15 32:21.70
14 52:51.60
20 35:34.60
13 24:36.60
-4.738
-6.638
-7.077
-9.648
-7.797
-8.137
151.645
130.513
150.112
155.537
129.467
109.043
31.2
17.7
23.0
25.0
57.0
28.0
4.9
5.2
5.4
5.1
4.8
5.5
04 45:49.85
15 04:50.90
-5.119
-6.422
147.722
131.240
33.0
62.0
5.4
5.0
July 17 1990
Sept. 29 1990
22 05:55.02
11 16:10.90
-7.746
-8.701
128.893
122.385
33.0
33.0
4.7
5.0
18
19
Oct. 18 1990
Dec. 16 1990
03 06:52.30
20 19:48.10
-7.348
-6.002
129.279
142.167
48.0
33.0
5.3
5.4
20
21
22
May 17 1991
May 7 1991
Sept. 26 1991
06 37:47.00
17 07:04.00
09 14:50.40
-9.941
-7.954
-9.283
119.780
124.716
158.620
21.0
30.0
26.0
5.2
5.3
5.2
23
24
25
26
27
28
29
Oct.
Dec.
Dec.
Dec.
Jan.
March
March
19
17
03
11
12
15
13
117.161
157.840
150.840
150.189
148.795
119.063
114.752
50.0
33.0
24.0
33.0
33.0
33.0
33.0
5.2
5.2
5.4
5.4
5.4
5.2
5.2
7 1991
27 1991
28 1991
28 1991
24 1992
29 1992
30 1992
10:21.80
58:14.07
33:40.39
12:14.20
58:07.95
13:59.80
11:39.72
56:21.10
14:30.70
40:31.40
05:20.30
09:24.10
12:30.60
42:19.10
positerecordsections
usingall availablerecordsfor both
SV andSH arrivals[Kennettet al., 1994].The process
is illustratedin Figure3a for an eventin the FloresArc
at 17.25ø;the two time windowsusedfor the analysis
of the seismogram
in Figure 3a correspondto $ wave
propagationin the mantle "lid" extendingdownto 210
km and to the arrivals from the mantle transition zone
-10.553
-9.093
-6.424
-6.325
-9.402
-10.161
-8.307
and synthetic calculationsconfirm that it is difficult
to induce much difference in the excitation
of the two
branchesunlessthe propagationpath leavesthe source
closeto a node in the radiation pattern. The general
consistencyof the splitting results obtained from the
analysisof many differenteventsarguesagainsta major
influence from this cause,but it could •nake a signific-
below410 km depth. The seismogram
hasbeenfiltered ant contribution to the scatter in the these results. The
with a band passfrom 0.05 to 0.5 Hz using a causal, most complex zone of pentuplication in upper mantle
four-pole,Butterworthfilter in orderto removesomeof arrival:•,near 23.5ø, is not sampledby our observations.
The phasevelocityof the differentclassesof arrivals
the incoherent,high-frequency,scatteredenergyassociated with the lithosphericarrivals. Figure 3b illustrates rangesfrom 4.6 km/s for the lithosphericarrivals,to
an event south of Java at 29.18 ø where the time win5.4-5.7 km/s for the refractionfrom beneaththe 410
dow corresponds
to wavesturned at the top of the lower km transition and to 6.3-6.6 km/s for the wavesthat
mantle, in this case,Earth has actedas a suitableband- turn at the top of the lowermantle beneaththe 660-km
discontinuity.The arrivalsin the 210 and 410 windows
passfilter.
The widths of the time windows used for subseque- are thereforepropagatingat phasespeedslessthan the
nt analysisare chosenso that they spanapproximately high P wavespeedat the surface,and in consequence,
four periodsin the waveform. As a result, thesetime the $V componentwill undergophaseshiftsin its interwindowsinclude the possibleinterferencebetweenthe action with the free surface and also to a lesser extent
waves reflected from or refracted beneath the 410- and
with the Moho and any crustal discontinuities. The
660-kin discontinuities. Differences between the waveinducedphaselags for SV wavesare small but mean
forms of the SV and SH waves could therefore arise
that the expectedpolarizationpattern in the horizonfrom a different distribution of energy between the re- tal plane for arrivalsin the 210 and 410 windowsis not
flected and refracted arrivals. However, the takeoff
purely linear. We have avoidednear-surfaceP wave
glesfor the reflectedand refractedwavesare w.•ryclose conversionproblems,with the hard rock recordingsite,
15,788
TONG
ET AL.:
SHEAR
WAVE
SPLITTING
I
210
410
IN THE
UPPER
MANTLE
three time windowsselectedin Figure 3. The top left
panel in each figure displays the selectedtime window
:
I
,,
for the horizontal componentsof the seismogramafter
,,
'
rotationto a radial/transverse
coordinatesystem.The
Z
correlation
•••'•...,• R
calculations
are carried out over the central
portion of the window as indicated by the triangles.
