PHYSICSOFTHE EARTH

L ANDPLANETARY_________ INTERIORSELSEVIER Physicsof the EarthandPlanetaryInteriors84 (1994)207—226

Broadbandobservationsof upper-mantleseismicphasesin northernAustraliaandtheattenuationstructure

in theuppermantle

0. Gudmundsson~, B.L.N. Kennett,A. Goody 1

ResearchSchoolofEarthSciences,AustralianNational University, Canberra,A.C.T 0200, Australia

(Received1 January1993; revision accepted8 December1993)

Abstract

Sourcesin the earthquakebelt throughIndonesiaandNew Guinea,recordedat a broadbandGuraip CMG3seismometerat theWarramungaarrayin the NorthernTerritory of Australia, give good coverageof propagationthroughtheupperpartof themantle.The midpoints of the propagationpathslie alongthenorthernmarginof theAustraliancontinent.Beyond18°thereis a significantdifferencein the frequencycontentof P andS waves.The Pwavesremainhigh frequency,but the S waves returnedfrom the transition zoneandbelow areof intermediateperiod(0.1—0.5 Hz), which would be difficult to recordwithout abroadbandinstrumentanda quiet site.The laterbranchesassociatedwith the410 and660 km discontinuitiesareclearlyseenin individual seismograms.TheS wavesrecordedon the radial (SV) andtangential(SH) componentsare of comparablequality, becausethe hard-rockrecordingsite minimizesthe influenceof coupling to P on theradial component.

We interpret theobserveddifferencein frequencycontentof P andS wavesat transition-zonedistancesas theeffect of alayerunder theseismicallyfastlid which possessesa large degreeof sheardissipation.We quantify theobservationin termsof thedecayrate of thespectralratio of S andP waves.That quantity canbe interpretedintermsof pathintegralsof thedifferencein inverseQ for S wavesandP waves,t’ — t~.We analyse22 seismogramsfrom the WRA broadbandinstrument and a further four from portable broadbandinstrumentsdeployednearWarramunga.The measuredslopeof the S/P spectralratio is consistentlysmall out to 18°epicentral distance,where it increasesdramatically.Assumingthat sheardissipation dominatesover bulk dissipation,thesemeasure-mentsareconsistentwith an averagequality factor for S wavesin thelid of theorderof Q~= 1400(Q~= 2800)ontop of a highly attenuativeasthenosphereof 200 km thicknesswith Q~= 100 (Q~= 200), which is underlainby atransitionzonewith Q~= 600(Q~= 1200). Thelow-Q zonein theasthenospherecanequallywell be modelledwitha thinner layer with a lower Q, e.g. a layerof 100 km thicknesswith Q~= 60. Frequencydependenceof Q mayrendertheaboveestimatesof Q about25% too large.Theseresultscomparefairly well with publishedresultsfrom0.3 Hz body-waveobservationsundertheEurasianshield.Notable exceptionsarethat we obtain asomewhatlowervalue for Q~in theasthenosphereandamuchhighervalue in the lid. Thelid undernorthernAustraliais unusually

* Correspondingauthor.1 Presentaddress:BHP PetroleumPty. Ltd., Melbourne,Vic., Australia.

0031-9201/94/$07.00© 1994 ElsevierScienceB.V. All rights reservedSSDI0031-9201(94)05034-U

208 0. Gudmundssonetal. /Physicsof theEarth and PlanetaryInteriors 84 (1994) 207—226

thick, 210km, andourdataconstrainthe low-Q zonein theasthenosphereto lie deeperthan 210 km. It is difficultto explain thelow observedvaluesof t~’— t~for pathscontainedin theseismiclid (lithosphere)in termsof artefactsrelatedto differencesin thesourceradiationof P andS wavesor scatteringeffects.We thereforeattributethem tointrinsic attenuation.Thehigh value inferredfor Q in thelid as comparedwith otherstudiesin asimilar frequencyrangecould representaregionalvariationanddifferencesbetweenindividual shields,but could alsobe explainedbya low QP/QS ratio (lessthanone),i.e. t~comparablewith t~’.A low QP/QS ratiocouldbeexplainedby asignificantcontribution from a thermoelasticdissipationmechanism.

