$OURNALOF GEOPHYSICAL
RESEARCH,
VOL.102,NO.B6,PAGES11,815-11,827,
J-kINE10,1997
Variationsof P wave speedsin the mantle transition zone
beneaththe northern Philippine Sea
MichaelR. Brudzinski
andWang-PingChen1
Department
ofGeology,
University
ofIllinois,
Urbana
RobertL. Nowack
Department
ofEarthandAtmospheric
Sciences,
Purdue
University,
WestLafayette,
Indiana
Bor-ShouhHuang
Institute
of EarthSciences,
AcademiaSinica,Taipei,Taiwan
Abstract.
Usingwaveforms
andtraveltimesfromdeepearthquakes,
weconstructed
16
seismic
profiles,
eachofwhichconstrains
theradial
variation
in Vpovera small
area
beneath
thenorthern
Philippine
Sea.Takentogether,
theazimuthal
coverage
of these
profiles
also
places
tightbounds
onthelateral
extent
ofaregion
ofanomalously
highVp
(upto3%fasterthanaverage
Earthmodels)
originally
suggested
bytraveltime
tomography.
Unliketraveltimetomography,
whichreliesheavilyonarrivaltimesof the
direct
P phase,
weutilizethewaveforms
andmove-out
of laterarrivals
thatmainly
sample
themantletransition
zoneof interest.Ourresults
identifythreeimportant
characteristics
of thenorthern
Philippine
Seaanomaly
thataredistinct
fromprevious
results.
First,beingapproximately
a subhorizontal,
laterallyuniformfeature,the
anomaly
islocalized
beneath
thenorthwestern
comerof thePhilippine
Sea,withina
region
ofapproximately
500x 500km2 immediately
eastoftheRyukyu
arc.Second,
the
anomaly
is wellconstrained
to occurin thelowerportionof thetransition
zone,
extending
allthewaydownto the660-kmdiscontinuity.
Third,thepresence
of sucha
distinct
anomaly
reduces
thecontrast
in V, across
the660-kmdiscontinuity
from
approximately
6%to 3%. Sucha configuration
•sconsistent
waththeinterpretanon
that
theanomaly
is caused
bya remnant
of subducted
slab,asnegative
buoyancy
should
rest
theslabjustabovethe660-kmdiscontinuity
whereresistance
to subduction
isexpected
froma negative
Clapeyron
slopeduringthespinel-Mg-Fe-perovskite
transition.
Introduction
large-scale depressionsextending over a width of
The fate of subductedlithosphereis one of the approximately 2000 km across the Kuril and northern
zones.Sucha featureisexpected
if cold,
fundamental
issuesin geophysics
andgeochemistry.
In Japansubduction
lithosphere
of thePacificis trapped
justabove
particular,the depth of penetrationof subducted subducted
lithosphere
constrains
the extentof heatandmasstransfer the660-kindiscontinuity.
beneath
the
between
theupperandthelowermantle[e.g.,Anderson, Recentimagesfromtraveltimetomography
western
Pacific
also
suggested
that
large,
subhorizontal
1989;Lay, 1994; Silver et at., 1988]. Severalrecent
ofanomalously
fastP wavespeed
(V•),interpreted
studies
showed
evidence
thattheextentof slabpenetrationregions
intothe lowermantleseemsto varylaterallyalong as subductedmaterial, exist in the transition zone of the
400
subduction
zonessuchthat at least somesubductedslabs mantle(i.e.,thezonebetweendepthsof approximately
and
660
kin)
[Fukao
et
al.,
1992;
van
derHilst
et
al.,
1991,
remainstagnant
in theuppermantle[e.g.,Glennonand
Chen,
1995a;
DingandGrand,1994],whileothers
plunge 1993]. The anomaly is most pronouncedunder the
portion
ofthePhilippine
Seaplate,
witha V•ofup
steeply
intothelowermantle[e.g.,Jordan,
1977;Creager northern
Earthmodels.In mapviewthis
andJordan,1984,1986]. For instance,
on thebasisof to 3% fasterthanaverage
long-period
(-20 s) converted
andreflected
bodywaves, feature seemsto extend westward for almost 2000 kin,
Shearer
andMasters
[1992]andShearer
[1991
] contendedfrom the tip of the Izu-Bonin Wadati-Benioffzoneto the
thatthetopography
of the660-kmdiscontinuity
shows backarc regionof the Ryukyu-Kyushutrenchwherethe
PhilippineSea plate subductsbeneathEurasia. If thisis
1 Alsoat Department
of Theoretical
andApplied
Mechanics,
true,boththemagnitude
of lateralvariations
in V•andthe
University
ofIllinois,Urbana.
Copyright
1997
bytheAmerican
Geophysical
Union.
Paper
number
97JB00212.
0148-0227/97/97JB.00212509.00
spatial extent of the anomaly beneath the northern
PhilippineSearegionare amongthe mostpronounced
in
the transition zone of the mantle.
In detail the geometryof this anomalysuggested
by
tomography
is peculiar. Insteadof restingjust abovethe
11,815
11,816
BRUDZINSKI ET AL.' NORTHEI•
120E
PHILIPPINE SEA ANOMALY
•
Japan
:&/--:..'..•_
•,,.,x•_,/,'Sea,-4-m
ß
.''•.......
,'
2
P8
PI0
ß::,,,::.:::.:-.:
.......
PI2
3
12
N. Philippine Sea
P14
• L{......
PI5
, • PI1
......
.....
Arc:
.
........... P16?
Figure 1. Map showingthe configurationandmain resultsof the northernPhilippineSea experiment.
Triangles mark the center of the CWB-TTSN short-periodarray and the locations of broadband
stationsof the Pre-POSEIDON arrayusedin this study. Crossesmark epicentersof 16 intermediateand deep-focusearthquakes
(Table 1) usedto constructthe seismicprofiles(P1-P 16). Ellipsesshow
approximatelocationsof the mantle transitionzone sampledby turningrays along the 16 seismic
profiles. Each ellipse is numberedaccordingto its associatedprofile. For regions5-12 (dark
shading),
V• in the lowerportionof themantletransition
zoneis approximately
2% fasterthanthe
iasp91averageEarth model,while V,•in regions1-2 and 13-16 (no shading)are closeto the iasp91
model.
V•inregions
3 and4 (lightsthading)
appear
tofallsomewhere
inbetween
thetwoextremes.
The azimuthalequidistant
projectionis centeredat thesymbolfor theCWB-TTSN array.
660-km discontinuity,
the mostpronounced
anomalies
are
reportedto occur between depthsof 490-570 km, in the
middle of the transitionzone [van der Hilst et al., 1993]. If
the anomalies are indeed causedby subductedPacific
lithosphere,the tornographic
imagesseemto imply that a
remnantof the slabeitherencountered
strongresistance
to
subductionin the middle of the transitionzone or may
even have becomepositivelybuoyantin a time periodof
the orderof only 10 million yearsaftersubduction.Either
scenario seems unusual.
To investigate
the natureof thisanomalyin W, we
utilized
waveform
and travel time data from two seismic
arrays near the edges of the Philippine Sea: the PrePOSEIDON broadbandseismicnetworkof Japanand the
short-period Central Weather Bureau and Taiwan
Telemetered Seismograph Network (CWB-TTSN)
(Figure 1). The Philippine Sea plate is surroundedby
several Wadati-Benioff zones of high background
seismicity. For many earthquakesat epicentraldistances
of approximately 15ø-25ø from the arrays (Figure 2),
turning points of rays that comprisethe P wave train
primarilysamplethe mantletransitionzonewherethemost
pronounced
lateralandradialvariations
in W havebeen
suggested
by tomography.
Unlike in traveltime tomography,
whichreliesheavily
on arrivaltimesof thedirectP phasereportedin bulletins,
we emphasizethe rich information containedin the
waveformandmove-out(i.e.,changes
in arrivaltimeswith
respectto distance)of laterarrivalsthatmainlysample
the
depthsof interest. This work complements
the previous
studiesby providingresolutionon severalaspects
of the
anomaly
thatareof particular
interest
to geodynarnics.
