JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 98, NO. B3, PAGES 4389-4405, MARCH 10, 1993
Investigationof Upper Mantle DiscontinuitiesNear Northwestern
Pacific SubductionZones Using Precursorsto sSH
ZHI ZHANG and TtIom•
LAY
C. F. RichterSeismological
Laboratory
andInstituteof Tectonics,
University
of California,SantaCruz
Teleseismic
long-period
Word-WideStandard
Seismograph
Network(WWSSN)tangential
component
recordings
of deepandintermediate
depthearthquakes
areanalyzed
for thepresence
of sS precursors,
denoted
sxS, signifying
underside
reflections
fromdiscontinuities
at depth
x belowthesS reflection
point
abovethesource.ThesesS precursors
canbe usedto placeconstraints
on uppermantlediscontinuities
in
thevidnityof subducfion
zones.Theclearest
precursor
is usually
thesinsphase,
theunderside
reflection
from the Moho, whichis observed
for a numberof eventsin the northwestem
Pacificfor pathsreflecting
undertheSeaof Okhotsk
andNorthKorea.Theamplitude
andtimingof s,•S relativeto sS aremodeled
usingsynthetic
seismograms
to determine
theshearwaveimpedance
contrast
at theMohoandthecrustal
thickness
in theseregions.Theresults
arecompared
withprevious
workonpP precursors
reflected
from
theMohofor similar
paths,withconsistent
crustal
thickness
beingfound.s,•S is strong
for continental
reflection
points
under
NorthKorea,which
haslaterally
varying
31 to36 kmcrustal
thickness
and31.2%to
18%shearwaveimpedance
contrast
at theMoho.In thenorthem
Seaof Okhotsk
region
thecrustal
thicknessis around
21 to 29 km with26%to 28%shearwaveimpedance
contrast
at theMoho.Stacked
sins
waveforms
for theSeaof Okhotsk
varyfromstrong,
isolated
arrivalsin thenorthto weak,poorlydefined
precursors
toward
thesouth,
reflecting
a transition
fromcontinental
to oceanic
crust.s,•S is notclearly
separated
fromsS foroceanic
paths
under
theIzuandJapan
regions,
andourprocedure
cannot
resolve
oce-
anicMohoproperties
unless
broadband
dataareused.Other
precursors,
sxS,aremuch
weaker
thansins
andaredifficultto identifyin individual
waveforms.
We useslantstacking
to enhance
the signalto noise
andsearch
forprecursors
withvarious
slownesses,
using
recordings
from13deepevents.
FourSII wave
reflectors
mayexistbelowtheMohoin ourstudyarea.Theshallowest
is the"80-km"discontinuity,
which
varies
in depth
laterally,
beingnear80 to 85kmbelowtheSeaof Okhotsk,
near66 kmbeneath
IzuJapan,
and near90 km underNorth Korea. A reflectornear200 to 210 km depthis indicatedby datafrom
Japan,
butthereisnoevidence
foranyshear
waveimpedance
contrast
around
thisdepth
under
NorthKorea
or the Sea of Okhotsk. Weak evidenceis found for a "330-km" discontinuityin theseregions,with the
depth
of a fewpercent
impedance
contrast
varying
from325to 335km. Relatively
strong
andconsistent
arrivals
indicate
thepresence
of a "400-km"
discontinuity,
withthedepthvarying
from380 to 400 kin,
perhaps
indicating
slight
elevation
oftheolivine•>•
phase
transformation
neartheslabs.
INTRODUCTION
impedancecontrastsat the Moho are about 29.2% and 32.6%,
respectively. Lerner-Lam and Jordan [1987] constructed
Investigation
of uppermantlediscontinuities
is an important models EU2 and PA2 for northern Eurasia and the western
step in understanding
fine structure,dynamicsand chemical
PacificOcean,respectively,
usingfundamental
andhigher-mode
evolutionof Earth. Of the variousremotesensingtechniques
Rayleighwaves. The shearandcompressional
impedance
conemployed,seismology
playsa prominentrole in studyingman- trasts at the Moho are 26.0% and 28.8% in EU2 and 36.8% and
tle structure. By the 1930s, the gross radially symmetric
38.4% in PA2, respectively.These modelsimply that the
seismicvelocity distributioninsidethe planet was established,
impedance
contrastat the Moho is significantly
lower in conshowingregionsof different seismiccharacter(crust, upper
tinentalthan in oceanicregions. However,there are many
mantle, lower mantle, inner and outer core) corresponding
to
regions for which Moho properties,particularlyfor shear
gross compositionaldivisions of Earth. With the advent of
waves, remain undetermined.
digital data and refinement of interpretationaltechniques,
The 410- and 660-km discontinuitiesare global features
modernseismologyhasrevealedthat the uppermantlehasgloexploredby precursorsto tvP ' [Whitcomband Anderson,1970;
bal layering associatedwith the Moho, "410-km", "520-km"
Husebye et al., 1977; Nakanishi, 1986, 1988], waveform
and "660-km" discontinuitiesand other regionally varying
modeling[Burdick and Helmberger,1978; Grand and Helmstructuressuch as the "60-km", "80-km", "220-km" and "330-
km" discontinuities.
berger, 1984a,b;LeFevre and Helmberger,1989], stackingof
reflectedand convertedphases[Bockand Ha, 1984; Wajeman,
The seismological
Moho, definingthe upperboundaryof the 1988; Paulssen, 1988; Bowman and Kennett, 1990; Richards
mantle, has been widely mappedbasedon the abruptincrease
and Wicks, 1990; Shearer, 1990, 1991; Vidale and Benz, 1992],
of seismicvelocity with depth inferred from earthquakeand
andmigrationof manfiereverberations
ScSn[Revenaugh,
1989;
controlled source seismology,mainly using PmP (reflections Revenaughand Jordan, 1987, 1989, 1991a,b]. Basedon these
from the top of the Moho) or Pn (Moho head waves). In the
previousstudies,averageP velocityjumpsof 5-6% at 410-km
preliminary reference Earth model (PREM) [Dziewonskiand
and 4% at 660-km, and S velocityjumpsof about5% at 410Anderson, 1981], the average shear and compressionalkm and 7-8% at 600-km are inferred all with a + 1-2% uncerCopyright1993by theAmericanGeophysical
Union.
Papernumber9ZIB02050.
0148-0227/93/9ZIB-02050505.00
tainty. The long-wavelength(>50 km) topographyfor these
two discontinuities
is on the order of + 20-30 km. Ringwood
[1975] suggested
that the 410-km discontinuity
is causedby the
phasechangefrom olivine to •-phasefor (Mg,Fe)2SiO4. The
4389
4390
ZHANaANDLAY: UPPERMAm'LEDiscosTiNUrrmsABOVEDEEPSLABS
670-km discontinuitycan be explainedby the transition),-spinel such as partial melting or anisotropicfabric. Resolvingthe
--> perovskite + magnesiowttstite[Liu, 1976, 1979; Jackson, natureof thesediscontinuitieswill undoubtedlycontributesub1983; Ito et al., 1984; Ito and Takahashi, 1989]. Gamets also stantiallyto understanding
the dynamicsof mantleconvection.
transformto perovskitestructurenear this depth with a two- Of course,no singlemethodor data set can hopeto resolveall
phase region several gigapascalsin extent (not an efficient propertiesof a given discontinuity,
and a varietyof methods
reflectorat high frequency).
mustbe usedto fully characterizethe uppermantle.
The 520-km discontinuityis potentially a global feature
In this paper we focus on analysisof precursorsto the
[Whitcomband Anderson,1970; Helmbergerand Engen, 1974; transversecomponemsSH phase(below we write S only for
Jones and Helmberger, 1990; Revenaugh,1989; Revenaugh the SH component,which we use exclusively)on long-period
and Jordan, 1991b; Shearer, 1990, 1991] but is very weak. recordingsfor deep earthquakes
to analyzethe regionalmantle
The shearimpedancecontrastat 520-km discontinuityis about structureabovethe sources. Our procedureis mostsensitiveto
3% [Revenaughand Jordan, 1991b]. The phasetransitionof mantle structure above and near to subductingslabs, where
[l-phase--> ),-spinelis a possibleexplanation
for the 520-km strong thermal and chemicalperturbationsmay affect mantle
discontinuity[Whitcomband Anderson,1970, Ringwood,1975; discontinuities. We determine crustal thickness and shear wave
impedancecontrastat the Moho usingsinS, the Moho underOtheruppermantlediscontinuities
havebeenproposed
based side reflection; and investigateupper mantle discontinuities
at x km
on seismological observations,such as the 60-km, 80-km usingsxS, undersidereflectionsfrom discontinuities
Weidner et al., 1984].
[Revenaughand Jordan, 1991c], 220-km [Jordan and Frazer, depth in the mantle above deep sourcesin the northwestern
1975; Hales et al., 1980; Drummondet al., 1982; Bowmanand Pacificregion.