1. Introduction age) and has proved to be very sensitive forshort-periodobservations.A three-componentset

The major earthquakebelt running through of broadbandseismometers(Guraip CMG-3) wasIndonesiaand New Guinealies at the appropri- installed in 1988, in a pit vault near the centralate distance range to investigateupper-mantle recordingfacility. The generallevel of microseis-structure, using recording stations in northern mic noise is low, with some seasonalincreaseAustralia. The Warramungaarray(WRA) in the when the northwesterlymonsoonsblow duringNorthern Territory of Australia has beenoper- the ‘wet’ from Decemberto March. The noiseatedsince 1965,with a 25 lun apertureset of 20 spectrumdropsrapidly for frequenciesabovetheshort-periodseismometersin two arms forming microseismicpeakto give very low noiseat 1 Hz,anL-shapedconfiguration.The site lies35 km to but also a good chancefor effectiverecordinginthe southeastof the small mining town of Ten- the intermediatefrequencyband from 0.2 to 1nantCreek.Thearray is installedon the granites Hz. Amplitudespectrafor a magnitude5.2 eventof the TennantCreekInlier (late Proterozoicin at 19.5°from WRA are displayedin Fig. 1, for

Radial component

io°a b

10 /v1,.ItA~

~- 10~~ \ ,/1j~J(\!~J\iJi~, .

\1 1\ ‘~‘~ V\ja ~ V Ici) 4a~

5

> ‘U

~ 106ci~ -~

S __

10: — P-waves — S-waves Ii10 noise P-coda10.10

I I I I I I I I I I I.1 -1 0 1

10 10 10 10 10 1

frequency [Hz]Fig. 1. Amplitude spectrafor the event displayedin Fig. 4(a); spectraare shownfor the P andS wavesaswell as theprecedingnoise, andthe P-wavecodajust beforethe onsetof S.

0. Gudmundrsonet aL /PhysicsoftheEarth andPlanetaryInteriors 84 (1994)207—226 209

the P and S waves returned from the upper somewhatuneven,but there are now sufficientmantle as well as a segmentof noise preceding datato beginto buildup three-componentrecordthe P onset,and a segmentof P codapreceding sectionsfor pathsfrom the IndonesianandNewthe S onset (the seismogramsfor this eventare Guineaearthquakebelt (Goody, 1991).About 40shown in Fig. 4(a)). The high-frequencycontent eventslargeenoughto generateobservableP andof the P-wave energy is clearly displayed, and S wavesat upper-mantledistancesare recordedthere is a shift for S to lower frequency.The at the siteeachyear.High-frequencySwavesareshear-wavespectrumpeakswithin the microseis- recordedat the onset of the S waveform formic band,but exceedsboth the noisepower and shallow eventsfor bothSV and SH wavesouttothe P-wave coda.This is generally the casefor 18°.At larger distances,the onset of S has aeventsaboveaboutfive in magnitude. much lower frequencycontent(0.1—0.5 Hz) and

The closest events from the Indonesianand suchintermediateperiodarrivalscharacterizetheNew Guineaearthquakebelt lie about 12°away S wavesreturnedfrom the 410 and 660 km dis-from WRA. Thereis a reasonabledistribution of continuities.eventsalongthe Floresarcwestwardsto Javaout As an illustration of the character of theto 30°and beyond,and also along the northern broadbandrecordings,we presentthree-compo-marginof the island of New Guineathrough to nent records for a selection of events from thethe Solomon Islandsto the east,againextending BandaSeathroughNew Guineato the Solomonto beyond30°.We arethereforeableto cover the Islands(130—160°E,3—9°S),which cover a broadfull spanof propagationassociatedwith the up- spanof azimuthsto the recordingsite at Warra-per-mantle transition zone. Even in an active munga(Figs. 3 and 4). We have choseneventsearthquakebelt the distribution of activity is (markedby stars in Fig. 2) that are shallow and

I ‘I I I I

I

100 110 120 130 140 150 160

longitude (degrees E)Fig. 2. Earthquakesfrom the Indonesiaand New Guinea regionusedin this study.The eventsfor which broadbandseismogramsare displayedin Figs. 3 and 4 are shown as stars.The remainingeventsusedare shownas filled circles.The locationof thebroadbandinstrumentat the Warramungaarray (WRA) is shownasa solid squareand locationsof portable instrumentsin thevicinity of WRA assomewhatsmallersolid squares.Opencirclesrepresentapproximatemidpointsof thepropagationpaths.Eventlocationsarefrom monthly PDE listings.