For
instance,
ourresultsindicatethatthe anomalyof fastV•
seemsto occurin the lower portionof the transitionzone,
reducing
thecontrast
inW across
the660-km
discontinuity
to approximately
3%. Thelateralextentof theanomaly
is
constrainedto be mainly beneaththe northwesterncorner
of the PhilippineSea, immediatelyto the eastof the
Ryukyu-Kyushu
trench. Moreover,if the fast W is
associated
with subducted
Pacific lithosphere,
cold
BRUDZINS• ET AL.: NORTHERNPHILIPPINESEA ANOMALY
we constructed16 seismic profiles over a range of
azimuths
(Figure1). Whileeachindividualprofilemainly
c
50
constrains
theradialvariation
in V• overa smallarea,the
B•
40
11,817
C•
azimuthalcoverageof the profilesplacesboundson the
m
lateral
extent
ofanomalies
in
C
•,•ON
•D
10
ß
,
•"
,
I"'.
We useddigitalsignalsthatoriginatedfrom deep-and
intermediate-depth
earthquakesaround the northern
Philippine
Sea(Table1). Thesesignalswererecorded
by
SAMPLED•--- B
two separate, regional seismic arrays.
•
.
,
,
,
I
,"
The Pre-
POSEIDONarrayof Japanconsists
of approximately
12
broadband(high-resolution)seismographs
near the
northernedgeof the targetregion(Figure1). The broad
bandwidthandhigh dynamicrangeof thesestationsare
highly effectivein recordingseismicsignalsfrom all
,
•2oo•'oo •ooo •8oo 2ooo2•'oo24002•oo e•oo
DISTANCE •m)
distanceranges.For thisreason,we wereableto usetravel
Figure 2. Triplicationof travel time curvesdue to time residualsfrom distantearthquakes
to removeneardiscontinuitiesnear the transition zone of the mantle, as
stationstructural
effects(stationcorrections).
For a given
predicted
bytheiasp91model.Traveltimecurves
fortwo observedseismicprofile we found that one-dimensional
representative
focal depths(h) are plotted. Eventsthat
ofradiallyvarying
V• aresufficient
toexplain
the
occurbelowthe 400-kin discontinuity
will exhibitonlyone models
observed waveforms.
These waveforms include all
triplication
caused
by the660-kmdiscontinuity.
Branches
withinthis triplicationare labeled accordingto standard
nomenclature:
branchAB, continuousrefractionturning
abovethe discontinuity;
branchBC, pseudo-reflections
off
the discontinuity;and branch CD, continuousrefraction
miningbelowthe discontinuity.Triplicatedbranches
from
the400-kindiscontinuity
arelabeledby primedletters.All
figuresdisplayingtravel time curves (2-3, 5-11) are
plottedwitha reductionspeedof 10 km/s.
relevantarrivalsfromtriplications
of thetraveltimecurves
thatare causedby discontinuities
in thetransitionzoneof
the mantle(Figure2). For this dataset,slownessof theP
waveat particulardepthsin the transitionzoneis directly
determined
by themove-outof firstarrivalsoverthearray.
Since data from most stations of the Pre-POSEIDON
array were not availableuntil May !993, we also useda
largenumberof short-period,
digitalseismograms
recorded
in Taiwanby the CWB-TrSN, availablesince1987. This
is a densearrayof approximately
75 stationsdistributed
in
and aroundTaiwan near the westernedge of our target
temperaturein the slab may not be sufficientto fully
account
for thelargemagnitudeof the anomaly,suggesting region(Figure1). Theprimarypurpose
of thisarrayis to
thepresence
of depletedmantlematerial.
recordregionaland local earthquakes.Data from few
teleseismic
eventsareretained. Consequently,
we cannot
obtain
Data and Analysis
station
corrections
to account
for
structural
heterogeneity
within the CWB-TTSN array. Insteadwe
To constrain
three-dimensional
variations
of V• in the utilizedthe relativetiming and move-outof later arrivals
manfietransitionzonebeneaththe northernPhilippineSea, with respectto thoseof the first arrivals.
Table1. Summary
of Source-Receiver
Geometry
Profile Information
Event Information
Date
July21, 1994
Mar.31, 1995
Aug.7, 1992
Jan.22, 1992
July7, 1995
Aug.29, 1992
Oct.11, 1993
Oct,30, 1992
Dec.12, 1987
JuneI, I992
May18, 1993
Dec.3,1988
Oct.2, 1987
July22, 1993
May4, 1988
Feb.10,1991
OriginTimea, Latitude
a, Longitude
a, Deptha,
UT
øN
øE
km mba
1836:32
1401:40
1111:42
0106:56
2115:19
1919:06
1554:21
0249:48
0451:51
1829:20
1019:34
0313:46
0738:28
1215:36
2347:02
1415:20
4234
38.21
35.73
38.47
33.97
33.19
32.02
29.94
29.69
29.74
19.91
27.80
2735
21.76
18.51
14.0!
132.87
135.01
135.15
14031
137.13
137.98
137.83
138.98
140.02
140.70
122.45
141.06
139.94
144.26
145.86
144.74
471
354
358
116
333
289
351
393
164
134
169
106
463
127
122
156
6.5
6.0
5.6
5.6
5.8
6.0
6.4
6.0
63
5.5
6.4
5.5
5.5
5.6
5.9
5.5
BackAzimuth,
Epiceritral
Distances,
Sampled
Depths
b, Preferred
Profile
deg
km
km
P1
25
37
43
44
51
55
58
66
67
68
40 e
74
75
91
99
110
2210-2520
1910-2256
1750-2060
2301-2610
1767-2097
1797-2097
1730-2024
1730-2120
1854-2103
I919-2207
2298-240 1
1948-2168
1810-2050
2300-2541
2564-2684
2609-2750
593-728
477-683
iasp
iasp
458-670
380-679
442-670
366-664
474-664
495-670
304-660
304--662
400-671
298-660
...
C34
C34
C34
C34
C34
(234
C34
C34
516-680
480-685
545-700
558-708
iasp
iasp
iasp
iasp
P4
P5
P6
P10
Pll
P12
P13
P14
P15
P16
,
Model
,
Values
ofparameters
arefromthemonthly
listing
ofPreliminary
Determination
ofEpicenters
(PDE).
Range
ofbottoming
depths
fortherays
comprising
thePwave
train,
including
allbranches
oftraveltime
triplication
sampled
byeach
profile.
Azimuth,
notbackazimuth,
istheappropriate
parameter
used
forthisprofile.
11,818
BRUDZINSKIET AL.:NORTHERNPHILIPPINESEAANOMALY
To combinedatafrom thesetwo arrays,our analysis Northern PhilippineSeaAnomaly
proceeded
asfollows.First,wesearched
forbroadband Broadband Data. In the shorttime periodsincePreseismograms
from the Pre-POSEIDONproject, POSEIDON
databecame
available,
profile11istheonly
concentrating
ondatafromintermediateanddeep-focusbroadband
datasetwe identifiedthatdirectlysampled
the
earthquakes
thatoccurred
approximately
20ø+5
ø away northernPhilippine Sea anomaly (Figure 1). Twelve
from the denselyspacedstationsin centralJapan stationsin the Pre-POSEIDON array recordedthe event.