Kennett, 1990; Revenaughand Jordan, 1991c; Vidale and
DATA
Benz,1992], and330-km [Wajeman,1988;Revenaugh
andJordan, 1991c] discontinuities,but the universalityof these
Seismic waves that encounterdiscontinuitiesin the upper
featuresis not yet established
nor is their underlyingphysical mantle spawn many phases. We use only transversecomexplanation.
ponents(SH waves) in this study since they have particularly
It is critical to determinethe propertiesof all uppermantle simpleinteractions,with no conversionof S to P energy. Data
discontinuities
and to establishwhether they are global or from 23 intermediateand deep eventsare analyzedin this study
regional structures. Propertiesthat seismologycan resolve (Table 1). Figure 1 showsthe locationof these23 earthquakes
include the sharpnessand size of velocity, density, and in the northwestern
Pacificregion. The depthsof theseevents
impedancecontrastsof these discontinuities.This information range from 115 to 580 km, and the magnitudesvary from 5.6
is neededfor mineral physicsexperimentsto determineif the to 6.0. Table 1 lists the Intemational Seismic Centre (ISC)
discontinuities
can be explainedby phasechangesor, failing sourceparameters,along with focal mechanismsfrom Gaherty
that, must be associatedwith compositionalor other changes and Lay [1992].
TABLE 1. Earthquake
Parameters
(ISC) for EventsUsedin This Study
No.
1
2
Date
March 18, 1964
Time,
GMT
Latitude, Longitude, Depth,
deg
deg
km
mb
Strike,
deg
Dip,
deg
48
84
04 37:25.7
52.56
153.67
424
5.6
28.96
128.23
195
6.0
Rake,
deg
-76
3
Sept.21, 1965
July 4, 1967
01 38:30.3
23 42:12.9
43.10
142.58
157
5.6
274
88
8O
4
Dec. 1, 1967
13 57:03.4
49.45
154.40
144
5.9
50
87
109
5
May 14,1968
14 05:05.4
29.93
129.39
162
5.9
March 31, 1969
19 25:27.0
38.49
134.52
397
5.7
38
75
230
52.28
151.49
560
5.7
3
72
-90
51.69
150.97
515
6.0
34
72
-110
-94
6
Sept.5, 1970
07 52:27.2
8
Jan. 29, 1971
21 58:03.2
9
May 27, 1972
04 06:49.0
54.97
156.33
397
5.7
24
85
49.47
147.08
573
5.9
15
17
47
28.22
139.30
508
5.9
317
72
-74
24
17
107
205
79
8O
7
lO
Aug. 21, 1972
06 23:48.6
11
Jan 31, 1973
20 55:54.2
12
Sept. 10, 1973
07 43:32.3
42.48
131.05
552
5.8
13
Feb. 22, 1974
00 36:54.6
33.17
136.98
391
5.9
14
Sept.21, 1974
15 54:59.1
52.19
157.44
119
5.7
15
Dec. 21, 1975
10 54:17.2
51.93
151.57
546
6.0
16
July 10, 1976
11 37:14.0
47.31
145.75
402
5.8
41
89
-87
17
Dec. 12, 1976
01 08:51.1
28.04
139.67
503
5.8
328
72
-74
18
May 23, 1978
07 50:28.3
31.07
130.10
160
6.2
19
June 21, 1978
11 10:38.7
48.27
148.66
380
5.8
288
32
32
20
Sept.2, 1978
01 57:34.2
24.81
121.87
115
6.0
34
28
138
21
Aug. 16, 1979
21 31:24.9
41.85
130.86
566
5.8
56
24
133
Feb. 1, 1984
07 28:28.7
49.05
146.63
580
5.9
231
85
83
April 20, 1984
06 31:10.6
50.12
148.75
572
5.9
257
16
-74
22
23
ZHANGANDLAY: UPPERMAs'n_•DISCONTINUITIES
ABOVEDEEPSLABS
[
} \
•1
500
d
12/01/67
09/1
400
01/29/71
\ o4nom•,
4391
__-
I'•08/16/79
•
.,•
30 o•
7
05/23fi8•-'
3 09/2v6s
ß.
iit3
200
I
120 ø
I
I
140ø
I
160ø
Fig.1.Earthquake
locations
(triangles)
for23events
inthenorthwestem
Pacific
forwhich
weanalyze
data.
Thenumbers
beside
each
location
gives
thedateoftheevent
(month/day/year).
Table1 indicates
theother
source
parameters.
contrast
abovethe
The sensitivity
of a reflected
seismicwaveto the vertical be anydepthat whichthereis animpedance
source.
Underside
reflections
from
the
Moho
are
given
the speextent(sharpness)
of thevelocityor impedance
increase
across
if thesourcedepthis 560 km,
a discontinuity
is a function
of thesignalwavelength
[Richards,cial names,•S. For example,
maybe generated
by theMoho,80-km,2201972].Largewavelength,
long-period
wavesvertically
incidentthenprecursors
if theyexist,
ona boundary
will bereflected
effectively
onlyif thechange
in km, 330-kin,400-km,or 520-kindiscontinuities
and
are
then
labeled
as
s,,S,
ssoS,
s22oS,
s33oS,
s4oosand
material
properties
is distributed
overa depthinterval
lessthan
In the teleseismic
distance
rangeof our
-1/4 wavelength.
Nearverticalincidence
reflection
coefficientsss20S,respectively.
areonlysensitive
to impedance
contrast
(A(velocity
xdensity
)). data, the core-mantleboundaryreflectionScS may arrive
in thes•S interval.Figure2b is a
Thus,if we analyzelong-period
wavestravelingnearlyverti- betweenS andsS phases
sketch
of
a
teleseismic
waveform
correspondingto the ray
callythrough
theuppermantle,
we canconstrain
gross
characof Figure
2a,suggesting
thatat some
distances
eachs•S
teristicsof uppermanfieimpedance
structure.We examine paths
be wellseparated
fromScSandthusobservable.
Figure
long-period
SH wavespropagating
upwardfromdeepearth- should
2c showstraveltime curvesfor S, sS, ScS, ands•S phasesfor
quakes
thatreflect
fromdiscontinuities
andtraveltoteleseismic
a sourcedepthof 560 km.
distances.
By usingonlylong-period
WorldWideStandardized
SeismicNetwork(WWSSN)recordsin this study(dominant
As an exampleof our waveformdataquality,we showthe
earthquake,that
periods
closeto 20 s andwavelengths
near100km)we are entire set of waveformsfor one representative
ableto imagemantleshearvelocityimpedance
contrasts
that of August16, 1979,in Figure3. The dataare globallydistri-
are distributedover as much as 25-30 km in depth. The cri- buted;thusthe wavessamplea coneof mantlematerialabove
teriafor choosing
ourdataarea highratioof signalto noise the source. The horizontal componentswere digitized and
andpolarityreverand clear directS (to use for a sourcepulse)and surface rotatedto give thesetangentialcomponents,
reflection
sS phases.
Theepicentral
distance
rangein ourdata salsdue to radiationpatternhavebeencorrectedfor. The sS
is from30ø to 95ø to avoiduppermantletriplications
andcore phaseon all tracesare aligned,andwe canseetheclear,large
amplitudearrivals of sS and S phases. The lines give
diffraction.
We defineprecursors
to sS as sxS,the upgoing
S wave
differential travel time curves calculated with the PREM velo-
showingthe arrivaltimesexpectedfor possible
reflectedfrom the undersideof a discontinuityabovethe source city structure,
s•S
precursors
s,,S, ssoS,S22oS,
andS4oos
between
S andsS.
at a depthof x km. Thesephases
arriveat teleseismic
diss,,S traveltime for a 36-km crustalthickness.
tancesafter directS but beforethe surfacereflectionsS. Fig- We computed
ure 2a showsthe geometryof ray pathsfor &S, wherex can Note that these curves are not based on actual arrivals. The
4392
ZuxsoANnL•Y: UPPER
M•n•
D•scoswm-m•Es
ABOVE
DEEP
SLABS
Surface
a)
1600
;s
1400
S{
cS
o 1200 op,.•
S
- 1000
b)
400
c)
860
Distance
(deg.)
Fig.2.(a) Geometry
ofraypaths
fors,•Sands•Sprecursors
tosSHalong
withS,sSand
ScSpaths
and(b)acartoon
of
a corresponding
waveform.
(c) Travel
timecurves
forS, sS,ScSandsxSphases
fora source
depth
of560km.
traveltimecurvefor ScSis alsoshownandclearlyhasassoci- enhance
the signalto noiseratiobeforedrawinganyconcluatedarrivals.In theplotwe canseea smallwiggleabout18 s sions. Sincethe Moho produces
the strongest
and most
beforethe sS phasein mosttraces,whichis the Moho under- coherentreflectionof the uppermantlediscontinuities,
we are
sidereflection
s,•S. Identifyings,•S in eachindividual
traceis ableto identifys,,,Sin thestacked
tracesreadilyandto actu-
sometimes
difficult,butit is evenharderto seeanyotherpre- ally model the waveforms to determinecrustal thicknessand
cursors
between
S andsS. Thenoiselevelsarequitelow for shearwaveimpedancecontrastat the Moho.
thisevent,andthedatacanbe usedto placebounds
on thesize
As shownin Figure3, underside
Mohoreflections
smScan
of anyuppermantlediscontinuities.