As we have noted above,the propagationdirectionfor
most of the S arrivals from the upper mantle deviates
significantlyfrom the verticaland sothe SV component
will be affected by the free surfaceof Earth.
Jepsenand Kennett[1990]and Kennett[1991]have
T
event 20
0
15
,
30
V•
45
•! 17.2
60
75
developeda procedurefor correctingthree-cmnponent
seismogramsfor the influenceof the free surface,which
has been successful
for studiesof regionalwave propagation. This approachcombines
the vertical(Z) and
radial(R) components
of motionto constructan estimate of the incident SV amplitude reachingthe surface. For a modelwith surfacevelocity•0 for S, the SV
contributionfor a planewaveof slowness
p is givenby
sv = Zppo+ •(•b
660
where
'
q•o = (•/•o• - p•)•/•
I
This transformationcompensates
for the amplification
effectsand the phasedistortionintroducedby the free
surface by a simple linear combination of the vertical
and radial componentswith real coefficients. Kennett
[1991]hasshownthat thecoefficients
in thetransformationfor SV wavesarerelativelyslowlyvaryingfunctions
of the slowness
p and the velocityof S wavesat the free
surface, so that the transformation is robust. For SH
waves,the equivalent transformation is a simple ampli-
•event
1 f•'•'••'9.•2
'"•T
tude scalingof the transverse(T) component
SH
-
For each of the selected
0
15
30
45
60
75
Time (seconds)
Figure 3. Three-componentshear wave recordsfor
a pair of events along the Indonesian Arc recorded at
0.5T.
time
windows
on the seis-
mograms we have adopted the slownessp appropriate
to the particular arrival and range based on the velocity model presented in Figure 2. For arrivals within
the 210 window the valuesfor p have the small range
from 0.206 to 0.214 s/km. Within the 410 window,p
rangesfrom 0.16 to 0.18 s/km and for the 660 window
from 0.14 to 0.15 s/km. We havethen transformed
the
WRA. (a) Event 20 in Table i at a distanceof 17.2ø three-componentseismogramsinto $V and $H traces
(b) Event I in Table I at a distanceof 29.2ø. The time using these simple linear combinationsof components.
windowsused for polarization analysisof shear wave The resulting S V and SH wavetracesare illustrated in
splittingare markedfor both the "lid" arrivals(210) the top middle panel of Figures 4-6. Since the surface
and the phases associatedwith the 410-kin transition
velocitiesare quite wellknown(•0 - 3.55 km/s for she-
(410) in Figure3a and for phasesturningunderthe ar waves,a0 - 5.8 km/s for compressional
waves),this
660-kindiscontinuity
(660) in Figure3b. The tracesin transformation will remove most of the phase (tistor-
Figure3a havebeenfilteredwith a bandpassfi'om0.05
to 0.5 'Hz.
at the expenseof a more complexpolarizationpattern
for $ waves.
For each of the selected time windows a correlation
tion associated with the interaction
of SV waves with
the free surface.The correctionis most significm•t,and
hence uncertain, for the 210 window but will also be
significantfor the 410 window. A comparisonwith Fig-
ure 1 of Kennett [1991]revealsthe sensitivityof this
transformationto the value of the slownessparameter,
analysiswas performedfollowinga similar procedure p. The relevant coefficientsfor reconstructingthe in-
to that proposedby Bowmanand Ando [1987]. This comingSV wave(Vsz and VsRin Kennett's[1991]noapproachis demonstratedin Figures4, 5, and 6 for the tation) arereasonably
smoothfunctionsof the slowness.
TONG ET AL.: SHEAR WAVE SPLITTING
a
IN TIlE
UPPER MANTLE
b
Tf
23O
238
c
SV
Sl
SH
S2
SH
S2
l
246
15,789
230
238
246
23O
238
246
d
Correlation
1 .o
-4.0
-2.0
0.0
2.0
coefficient
o.o
-1 .o
4.0
Time lag (seconds)
Figure 4. Correlation analysisto determine the tinheadvanceand rerotation angle from the
radial/transverse
geometryfor the "lid" arrivalsin Figure3a. (a)Vertical (Z), radial (R)and
tangential(T) traces.(b) $V and $H tracesincorporating
compensation
for the influenceof the
free surface. (c) Rotated and time-shiftedtracesfor elementalshearwavesfor whichthe cross
correlationis optimized.d) Correlationbetweenorthogonalshearwavecomponents
asa function
of time shift and rerotation angle; the crossindicates the maximum correlation found.