(Figure
1). In thisepicentral
distance
range,
turning
points Among them, excellent data with high signal-to-noise
of raysthatcomprise
the P wavetrainoccurin the ratios(S/N) areavailablefromsix stations
nearTokyo,
transition
zoneof themantle(Figure2). We selected
the
with an averagespacingof only20 km overanaperture
of
azimuthal
rangeof profiles
suchthatthese
turning
points 100km (Figure3). The source-receiver
geometry
is
fallneartheregion
inwhichthemostpronounced
anomaly approximately
linear,within10øin backazimuthal
range
of fastV•beneath
thenorthern
Philippine
Seahasbeen from the source(Figure1).
suggested
bytraveltimetomography
(Figure
1). Byusing Data shownin Figure3 have been correctedfor static
only datafrom earthquakes
deeperthan !00 km we time shifts due to near-station structure. Waveforms have
minimized
anypotential
effectsof structural
complexitybeendeconvolved
to removeeffectsof a finiteearthquake
abovethesource.For eachprofile,observed
waveforms source whose parameters,in turn, were precisely
werethenmatchedby synthetic
seismograms
to generate
a determinedby the inversion of teleseismicwaveforms
working
modelof V•. Wefound
thatsimple,
laterallyfollowing a proceduredescribedby Glennon.and Chen
homogeneous
modelsare sufficientto explaineach [1993, 1995b]. During deconvolutionthe data are bandobserved
profile,andthereis noevidence
to suggest
that passfiltered between0.75 and 0.1 Hz. The layoutof the
morecomplex
models
arenecessary.
Obviously,
eachof final datasetplacesan arrival from a forwardbranchof a
thelaterallyhomogeneous
modelsis appropriate
onlyin
thevicinityoftheregionsampled
bya particular
profile.
Next we turnto profilesconstructed
fromthe CWBTTSN short-period
datathat sampleapproximately
the Pll
sameportionof themantleas do broadband
waveforms
5O
C34
obtainedfrom the Pre-POSEIDON seismicarray. Along a
givenprofileusingCWB-TrSNdata,thefirstarrivaltime
at eachstationis manuallypickedandthentimecorrected
to thatpredicted
by the workingmodelon thebasisof
broadband
(Pre-POSE!DON)
data. Observed
arrivaltimes
of later arrivalsand move-outwith respectto the first
arrivalsare thencomparedwith thosepredictedby the
working
model.Whenever
possible,
observed
short-period
waveforms
arealsocompared
with synthetic
seismograms
generated
fromthisworkingmodel.Eachworkingmodel
originally developedfrom Pre-POSEIDONdata is
30
2350
2300
2400
2450
2400
2450
modifiediteratively until syntheticseismograms
and
predicted
arrivaltimesgenerated
fromthe modelmatch
boththebroadband
andshort-period
profiles.Thisentire
processis repeatedfor different geographicregions
sampled
by distinctprofiles. Additionaldetailson the
processingof broadbandand short-perioddata are
presented
in Appendices
A andB, respectively.
a) 22002250DISTANCE
(kin)
Results
• 40
In the region sampledby our data where Vp is
anomalously
high,one-dimensional
modelsaresufficient
to explaineachseismicprofile. Furthermore,
a single,
uniform model can adequately represent the entire
anomalousregion as a laterally homogeneous
structure,
referredto asthenorthernPhilippineSeaanomaly.Within
thisanomaly
ourdataplacedconstraints
ontheslowness
of
50-
C27
.
ß
'2.•
30
2250
2:300
2:350
DISTANCE (kin)
theP wave,theverticalgradient
of Vpin thetransition
3. Comparisons
between
observed
(solidtraces)
zone, and the changein V• acrossthe 660-kin Figure
and
synthetic
(dash-dotted
traces),
broadband
seismograms
discontinuity. The lateral extent of the anomalyis
fromthePre-POSEIDON
arrayalongprofile11. Smooth
constrained
by surrounding
profilesthatcanbe explained curves
showpredicted
traveltimes,whilesoliddotsshow
well by averageEarthmodels(Figure1). Sinceit is not
the purposeof this studyto investigate
averageEarth
models,in the followingdiscussions
we have usedthe
iasp9!model[KennettandEngdahl,1991]thathappens
to
be suitable
for profilessurrounding
thenorthern
Philippine
visuallypicked first arrivalsfrom unprocessed
seismograms.
(a)Thesimple
model
C34(Figure
4). (b)
Seaanomaly.
the waveform.
ModelC27witha 520-kmdiscontinuity
(Figure
4). Atthe
nearendofthearraythecusp(B")produced
bythe520-
kmdiscontinuity
provides
a good
fit tothebeginning
of
BRUDZINSKI ET AL.: NORTHERNPHILIPPINE SEA ANOMALY
11,819
triplication
asa peakof anacausal,
symmetric
wavelet. is adequately
represented
by theiasp91model.As a result
Forcomparison,
visually picked first arrivalsare also wehavevaried
onlyvalues
of Vpandthethickness
of the
plotted
(Figure
3). Thedeconvolved
datasetisused
for transitionzonein ourmodeling. In the simplestcaseof a
waveformanalysis. Details of data processingare singlelayerin the transitionzonesuchasmodelC34, there
presented
inAppendix
A.
areonlyfourfreeparmeters:depths
to andcontrasts
in V•
Giventhe approximate
lineargeometryof eachprofile acrossthe 400- and 660-kin discontinuities,all subjectto a
inFigure1,wecalculated
synthetic
seismograms
usingthe priori constraints
discussed
earlierin this section.
WKBJalgorithm[Chapman,1978;Chapmanet al., 1988].
Under these conditions a steep linear gradient of
Weperformedboth forwardmodelingand waveform approximately
5.35_+0.3
x 10'3 km/s/km appearsto be
inversionto find the best fitting models,and the results requiredto explain the data, and model C34 is our best
frombothapproaches
aresimilar.Eachmodelof V•is fitting model. Overallfeaturesof observedwaveformsare
subject
to a prioriconstraints
regarding
its gradientwith matchedby syntheticseismograms
generatedfrom this
respect
to depthin thetransition
zone,contrasts
in Vp model,with a maximummismatchin timing of only 0.2 s
acrossmajor discontinuities, and depths of such (Figure 3a). The only noticeable mismatch in the
discontinuities.We usedboundson theseparameters waveformsis at the nearend of the arrayat a distanceof
summarized
by Kennett[1993]. The bestfittingmodelis 2300 km. For this station a prominentpeak arrives
then perturbed by forward modeling to estimate approximately1 s after the first arrival. Since no such
uncertainties
of parameters.
arrival is expected if the entire transition zone is
In Figure3, move-out of first arrivalsacrossthe Pre- representedby a single layer, the most straightforward
POSEIDON
arrayis approximately
10km/s(in a spherical interpretationof this mismatch is to introducea small
Earth). This apparentspeedand the distancerangeover discontinuityin the middle of the transitionzone. This
whichthe speedis measuredindicatethat thesearrivals procedureresultsin two additionalfree parameters,the
mainlysamplethe middleto lower portionof the transition contrast
in V•andthedepthto thediscontinuity.
zone. Indeed, results of modeling the waveformsshow
Following the work of Shearer [1990, 1991] and
thatthelargestarrivalsare from refractionsturningin the Revenaughand Jordan [ 1991], we experimentedwith
middleof thetransitionzone(branchAB, Figure3a). The
precisedepthsfor the turningpoints of rays are model
dependent.
We foundtheVpto be 10.0-•_0.13
km/sat a
Vp(km/s)
depthof approximately
580-+15km.
With a Nyquistfrequencyof 10 Hz in the raw datathe
highS/N of the data can resolve differencesas small as
0.2-0.3 s in travel timesbetweenobservedand synthetic
5
6
7
9
10
11
'1 ;
waveforms. Thus, observedfirst arrivals are sensitiveto
changes
inthegradient
of V•withrespect
todepth.Given
:
-lOO
thelimitedapertureof thePre-POSEIDONarray,however,
a morerobustconstrainton the gradientcomesfrom the
nearend of the array, where a conspicuous
secondary
arrival is identified
8
Velocity
Models
to be the caustic arrival from the
400-kin
discontinuity
(cuspB',Figures2 and3).
In addition
to thefactthatsynthetic
seismograms
match
observed
waveformsfrom this cusp(Figure3b), the
identification
of thelocation
of cuspB' is corroborated
by
short-period
seismograms
fromTaiwanalongprofiles10
and5 to be discussed
in Figures5 and11, respectively.