However,sincethemantle
beseenin sometraces,
butit is hardto identify
thephase
in
discontinuities
(abovethe sources)
areexpected
to be lessthan others.Theonlysignificant
difference
between
sS andsmSis
half the size of the Moho arrival, we will stack the traces to
thepaththrough
thecrustforsS. If thecrust
haslaterally
uniform propertiesabove the source,then all of the arrivalsat
differentazimuthsshouldhave similarwaveforms.Our events
havestable
upgoing
SH radiation
patterns,
sothewaveshapes
areverysimilar. To increase
thesignalto noiseratio,we stack
the signals.We alignthe tracesfor a giveneventon thesS
phaseandnormalize
thepeakamplitudes
to unity,thenstack
the arrivals.This is validbecause
we expectlittlemoveout
between
s,.S andsS at teleseismic
distances.
Figure4 shows
stacked
waveforms
forall 23 events,
someof whichhavevery
clearandisolated
s.,S, suchastheevents
onFebruary
1, 1984,
April20, 1984,August
21, 1972,August
16,1979,September
5, 1970,andSeptember
10,1973,whilesomelacka clearSmS.
Notethatalignment
onsS causes
earlierphases
likeS andScS
or deeper
&S phases
to stackincoherently
giventhelargedisrance range spannedfor each event. A total of 451 individual
seismograms
is includedin Figure 4. Since oceaniccrust is
expected to be less than 10 km thick, the Moho underside
reflections.,S will overlap the sS arrival in long-period
waveforms;therefore,it is very hard to identifys,,,S from oceanic Moho, as for events on March 31, 1969, December 12,
0
150
300
Time (s)
1976,andFebruary22, 1974,(seeFigures1 and4).
Otherprecursors,
sxS,axemuchweaker
thans,•S,andthey
haveslightlydifferentslownesses
as indicated
by the travel
Fig.3. Waveform
distribution
foranevent566kmdeeponAugust
16, time curvesin Figure 3, so later we will slant stack the
1979.Thecurves
aredifferential
traveltimesindicating
timesat which
at variousslownesses
to seekevidencefor any
sxS, S, andScS phases
wouldbe expected
ff thereare mantle waveforms
impedance
contrasts
at depths
x.
uppermantlediscontinuities
in thesS precursor
window.
LAY: UPPERMmqTLE DISCOS'rINurrIESABOVEDEEP SLABS
4393
isolateddowngoingS phasecontainsthe attenuationcontribution from the lower mantle and near the receiver,along with
any receiverreverberations.Upgoingphases,such as sS, have
additionalattenuationand accompanying
multiple arrivalsfrom
undersideand topsidereflectionsfrom any discontinuities
above
the source.To model the sS phaseand its precursors,we use
the S phaseas a sourcewaveletcontainingthe deepmanfie and
receiver effects, the source radiation, and the instrument
response.The additionalfactors that have to be accountedfor
thus include the extra attenuationof the sS phases and the
interactionswith any velocity discontinuitiesabove the source.
We determineprecursorcharacteristics
relative to sS, so scalar
radiation pattern differencesof the downgoingphase are not
important,being eliminated by normalization. The paths of
s,,S and sS in the mantle are very similar, the difference
between the two arises near their reflection points. For deep
foci, the incident wave at the base of the crustis nearly planar,
and a simpleplane wave reflectivitymethodis sufficientfor the
calculationof synthetics,,S and sS seismograms.There are
many parametersthat affectthe syntheticmodeling,suchas ray
parameterp; sS differential attenuationrelative to S, Atss*;
crustalthicknessh; and shearvelocity and densitycontrastat
05.23.?8
I
I
I
I
I
I
the Moho, AI$andAp; but whichplaysan importantrole in the
1
2õ0
Time (s)
Fig. 4. Event-stackedwaveformsfor all 23 events,alignedand stacked
computation? We discussthis at some length since our procedurediffers from previousmethodsused to study underside
Moho reflections[e.g., Revenaughand Jordan, 1989].
onthesS phase.
Parameter
Search
We first test the sensitivityof syntheticwaveformsto the five
ANALYSIS
OFs,nS PHASES
TO DETERMINE
PROPERTIESOF THE MOHO
parameters:
p, At•s*, h, A•, and Ap. For eachtest,only one
parameteris varied. For the rangeof epicentraldistanceof our
We use the September5, 1970, deepKurile slabearthquake
sS data (September5, 1970 event)from 50ø to 90ø, the param(Table 1) to developour modelingmethodand to estimatethe
averagecrustalthicknessand shearimpedancecontrastat the etersp and Atss* were computedfor PREM: p changesfrom
Moho near the s,nS reflectionpoint under the northernSea of 9.5 to 14.5 s/deg,and Atss*variesfrom 1.5 to 3.0 s. The averOkhotsk.We have a total of 58 digitizedWWSSN transverse agep and At•s* for our data samplesare 11.81 s/degand 2.56
componentrecordsavailablefor this event. Twenty tracesof S
phasesand 34 tracesof sS phasesare chosenfor stackingon
each phasefor this event, at distanceswhere the phasesare
well isolated.We separatelystack the data alignedon the S
and sS phasesto enhancethe ratio of signal to noise. The
stackedS pulseis usedas a sourcepulseand the averagedsS
waveform is modeledwith syntheticwaveforms. Clearly, this
stackinginvolveslateral averagingover the regionsampledby
sS, as well as the intrinsicaveragingof the long wavelength
waves. However, the gain in signal stability gives us
confidence that the average structure can be determined
robustlyfrom the event-averaged
data. Later we will consider
subdivisions
of the databasedon reflectionpoint grouping.
In syntheticmodeling, the computedseismogram,S(t), is a
convolutionalproductof time series:
S (t)=X (t)*E (t)'I (t)
(1)
where X(t) is the sourcetime history,E(t) is the Earth transfer
function,andl(t) is the instrumentimpulseresponse.For a simple rupture,it is sufficientto treat the sourceas a point source
for which the spectrumof upgoing and downgoingradiation
from the sourceis the same,with separatescalingfactorsdue
to the differencein radiationpatternfor eachray parameter.At
s, respectively. For the numericaltests,we prescribea shear
wave surfacevelocity • = 3.2 km/s and surfacedensitypo=
2.6x103kg/m3, uppermost
manfieshearvelocity•,,, = 4.7
km/s anddensity
p,,,=3.38x103
kg/m3, crustal
thickness
h=
25 km, At•s* = 2.56 s,p = 11.81 s/deg,A• = 0.7 kin/s, and
Ap= 0.4x103
kg/m3,wheneachparticular
parameter
is not
being varied. The crusthas a linear velocity m•d densilygradient from the surfaceto the Moho, approximatedby many thin
layersin the reflectivitymodeling.
In Figure5a the effectof variableray parameter
p is shown
in superimposed
waveformsfor the rangep = 9.5-14.5s/deg;
the variation interval is 0.5 s/deg. All other parametersare
fixed at the values listed above. These signalsshow only a
small changein waveformas ray parametervaries,indicating
the possiblesmoothing
effectof stackingthe data. Sincethe
dataare actuallyconcentrated
in a limitedrangenear75ø, this
is a minor bias. Thus, in the following analysiswe fix an average value of p = 11.81 s/deg,which is an appropriate
average
for our shacked sS data.
The waveformsensitivityto At•s* is shownin Figure 5b.
At•s* variesfrom 1.5 to 3.0 s, with intervalof 0.1 s. Although
teleseismicdistancesthe Earth transfer function is essentiallya
the changeis visible, it is quite small. Comparisons
with
sequenceof arrivals with different geometricspreadingand
observedwaveformswill only be affectedif we comparewith
the entire sS waveform, includingits instrumentalovershoot.
attenuation
filters convolved
with
the receiver
function.
The
4394
ZHANGANDLAY: UPPERMANTt• DISCONTINUm•ABOW-DEEPSLABS
Varying Ray Parameter
c)
1.0
Varying Crustal Thickness
1.0
0.5
0.5
p=14.5s/deg ]
0.0
-
0.0
l=30
-0.5
-0.5
-1.0
-1.0
0
10
20
b)
30
40
50
60
km
h=15
I
I
I
I
I
I
I
0
10
20
30
40
50
60
Varying At.S*
d)
1.0
km
Varying Density Contrast
1.0
At•s*=3.0s
Atss*=1.5 s
0.5
0.5
0.0
0.0
Ap=50 kg/m
-0.5
-1.0
-0.5
I
I
I
I
I
I
I
0
lO
20
30
40
50
60
-1.0
Time (s)
Ap=750 kg/m 3
I
I
I
I
I
I
I
0
10
20
30
40
50
60
Time (s)
Fig.5. Synthetic
waveforms
of sS superimposed
waveforms
(a) forp =9.5to 14.5s/deg
withaninterval
of 0.5s/deg;
(b)
for Atas* = 1.5 to 3.0 s with an intervalof 0.1 s; (c) for h = 15 to 30 km with a 1-kminterval;notethattraveltime
difference
between
sS andsinsincreases
withincreasing
h' and(d) forAp= 0.05to0.75X103
kg/m3 withaninterval
of
0.05x103
kg/m3;
note
that
Apinfluences
theratio
ofsmS/sS.
Other
model
parameters
areheld
fixed
atthevalue
given
in
the text.