210 0. Gudmundssonetal. /PhysicsoftheEarth andPlanetaryInteriors84 (1994) 207—226

have body-~iavemagnitudesof 5—5.5, with the then at least regionally. It is associatedwith aobjectof trying to secureassimplea waveformas low-viscosity asthenosphere,which may be par-possible. The midpoints for propagationfrom tially molten(Andersonand Sammis,1970;Chantheseeventslie beneaththe Australiancontinent et al., 1989), or at leastrelatively hot (Karatoandor continentalshelf,with the turningpointsin the Spetzler,1990).A low-Q layer in the upperman-mantlelying mostly beneaththe northernmargin tle is a common feature of global Q modelsof the Australiancontinent(Fig. 2). (Anderson and Archambeau,1964; Mills, 1977;

The change in the frequencycharacterof S Anderson and Hart, 1978a,b; Sailor andwavesrelative to P waves at about 18°can be Dziewonski, 1978;AndersonandGiven, 1982). Itqualitatively explainedby a layer of high shear is found underoceans(CanasandMitchell, 1978)attenuationat depth, on the basis of visual in- where its depth is shallow and the degreeofspection of the seismograms.The existenceof attenuation reduceswith age of the overlyingsuch a layer is generallyaccepted,if notglobally oceanic lithosphere(Canas and Mitchell, 1978;

Latitude -6.42 Longitude 131.24 Depth 62Magnitude 5.0 Distance 13.8

bb

sp

bb

sp

bb

sp

(a) Time duration 5 minutesFig. 3. Three-componentrecordsfor events in the BandaSeaand Irian Jaya (rangeless than 18°),which show high-frequencyS-wavepropagationto the broadbandstation atWRA. The uppertrace for eachcomponentis the unfilteredbroadbandrecord,and thelower trace is the responseof a simulatedshort-periodinstrument.

O. Gudmundsson et al. / Physics of the Earth and Planetary Interiors 84 (1994) 207-226 211

Chan et al., 1989). It is also found under shield areas, where it appears to be deeper and less pronounced than under the young oceans (Der et al., 1986). For a comprehensive review of seismic measurements relevant to attenuation, the reader is referred to Dziewonski (1979) and Minster (1981), and for the physics of attenuation, to Anderson (1989) and Karato and Spetzler (1990).

2. Broadband observat ions out to 18 °

Events from the front of the Banda Arc recorded at the Warramunga array are character-

ized by strong short-period P and S with a time separation of just over 2 min. Both the P and S arrivals are followed by an extended coda. The addition of the broadband data confirms the gen- erally high-frequency character of the S-wave propagation, but also reveals a structure unsus- pected from short-period results (Kennett et al., 1994).

In Fig, 3, three-component seismograms for events in the Banda Arc (Fig. 3(a)--epicentral distance 13.8 °) and Irian Jaya (Fig. 3(b)--16.9 °) are shown; the horizontal components are ro- tated to the radial and transverse directions rela- tive to great-circle propagation. The upper trace

Latitude -3.36 Magnitude 5.5

Longitude 137.62 Distance 1 6.9

Depth 64

bb

sp

bb

sp

bb

sp

(b) Time duration 6 minutes Fig. 3 (continued).