Timingof thiscuspgivesan estimate
of theroot-mean-
iasp
-2oo
C34
' -300
C27
................
i
square
(RMS)Vpabovethe400-kindiscontinuity.
Taken
together
withthetimingandmove-outof thefirstarrivals,
theobserved
waveforms
canonlybematched
witha steep
• -400
gradient
in V•across
thetransition
zone.Thisfeature
is
illustrated
•hen onecompares
the iasp91model,a
commonly
usedone-dimensional,
radiallyvaryingmodel
ß
-500
representing
theaverage
VpoftheEarth,
withourpreferred
model,C34, which is used to generatesynthetic
'
,
seismograms
in Figure3a (Figure4).
Noticethatthe identification
of cuspB' in thedata
TRANSITION
'
ZONE
,,
-600
constrains
only the RMS V• abovethe 400-km
discontinuity.
IChus
inmodel
C34,
variations
inV•above
ß
depths
of 400kmaresimply
takenfromtheModel
of
-700
Erdogan
andNowack[1993],themostrecent,average
model
forthePhilippine
Seaareaasa whole.Similarly, Figure 4. Comparisonof threemodelsfor V. in theupper
since
thereis noindication
fromtraveltimetomography
mantle.Bothmodels
C34andC27arechar•tcterized
bya
that
anyanomaly
inVpexists
inthelower
mantle
beneath
high V• in the lowerportionof the transition
zone,
theentire
Philippine
Sea[Fukao
etaL,1992;
vanderHilst approx!mately2% fasterthan that of the iasp91average
etaL,1991,
1993],
weassumed
thatVpinthelower
mantleEarth
model.
11,820
BRUDZINSKI ET AL.' NORTHE•
PHILIPPINESEAANOMALY
(branchBC) in the transitionzone. Sucharrivalsoccur
modelsthat includeda small (1-2% contrastin
(Figure
5a),approximately
discontinuity
neara depthof 520km. Underthiscategory within5 s of thefirstarrivals
based
ontheiasp91
model
of models,C27 is ourbestfittingmodelthatmatches
the 2-3 s earllerthanpredictions
seismogram
ata distance
of2300kmindetail,particularly(Figure5c). In model C34 the early arrival timesarea
of fastVpin thelowertransition
zone.
regarding
thelocation
of cuspB" (Figure3b). Forthe directconsequence
interpretation
thatVpjustbelowthe660-kin
otherstationsalongthis profile, syntheticseismogramsThealternative
discontinuityis anomalouslyslow is inconsistent
withthe
generated
frommodels
C34andC27matchobservations
equallywell(Figure
3). Witha smalljumpin Vpin the move-outof observedfirst arrivalsfrom broadbanddata
alongprofile 11 (Figure3).
linear
gradient
inVpto4.86+0.2
x 10-3km/s/km
ineach
of Finally,judgingfrom the simplepulseshapeof thefirst
thetwolayersstraddled
by the520-kmdiscontinuity.
To arrivalat the nearendof the array,the total durationof the
matchthe observedwaveforms,resultsof modelingshow sourcetime function is approximatelyonly 1 s or less
of 2075-2200kin,large
that the discontinuitycan occur over depthsup to (Figure5a). Thusat distances
10-15 km, but no morethan20 km. Otherwise,
models arrivalsassociatedwith the cuspB' are interpretedto be
middle of the transition zone, model C27 decreasesthe
of gulsesthatoccurwithin5 s of the
C27andC34 arequitesimilar(Figure4). In particular,
in amongthesequence
first
arrivals
(Figure
5a). The assertionthat cuspB'
thelowerpartofthetransition
zone,
Vpinbothmodels
are
with
approximately
2%fasterthantheiasp91
velocity
model. terminatesbeyonda distanceof 2200 km is consistent
The existenceof the 520-km discontinuity
as a global
featureis controversial[cf. Shearer,1996; Benz and
Vidale,1993;Bock;1994; Cumminset al., 1992;Joneset
al., 1992]. AlthoughmodelC27 seemsto explaindata
alongprofile 11 betterthanmodelswithouta 520-kin
discontinuity,
wehavenootherevidence
to corroborate
the
existenceof sucha discontinuitybeneaththe northern
our identificationof this cuspat the very nearendof the
broadbandprofile (Pll) near 2300 km (Figure3). The
locationof thiscuspwasalsocorrectlypredictedby model
C34 alongprofile 5, to be discussed
in Figure 11.
For the short-periodprofiles in general,the average
spacingbetweenstationsis only 20 km, and the arrival
time of each phasecan be determinedto within _+0.5
s.
Philippine
Sea. Therefore,
we shallusethesimplemodel
C34 to describethe northernPhilippineSea anomaly.
Giventhelimitedapertureof the broadband
profile,one PlO
mayquestion
whetherall featuresof the transitionzone
5O
associated
with modelC34 canbe confidentlyresolved.In
the next few sectionswe shall revisit thesefeaturesas they
areborneoutby short-period
data.
Short-perioddata. Usingshort-period
datafromthe
CWB-TTSN arrayin Taiwan,we find thatportionsof the
manfietransitionzone sampledby severalprofiles are
closeto theareaexaminedby profile! !. In particular,
the
source-receiver
geometryfor profile10 is approximately
thereverseof profile11,minimizing
thepossible
effectof
dippingstructures
(Figure1). Indeed,similarresultsfrom
profiles10 and 11 supportthe notionthat a laterally
homogeneous
modelis sufficientto explainthe northern
PhilippineSeaanomaly.Data andresultsof ouranalysis
for profile10 are shownin Figure5, whoselayoutof
seismograms
is similar to that of Figure3. Details of
processing
andselectionof datafor the CWB-TTSN array
>
a) 1900
,
,
,
,
2100
,
22O0
b) •o0o"" ' •'o'oo
' ' "21Iøo' '
DISTANCE (km)
appearin AppendixB.
In Figure 5, both the relative travel times and
amplitudes
of laterarrivalswith respectto thefirstarrival
are well explainedby syntheticseismogramsgenerated
2000
C34 (PREFERRED)
• 50
iasp
50 c
s
from model C34. Notice that first, no late arrivalsof large
• 40
-"
40
amplitudeare observedcloseto the nearend of the array
(distance of--1950 km; Figure 5a). The only likely C) 1900 2000 2100 2200 1900 2000 21002200
DISTANCE(km)
DISTANCE (km)
candidatesfor large, late arrivals near a distance of
forprofile10,approximately
sampling
2000 km arephasesassociated
with cuspC, at thenearend Figure5. Results
sourceof the triplication due to the 660-km discontinuity the sameregionasprofile11 butwith a reversed
receiver
configuration
(Figure
1).
(a)
Observed
(Figure5c). Thustheshortspanof branches
BC andCD is
(solidtraces)overlain
b5/predicted
travel
consistent
with a smallcontrast
in Vpacross
the660-kin seismograms
times
of
the
preferred
model
C34
(smooth
curves).
(b)
discontinuity, characteristic of model C34 but
Syntheticseismograms
(solidtraces)and traveltimes
approximatelyhalf the contrastpredictedby the iasp91 predicted
bymodel
C34.Both'the
timing
andamplitude
of
averageEarthmodel(Figures4 and5c).
syntheticwaveforms
matchobservations
shownin
Second,at distances
largerthan2000 km the effectof a Figure
5a. (c)Comparison
oftravel
timecurves
predicted
smallcontrast
in Vpacross
thebottom
ofthetransition
zone by models
C34 andiasp91.Noticedifferences
in the
is alsoobservedfrom the arrivaltimesof phasesassociated locations
of cusps
C andB' andin thetimingof BC-CD
with turningrefraction(branchCD) andpseudo-reflectionbrancheswith respectto first arrivals.