Thereforewe choosean average
Atss
* = 2.56s in themodeling
below
and use a short window
waveform
which excludes the sS
We alsotestAll for waveform
sensitivity.
If wekeepthe
averagecrustal velocity constant,identicalwaveformscan be
overshoot.
produced
by varyingAll andAp sincethereflection
coefficient
Figtire5c showsthe waveformsensitivityto crustalthick- at near-verticalincidence is primarily sensitiveto the
nessh at the reflectionpoint, which variesfrom 15 to 30 km.
impedance
contrast
A(p[3).Thes,•S/sSamplitude
ratiopro-
The substantial
waveformvariationshownin thisfigureis due videsour estimateof the impedance
contrast.
to the travel time difference between sS and s,.S, which
increaseswith increasingcrustalthickness.The ratio of sS and
Best Model Search
s,.S amplitudealso changesdue to the variable interference,
but this is less pronotmcedthan the time difference effect.
The testsaboveshowthattheparameters
h, A•, andAp
Crustalthickness
is mainlydetermined
by the difference
in affectthewaveform
modeling
muchmorethantheparameters
travel time betweensS and s,.S. It is clear that we should At•s*andp. Thuswe useaverage
valuesof p = 11.81s/deg
dataanalysis.
As Ap and
avoidstacking
waveforms
thatsample
regions
withvarying
cru- andAt•s*= 2.56s in thefollowing
we combine
thesetwoto
stalthickness.
Thisprevents
usfromstacking
multiple
events, All tradeoff directlyin themodeling,
astheeffective
footprint
of sinsbecomes
toolarge.
determinethe impedance.contrastA(p•) (A(p•) =
in thefollowing.
Sincethevelocity
Figtire5d is anexamination
of thewaveform
sensitivity
to 2(p1•1-P2•2)/(p1•+P2•2))
density
variation
ApattheMohoreflection
point,which
rangesanddensityarelikelyto be relatedto eachother,we specify
from 0.05x10
• to 0.75x103
kg/m3, with a step of the absolutedensityassociated
with a givenvelocitymodel
0.05x103kg/m3. The surfaceandMohodensities
arefixedat usingtheNafe-Drakerelation[GrantandWest,1965]'
po= 2.6x103
kg/m3 andp,. = 3.38x103
kg/m
3,respectively,
andthecrusthasa lineargradient
from2.6x103
kg/m3 at the
pc---0.1730cc
+1.695
(2)
stirface
to thedensity
at thebaseof thecrustgivenbypm-Ap wherePc is densityin thecrust,andOCc
is theP wavevelocity.
kg/m
3. Theamplitude
ratios,.S/sSincreases
asthedensityWe assume
oct= '/J[[,where[1is theshear
velocity
in the
contrast
Ap increases
as shownin the superimposed
tracesfor crust.We asstune
a linearcrustalvelocitygradient
throughout
different Ap models. There is no travel time difference the crust and do not have any midcrustaldiscontinuities.
between
sS ands,.S dueto varyingaverage
density,
sincewe Therefore,
thefreeparameters
in thesynthetic
computations
are
do notperturbthevelocities,
although
theapparent
risetimeof
thes,,,S phasevariessubtly.
the surfacevelocity{]o,the Mohovelocity{3,,,,Atss*,thecrustalthickness
h, andtheimpedance
contrast
A(p{])at theMoho.
LAY: UPPERMAN"ILEDIsco•m•
0.0
ABovE DEEP SLABS
4395
different At•s*. For the best model, the crustal thicknessis 24
km, and the shear wave impedancecontrastat the Moho is
27%. The correlation coefficient is slightly lower for
At•s*=1.98 s than for At•s*=2.56 s.
20
Comparisonfor IndividualStations
22
Since some individual records have clear s=S arrivals, we
24
can assessthe variabilityin the Moho parametersby fitting single stations,while recognizingthat thesehave a much higher
noise level but involve less horizontalaveragingof the Moho.
26
We
28
ß
.
chose a data set from four individual
stations with con-
..
30
..
.. :::. :.
38.6
1.0
44.
Impedance
contrast
(%)
Fig. 6. Contourof cross-correlation
coefficients
betweensynthetics
and
the stackeds,•S +sS waveformfor September5, 1970, for a model
sistentlyhighsignalto noiseratiosfor detailedcomparison
with
synthetics.
Thesefour WWSSN stationsare locatedin Europe
(KON, TRI, STU) and North American (SCH). Since they
have differentpath effects,we use the corresponding
S phase
at each stationas input signalsfor the modeling.We use the
same search method as described above, i.e., for each station,
we compute400 tracesto comparewith the observeddata at
with [•0= 3.2 km/s,[•,, = 4.7 km/s,andAt•s* = 2.56 s, for varying fixed (p[3)o and (pl3),,, and choosethe best fitting one. The
crustalthicknessand Moho shearimpedancecontrasts.
correlationcoefficientsdiffer for variouschoicesof (pl3)oand
(p[3)=. Table 3 gives the resultsand Figure 7 showsthe
We allow •o, •=, andAtss*to vary over a limitedrangecom- observedand syntheticwaveformcomparisons
for eachstation.
patiblewith reasonablecrustaland mantlevelocities,perform- The waveformcomparisonfor the event stack is also shown,
ing completesearches
of h andA(p[•)for eachcase.
with the syntheticparameterscorresponding
to the best fit
First, we vary h over a large range,findingthat the correla- foundin Figure 6. The resultsshowthat iX(p[5)variesfrom
tion coefficientsbetweendata and syntheticsfor crustalthicknessless than 14 km or more than 33 km for the September5,
20% to 36%, and h varies from 23 to 25 km for optimal cru-
1970 test event are less than 0.2.
stablebecausethe waveformcorrelationprocedurein the pres-
For each model suite in the
final search h varies from 14 to 33 km at a 14:m interval.
For
stal structures. The crustal thickness estimates tend to be more
ence of noise is more robust for differential times than for rela-
someeventswe explorethickercrusts.A(p[3)has20 intervals tive amplitudes.As expected,the parametersfound for the
between 1% and 46%. Our absoluteMoho depth determina- stacked trace are intermediate to the individual observations.
tionsthusdependon how well this simplemodelrepresentsWe feel that the parametersdeterminedfor the stackedtraces
averagecrustalvelocity. For eachmodelsearchwith a arethe mostreliable,giventhehighersignalto noise.
specified
13o,
[3=,andAt•s*,wecompute
400waveforms
which
we comparewith the observed
one by computing
the cross AverageMoho PropertiesUnderneathsS ReflectionPoints
correlationbetween the observedstackedsS waveform and the
synthetics.
We correlate
overan intervalcontaining
thes,,S
From the discussion of our method, it is dear that we can
arrivaland the first half cycleof the sS arrivalto reducethe determine laterally averagedcrustal thickness(for specified
averagecrustalvelocity) and shearwave impedancecontrastat
sensitivityto At•s*.
the
Moho from clear and isolateds=S phasesprecedingthe sS
A contourplot of the correlation
coefficients
obtained
for
reflection.
This is becausewe use waveform modelingto
fixedvaluesof (p[3)oand(pl3)=andnumerous
combinations
of
determine
the
structure. Figure 8a shows the surface sS
h andA(pl3)(Figure6) indicates
thatthesearch
algorithm
does
reflection
point
locationsfor the event of September5, 1970.
definean optimumchoiceor rangeof choices
of parameters.
The
actual
reflection
for s=S involvesa horizontalregionwith
There is some couplingbetweencrustal thicknessand
scale
defined
by
the
Fresnel zone, which dependson the
impedance
contrast
because
we assume
a linearvelocitygradientin the crustbetweenthe surfacevelocity13oandthe velo- wavelength and the source depth. The inversionresults for
A(p[3)and h are thus averagesover this zone and not local
city at thebaseof thecrust,I3,,-AI3.Thisparameterization
avoidssolutionswith unrealistically
high mantle velocities. values at any reflectionpoint. The Fresnelzone radiusfor our
Table2 showsthebestfittingmodelfor theSeptember
5, 1970, long-periodS wave data is about170 km for a sourcedepthof
event,with velocities[3o=3.1km/s and [3,,,--4.65
km/s and 560 km and wavelengthsof about 100 km. Most of the data
TABLE 2. Parameters
for BestFittingModelsfor the Eventof September
5, 1970
•3o' era'
km/s
km/s
A½, A(la[•),
Thickness,
At•s* Correlation
%
km
Coefficient
km/s
3.1
4.65
0.81
27
24.0
2.56
0.993520
3.1
4.65
0.81
27
24.0
1.98
0.992514
4396
ZUANOASOLAY: UI,PF• MineroE D•SCONT•NUmES
ABOVEDEEPSLABS
TABLE 3. Parameters
for BestFittingSingleStationModelsfor the Eventof September
5, 1970
NalTle
150, 15
m,
km/s
km/s
AI3, A(p[5),Thickness,
at,s* Correlation
km/s
%
km
Coefficient
KON
3.4
4.4
0.85
30
25.0
1.98
0.973723
SCH
3.2
4.7
1.10
36
25.0
1.98
0.969731
STU
3.9
4.4
0.60
20
24.0
1.98
0.971232
TRI
3.2
4.7
1.05
34
23.0
1.98
0.983613
To explore for lateral variations on a smaller scale, we
for the event of September5, 1970, samplethe crustnorth of
the source,over an area spanning4.5ø. Only a few tracessam- separatesignalswith reflectionpointsinto threegroups(Figure
ple structuresouth of the event. Thus, stackingthe data and 8a): groupA with reflectionpointsat azimuthsto European
stathe intrinsicFresnelzone limitationsinvolve smoothingover a tions (shadedtriangles),group B with reflectionpoints at
region at least 900 km across. Therefore, the estimatedcrustal azimuthsto North Americanstations(opentriangles)andgroup
thicknessand impedancecontrastare appropriateaveragesover C with reflectionpointsat azimuthsto Southeast
Asia andAusthe entirereflectionpoint area.
tralianstations(solid triangles). Then we separatelystackdata
for eachgroupand performthe waveformmodeling. GroupC
a)
only has four tracesall with oceanicreflectionpoints,and can0.4
not be modeled
0.0
from modelingof groupsA and B. The crustalthicknessestimatesrange between23 and 26 km, while the shearimpedance
contrastsat the Moho vary from 30.8% to 27% from groupA
to groupB. Other modelingparametersare shownin Table 4.