Only in the caseof the scalingof the radial component records can be representedby a simple •nodel for possible shearwave splitting. We considera superpositionof
exceed 10%. Thus this transformation is a robust one
two orthogonal shear wavesrotated by an angle • from
and should yield an accurate result if the wave field is the SV/$H polarizationreference,
separated
in time by
in fact characterizedby a single incoming plane wave. •t
The presenceof coda from P wavesobscuresthe transSV(t) = A cosel(t) + B sin½f(t + •t).
formation to somedegree. A merit of the transformaSH(t) = Asin el(t) - B cos½f(t +
tion processis that information is used from both the
vertical and radial componentsin assessingthe d•arac- The amplitudes of the two wavesare assumedto be arteristics of the S V waves. We should note that there
bitrary, but the waveformsimposed by sourceprocesses
For each time window
is no simple relation between the apparent polarization are assumed to be identical.
expressedby plotting these SV and SH traces on or- the parameters ½ and •t are estimated using a crossscheme which is restricted
to the central
thogonal axes and the original polm-izationsin the hor- correlation
izontal plane displayedin the top left panel of Figures portionof the time window(the portionusedis indi4-6. The length of the windowsusedin the plots is such catedby the trianglesin Figures4-6). For a spanof
that the simple model of a single slownessmay not be time offsetsSt from -4 s to +4 s and rerotation angles
adequate for arrivals at the extremities of the wi]tdow. .½ between-90 ø and 90 ø we have decomposedthe $V
Oncethe transformationof the originalthree-compon- and $H traces into two contributionsincorporating the
for the 210 window(Vsn) is the uncertaintylikelyto
ent records for each time window into $H and $V traces
rotation
has been accomplished,we assumethat the transformed
cross-correlation
and time
offset.
between
We have then calculated
these two trace
estimates
the
of
15,790
TONG ET AL.: SHEAR WAVE SPLITTING
a
IN THE UPPER, MANTLE
b
c
SV
Sl
SH
S2
S2,
T1
R
248
d
256
248
264
256
264
248
256
264
9O
60
Correlation
30
1.0
coefficient
0.0
-1.0
0
-30
-60
-9O
-2.0
0.0
2.0
4.0
Time lag (seconds)
Figure 5. Correlation analysisto determine the time advanceand rerotation angle from the
radial/transversegeometryfor the transitionzonearrivalsin Figure 3b. For explanation,see
Figure 4.
the elementalshearwaves(S1, S2)for each½,5t combination and contouredthe resulting correlationdiagram.
The combination of parameters which gave the largest
positive or negative correlation was then adopted as the
best estimate of the splitting parameters. This procedure was found to give a closecorrespondencebetween
5t and the time offsetsestimated by visual analysis.
In the bottom panel of each of Figures 4-6, the contoured correlation diagram correspondingto the selected time window is displayed. We recall that only
the central part of the window indicated by the triangles is used in this analysis. Superimposedon each
contour diagram is a crossindicating the combination
of splitting parameters •b and 5t which yield the high-
It is worth noting that in the contouredcorrelationdiagramsof Figures 4-6 there is somedegreeof nonuniquenessbecausethe rotation angle •bspanshalf a circle. A
rotation by 90ø is equivalentto interchangingthe traces
while changingthe polarity of one. Thus, for eachpositive correlation peak there is a correspondingnegative
correlation peak at a time shift reversedin sign and a
90ø shift in rotation angle. In Figure 4 the peak correlation is found at q• = 65ø, 5t = 0.3 s, but there is
an equivalentnegative peak at q• = -25 ø, 5t = -0.3s.
It is only becausethe correlation is computed within a
time window of finite length that this equivalenceis not
perfect for the computed correlation function.