BRUDZINSKI ET AL.: NORTHERNPHILIPPINESEAANOMALY
11,821
a move-outis preciselythatpredictedby the iasp91model
for refraction (branchCD) and pseudo-reflections
(branchBC) bottoming
in the transitionzone. Werethe
lowerportionof thetransition
zoneto haveanomalously
highVp,suchas in modelC34, thesedelaysmustbe
considerably
smallerthanthoseobserved
(Figure7b).
Notice that in Figure7b, move-outs of arrivals
associated
with the triplication due to the 400-km
discontinuity,
if any, will showa positiveslope(e.g.,
1850
1900
1950
2000
2050
2100
branchB'C';Figure2). Sucharrivalsareabsentin thedata
DISTANCE (kin)
(Figure7a). Theabsence
of cuspB' in profile14indicates
thatthisprofilesamples
themantletransition
zoneoutside
C34 (PREFERRED)
iasp91
the southern
boundary
of the northernPhilippineSea
• 50
anomaly
(Figure1). In contrast,
datafromprofile12can
• 40
'•••__•]1'40
by modelC34 butareclearlyincompatible
•
Bø be explained
withmodeliasp91.
The southern
limit of theanomalyis furtherconstrained
900
2000
2100
1900
2000
2100
b)
DISTANCE (krn)
DISTANCE (kin)
by profile13,whosesource
occursonthesouthern
edgeof
the
Izu-Bonin
arc,
and
by
profiles
15
and
I6,
whose
Figure6. Resultsfor profile9, samplinga regionbetween
sources
are
events
along
the
Mariana
arc
(Figure
1).
profiles10 and 11 within the northernPhilippineSea
nocuspsarepresent
in the waveforms
sampled
anomaly(Figure !). (a) Observed seismograms(solid Although
traces)
overlainby predictedtravel timesof the preferred alongprofile 13, the observeddifferencesin timing
½
50c
!
i' ,,i'',i,
modelC34 (smoothcurves). (b) Comparisonof travel
timecurvespredictedby modelsC34 andiasp91. Notice
differences
in the locationof cuspC andin the timing of
latearrivalswith respectto first arrivals.
between later and first arrivals are consistent with the
iasp91model(Figure8). Thesedifferencesdecreasefrom
approximately
3 s at thenearendof the arrayto slightly
lessthan2 satthefarend.In contrast,
modelC34predicts
a nearlyconstant
intervalof approximately
1 s acrossthe
entire array (Figure8b). Similarly, data from all three
The latter is equivalent to an uncertainty of +_0.7s for
with the iasp91
differentialtravel times. Sucha resolutionin spaceand profiles(13, 15, and 16) are consistent
model,
with
no
indication
of
an
anomalously
fastV•in the
timeis comparableto that expectedfrom the uncertainties
transition
zone
south
or
east
of
profile
12. These
citedearlierbasedon modelingof broadband
waveforms.
observations
leadusto concludethatthesoutheastern
edge
Along profiles 5-9 and profile 12, model C34 also
of thenorthernPhilippineSeaanomalyterminates
between
provides a satisfactory explanation for observed theregionssampled
by profiles12 and13 (Figure1).
seismograms.
Figure6 showsanotherexample,profile9.
Northern termination of the anomaly. The
Theapertures
of profiles9 and 10 aresimilar,butprofile9
northwestern
limit of the northernPhilippineSeaanomaly
incorporates
datafrom smallerepicentraldistances.The
termination
of cuspC near the distanceof 1950km is
particularly
clear in this case(Figure6), corroborating
a P14
smallcontrast
in Vpacross
the660-kmdiscontinuity
in
50
modelC34. Alongprofiles5-12 the turningpointsof rays
sample
a regionof approximately
500x 500km2 within
whichVpof the transition
zoneappears
to be laterally
uniform'
(Figure1). This resultcorroborates
that the
northern
PhilippineSeaanomalyis mainlya subhorizontal
feature[Fukao et al., 1992; van der HiIst et al., 1991,
19931.
40
a)
23'00
2350
240O
2450
DISTANCE (km)
LateralExtentof theAnomaly
C34
Southernterminationof the anomaly. The anomaly
terminates
to the southeast
andsouthof theregionsampled
•
byprofile
12,asnoanomalous
V•inthetransition
zone
is
•
detected
alongprofiles13-16(Figure1). Forinstance,
the
horizontal
separation
between
regions
sampled
byprofiles
12and14is approximately
300km,yetobservations
along b)
profile14 canbe fully accounted
for by theiasp91model
50
50
'
no
ol,....., ,
2300
235024002450
DISTANCE (km)
"
2300
235024002450
DISTANCE (kin)
Figure 7. Resultsfor profile 14, samplingthe mantle
andareclearly
inconsistent
witha fastV•in thetransitiontransition
zone due east of Taiwan beneath the northern
zone.
Philippine Sea (Figure 1). (a) Observed seismograms
Dataplottedin Figure7a showthatin thisparticular (solid traces)overlain by predictedtravel times of the
layout,
move-outs
of all laterarrivals
havenegative
slopes preferrediasp91model(smoothcurves). (b) Comparison
overtheentirerangeof profile14. Timedelaysbetween of traveltime curvespredictedby modelsC34 andiasp91.
secondary
and first arrivalsdecreasefrom approximately Notice differencesin the locationsof cuspB' and in the
4 s at a distanceof 2300 km to about2 s at 2450 km. Such timingof laterarrivalswith respectto firstarrivals.
BRUDZINS•ETAL.'NORTHERN
PHILIPPINE
SEAANOMALY
11,822
P13
t
30
• 20
!
lO
20
a)
1800
1850
1900
1950
2000
2050
DISTANCE (kin)
iasp91(PREFERRED)
• 30
'
2300
2400
",
,
,
i
1900
,
,
,
,
,'"
2000
DISTANCE (km)
,
,
1800
1900
2000
DISTANCE (km)
C34
•B)
20
t
20
i'
800
2500
DISTANCE (km)
•'20
•-•
c
b)
2200
30 iasp91(PREFERRED)30-
C34
30
• 20
a)
b) •00DISTANCE
2300
2400
2500 22'06
'DISTANCE
•3'0•
' •4'0•
' •5'0•
'•
(km)
(kin)
Figure 9. Resultsfor profile 1, samplingthe transition
zone in the back arc region beneath South Korea
(Figure 1). (a) Observed seismograms(solid traces)
overlainby predictedtraveltimesof the preferrediasp91
(smoothcurves). (b) Comparisonof travel time curves model (smooth curves). On the basis of detailed
predicted
by modelsC34 andiasp91. Whencompared investigationof the source-timefunction [Chen et at.,
with the observations,
time intervalsbetweenlaterandfirst 1996]we usedthe arrivaltimesof the second,largerpulse
arrivalspredictedby modelC34 aretoosmallandnearly of observedsignals. (b) Comparisonof travel time curves
predictedby modelsC34 and iasp9!. No trip!icationis
constantacrossthe array.
predictedby modelC34 at distancesgreaterthan2175kin.
Figure8. Resultsforprofile!3, sampling
a regionjustto
theeastof thenorthern
PhilippineSeaanomaly(Figure1).
(a) Observedseismograms
(solid traces) overlainby
predictedtravel timesof the preferrediasp91model
is largelyconstrained
byprofiles1,2, and5 (Figures
1 and are clearly incompatiblewith thoseof model C34, which
9-11).
The source for profile 1 is a recent, large
earthquake
whosesourcetime functionconsists
of two
bursts of seismic moment release. Chen et al. [1996]
predictsa delayof only 1.5 s at thisdistance(Figure10b).
In contrast, profile 5 indicates that the northern
Philippine Sea anomaly does extend beneath the
studiedthis event in detail and reportedthat eachof the
southernmosttip of Japan near a latitude of 30øN
twobodywavepulseshasa durationof approximately
4 s, (Figure 1). At distancesnear 1800 km, arrivals from
with a timeseparation
of approximately
4 s betweenthem.