212 0. Gudmundssonetal. /Physicsof theEarth andPlanetary Interiors 84 (1994)207—226

for eachcomponentshowsthe unfilteredbroad- The S-wave arrivals also contain substantialband data; the lower trace has been filteredto high-frequencyenergy,as indicatedin the short-simulate the responseof a short-periodinstru- periodrecordsfrom the array,butshow ashift toment. The successof the rotations is demon- lower frequenciescomparedwith P. The lowest-stratedby the slow build-up of the transverse frequencyarrivals tend to occurnearthe onsetofcomponentafter the P onset.Formany eventsin S, with high-frequencyenergyfollowing immedi-theseregions,WRA is close to a node in the ately; this feature is particularly clear on theradiationpattern,so that the P-wavearrivalsare transversecomponentin Fig. 3(b). A Similar ef-oftenseenas high-frequencyriders on the micro- fect is also seenfor P waves,often at somewhatseismicnoise, whichbecomemore prominent in higher frequencies. Thismay be causedby scat-the short-periodrecords (see Fig.3(a)). tering,which tends to broadenthe initial arrival

Latitude -4.38 Longitude 146.38 Depth 42Magnitude 5.2 Distance 19.5

~ ~

sp ~ —~

bb ~I Radial

s p ~-~-~--~ ~ —~—~—~ ~—~‘

bb

(a) Time duration 6 minutesFig. 4. Three-component recordsfor more distantevents(rangegreaterthan 18°)in New Britain and theSolomonIslands,whichshow clearS wavesreturnedfrom the uppermantlewith a muchlower frequencycontentatWRA thanthe seismogramsof Fig. 3.The uppertrace for eachcomponentis the unfiltered broadbandrecord, andthe lower trace is the responseof a simulatedshort-periodinstrument.

O. Gudmundsson et al. / Physics of the Earth and Planetary Interiors 84 (1994) 207-226 213

and generate a somewhat higher-frequency coda (Richards and Menke, 1983).

The first arrivals out to 18 ° are associated with interaction with structure near 210 km depth (Hales et al., 1980; Leven, 1985; Bowman and Kennett , 1990; Dey et al., 1993). The detailed structure of the P-wave arrivals is best explained by a velocity increase just above 210 km, followed by a slight low-velocity zone. The high-frequency S arrivals are probably trapped in the lithosphere above this small 210 km discontinuity. The lower-frequency S arrivals beyond 18 ° are waves which feel the influence of the velocity inversion, which must be accompanied by higher attenua-

tion to produce the change in frequency charac- ter.

3. Broadband observat ions beyond 18 °

For shallow events the refracted arrivals from below the 410 km discontinuity overtake energy returned from shallower depths at a distance between 18 ° and 19 ° . Beyond this crossover, the S-wave arrivals are characterized by intermediate frequencies compared with the high frequencies which are still present in the P waves. Three events at distances of 19.5 °, 22.0 ° and 25.1 °,

Latitude -5.26 Magnitude 5.3

Longitude 151.27 Distance 22.0

Depth. 10

bb

sp

bb

sp

bb

sp

;rtical

~dial

nsverse

(b) Time duration 7 minutes Fig. 4 (continued).

214 0. Gudmundssonetal. /Physicsof theEarth andPlanetaryInteriors 84 (1994)207—226

through New Britain and the Solomon Islands, the S waves are very clear in the broadbandare shown in Fig. 4. The configurationof Fig. 4 records,with a sequenceof well-definedarrivalsmatchesthat of Fig. 3, the horizontal-component correspondingto the different propagationpathstracesare rotatedandthe broadbandrecordsare within the uppermantle. On a few records,en-accompaniedby a simulation of a short-period ergyreflectedbackfrom the 670 km discontinuitytrace. appearsto have higher frequencythan the re-

For the events in Fig. 4, the P wave is well fractedarrival turnedbackby the velocity gradi-representedin the short-periodtraces,but the S ent abovethe discontinuity.The higher-frequencywavesareweakand comparablewith the P coda. reflectedarrivalssuggesta reflectionfrom a sharpAs a consequence,it is very difficult to correlate discontinuity, but it is not easy to explain theS-wave arrivals across short-periodrecord see- differencein frequencycontentwith therefractedtions beyond 18° from the source, even when arrivals as the ray paths would be similar forrecordsare available from multiple sourcesand much of the propagationpath. Interferenceofreceivers(Dey et al., 1993). On the other hand, arrivals associatedwith the presenceof a weak

Latitude -9.09 Longitude 157.84 Depth 33Magnitude 5.2 Distance 25.1

bb

Verticalsp

bbala

sp

bbr nsverse.

sp

(c) Time duration 7 minutesFig. 4 (continued).