Each of the correlation functions presentedin Fig-
est correlation.The elementalshearwavetraces(S1, ures 4-6 have a clear maximum at a correlation with
S2) incorporating
the rotationand time shift fi)r the amplitude approaching0.9. The widths of the correlaoptimal parametersare displayedin the top right panel
in Figures 4-6. For the central portion of the window
we would expect linear polarization and this has been
achiewMquite well. Comparisonof the splitting results
in Figures 4-6 with the correspondingthree-component
seismogramsof Figure 3 givesgood visual confirmation
of the choiceof optimal splitting parameters.
tion peaks vary considerably.The width along the time
axis is in large part controlled by the dominant period
of the waveforms. Taking the half width of the chosen
correlation peak along the time axis as an uncertainty
of the measurement of time shift, the value would be
about 1 s for the 210 and 410 branches(Figures4 and
5) and 1.5 s for the 660 branch(Figure 6). The next
TONG
ET AL.:
a
SHEAR
WAVE
SPLITTING
IN THE
UPPER
b
MANTLE
15,791
c
S2,
Sl
314
330
314
346
330
346
314
330
346
d
Correlation
1 .o
coefficient
o.o
-1 .o
%•:i .•::..•.•-.,:,•.-:::::::•:•:::::•:::::•:.::•:•:::•:•:•::_?•:--:---.-
0
i.....
•
,i•i•!i:i:•?
-•:'.:-!•:::::::'-::--::::'..-:::::::::::::::--:.-::---:':--:--•-ß-•'
-
-30
-iO0
-4.0
-2.0
'"'""'
'!"""'i!•:½""
""•ii½ii½
0.0
2.0
4.0
Time lag (seconds)
Figure 6. Correlation analysisto determine the time advanceand rerotation angle from the
radial]transversegeometry for the upper mantle arrivals in Figure 3b. For explanation, see
Figure 4.
most significant correlation peak is normally of opposite sign to that selectedand correspondsto a skip of
half a cycle. The width of the correlation peak along
the angle-of-rotationaxis is primarily controlled1)ythe
rotation kernels, cosqband sin qbas is evident from the
fact that in general,the correlationfunction sI)ansone
cycle in the interval from qb= -90 ø to qb= 90ø. It
is not obvious
that
we can translate
the half width
of
the correlation peak into an absolute measure of uncertainty in this case, but it is clear that the uncertainty
in the determination of the optimal rotation angle is
considerable,perhaps of the order of 4-20ø.
It is difficult to developa more satisfactoryerror analysis for the estimates of the rotation angle and time
shift, becduseeachevent and time windowpresentsa
during passagethrough an anisotropic zone is admittedly simple, but the present data do not warrant a
more complex interpretation.
Results
The results of the splitting analysis are summarized
in Table 2 and Figure 7 for 34 time windows from 29
events, out of the 200 windows for which such analysis
was attempted. A measurement from the correlation
procedure was deemed useful when a clear arrival was
seenin a seismogramfiltered with a low passbelow 0.5
Hz, the signal-to-noiseratio exceededa value of 2.0, and
correlationand polarization diagramssuchas thosepresentedin Figures 4-6 were unambiguouslyinterpretable
different noise environment. During the time interval (a maximumcorrelationpeak with amplitudeexceedfor whichbroadbanddata are availableit hasnot proved ing 0.8 and a linear polarization achievedthroughout a
possibleto achieveduplicationof propagationpathsto significantportionof the main arrival).
provide further confirmation of the results.
The model
which
we have used as the basis of the
shear wave splitting estimation comprisingtwo orthogonal shear waves subjected to rotation and ti.mc shift
In some cases, arrivals on the shallow "lid" branch
contain little energy below 0.5 Hz above the P coda
but displaya high level of energyaround i Hz where
scattering effects destroy coherencybetween tht: three
15,792
TONG ET AL.: SHEAR WAVE SPLITTING IN THE UPPER MANTLE
Table 2. A Summary of the Resultsof Polarizationand Splitting Measurements.
Event
A, deg
12
16
18
13.03
13.29
13.51
9
15
7
21
19
Jr, s
•b,deg
R
branch
0.3
0.0
-0.3
......
-25.0
......
0.89
210
210
210
13.81
13.85
15.09
15.19
15.87
-0.5
-0.3
0.3
0.0
0.0
......
......
......
......
-15.0
17
16.13
-0.5
......
5
16.50
0.2
......
20
27
3
23
20
17.25
17.48
17.86
19.03
17.25
0.3
0.0
0.0
0.0
-2.0
65.0
......
......
......