Consequently,
we havepickedthe firstarrivalsaccording
to the onsetof the second,largerburstof momentrelease
P2
in Figure 9a.
Since the sourcedepth is well below the 400-kin
discontinuity(Table 1), only the triplicationdue to the
• 30
660-kin discontinuitycan be expectedfrom this profile
(Figure2). A prominent
secondarrivalis clearlyobserved
over the entiredistancerange of this profile (Figure9a).
Delay timesandmove-outof this secondphasein relation
to the fu:starrivalsarewell explainedby the iasp91model,
placingcuspB at a distanceof approximately2500 km
a) 9O0
(Figure9a). Incidentally,like profile 10 (Figure5), shortperiod waveformsfrom this event can be matchedby
synthetic waveforms. These observationscannot be
explained
bya highWin thetransition
zone,because
such
an anomaly
wouldreducethe contrast
in Vpacross
the
ß
1950
2000
2050
2100
i
2150
,
22OO
2250
DiSTANCE (km)
.-, c•aSp91
(PREFERRED)
C34
660-krndiscontinuityand place cuspB at a distanceof
several hundred kilometers
smaller than that observed
(Figure9b). This particularresultis consistent
with thatof
Tajima and Grand [1995], who studieda similarproblem
beneaththeJapanSea.
In fact, profile 2 providesevidencethat the anomaly
doesnot extendbeyondthe west coastof southernJapan
(Figure 1). At the near end of the array (-!900 km),
arrivalsfrom branchesBC and CD are delayedby more
than4 s fromthe first arrivals(Figure10a). Thesearrival
timesagreewell withpredictions
of theiasp91modelbut
....
ß ............
b) •000 DISTANCE
•000 2100
2200 1900DISTANCE
2000 2100
2200
(km)
(kin)
Figure10. Resultsfor profile2, sampling
themantle
transition
zonein thebackarcregion
justwestof Kyushu
(Figure1). (a) Observedseismograms
(solidtraces)
overlainby predicted
traveltimesof the iasp91model
(preferred,
smooth
curves).(b) Comparison
of traveltime
curvespredictedby modelsC34 and iasp91. The
difference
in predicted
move-outs
of laterarrivalsis most
conspicuous
at thenearendof thearray(!900 km).
BRUDZINSKI ET AL.: NORTHERNPHILIPPINESEA ANOMALY
branches
BC andCD are observed
within4 s of thefirst
arrival, as predicted by model C34 (Figure11a).
Meanwhile,
modeliasp91predictstoo large a delayof
more than 6 s at the same distance (Figure 11b).
11,823
••4o
Moreover,
arrivalsfrom cuspB' predictedby modelC34
arealsopresentin the datanearthe distanceof 1775km
• 30
(Figure
1!a).
Thusthe terminationof the northernPhilippineSea
anomaly
musthave occurredwithin the 300km distance
betweenthe regions sampled by profiles 2 and 5
a) 1750 1800 18'501900 1950 2000 2050 2100
(Figure
1). However,the exactnorthernlimit of the
anomaly
cannotbe furtherrefined.Alongprofiles3 and4,
DISTANCE (km)
predicted
differences
in arrivaltimesandmove-outs
between
modelsC34 andiasp91aresmall.Consequently,
wecannot
confidenfiy
resolvewhetherregionssampled
by
profiles
3 and4 arepan of thenorthern
Philippine
Sea
anomalyor a transitionalregion near the edge of the
anomaly.
In summary,resultsof 16 seismicprofilesindicatethat
thelateralextentof the anomalyseemsmuchsmallerthan
thatinferredfrom traveltime tomographyof first arrivals
[cf.Fukaoet aI., 1992; van der Hilst et al., !991, 1993].
The most pronouncedanomaly in the transitionzone is
mapped
outby profiles5-12, approximately
centered
just
eastof the Ryukyu are beneaththe northwesterncomerof
thePhilippineSea (Figure 1). Within this region the
anomaly
canbe approximated
as a subhorizontal,
laterally
Figure 11. Resultsfor profile5 nearthe northernmost
limit of thenorthern
PhilippineSeaanomaly(Figure1).
(a) Observedseismograms
(solid traces)overlainby
predicted
traveltimesof modelC34 (smooth
curves).(b)
Comparison
oftraveltimecurves
predicted
bymodels
C34
andiasp91.Notethatthepresence
of cuspB' is predicted
correctlyby modelC34.
If the northernPhilippineSea anomalyis due to
lithosphereof the Pacific plate, an obvious
centeringnear a depth of approximately530+40 km subducted
[Fukaoet al., 1992; van der Hilst et al. , !991, 1993], fast interpretation
is thatthe high Vpis a resultof cold
associated
with subducted
lithosphere.We
V•appears
to occurin thelowerportionof thetransitiontemperatures
uniformfeaturein the transitionzone. However, insteadof
thetemperature
difference,AT,m, betweenthe
zone,reducingthe contrastof V• acrossthe 660-kin estimated
coldest interior of the slab and the surrounding
discontinuity
from approximately
6% to 3% (Figure4).
asthenosphere
by ananalytical
approximation
of McKenzie
[!970].
Since
we
are
interested
only
in
the
temperature
Interpretationand Discussion
differencebetweentheasthenosphere
andsubducted
slab,
Ourresultsbolsterthe interpretation
that the northern adiabaticheatingandlatentheat associated
with phase
PhilippineSea anomaly is caused by remnantsof transformation
haveno effecton AT,nax.We specifieda
subducted
Pacificlithosphere,even thoughwe disagree slablengthof 1500km, slabthicknessof 100 km [ran der
withtheassertionof Fukaoet al. [ 1992]and vanderHilst HilstandSeno,1993],andconvergence
rateof 45 mm/yr
etal. [1991,1993]thatthe anomalymainlyoccursin the [Senoet al., !993] in our calculations.Values for the other
middleof the transitionzone. The reasonis twofold. First,
parameters are assumed to be the same as those of
sincethethermalconstant
for old oceaniclithosphere
is
Molnar et al. [1979]. The estimatedAT,• is 300øK.
approximately
80 Ma [e.g., Parsonsand Sclater, 1977], Thus, on average, the suspectedslab remnant is
coldPacificlithosphere
shouldremainnegatively
buoyant approximately
150øKcooler(AT)thantheasthenosphere.
fortensofmillionsof yearsaftersubduction.
It is importantto bearin mind that evenat relatively
Second,sincethe phasetransitionacrossthe 660-kin shallow depths, all estimatesof AT are subject to
discontinuity
hasa negative
Clapeyron
slope[e.g.,Ito and considerable
un.certainty
[e.g.,Peacock,1996]. Using
Takahashi,
1989],thisdiscontinuity
presents
a resistanceparameterswhosevaluesare similar to thoselistedabove,
to slabpenetration[Turcotteand Schubert,1971]. As
we comparedthe resultsfrom differentschemesof thermal
subduction
continues
alongthe!zu-Bonin[vanderHiIst modeling.For instance,for a 60ø-dipping
slabat a depth
andSeno,1993],negative
buoyancy
wouldresttheslab of 400 km where a number of publishedresults are
justabovethe660~km
discontinuity
whereconsiderable
available,the analyticalsolutionseemsto underpredict
the
resistance
to slabpenetration
is expected
[Turcotteand AT. The amountof underprediction,
however,rangesfrom
Schubert,
1971].The presence
of a coldslabshould as small as 20% up to nearly 60%, whencomparedwith
increase
thedepthto the660-kmdiscontinuity.