0. Gudmundssonet al. /Physicsof theEarth and PlanetaryInteriors84 (1994)207—226 215

520 km discontinuitymight help to producesuch around 1 Hz, and at greaterdistancesa shift inan effect. the dominantfrequencyof the Swavesto about

0.2 Hz. To encompassthis frequencyrangewecompute the discrete Fourier transform within

4. Theoryand data analysis time windowsof 51.2s length(1024 samplesat 20sampless~).This gives512 samplesof the power

The changein the frequencycharacterof S spectrumup to the Nyquist frequency, 10 Hz,wavesrelative to P waves,which occursbetween with a spectral resolution of 0.02 Hz. The time16.9°and 19.5°epicentraldistance,as is evident windows analysedare the 51.2 s following a timefrom Figs. 3 and4, maybe explainedby a layerof pick of the earliestarrival of eachwave type ashigh shearattenuationat depth.To quantify the well as the 51.2 s preceding the pick for anobservation,weuse the spectralratio of S waves estimateof the appropriatenoise spectrum.Be-andPwaves.The logarithmic slopeof this spec- causeof the length of the time window, multipletral ratio maybe relatedto the differencet’ — t~. arrivalsare includedat triplication distances.Be-

The useof the spectralratio to equalizeobser- fore spectrallyanalysingeachset of three-compo-vationsof body wavesin the frequencydomain nentdatawe decomposethem into P, SV andSHwas introducedin the mid-1960s(e.g. Phinney, wavesas opposedto Z, R and T (vertical, radial1964). The merit of the method lies in the can- and transverse)usingthe approximateschemeofcelling of unpredictable,common, frequency-de- Kennett (1991). This is simply a linear combina-pendentfactors by means of the spectralratio tion of the observedseismogramsand has noandthe avoidanceof geometricaleffectson abso- effecton the shapeof the amplitudespectra,butlute amplitudeby measuringthe logarithmic fre- slightly reducesthe relativelevel of noisewhich isquency-derivativeof the spectralratio. The loga- incoherentin time acrosscomponents.We thenrithmic slopeof the spectralratio is interpretable computethe powerspectrumfor P waves,P~(r’),

in terms of a difference in t’~,which is linearly a noise spectrumfor P waves, P0~(v),and therelatedto the inverseof the quality factor, Q. In same for SV and SH waves, P~~(v),~sH(~),

the following, we outline themethodby whichwe P,~\,(v), and ~~SH(~), where ~ stands for fre-measurethe spectralratio of S andP wavesand quency.We do not apply any windowing in theits logarithmic slopeon a singleseismogram,and power-spectral estimation because the high-review the formulation of the spectral-ratio frequencycontentis clearlynot stationaryin timemethod briefly to stateclearly the assumptions within eachtime window. Windowing could thusmade.Someproblemswith modellingour databy biasan estimateof the decayrateof the spectrumthis method will becomeapparent;nevertheless, or a spectralratio. Consequently,we must livethe logarithmic slope of the spectral ratio of S with significant potential side lobesin the spec-andP wavesrepresentsa usefulquantificationof trum, which decay as 1/v in amplitude. Thethe observationspresentedin Figs. 3 and 4 and differencesdiscussedabove.A comparisonof SandP spectra P (v)=P (v)—P (v)andfinds common use in sourcestudies. Rautian et P P nP

al. (1978)comparedthe spectraof S andP waves P~(v)=P~~(~)+PSH(v)—P~~~(~)—~~sH(~)

in a combinedsourceand Q studyin the Garm (1)regionof Tadjikistan.Ulug andBerkhemer(1984)usedthe S/Pspectralratio to infer thefrequency removethe contributionsof noise to the spectradependenceof Q in the mantle from distant upon the assumptionthat the dataandthe noiseteleseismicevents recordedat Station GRF in areindependentin phase.On the otherhand,theGermany. SH and SV wavesareassumedto be completely