-20.0
27
17.48
-1.3
10.0
0.87
0.86
28
17.69
-0.6
-10.0
0.88
23
19.03
-3.0
50.0
0.96
10
26
25
19.97
20.53
20.95
-0.9
-3.0
-3.5
60.0
-15.0
0.0
4
29
11
22.05
22.24
22.88
-1.9
-1.6
-4.4
-25.0
30.0
-15.0
0.92
0.97
0.92
0.96
0.83
0.94
24
6
14
29
8
2
24
22
13
i
25.20
18.15
19.71
22.24
22.68
25.10
25.20
25.74
27.18
29.18
-3.2
-2.6
-0.4
-1.6
-0.9
-0.7
-2.0
-0.3
-3.3
-3.5
-15.0
-35.0
-40.0
30.0
5.0
-20.0
0.0
-30.0
30.0
-30.0
0.95
0.94
0.93
0.83
0.94
0.94
0.88
0.92
0.86
0.89
0.89
210
210
210
210
210
210
210
0.88
210
210
210
210
410
410
410
410
410
410
410
410
410 *
410
410
660
660
660 *
660
660
660
660
660
660
A is epicentral distance in degrees, dt is the measured time shift, •b is the
measured rotation angle of the slow polarization clockwiseaway from the radial
direction, and R is the peak correlation for each measurement. Blank entries
the polarization direction and correlation indicate where measurementswere made
directly from high-frequencyseismograms
without polarization analysis.
* Two interferringarrivalscannotbe separatedwhilea goodpolarizationmeasurement
can still be made.
recorded components. In such casespolarization analysis was not undertaken. Instead, travel time picks were
made from both the radial and vertical componentsfor
$V and the transverse componentsfor $H whenever
this could be achieved with reasonable accuracy. The
time shift was then definedas the timing differencebe-
small. In all we have 14 measurementsaveraging 0.0
s, with a standard deviation of 0.3 s and no systematic
trend with epicentral distance. Clearly, no resolvable
anisotropiceffect is seen here. This is at odds with
tween the onset of the $H wave and onset of the $V
wave. Entries in Table 2 with blanks entered for the
tween $H and SV waveson this branch basedon only a
few observations.For suchvery small time offsetsthere
tentativestatementsmadeby Goody[1991]and Dey et
ai. [1993]aboutobservations
of timingdifferences
be-
rotation angle and the correlationcoefficientrepresent is very little control on the angle of orientationof the
measurementsof this type. For thosecaseswhere corre- anisotropy,and so the rotation anglesfor the lid arrivals
lation analysiscould also be achieved,there was a close are not used in the subsequentanalysis.
For ghe deeper branches,in all but a few casesthe
correspondencebetween the estimates of tirne shift by
the two methods. For all the lid arrivals(210 branch) fast polarization is within 30ø from the transversedithe time shift between the two S componentsis very rection. Since the likely uncertainty of measurement
TONG ET AL'
SHEAR WAVE SPLITTING IN THE UPPER. MANTLE
shift I• I •
2time
(secønds)
1:5,793
660
410
210
I
I
I
'
10
'
I
•
•
•
•
15
I
,
,
I
20
I
I
25
I
=
'
'
I
30
epicentraldistance(degrees)
Figure 7. A summary
of measurements
of timeshiftasa functionof epicentral
distance
and
branch on the travel time curve.
is 4.20", deviationsfrom a purelySH/SV splittingge- excludethe possibilityof a half cycleskip producing
ometry are not significant. The measurementsof time a high and visuallyappealingcorrelation.A further
delay 5t display a fair degreeof scatter along each of contributionis possiblefrom a modificationof the apthe 410 and 660 branches. Eleven measurements from
parentwaveformsof the SH and $V waveformsdue to
arrivalsreturningfrom the transitionzone(410branch) differencesin the excitation of interfering arrivals.
In Figure8 we presenta geographical
summaryof the
havean averagetime shift of 2.3 s and a standm'ddeviation of 1.2 s. Nine measurements from arrivals returned
results;we have only plotted thoseeventsfor which a
is availablefor eitherthe 410 or
from the top of the lowermantle(660 branch)havean splittingmeasurement
averagetime offset of 1.7 s and a standard deviation the 660 branch. The events are connected to the WRA
of 1.2 s. We should note that the set of observations
recordingsiteby a thin line to indicatethe propagation
alongeachbranchare built up from a numberof differ- path and the radial directionfor eachevent.The shear
ent ew•nts and mix data from the two source clusters
wavesplittingresultsare shownas orientedtime bars.
The lengthof eachbar is proportionalto St, andthe bar
alongthe Indonesianarc and throughNew Guinea.
Figure 7 summarizesthe time shiftsbetweenthe two is orientedalongthe inferredfast polarizationdirection.