Shearer the finite elementmodel of Davies and Stevenson[1992]
[!99!] reported
thatonaverage,
thedifference
in depths
to and a finite differencemodel of Steinand Stein [1996],
thisdiscontinuity
between
large-scale
tectonic
provinces
is respectively. Thus our estimatedAT for the northern
oftheorderof 10km. However,we cannot
provethat PhilippineSeaanomalyis likely to be uncertainby at least
such
a depression
of the660-kindiscontinuity
exists
under the samemagnitude.
thenorthern
Philippine
Seaanomaly,
because
thisvalueis
Using a temperaturederivativeof-0.4 to -0.5 m/s øK
approximately
the limit of resolutionin our datafor this [e.g.,Kern, 1982; CreagerandJordan,!986], we expecta
particular
parmeter.
AT of 1500Kto raisethe V• by 0.06-0.075km/swhen
11,824
BRUDZINSKIET AL.: NORTHERNPHILIPPINESEAANOMALY
variations
in Vpassociated
withthenorthern
averaged
overthethickness
of theslab.If thetemperaturedimensional
anomaly
is60%larger,
assuggested
bythemodels
ofStein Philippine Sea anomaly. Azimuthal coverageof the
thatthelateralextentof theanomaly
is
andStein[1996],
V•willincrease
by0.10-0.12
km/s.In profilesindicate
thiscase,thermaleffectalonemaybesufficient
to explain much smaller than that inferred from travel time
usingmainlyfirst arrivals. Nonetheless,
the
theanomaly
in V•observed
atadepth
near580kmunder tomography
thenorthern
Philippine
Sea.Ontheother
hand,
if theATis anomaly
doesstandout as a subhorizontal,
laterally
only20%greater,
asis suggested
bythecalculations
of uniform feature in the transition zone beneaththe
northwestern
comerof thePhilippineSea(Figure1).
On the basisof seismicrayswhoseturningpointsoccur
actual
value
ofVpisatthelowest
possible
bound
allowedwithin the anomaly, an important characteristicof the
DaviesandStevenson
[ 1992],petrological
effectsmaybe
needed
to explain
theobserved
anomaly
in Vp,unless
the
is thatfastVpoccurs
in thelowerportion
ofthe
byuncertainties
in ourmeasurements.
It is interesting
to anomaly
notethat a AT in excessof 200øKis expectedto depress transition
zone,reducingthe contrast
in V• across
the
3% (Figure4).
thedepthof the660-kmdiscontinuity
by approximately660-km discontinuityto approximately
20 krn[Ito andTakahashi,
1989;Shearer,1991],aneffect Althoughthesefeaturesdiffer from imagesof traveltime
we did not observe.
tomography,
constraints
presented
here actuallyhelp
Mineralsexpected
to be stableundertemperature
and bolsterthe interpretationthat the northernPhilippineSea
with subducted
Pacificlithosphere.
pressure
conditions
in thetransition
zonefall intothree anomalyis associated
categories
[DuffyandAnderson,
1989].Firstis [•- and?- On the basisof estimatesof temperaturefields associated
Vpof stablemineralassemblages
under
spinel,
bothbeinghigh-pressure
formsof olivinewithfast withsubduction,
Vp. Secondis a garnet-majorite
solidsolution,mantleconditions,andexpectedlayeringwithinsubducted
characterized
byVpvalues
slower
thanthose
of spinels.slabs,we interpretthat a remnantof subductedPacific
Thirdis a minorcomponent,
jadeitc,a clinopyroxene
with lithosphereis trappedin the transitionzonejust abovethe
veryslowV•. If thetemperature
effectassociated
witha
660-kin discontinuity.
c61dsubductedslab is much less than 250øK, the high
valueof observed
V• associated
with the northern Appendix A: Processingof Pre-POSEIDON
Philippine
Seaanomaly
suggests
that[5-andT-spinel
are Broadband Data
the only candidates
that can accountfor an additional
increase
in Vp.If so,thelowerportion
of thetransitionStation
Corrections
zoneunderinvestigation
mustbeenriched
in high-pressure
The Pre-POSEIDON stationsare locatedin Japannear
the junction of the Pacific, Philippine Sea, and Honshu
In generalterms this petrologicinterpretationis plateswherethe geologyis complex[e.g.,DeMets,1992].
consistent
with the notionthat this anomalyis a resultof
On the basisof short-periodP arrivals,stationcorrections
subductedPacific lithosphere,trappedjust abovethe
(statics)of the order of _+1s have been reportedfor this
660-kindiscontinuity.
First,bothtraveltimetomography
generalarea [e.g., Hager and Clayton,1989]. Azimuthandourresultsindicatethatthe anomalyis a subhorizontal
dependentstationcorrectionsof similar magnitudehave
feature,probablyextending
continuously
westward
from
alsobeenreportedacrossthe Gr•ifenbergbroadband
array
the tip of the Izu-Bonin Wadati-Benioff zone in Gemany [Weber,1994]. We determined
statics
forthe
[Fukaoet aI., 1992;vanderHilst et al., 1991,1993]. This
Pre-POSEIDONstationsby taking weighedaverages
of
geometrysuggests
that originallayeringof subducted travel time residualsfrom earthquakesat teleseismic
Pacificlithosphere
wouldbe largelyintact. The bulk of distances(Table A1).
subducted
lithosphere
is expectedto be depletedmantle
Withinthe 1-yearperiodof May 1993to May 1994we
material(i.e., relativelyenrichedin olivine),overlainby a
were
ableto determineprecisearrivaltimesfromeight
veneerof enrichedeclogitecrust. A remnantof thick,
largeteleseismicearthquakes
whoseepicentraldistances
formsof olivinerelativeto the averageEarth.
depleted
lithospheric
mantleis a suitable
candidate
for fast varied from 30 ø to 90 ø.
In this distance range,
Vpinthebottom
portion
ofthetransition
zone.
complications
dueto triplications
in theuppermantle
and
Second,O'Neill et al. [1993] reportedthat hydrous,
phases
interacting
withthecoreareminimized.Foreach
calcium-rich garnet, likely to be stable under high
pressures,
hasanextremely
low seismic
wavespeed.Thus
a veneerof hydrothermally
alteredoceaniccrust,whichis TableA1. Hypocenter
Information
of Calibration
Earthquakes
expectedto turn into ec!ogite (mainly garnet and
clinopyroxene)if it is subductedwith the rest of the
OriginTime, Latitude, Longitude,Depth,
Date
UT
øN
øE
km
mb
lithosphere,
maybe sufficientto account
for therelatively
slowVpintheupper
portion
ofthetransition
zonebeneath
the northern Philippine Sea.
This interpretation is
May11,1993
1826:51.3
7.22
speculative,
and the relativelyslowVp over a large May15,1993 0155:38.8 7.23
thickness
in the upperportionof the transitionzoneis Aug.4, 1993 1131:18.0 -1.63
difficultto explain.
Conclusions
Using 16 seismicprofiles, eachincludingrelevant
arrivalsfromtriplications
of traveltimecurvesassociated
with the mantle transition zone, we constrained three-
Aug.29,1993 0957:54.9 -7.01
Sept.
1,1993 1148:38.4 -4.33
Sept.
29,1993 1116:03.5 0.49
Dec.9, 1993
Dec.9, 1993
0432:19.5
1138:27.9
0.49
0.43
126.57
126.64
99.62
!29.56
102.57
121.53
126.00
!25.89
59
31
32
147
71
97
15
!6
6.1
5.6
5.9
5.8
5.8
6.1
6.5
63
Valuesof parameters
arefromthemonthly
listingof Preliminary
Determination
of Epicenters
(PDE).
BRUDZINSKI ET AL.: NORTHERN PHILIPPINE SEA ANOMALY
11,825
Station Corrections
1,5 --
1.0--
-1.5
2250
[
•
2300
2350
•
2400
2450
DISTANCE (km)
FigureA1. Traveltimeresiduals
at five Pre-POSEIDON
stations
(solidcircles)usedin Figure3
relativeto globalstationMAJO, whoseabsolute
residualis closeto zero. At eachstationthe solid
square
shows
thestation
correction
thatis a weighed
average
ofresiduals.