Explorationof model trade-offs suggestsabout + 2 km uncer-
-0.4
K0N
-0.B
0
5
10
15
20
tainties
0
0
-0
-0
C'
0
5
10
15
20
25
STU
0
5
10
15
20
0.4
0.0
\\
in
due to the thin crust.
crustal
thickness
and
+
Table
5%
4 shows results
uncertainties
in
impedance contrast for the two groups separately. There
appearsto be a slight thickeningof the crust and decreasein
Moho shearimpedancefor paths under the Kamchatkapeninsula (groupB); however,thesedifferencesare only marginally
resolvable,given the decreasein numberof tracesavailablefor
the separatestacks.
From this analysisof s,,S, we infer that when largedatasets
are availablewith sufficientsignal to noiseratio, we can determine average crustal thicknessand impedancecontrastat the
Moho under sS reflectionpoints for reasonablechoicesof average crustalvelocity, with uncertaintiesof about+ 2 km for crustal thicknessand + 5% for impedancecontrast. We will now
considerhow well we can hope to resolvepropertiesof deeper
discontinuities,which we expect to be far weaker than s=S,
and then we will presentresults.
//
SLANTSTACKING
Sx TO SEEKEVIDENCE
FOR UPPER MANTLE
-0
e
0
5
10
15
20
0.0-0.4
Event
Stack
-0.[3
I
i
5
10
15
2O
Time (s)
DISCONTn•.nTmS
As shown in Figure 3, it is very hard to identify ,any
coherentundersidereflectionphasesotherthan s,,,S betweenS
and sS phasesin the raw data, suggestingthat only weak
impedancecontrastsexist above the source. Isolated arrivals
observedat only a singlestationcannotbe safelyinterpreted
in
terms of mantle layering. ScS arrives betweenS and sS
phasesover much of the distancerange,thus we must avoid
time intervalsin which the strongScS arrival would obscure
any sxS phase. After someeditingof the datato accomplish
Fig. 7. Comparisons
betweenthe bestfittingsynthetic(solidline) and this, we stack all the waveforms on different slownesses. We
observed(dashedline) sS and s,•S signalsfor individualstations: considerall possibledepthsfor a reflectingboundary.
KON, SCH, STU, andTRI andfor the eventstackfor September
5,
1970,eard•quake.The structural
parameters
for eachcasearegivenin
Table 3.
Becauseof possiblelateralvariationsin reflectordepth,the
sharpness
andimpedance
contrastof someuppermantlediscon-
ZI-IANGAND LAY: U•
MANTt• DISCO•rnNUmES ABOVV.DEEP SLABS
4397
b)
,../•/10P3
40 ø
50 ø
160 ø
120ø
130ø
140ø
Fig. 8. (a) Locations
of sS reflection
pointsfor theeventof September
5, 1970.Theasterisk
indicates
thesource
locationand
trianglesare reflectionpointsfor pathsto the labeledstations.We separate
reflectionpointsinto threegroups:groupA,
reflectionpointsat azimuths
to European
stations
(shaded
triangles);
groupB, reflection
pointsat a•muthsto NorthAmerican
stations
(opentriangles);
andgroupC, reflection
pointsat azimuths
to southeast
AsiaandAustralian
stations
(solidtriangles).
(b) Similarplotfor theNorthKoreaneventof September
10, 1973.
tinuities may be o•scured in a global analysis like that of
Shearer [1990, 1991]. Our stackingexplorationfor sxS precursors reveals relatively localized upper mantle propertiesabove
the deep sourceregionsnear the northwesternPacific subduction zones. The same Fresnel zone and laterally averaging
effectsas describedfor the s,,,S analysisapply for this portion
of the study,so if the boundaryvariesover a 1000-kmscalewe
will obtainedonly locallyaveragedproperties
of the structure.
Differing distance ranges and source depths of the earthquakesaffect the arrival times for a certainsxS phase.We line
up all the data for a given event on the sS phaseto provide a
clear referencephaseand to eliminatestationstatics. We also
separatethe data into three groupsto make the depthinfluence
as small as possible. These three groupsare for differentfocal
depthranges:(1) 115 km < H < 195 km, (2) 348 km < H <
424 km and (3) 503 < H < 566 kin. Given possiblelateral
variations in our study area, as indicated in analysis of s,,,S
phases,we divide the data into three regions for each depth
group;(1) the Sea of Okhotsk,(2) North Korea, and (3) Japan,
RyukyusandIzu.
By aligningthe traceson sS, we set the corresponding
slowness of the sS
arrivals
to zero.
The
slowness
for different
undersidereflectionphasesprovidesa meansfor identifyingthe
s•S arrivals, if there are any, relative to random or signalgeneratednoise. Slant stackingon a particularslownesscreates
interferencebetweenthe signals,which is destructiveexceptfor
a phase with the correspondingslowness.Such phaseswill
appear with a maximum amplitudeat a certain time on the
stackedtrace. The confidencethat one has in the stackedsignal dependson the degreeto which noiseis suppressed
in the
overall stack and consistencyof the timing of the arrival with
the depthexpectedfor a particularslowness.Figure9 givesan
exampleof slant stackingat differentslownessvaluesfor the
event of August 16, 1979. For slowness As = 0.00, which
corresponds
to a stackwith the sS phaselined up, we can see a
very strongsS phase and a weak incoherentS phase. As As
decreasesfrom zero, the sS phasebecomesweaker and distotted, while the S phasebecomesstrongerand more coherent.
The energybetweenthe S andsS phasesalsochanges.For As
= -0.0065 s/km, the S phaseis aligned and a singlepulse is
Upper mantle discontinuities
can have differentreflection retrieved. Figure 3 showsthat a singledifferentialslownessis
coefficients,but most of them are expectedto be very weak. not ideal for aligningS, but will be an excellentapproximation
For example, shearimpedancecontrastsof 2-15% have been for any s•S phases. Therefore,when slant stackingover a
suggestedfor the 80-, 220-, 410-, 520-, and 650-km discon- range of slowness,any phase with a particularslownesswill
stacks.
tinuities,explainingin part why it is difficult to identify s,S have maximum coherencefor the corresponding
Since
we
exclude
traces
with
ScS
between
S
and sS phases,
precursorsin the individualtraces. We do know that different
the
traces
which
can
be
used
for
slant
stacking
are
fairly sparse
phaseswill have differeraslowness.
TABLE 4. Parameters
for BestFittingModelsfor DifferentReflection
RegionsUndertheSeaof Okhotsk
Sept.
5,1970
•m,
A•,
km/s
P0,
km/s
km/s
Thickness, A(Q•),
km
A
3.15
4.8
1.05
23
30.8
0.993747
B
3.1
4.8
0.9
26
27
0.992433
%
Correlation
Coefficient
4398
ZHANOANDLAY: UPPERMAmLE DISCOmUNUmES
ABOVEDEEPSLABS
0.0000 s/km
2, 1978, becauseof their shallow sourcedepths. Since the
periodof our data is about20 to 25 s, it is difficultto isolate
the s,nS phasebetweenS and sS for theseevents. Allowing
for substantial
uncertaintyin the s,•S phasefor eventson Septe;aber 21, 1974, and September2, 1978 (Figure 4), the
waveformmodelingindicatescrustalthicknesses
of 19 and 30
kin, respectively,and 21% shearimpedancecontrasts
for these
• 0.0007
s/km
two events. The crustappearsto be thickerunderTaiwan than
- 0.0020 s/krn
under Kamchatka
if these numbers are reliable.
It should be
possibleto improve the resolutionwith broadbanddata as it
becomesmore extensive. The depthsof eventson May 23,
......
I
,
,
0
,
•-_
I
100
,
,
,
I
,
200
i
1978, May 14, 1968, and September21, 1965 along the
Ryukyuarc rangefrom 160 to 195 km (Table 1). The modeling (Figure10 andTable5) givescrustalthicknesses
of 25, 20,
and 20 km and shearimpedancecontrastsof 21%, 29% and
29%, respectively.