S wavesfrom Table 2 in terms of the branch of the travel A scalingbar is shownfor reference
in the lowerleft of
time curve, the epicentral distanceand the geographic the figure.The splittingresultsfor the 410 branchare
location of the source. As in Figure 1, open symbols plottee[in grey at one third of the epicentraldistance
resultsfor tl•e 660
are used to indicate events along the Indonesian arc, awayfrom WRA. The corresponding
and solid symbols are used for events in New Guinea branchare plotted in black halfwayalongthe ray. The
to separatethe two
and the Solomon Islands. It is clear that a significant positionsof the time barsare chosen
classes
of
measurements
in
the
figure
and haveno other
level of shear wave splitting is found for the 410 and
sinceall ofthe measurements
aremadefrom
660 branches, while no discernible anisotropic effect is significance
found on the shallow 210 branch. The results fi)r the
recordingsat the WRA site.
410 and 660 branches show a hint of a weak increase of
Figure 8 highlightsthe differences
in the resultsfor
5t with epicentral distance. In each casea linear trend the two classesof eventsused(the Indonesianarc and
Islands).The absolutepolarizaof d5t/dA = 0.1 s/degis estimatedby linearregression; New Guinea/Solomon
with the extraction of the trend, the standard deviation tion is fairly consistentwithin eachclusterbut very diffof the scatter
From
this
is reduced
from
1.2 s to 1.1 s.
set of observations
of $ waves returned
from the upper mantle we have a clearindicationof an
anisotropiceffectthrough splitting of the differentpo-
erent for the two clusters. The fast polarization is close
to the transverse direction to each ray, particularly so
for the cluster of events in New Guinea and the Solomon
larizations of 5'. Even with a fairly favorable geometry
Islands. The relative values for the time delays of the
two branchesare different in the two regions. The aver-
of sourcesfor the recordingsite at WRA we have only
agetime delayis aboutthe sameon the 660 branchas
a limited
on the 410 branch for the Indonesian
data set available from the broad bmod in-
events but varies
strumentwith patchy coverageof the major travel time
branches.The limited data set helps to accentuatethe
scatter amongthe delay time measurements,which are
by almost a factor of 2 for the New Guinean events,
where the splitting for the 410 branch has larger values. Unfortunately, the number of observationson each
carried out in the coda of other arrivals. The correlation
branch from each cluster is still quite low.
procedureusedfor the analysishasexpecteduncertain-
In summary,no indicationsof anisotropiceffe.
cts on
the propagationof shearwaveswithin the mantle "lid"
ties of the order of 4-0.5-
1.0 s. However, we cannot
15,794
TONG ET AL.' SHEAR WAVE SPLITTING IN THE UPPER MANTLE
-lO
5
seconds
-2o
'0
1 O0
110
120
130
140
150
160
170
longitude(degrees E)
Figure 8. A summary of polarization and time shift measurements.The result of eachsplitting
measurement
is represented
by a time bar orientedalongthe fast polarization(as measuredat
the recordingsite). The lengthof the bar is proportionalto the measured
time shiftasindicated
by the bar in the lower left-hand corner. Only earthquakescontributinga measurementon the
410 and 660 branchesare included. Measurementson the 410 bra•ch are plotted in gray at a
quarter of the distance of each event from WRA. Measurementson the 660 branch are plotted
in black at the midpoint between eachevent and WRA.
were found. However, for paths through the lower
parts of the upper mantle, we have shear wave splitting with time delays of 2 s or more which has to be
imposedsomewherealong the propagationpath. The
wave velocities are generally faster for transversepolarization and slowerfor the radial component. The time
shifts 1)etweenthe two polarizations of S may increase
slightly with epicentraldistanceon the 410- and 660-kin
branches.The time shift is on averagesomewhatlarger
on the 410 branch than the 660 branch, particularly for
eventsin the New Guinea region.
Discussion
and
Conclusions
In the previous sections we have demonstrated the
presenceof shear wave splitting in the propagation of
shear waveswhich are refracted through the lower part
of the upper mantle. What remains is to isolate the
sourceof this anisotropic effect, which unfortunately is
a poorly constrainedand non unique exercise.We can,
howew;r,make use of idealized modelsof anisotropyand
the layering of the upper mantle which is a manifestation of temperature and mineralogy.
There are three possible assumptionsfor the location of the anisotropy which induces the observed she-
plausibleas the dominant cause,so that we must seek
the major influenceof anisotropysomewherealong the
propagation path.