The seismogramsin Figs. 3 and 4 show high- coherentin time. In effect, the aboveremovaloffrequencyP and S wavesat short distances(less the noisecontributionservesto identify rangesinthan 18°)with a dominant frequency content frequencywhere the signal doesnot significantly

216 0. Gudmundssonetal. /PhysicsoftheEarth andPlanetaryInteriors 84 (1994)207—226

exceedthe noise, renderingthis estimateof the andmeasurethe logarithmic slopeof the spectralpowerspectrumnearzero or negative.This gen- ratio by fitting a straightline throughloge(R5p(v))erallyoccursat the lowestfrequenciesincludedin by linear regression.In so doing,we stay clearofthe analysis(0.02—0.2 Hz) and beyondabout 0.5 thoseportionsof the spectrumwhere a 1/v de-Hz for eventsbeyondan epicentral distanceof cay is identified. This generallyoccursbeyond 1about 18°.Using the rangebetween,we define Hz. Thus,our measurementsof the decayrateofthe spectralratio by the spectral ratio apply to frequencies in the

2 range 0.2—1.0 Hz at distancesout to 18°and

R~~(v)= [1’~(t’)/1~~(u’)I (2) 0.2—0.5Hz at greaterdistances.No smoothingof

1~ ~15~6°~ event 8 18.70

o x P waves 0 . ~ P wavesx°

x ,oc° ‘¼êStc~, x

x ~ xx

x “

—1 —1

-2 ~ -2ci~~0 I

.~ I

~ 0 . S waves 0 S waves~ xx ~

Cl) ‘x~

a)~

-1~ xx,,,. ~1. xx

E xx xxxxx~

a)N~ -2 ....I.... -2

o

0) 0 x x SIP ratio 0 S/P ratio2 ~ -2

frequency [Hz]Fig. 5. Examplesof P- and S-wavevelocity—amplitudespectrain the frequencyrangefrom 0.2 to 1.0 Hz. (Note the amount ofcommonstructurein the S andP spectraand the linear behaviourof the spectralratio.)

0. Gudmundssonet al. /Physicsof theEarth and PlanetaryInteriors84 (1994)207—226 217

the spectrais done before the linear regression deviation of the P-wave corner frequencyfromthroughthe spectralratio. No sign of a changein that for Swaves~‘ithin about50%.Of course,theslope of the spectralratio is seenin our data,but source spectra for P waves and S waves maysuch a changemay be obscuredby artefactsof differ in moresubtlewaysthanthat characterizedthe power spectral estimation at frequencies by the cornerfrequency.One can speculatethathigher than 1 Hz. small-scalegeometricalheterogeneityon a fault

When applying the spectral-ratiomethod,it is may act as localized volume sourcesand emitpostulatedthat the amplitudespectrumof a wave compressionalwavesmore efficiently than shearmaybe decomposedas follows: waves.Acknowledgingthis limitation, we assume

identicalsourcespectrafor P andS wavesat thisA~(v)= I~(v). E(v) . R,(p

1, ~.) . G,(v) point, andwill return to this issue in the discus-S1(p~,~.) . C~(v) (3) sion section. The examples shown in Fig. 5

demonstratethe existenceof apparentstructurewhere p standsfor ray parameter,~ represents in the spectrawhich is commonto P andS wavesazimuth,and i indexesthe particularobservation.

(notethat thesearevelocity spectra).This impliesI is the transferfunction of the instrument,F is

thatour assumptionis sound.Although the inter-the sourcespectrum,R is the radiation pattern, ference of multiple arrivals within the analysedand G is the elasticimpulseresponse,which may time window could causesuch structurein theinclude a crustal responsefunction and other spectra(Ganleyand Kanasewich,1980), it wouldmultipathing effects. S describes geometric

not occur at the samefrequenciesfor S and Pspreadingeffects and is strictly a part of the

waves.elastic impulse response,and C describestheeffect of attenuation (Teng, 1968; Ulug and From Eq. (4), it follows that the logarithmic