Sincethe sins phasesare not well isolated,
relatively low correlationcoefficientsare found, as listed in
0.0065
s]km
,
I
300
,
i
,
I
400
Time (s)
Fig. 9. Slantstacked
waveforms
for relativeslowness
variations
of As
Table 5. Nonetheless,there is some indication of crustal thick-
= 0.0, -0.0007, -0.002, -0.0065 s/kmfor the eventof August16, 1979,
erring along the arc toward the Japaneseislands. Event on
March 31, 1969 underthe Sea of Japanand eventson February
22, 1974, January31, 1973 and December12, 1976, underthe
whichhasa depthof 566 km. thirty-fourwaveforms
areincluded
in
the stacks. The relative slownessfor sS is 0.0, while for S it is
-0.0065 s/km.
for eachevent. Therefore,we combinenearbyeventsthathave
similardepthsinto groupstacks.This is possible
because
the
North
eventsall have simple,similar waveforms. In the slant stack-
o
1679••
ing, we firstline up all the traceson thesS phases,
flip any
tracesthat have reversedsS polarity, and then normalizethe
tracesto the peakof thesS phase.Giventheveryweakprecursors
sxS, we plot the logarithmof the envelope
of thetrue
slantstacksusingthesamemethodasVidaleandBenz[1992].
Very weakvariations
canbe seenin thelogarithmic
scale.In
our slantstacking,we considerrelativeslowness
variationfrom
-0.015 to 0.015 s/km, a rangespanning
any plausiblesxS
Korea
Sea of Okhotsk
031864
08
/
090570
Taiwan
122175
arrivals.
For shallowerfocaldepthevents(115 km < H < 195 km), it
O9O
012971
is very difficult to separateprecursorsfrom S and sS in the
long-period
data. We did testson shalloweventson September
21, 1965,July4, 1967,May 14, 1968,May 23, 1978.Because
of the shallow depth for those events,it is very hard to
confidently
identifyany energybetweenS andsS phases,so
we only discusseventsin the depthrangeof 348 to 566 km in
the investigation
of sxS phases.
042084
/
Ryuk•_L.Arc
092165-•
051468•
RESULTS
ResultsFrom Analysisof smS
020184
092174
05237•
Figure10 showscomparisons
betweenobserved
(dashedline)
and synthetic(solid line) waveformsfor all eventsthat have
identifiables,•S phasesthat could be modeled. Table 5 lists
resultsfrom the modeling.Table 5 indicatesthat crustalthick-
082172
I
I
I
I
I
J
0
10
20
30
40
50
Time (sec)
impedance
contrastat the Mohois about18%. The relatively Fig. 10. Comparisonsbetween synthetic(solid line) and observed
for eventswhichhavemodelable
s,,,S phases.
earlys,•S arrivalsin thisregioncanbe seenin Figure10. As (dashedline) waveforms
ness near North Korea is about 36 km, and the shear wave
shownin Figure 4 and Table 1, the durationbetweenS andsS
The eventsare arranged
in the NorthKorea(September
10, 1973,and
August 16, 1979), Taiwan (September2, 1978), Ryukyuarc (Sep-
is lessthan50 s for the Kurile Islandeventson September
21, tember21, 1965, May 14, 1968, and May 23, 1978) areasand the Sea
1974, December1, 1967, and the Taiwaneventon September of Okhotskby sourcelocationfrom northto south.
Znn•o ANDLAY: UPPERMmCTLEDISCONT•NUmEs
ABOVEDEEPSLABS
4399
TABLE 5. CrustalThickness
andImpedance
Contrast
for SomeEvents
Latitude, Longitude,
(deg)
(deg)
Date
Depth,
(km)
mb,
Thickness, A(p•),
(%)
(km)
Correlation
Coefficient
North Korea
Sept.10, 1973
Aug. 16, 1979
42.48
131.05
552
5.8
36
18
0.99528O
41.85
130.86
566
5.8
36
18
0.994874
Taiwan
Sept.2, 1978
24.81
121.87
115
6.0
30
21
0.973122
Sept.21, 1965
28.96
128.23
RyukyuArc
195
6.0
20
29
O.975749
20
29
O.979251
6.2
25
21
0.938021
5.6
29
12
0.989448
24
26
0.993520
22
46
0.985712
0.991975
May 14, 1968
29.93
129.39
May 23, 1978
31.07
130.10
March 18, 1964
52.56
153.67
162
5.9
160
Sea of Okhotsk
Sept.5, 1970
Dec. 21, 1975
52.28
51.93
424
151.49
560
151.57
5.7
546
6.0
Jan. 29, 1971
51.69
150.97
515
6.0
23
28
April 20, 1984
Aug. 21, 1972
50.12
148.75
572
5.9
22
28
0.995713
49.47
147.08
573
5.9
21
26
0.991805
Feb. 1, 1984
49.05
146.63
580
5.8
22
26
0.996441
Sept.21, 1974
52.19
157.44
119
5.7
19
21
0.966426
Izu arc, all reflectunderoceanicstructureas shownin Figure4 contrast.This eventappearsto be unreliableand is the only
whena 30-s
and have unmodelableweak s,•S waveformeffectsso we can- eventfromtheearlydaysof theWWSSNsystem
pendulum
periodwasbeingused.Thesetwoanomalous
results
not quantifythe structure
there.
The crustal thickness beneath the northern Sea of Okhotsk
may represent
locallycomplexuppermost
mantlestructure
or
varies from 21 to 29 km, and the shearwave impedancecon- simplynoisefor theseevents.Figure11 plotsstacked
trast varies from 26% to 28% for all events except events on waveformsfor differenteventsin the Sea of Okhotsk,showing
s,•S phases
areobserved
in thenorthof
December21, 1975, and March 18, 1964, which give extreme thatclearandstrong
5, 1970,December
21, 1975,
estimates
of shearwaveimpedance
contrast
of 46% and 12%. thisarea,for eventson September
29, 1971,April2, 1984,August
21, 1972andFebruary
Lookingat the datafor eventon December
21, 1975(Figure January
at eventsfartherto the
10), we seethattheonsetof s,nS is unclearandthe synthetics 1, 1984,while weaks,•S are observed
givea moreimpulsive
arrivalthanobserved,
whichmightbe south:June 21, 1978, and July 10, 1976. We do not report
for the lattertwo eventsbecause
they are
one reasonfor the high valueof A(p[•). It is hard to identify Moho parameters
fromnorthto southprobably
s,•S for eventon March18, 1964(Figures
4 and10) because poorlyresolved.Thisvariation
to oceanic
the amplitude
is small,yieldinga low estimate
of impedancereflectsthe crustalvariationfrom quasi-continental
60
ø
02/0/84
., AVll
\F
,,
a•
120ø
130ø
140ø
rlII\F t'•06•1.8
/
150ø
0710/76
160ø
170ø
Fig.11.Stacked
sS waveforms
fortheSeaof Okhotsk
events
ploued
fromnorthto south.Thes,•S arrival
weakens
forthe
southern events.
4400
•o
mgDLAY: UPPERMm½mEDISCON•NUmES
AnOVEDEEPSLAnS
stxucturein this area. We only have six tracesfor the Kurile
event on May 27, 1972, so the stack is less reliable for this
event and we did not model it.
ResultsFrom Slant StackingsxS
Plate 1 shows resultsfrom slant stackingsxS phasesfor
source depths varying between 503 and 566 km in three
regions. The plot showsthe logarithmicamplitudeof the stack
envelope as a function of relative slownessand travel time.
The color scaleis a logarithmicamplitudethat variesfrom 0 to
2, which meansthe true amplitudescalewill variesby a factor
of 100 relative to the stackedaS amplitude. Black indicates
less than 1% amplituderelative to sS. Any arrivals with
amplitude2% to 10% of aS can be readily detected. Some
clear artifactsof slant stackingare apparentin the streaking.
Plates la, lb and lc are slant stacksof deep eventsin North
Korea, the Sea of Okhotsk, and Izu Japan,respectively. The
slownessof any sxS precursorsfrom horizontallayerswill vary
between the slownessesof S and sS phases (Figure 3). If
energy is found betweenS and sS with correctcorresponding
slowness,it is likely to be an &S arrival, but the limited ray
parameterresolutionmust be allowed for. In each case, sS is
the strongestarrival at a relative slownessof zero. As the relative slownessvaries, the sS energy will become less coherent
in both slowness variation
directions.
The extent to which the
sS arrival is localizedin time andray parameteris indicativeof
how well any arrival can be isolated, and real sxS arrivals
should streak as much as sS.
A very clear stackis foundfor two deepeventsbelow North
Korea (Plate la). We can see that relatively strong energy
occurs in front of the aS phase with very subfie slowness
differencefrom aS. This correspondsto energyreflectedfrom
the Moho and the amplituderatio of sins to sS is about 8%.
Ahead of sinS, other energy is seen which is consistentwith
reflectionfrom a 90-km-deepboundary,with about6% ampli-
500 < H < 570 km
a)N. Korea
0.015
.
.