Theevents(TableA1) are
chosenwith backazimuthsappropriate
for thegeometry
of profile1!. Traveltimespredictedby
iasp91areusedasthereferencemodel.
earthquake
thebackazimuths
arewithina +_20'
rangefrom sense. This algorithm was originally developedby
the source-receiver
geometryof profile ! 1. We then
calculatedtravel time residuals, using the hypocenter
informationof the Monthly Listing of the Preliminary
Determination
of Epicentersand the iasp91traveltime
curves[KennettandEngdah!,1991].
AssumingGaussianstatisticsand takinginto account
uncertainties
in readingarrivaltimes,we calculated
the
correction
for eachstationas weightedaverages
of travel
timeresidualswith respectto stationMAJO, a longstanding,
standardized
globalseismograph
stationwhose
arrival times have often been included in building
reference
Earthmodels(FigureA1). Theresulting
station
Ndbelek [1984] to investigate shallow earthquakes.
Glennon and Chen [1993, 1995b] and Chen [1995]
modifiedN•ibelek's
routinesto studydeepearthquakes,
and
we haveusedthe sameprocedurehere. The readersare
referredto thosepapersfor technicaldetails.
FigureA2 showstheresultof theinversion.Thesource
depthof this eventis determined
to be approximately
169-•_5
kin. Thisis a necessary
stepin ouranalysisbecause
to first order,erroneous
sourcedepthmapsinto erroneous
epicentral
distances
across
theentirearray.Inspection
of
directP phases
at teleseismic
distances
indicated
thatthe
main burstof momentreleasewas precededby a small
(FigureA2). Consequently,
we enhanced
the
corrections
areapproximately
+_1s, consistent
withvalues precursor
high-frequency
component
of
the
signals
recorded
at
Prereported
by Weber[1994]andHagerandClayton[1989].
Notethatif stationcorrections
werenotapplied,themove- POSEIDON stationsby usingthe teleseismic,directP
recorded
atCOLasthebasisfordeconvolution.
The
out of first arrivals shown in Figure3 would have phase
signal
at
COL
was
chosen
because
this
station
lies
in
the
exceeded 13.Skm/s (in a spherical Earth), an
same
general
azimuth
of
the
source-receiver
configuration
unrealistically
fastV•forthetransition
zoneorthelower
for profile11. Any effectof directivity
for thisstation
is
manfie.
thesameastheeffectsat Pre-POSEIDON
Finally,to matchthe absolutearrivaltime of cuspB' approximately
shown
in Figure3, thethin(!.4 kin)veneer
of sediments
in stations.
In fact, strongdirectivityin the generaldirectionof
themodelof ErdoganandNowack[1993]is replaced
by
southis evidentfrom the narrowpulsewidth at CTAO
extending
theiruppercrustal
layertothesurface.
(FigureA2). The sourcemodelusedin FigureA2 is a
Haskell line sourcewith a rupturevelocityof 4 km/s
SourceParametersof the Event onMay 18, 1993
towardsouth-southwest.
Consequently,
onlythedirectP
at COL hastheappropriate
azimuth
to be
Usingthe P wave train from the nearreal time data pulserecorded
used
as
a
basis
for
deconvo!ving
the
Pre-POSEIDON
data
retrieval
system
SPYDERof theDataManagement
Center
of theIncorporated
Research
Institutions
for Seisinologyalongprofile11.
in termsof tectonicsthis earthquake
is unusual.The
(IRIS DMC), we determinedthe focalmechanism,
the
Wadati-Benioff
zone
of
the
source
region
is associated
depth,the scalar seismicmoment, the duration,and the
slabof theSouthChinaSeaunder
the
shapeof thefar-fieldsourcetimefunctionof thiseventby withtheeastdipping
waveform
inversion.Thistechnique
analyzes
P andSH Luzonarc [e.g.,Hamburgeret al., !983]. The focal
of the earthquake,
strike203', dip 44ø,and
wavetrains by minimizingthe differencesbetween mechanism
rake
-150
ø,
has
an
east-northeasterly
plungingP axis
observed
andsynthetic
seismograms
in a leastsquares
11,826
BRUDZINSKI
COL P
ET AL.: NORTHERN PHILIPPINE SEA ANOMALY
•
KIP P
KONO
""
P
•
KON
ANT
0
Source T•e
10
s 0 BB
Function
•CTAO
Figure A2. Comparisonbetweenobserved(solid traces)and syntheticbroadbandP wave trains
(dashedtraces)at selectedteleseismic
stations.After normalization
to an epicentraldistanceof 50ø,
amplitudes
of bothobservedandsyntheticgrounddisplacements
areplottedwith a commonabsolute
scale. Orientationsof nodalsurfacesfor directP phasesandlocationsof observations
on the focal
sphereareshownin equal-area
projectionof thelowerhemisphere
of thefocalsphere.Verticalbars
on seismograms
indicatetimewindowsusedin theinversion.The shapeof the source-time
functionis
alsoplotted.Noticethenarrowpulsewidthat CTAO, indicatingstrongdirectivitytowardthegeneral
direction of south.
(Figure A2), indicating downdip compressionin the
subducting
lithosphere.Sucha patternof strainis rare at
intermediate
depths[e.g.,Kao and Chen,1991].
The selecteddataaredown-sampled
to 25 Hz afterantialiastiketing. For displayandmodelingpurposes
wethen
applied a zero-phase(acausal),sixth-orderButterworth
filter, with bandpasscomer frequenciesset at 1.0 and
0.1 Hz. Without reliable estimatesof stationcorrections,
AppendixB: Processing
of CWB-TTSN
Short-Period
Data
The presentconfiguration
of the combinedCWB-TTSN
arrayincludes75 three-component,
short-period
stations
densely located over an aperture of approximately
3.5øx 1.5ø The arrayhasbeenoperated
jointly by the
Central
Weather
Bureau
and the Institute
of Earth
averagetime shiftsbetweenobservedand syntheticfirst
arrivalsare typically near 3 s, with occasionalvaluesas
smallas 1 s to aslargeas 5 s. In nearlyall casesthereis
redundancy
in data coveragewhenseismicprofilesare
plotted.To avoidclutter,we showedonlya smallnumber
of tracesthat more or less uniformly coverthe entire
apertureof each profile in Figures 5-9. In addition,
stationslocatedin sedimentary
basinswere notused,as
long coda of the first arrival often obscuredlater arrivals
on the sameseismogram.
Sciences,
AcademiaSinica. The TTSN networkbegan
digital recording in July 1987, with only vertical
componentseismographs
sampledat 100 Hz [Wang,
1989]. In March1991theCWB network
beganrecording Acknowledgments. Chu-YungChen and J. D. Bassguidedus
mantle petrology,and we benefitedfrom discussion
with
three-component
dataat thesamesampling
rate. Forboth through
S. Grand,B.-Y. Kuo, T. Lay, and J. Vidale. In particular,S. Grand
networks, data are archived for a 3 min duration when
offeredmanyspecific
advises,
andS.Steinlenthelpin themodeling
of
triggeredby an event. Sincethe mainpurposeof the thermalstructures.W.-J.Su, Y. J. Chen,andan anonymous
reviewer
for
CWB-TTSN array is to record local and regional offeredhelpfulsuggestions.We are gratefulto H. Kawakatsu
earthquakes,
datafor few teleseismiceventsareretained.
We
searched
all
available
data
and
selected
answering
manyquestions
regarding
Pre-POSEIDONdata. We also
thankK. Shimazaki,
T. C. Shin,andY. H. Yet forpermitting
usaccess
to
Pre-POSEIDON and CWB-TTSN data. Teleseismicdata were collected
seismograms
that satisfythe followingcriteria: (1) on- from the IRIS DMC. This researchwas supported
by NSF grants
scalerecordings
withhighS/N ratios;(2) signals
produced EAR94-04809andEAR93-16012(W.-P.C.)andgrantEAR94-05167
by intermediate-and deep-focusearthquakes
to avoid (R.L.N.).
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from pP andsP or any effectsof
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