There is a small differencein the take off angle froin
the source between the 210 branch, for which waves
which travel within the mantle "lid" out to about 18ø,
and the 410, 660 branches,for which waves dive deeper
into the upper mantle. For the "lid" arrivals we have no
indication of significantsplitting which arguesagainst
a source effect. There is, however, limited overlap in
the patterns of observationsbetweenthe shallowerand
deeperpropagationpaths. The eventswhich yiel(l data
at shorter epicentral distances, 12-18ø, come i¾omthe
Banda sea and the eastern end of the Flores arc, while
observations of the deeper penetrating branches are
fromgeographically
distinctareas(New Guinea,Java).
However, there is a relative conformity of the polarization within the two clusters of events in Figure 8 and
there is no apparent relationship with the orientation'
of the island arcs where the events occurred.
All the
eventsoccurredin the fore arc region of variousisland
arcs(Solomon
Islands,NewBritain,Indonesia),
but the
ray paths bear no uniform relation to the strike of the
arcs. ]'he sourceregion is unlikely, therefore,to 1)ethe
dominant region for the impositionof shearwave splitar wavesplitting:(1) anisotropynearthe sources,
(2) ting on the refracted arrivals.
anisotropy
neartherecording
station,and(3) anisotropy The difference in the absolute polarization for the
along the propagation path. We will be able to show two clustersof eventsalong the Indonesia arc and froin
that near-sourceand near-receiver anisotropy are im- New Guinea/Solomon
Islands,whichis clearlydemon-
TONG
ET AL.: SHEAR
WAVE
SPLITTING
strated by Figure 8, cannot be consistentwith a major
near-receiver contribution to the shear wave splitting.
Anisotropy in the upper lithosphere in the immediate
vicinity of the WRA site is thus not a major contributor
to the observed shear wave birefringence.
We
therefore
need to examine
the influence
of the
different aspects of the propagation path between the
sourcesand the recording station at WRA. A shear
wave traveling nearly vertically will be most sensitive
to anisotropy in the horizontal plane, and similarly for
a refracted wave traveling nearly horizontally in the upper mantle the greatest sensitivity will be to the properties in a vertical plane perpendicular to the propagation
direction.
Because of the nature
of the refracted
wave
paths (Figure 2) the influenceof anisotropywill vary
with different portions of the path.
We considerfirst the influenceof the lithosphere. The
absenceof splitting for the mantle "lid" arrivals is incompatible with a simple model of transverseisotropy
with a vertical symmetry axis as is often used to reconcile Love and Rayleigh wave dispersionalong a particular path. For transverseisotropy we would expect to
acquir• shear wave splitting along the nearly horizontal part of the path due to the difference between the
wave speeds for horizontal and vertical polarizations.
The w•.ry thick lithosphere in northern Australia is a
zone of low attenuation and intensescattering, and it is
possiblethat heterogeneity may mask the influence of
anisotropy. Unfortunately, the distribution of sources
for which we can sample the lid zone at WRA allowsno
assessmentof any azimuthal effects.
The paths of the refracted wavesreturned from (teeper
in the upper mantle are at a significantangle to the ver-
IN THE
UPPER. MANTLE
15,795
The most likely location for the imposition of the major componentof shear wave splitting for the refracted
wavesfrom deep in the mantle is therefore from the portions of the paths well away from the endpoints. This
leavesthe segmentin the asthenosphereand in the transition zone or lower mantle.
The lack of significantly anisotropicminerals which
are stablebelowa depth of about 480 km [Mainprice
and Silver, 1993]constrainsthe regionof likely anisotropy from below. Olivine is the most anisotropic of
the most abundant minerals in the upper mantle and
apparently the most ductile mineral, i.e., the mineral
whichmosteasilyalignswith flow [Anderson,1989].Its
anisotropy and alignments of its orientation are generally believed to cause the azimuthal anisotropy which
has been observedin Pn wavesand vertical S, ScS, and
$KS
waves. It is also believed to be the main cause of
the anisotropy which manifests itself in the inconsistency between the isotropic models needed to match
the dispersionof Love and Rayleighwaves[Anderson,
1989].At or nearthe 410 discontinuity
the low-pressure
form of olivine is transformed into a spinel-like structure, •-olivine.
It is almost as anisotropic a mineral
as the low pressureform of the mineral, a-olivine, but
nothingis knownaboutits petrofabrics[Mainpriceand
Silver,1993].The olivinecontentof the transitionzone
may be lowerthan nearerto the earth'ssurface[Duffy
and Anderson,1989].Nevertheless,
•-olivine mustbe
conside,reda plausible source of seismic anisotropy in
the mantle. Below a depth of about 480 km/•-olivine
transformsinto -/-spinel [e.g,. Ringwood,1991]which
is only slightly anisotropic[Anderson,1989]. Thus
the lower part of the transition zone is not a candi-
tical in the lithosphere(at least 45ø) and so will have
date regionfor anisotropy.Mainpriceand Silver[1993]
path lengths of rather greater than 200 km within the
lithosphere. We have pointed out above the difficulty
of reconciling the polarization characteristicsof the arrivals from the two different sourceclusterswith a major
effectnear the receiver,but the substantialpath lengths
in the lithosphere could introduce some portion of the
splitting.