"4
-0.015
,
.
b)Sea of Okhotsk
0.015
-0.015
c) Izu-Ja an
0.015
0.000
-0.015
0
200
Time
o.o
300
s
......
2.0
Log(Amplitude)
Plate1. Slantstacksections
for eventswithfocaldepths
of 503 < H < 566 km below:(a) Noah Korea,(b) theSeaof
Okhotsk,and(c) Izu Japan.Seetextfor details.
300<H
4401
< 500 km
o
a)Izu-Japan 9
o
,
0.015
0.000
-0.015
b) Sea of Okhotsk
0.015
0.000
-0.015
.
0
110
200
.
300
Time (s)
0.0
2.0
Log(Amplitude)
Plate2. Slantstacksections
for eventswith focaldepthsof 348 _<I1 < 424 km below:(a) the Seaof Okhotskand(b) Izu
Japan.
tuderatio of s9oS/sS. There is no evidencefor a s220Sphase. 26%. This is again consistent with the results of s,•S
In fact, the very low amplitudein the corresponding
time win- waveform modeling. Since the crustal structurevaries from
dow limits any sharpdiscontinuityto be less than 1% shear north to southin the Sea of Okhotsk,the wide energyrange
impedancecontrast.There is a weak,but still distinguishableassociatedwith s,•S may reflect the transitionfrom continental
arrivalcompatiblewith a 330-kmreflector,with the amplitude to oceanic structure. At the time for reflectors near 85-90 km
ratio of S33oS/sS
being about5%. There is a strongerarrival and 160 km depth there are very weak arrivalswith 11.4% and
closein time to a reflectionfrom a 400-km discontinuitywith 8.4% impedancecontrast. But thesetwo arrivals are very narthe amplituderatio sax•C/sSbeing about 6%, althoughthe row, and they may be noise or artifacts. There is again no
slownessis somewhatanomalous.We convertamplituderatio clear reflectionfrom any structurenear 210 km, althougha 2%
to impedancecontrastby consideringthe geometricspreading relative amplitudearrival cannotbe precluded. There is some
and attenuation
usingreferencemodelPREM. The impedance energynear the time for a 330-km discontinuitywith about8%
contrasts for the Moho, 90-, 330-, and 400-km discontinuities impedancecontrast,but becauseof the weak signal we do not
in North Korea•are abbut16%, 11.4%, 9%, and 10.8%,respec- have great confidancein this discontinuity. A strongerarrival
tively. The Moho impedan6econtrast(q6%) inferred from the is seen near 400 km depth with 11.2% impedancecontrast.
slant stackis compatiblewith the 18% value foundby model- While these do not have very discretearrivals and lack much
ing the s,•S phases. Strongattenuationin the mantlewedge ray parameterresolution,thesearrivalsare consistentwith those
abovethe slab couldreducethesevalues,but radiationpattern observedfor pathsunderKorea.
effects may compensatesomewhat. Overall, we estimate a
In the areaof Izu Japan(Platelc), whichhasan oceaniccru_+3%uncertaintyassociatedwith deeperdiscontinuities
and +1- stal structure.there is no separationof s,•S energy in our
2% for structures
above100 km for shearimpedance
contrasts long-period
data. The slantstackfor thisregionis in factvery
and+_5-10%uncertaintyfor estimationof discontinuity
depths. clean. There is weak energy aheadof sS which may come
The region under the Sea of Okhotskyields the slant stack from a depthnear 66 km, with about12.6% impedanceconshown in Plate lb. The particulardata distributionleads to trast. Near a depthof 210 km, we can seeweak energy,but
some strongstreakingin the slant stack. We see very strong the slownessis shiftedto more negativevaluesthan we expect.
sS energy,and there is visible energyjust aheadof sS associ- If thisenergycomesfrom a reflector,it shouldbe near210 km
ated with s,•S which has an impedancecontrastof about 16- depth,with impedance
contrastof 10.4%. Anotherweak and
4402
Z•so
ANDLAY: UPPER
MAN'II,EDISCONTINUITIES
ABOVEDEEPSLABS
TABLE 6. Parameters
for BestFittingModelsfor DifferentReflection
RegionsUnderNorthKorea
•m,
A•,
Sept.10,1973
Aug.16,1979/ •0,
km/s
km/s
km/s
km
%
A
3.25
4.6
0.69
35
21
B
3.3
4.6
0.99
31
31.2
narrow arrival is suggestednear a depth of 325 km with 9%
impedancecontrast,but it too may be noise. Energyfrom near
the 400 km discontinuityis strongerthan that of the otherprecursors,with 5.6% amplituderatio, but becauseof focal depth
rangeof events,this is very closeto the S phaseand may be
contaminated
by S wave coda.
Plate 2 showsslantstacksfor focal depthsvarying from 348
to 424 km for two regions:Izu Japan(Plate2a) and the Seaof
Okhotsk(Plate 2b). In the slantstackfor Izu Japan,we again
cannotidentify energyfrom the Moho becauseof the oceanic
crustal structure there. But there is some energy that may
come
from
reflectors
around
66
and 210
km
with
the
Thickness,A(p•),
Correlation
Coefficient
'0.991637'
0.982888
the crust in Korea appearsto thicken from 27 km (in the
reflectionpoint regionat azimuthsto Europeanstations:
TOL,
MAD, PTO, and STU) see Figure 8b to 32 km (in the
reflectionpoint region towardAfrica stations:AAE and NAI)
with theP impedance
contrastat theMohovaryingfromabout
23.8 to 17.1%. We have two eventsthat samplethe crust
slightlynorthof theirpP paths. We combinethesetwo events
and sort them into threegroupsby surfacereflectionpoints
(Figure8b). Figure8b is the locations
of sS reflection
points
for eventon September
10, 1973,as an exampleof thesetwo
events.The first,groupA, is composed
of pathsto European
stations,group B includesonly North American stations,and
ever, the noise level is high so this identificationis very tenta-
groupC includesthe moresouthern
stationsincludingthosein
Africa. Sincethesetwo eventsare closelylocatedand have
almost the same depth (Table 1), we combinethem in the
stacksfor eachgroupand applythe s,•S waveformmodeling.
tive.
Unlike Schenk et al. [1989], stationsAAE and NAI are not
impedancecontrastbeing7.6% and 10.4%,respectively.It is
hard to determinethe amplituderatio of s33oS/sSbecausethe S
wave is very close,but thereis energyin the window. HowPlate 2b shows the slant stack for the Sea of Okhotsk,
whichhasstrongartifacts.We can still identifysomepossible includedin the presentstudy'sdata set, and most groupC
arrivalsbetweenS andsS phases,whichsuggest
discontinuities reflectionpoints lie in the Sea of Japan,giving poor s,•S
near the Moho, 90 and 335 km with impedancecontrastsof arrivals, so we did not attemptto model group C records.
8%, 4.7% and 5.5%, respectively,but thereis little confidence Table 6 summarizesthe resultsof modelinggroupsA and B.
variesfrom 35 km in groupA to 31 km
in thesearrivals,otherthenas supportingthe evidencefor weak The crustalthickness
in
group
B,
thinning
slightly toward the coast, and the shear
reflectorsseenin Plate 1. There is againno evidencefor any
impedancecontrastat the Moho rangesfrom 21% to 31.2%.
reflector near 210 km beneath the Sea of Okhotsk.
For the total stacksfor August 16, 1979, and September10,
1973, we determinedthe averagecrustalthicknessin the area
DISCUSSION
to be about36 km and the shearwave impedancecontrastis
From seismicprofiling, refraction,and deep seismicsound- about 18%. The averagecrustalthicknessis slightlygreater
ing, Gnibidenko [1985] pointed out that the principal thanthat from regionsA and B in part becausemodelingstageomorphicelementsof the Sea of Okhotskare the continental tion CHG (group C) gives a 37-km-thick crust. The CHG
shelf in the north, the continentalslopeareaof the centralpart reflectionpoint locatesnear the area whereSchenket al. [1989]
of the sea, and the deep-seabasinin the south.They reported found thicker as well. There is some trade off between
that crustalthicknessvariesfrom 12 to 35 km. From Figure 11
we see that the s,•S waveform varies from north to south,
althoughwe were able to determinethe crustalthicknessand
shear wave impedancecontrastonly in the north where we
have clear s,•S phases. The crustalthicknessof the northern
Sea of Okhotskvaries from 21 (August 21, 1972) to 29 km
(March 18, 1964) in our study. Our resultsfor the Sea of
Okhotsk are consistentwith Gnibidenko's[ 1985] results.
Schenket al. [1989] analyzedone of the sameeventsas we
did (January29, 1971) under the Sea of Okhotsk,lookingfor
pP Moho precursors.Their model has a crustalthicknessof
21-25 km and a P wave impedancecontrastat the Moho from
27% to 31%, while we estimatean averagecrustalthicknessof
23 km and an averageshearwave impedancecontrastof about
28% for this event. The comparisonshowsvery consistent
resultsfrom studyof pP andsS precursors.Futurestudieswill
pursuesuchsimultaneous
measurements.