The influence of the lithosphere will be rather dif-
estimate the maximum shear wave anisotropy of the
main constituents of the lower mantle, perovskite and
ferent
for these refracted
waves than
for SK$
waves
traveling near vertically. There are only a limited number of $K$ splitting observationsavailablewithin Australia. The nearest station CTAO, in Queenslandnear
the edgeof the craton, has an $KS splitting of about
i s with a polarization direction at an azimuth of 60ø,
wustitc, to be around 10%. They point out, however,
that little is known about the petrofabrics of these minerals, and that indications are that deformation does
not inducepetrofabric[KaratoandLi, 1992].The lower
mantle is thus a weak candidate for the anisotropy we
are seeking.
The measured time delays between the two shear
wave components for the 410 and 660 branches do not
display a significant dependenceon epicentral distance.
It is unlikely that the splitting occursin the region of
turning for either branch becausewe would then expect a dependenceof the length of the turning segment.
and similar
results are obtained for NWAO
in WestMainpriceand Silver [1993]argueagainstperceptible
ern Australiaand TAU in Tasmania[Vinnik et al. , anisotropy in perovskite at the top of the lower man1992].However,the stationCAN in southernstern
Aus- tle. However, it is not trivial to rule out anisotropy in
tralia is reported as having no perceptiblesplitting for
the/•--olivine regime at the top of the transition zone.
$KS arrivals[Vinnik et al., 1989].Despitethe tectonic In order to predict an observation where the time shift
stability of Australia, there is considerablecomplexity
in the assemblageof the craton and it is not easy to
extrapolate the $KS results to WRA, but to achievea
delay time of about I s between the two shear wave
components requires the presence of some azimuthal
anisotropy which is normally associatedwith the up-
per mantle[ e.g.,MainpriceandSilver,1993].
does not increase with epicentral distance on the 410
branch, the level of anisotropy would have to decrease
sharply with depth. Allowing for a 70-km-thick layer at
the top of the transition zone with a linearly decreasing
level of anisotropyresults in about 10% anisotropyat
the top of the layer in order to satisfy the 1.7-s delay on
the 660 branch. The maximum level of shear anisotropy
15,796
TONG ET AL.: SHEAR WAVE SPLITTING IN THE UPPER MANTLE
in /•--olivineis 14 % [Anderson,
1989]whichassumes lithosphere,the geometry of availablesourcesdoesnot
perfect.alignmentin an aggregateof 100%/•-spinel placesignificantconstraintson lithosphericproperties.
and samplingof the maximalpolarization.Tenpercent The level of anisotropywe have proposedfor the asanisot,'opy
is thereforeunreasonable.
Anisotropy
at the thenosphere beneath northern Australia is much lower
top of the transitionzone cannotbe the major con- thanthe2.5%anisotropy
(ofunspecified
type)indicated
tributorto our splittingmeasurements,
althoughsome by Gahertyand Jordan[1993]for pathsto the station
contribution is possible.
NWAO in westernAustralia. A modestanisotropyin
We are then left with the depth intervalbetween210 the asthenosphere
wouldnot vitiate their suggestion
and 410 km as the sourceregionfor the anisotropic
ef- that the Lehmanndiscontinuity(210 km deepin our
fect. Werecallthat thisasthenospheric
zonehasslightly region)marksa mechanical
boundary.
reduced shear velocity comparedwith the mantle lid
and alsoenhancedattenuationof shearwaves.Rays
Acknowledgments.
The constructivecriticism of the
correspondingto arrivals on the 410 and 660 branches reviewers,AssociateEditor, and membersof the RSES Seistraversethis regionat an angle of about 65ø and 50ø inologyGroup were very helpfulin preparingthe revised
from the vertical,respectively.Thus the polarizationof
the SV waves is 65ø and 50ø from the horizontal. As-
version of this paper. I.J. Weekes assistedwith the data
extractionfrom the WRA tapes.
suminga simplevariationof velocitywith polarization References
anglefrom the horizontal,8, of the form
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(Received
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31, 1994;
acceptedFebruary14, 1994.)