Usingan eventin North Korea,Schenket al. [ 1989] find that
impedancecontrastand crustalthickness,but we tendto prefer
the averageparametersbecausethe stackshave the highestsig0
• 20
t•okhot•k
/21
30
35[
/
40 ß
•
2
3
[ akorea
Pkorea
• ••korea
-• •]
•
•
•
4
5
6
7
8
9
Density (10a kg/m a) and Velocity (km/s)
Fig. 12. Vel•ity anddensitym•els fr• •r seismogmm
m•el•g •
•e S• of Okh•sk and No• Ko•, along•
Schenket al. [1989]
P vel•i W m•els for •e sameregions.
•o
A•,a)l•v: UPPERM•,rrLE DISCONTINUITIES
ABOVEDEEPSLA/•S
nal to noise. Comparingthe resultsof thepP and sS precursor
studies, the estimates of crastal thickness are in reasonable
TABLE 7. VelocityStructure
Region[•o' A[•, [•m' hly rto,
agreementsince our eventslocate slightly north of the event
they used. If we comparewith other models for continental
Okhotsk
regions,such as EU2 [Lerner-Lainand Jordan, 1987], the S
Korea
and P impedancecontrastsappear somewhatlower than in
other continentalregions.
In our study, the averageshearimpedancecontrastsat the
Moho
are 18% beneath North Korea and 26-28%
under the
4403
km/s
km/s
km/s
Art, rt
h
km
km/s
km/s
km
km/s
3.1
0.9
4.8
23
6.2
1.5
8.11
22
3.25
0.84
4.6
33
6.5
1.2
8.11
30
thermalandpetrological
modeling
of the crustalevolutionin
north of the Sea of Okhotsk. We show our preferredcrustal different tectonic environments.
modelsin Figure12 andTable7, alongwith theP resultsof
Schenket al. [1989]. The shearwave impedancecontrastof
the continental structure is found to be smaller than that of the
Different shallow structuresuch as oceanicor continental
crestshouldreflecttheunderlying
uppermantlestructure
in the
lithospheric
lid. It is generally
accepted
thatthelithosphere
quasi-continental
to oceanictransition
structure
underthe Sea structure
andthickness
varieswithdifferent
tectonic
region.As
of Okhotsk(Table 5). This informationmay contributeto the
pointed
outby Revenaugh
[1989],thebaseof thislid maybe
2.0
0.0
•
nn _•
c)
0.015
• -0.015
,•d)2.0
"'
0.0!
.......
I
0
,
, ,
,
! ........,
1•
......
i
.... ,,,
200
•
•,,,
I
,,•,
t..
300
Time (s)
Plate
3.Slant
stack
forevents
August
16,1979,
andSeptember
10,1973.
(a) Waveform
stack
atslowness
s90S,
(b)envelope
of thestack
atslowness
s90S,(c) slantstack
(seedetail
in PlateI) withtwoarrows
pointing
atslownesses
for90-and330-
kmdiscontinuities,
(d) envelope
ofthestack
atslowness
s330S
and(e) waveform
stack
atslowness
s330S.
Dash
lines
point
corresponding
arrivals.
4404
ZHA•OANDLAY:UPPER
MAl•rt• DISCONT•m• ABOVEDEEPSLABS
markedby an abrupt5-8% decreasein shearwave impedance
at an averagedepthof 67 km in the majorityof deepsubduction zones and an average depth of 86 km in the western
Pacific, which he attributesto the lid low-velocityzone (LVZ)
transition. In our studyregionof the northwestern
Pacific,we
find evidencefor a discontinuitynear 66, 80 to 85 and 90 km
in the Izu Japan,Sea of Okhotsk,and North Korea regions,
respectively,with about 11.4% averageimpedancecontrast.
Our basicprocessingis insensitiveto whetherthis involvesa
velocityincreaseor decrease.We can attemptto determine
polarityof the reflection. Plate 3 showsdetailsof the slant
stack for North Korea. Plate 3c is the same as Plate la with
3c, 3d, and 3e suggestthat the polaritiesof s330Sand s,•S are
the same. Since this weak discontinuityis observedonly on
profiles sampling regions of active subduction,it may be
causedby hydrafionreactions,occurringin parts of the mantle
that have been volatile charged by subductinglithosphere
[Revenaughand Jordan, 1991c]. We will seekmore explanations of these discontinuities
in future studies that will
extend
the coverageof subductionzone mantlestratification.
Table 8 summarizesof our results along with estimatesof
impedancecontrastsat x km discontinuities.We include crustal information in Table 8 from the resultsof s,•S waveform
modeling.
two arrows indicatingstack slownesses
at 90 km (s90S) and
330 km (S33oS)discontinuities.
Plate3a, 3b, 3d, and 3e are the
corresponding
stackedwaveformsand envelopes.The polarity
of s,•S and s90Sappearsto be the same,suggesting
reflection
CONCLUSIONS
We have establishedthat precursorsto sS can be used to
determine
mantle
stratification
above
subduction
zones.
For
from a velocityincrease
near90 km ratherthanfromthetopof our northwesternPacific study area, we find oceanic crust
a low-velocityzone. The lid discontinuity
is slightlydeeper beneathIzu Japan,continentalcrustunderNorth Koreawith 36
under Korea than under the Sea of Okhotsk, and we again
km crustalthicknessand 18% shearwave impedancecontrastat
slightlyprefera velocityincrease
near85 km in thelattercase. the Moho, 20-25 km thick crust under the Ryukyu arc with
Thus the transitionalcontinentalmargin structuremay involve
21-29% impedancecontrast,and beneaththe Sea of Okhotsk
somethingother than the onsetof partialmelting. The nar- the crustal structurevaries from quasi-continentalin the north
rowbanddata andweak signalmakethis a tentativeconclusion. with 21-29 km thickness to oceanic tcward the south. Our
The Izu Japanregionhasoceaniccrustalstructure,
andherewe study indicatesthat the shear wave impedancecontrastis
find a relativelyshallowdiscontinuity
at 66 km. The signalis smaller for the continental Moho than for the transitional structoo weakto makeany statement
aboutits polarity,so it may in ture. Lateralvariationsalsoexist in the uppermanfiefor these
fact be associatedwith the lid-LVZ boundary.
regions. Evidencefor an 80-km discontinuity
was found,with
In our studyof sxS precursors,
we only foundevidencefor a lateral variation in depth. The data suggesta discontinuity
200 to 210 km discontinuityunderneaththe Izu Japanarea,
which has oceaniclithosphere.Recently,Vidale and Benz
[1992] found evidencefor a P wave impedancecontrastnear
structurenear 66 km underIzu Japan,80-85 km underthe Sea
of Okhotsk and 90 km under North Korea, all with average
impedancecontrastof about11.4%. We only foundevidence
210 km depthnearthe Bonin,Mariana,andTongasubduction for a 200-km discontinuity under the Izu Japan area, below
zones,also beneathoceaniclithosphere.The 200-km discon- oceaniclithosphere,with depth near 200 to 210 km. No evitinuity may exist underboth continental
and oceaniclitho- dencefor this discontinuityis found in the other two regions.
sphere,but thelateralvariationmustbe verylarge,because
we Consistentevidencefor a 330-km discontint,
ity is found in all
did not find evidence for this structure under North Korea or
threeregions.The depthrangevariesfrom325 to 335 km, and
the Sea of Okhotsk. Therefore our data do not supportthe the averageimpedance
contrastis about9%, but we do not yet
hypothesis
thatthe 200-kmdiscontinuity
is a globalfeatureas have great confidencein this feature. Slant stackingresults
advocatedin some studies [Anderson,1979; Dziewonskiand also show consistenceevidencefor a 400-km discontinuityin
Anderson,1981; Drummo• et al., 1982]. This is supportedby our studyarea;underNorth Korea and the Seaof Okhotskthe
the absenceof strongreflectionsfrom a depthof 200 km in the depthis around400 km, andunderIzu Japanit is 380 km. The
globalstacksof Shearer[ 1991]. A searchfor pP precursors
in averageimpedance
contrastis about10.8%.
theseregionsis neededto test whetherthis is a resultof there
beingno structureor only lack of S wave structure.
Acknowledgments.
We thankJohnVidalefor his help with the slant
A few studies [Revenaughand Jordan, 1991c; Wajeman, stackprogram
andfor manydiscussions,
aswellasJohnVidale,Justin
andSusanSchwartzfor makingsuggestions
to improvethe
1988] have found evidencefor a 330-km discontinuity.In our Revenaugh
andXiao-biXie for hisveryusefuldiscussions.
We thank
study,we found consistentbut very weak evidencein all three manuscript
PeterShearerand Mark Richardsfor helpful reviews. This studywas
regionsfor a reflectornearthisdepth,but we needmoredatato supported
by NSF grantEAR-9104764
andUSGSgrant14-08-0001increase confidence in the identification of this structure. Plates
G2079.
TABLE 8. Results
for UpperMantleDiscontinuities
Region
Izu-Japan
North Korea
North Okhotsk sea
Possible
Impedance
Contrast
Moho
Oceanic
80km
66
220km
200-210
330km
400krn
325
380
36
90
No
330
400
22-24
80-85
No
330-335
400
-8%
-5.7%
-5.5%
'4.5%
'5.4%
ZHANOASD I•Y:
U•
Mnmx•
Diseol•mmm'i•
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(Received
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