slopeof the spectralratio isBerkhemer,1984). Thus, the ratio of two ampli-tude spectracancelscommon frequencyeffects. d 1 dC 1 dC0If the velocity ratio vs/vp for S and P waves ~[loge(Rsp)} = ....~. . — . _~. (5)remainedconstantin the Earth we would haveG1(v) = G~(v)andidenticalray pathsfor the two According to the definition of the quality factor,wave types,S andP. This is not the case,butwill Q, the attenuationeffect is describedbybe assumedhere nevertheless.For the simple C ex~( ds~.vf —J (6)interpretationthat follows, this is probably a sat- = — pathQv /isfactory assumption.If we further assumethatthe source spectrumis the samefor S and ~ where v is velocity. If Q and v are independentwaves,the spectralratio of S andPbecomes of frequency,the derivativebecomessimply

dC dsR(p~,~~)~ C1(v) ~ (7)

R5~(v)= R~(p~,~) S0(p~,~) C~(v) (4) dv pathQV

wherebecausebothwave typesareobservedon identi- d~ dt

cal instruments.The only remaining frequency- t’~= fdependentcontribution is then the effect of at- pathQ V = ~ath Q

tenuation.The sourcespectraarenot necessarily and thus the logarithmic slope of the spectralthe samefor S andP waves(Ranks,1981). This ratio isissuereceivedsomeattentionin the 1970s.Ranks d(1981)gavea review of comparisonsof the corner -~—(loge(Rsp))= — ~.. (t~— t’) = — ~,.. spfrequenciesof S andPwavesandconcludedthat,in mostcases,the cornerfor P is 20—70%higher (8)in frequencythan that for S. The dynamic crack wherewehavedefinedthedifferential t’~(t’ — t~)

model of Madariaga(1976)predictsa directional as ~

218 0. Gudmundssonet al. /Physicsof theEarth and PlanetaryInteriors84 (1994) 207—226

5. Modelling and results

The results of 26 measurementsof the loga-rithmic slopeof the S/Pspectralratio, ~ are 2summarizedin Fig. 6(a) (four measurementsof 2the last event in Table 1). Details of the events •

used are given in Table 1 and Fig. 2. 5t~is __plottedagainstepicentraldistancewith onestan-darddeviationerrorestimates.The main feat 0uresdisplayedby thesedata are: (1) the consistently

I S.

low valueof 5t~out to a distanceof 18°;(2) the *

suddenincreaseof 6t~betweendistancesof 18°and 20°;(3) a hint of a gradualdecreasein ~beyondthe distanceof 20°.The first two of these .

points are of course anticipatedfrom observing 0the raw seismograms(Figs. 3 and4).

To translatethesedatainto a model of attenu- 10 15 20 25 30ation in the Earthwemakethe following simplifi-cations.Again, we assumethat the pathsof Sand distance(degrees)P wavesare the same.Thus,we write

ds ds 0 — III, II

SPJ~athQs~Vs ~athQp~Vp Qs=1400

Vs Vp

~ath( 1 1 \ 200 ____________________________________= Ids~ Qs=100

I ds ~‘400

path\ Q~ v~/ Q~V~ Qs=600y~ds .~ 600

= ‘~athQ~V~ (9)

wherewe havedefinedy accordingto 800 Os= 1300

I Q~ V~\ b(10) _________________________

~ Q~, Vi,) 10003 4 5 6 7 8 9 10 11 12 13

It is reasonableto assumethat the velocity ratio velocity (km/s)V

0/V~ takes a value near 3i/2 and varies littlealong the path. If dissipationis all in the shear Fig. 6. Differential t~K data and a model constructedto

explainthem.(a)Plot of 26 measurementsof the logarithmicmodulusthe ratio Q0/Q5takesa value near9/4. slopeof S/P spectralratios in thefrequencyrangefrom0.2toThis ratio is usually near two, but could be less 0.5 Hz (1.0 Hz). Eachdot representsa single datumwith a 1

thanone if the dominantdissipationmechanism SD error bar basedon the quality of the linear fit to the

is thermoelasticor is dueto relaxationin the bulk spectralratio. The solid curve representsthe data fit pro-

modulus associated with a first-order phase duced by the Q model (QNA1) presentedin (b). The dashedcurves representbounds on data predictions producedby

change(Anderson,1989).ForQP/QS in the range allowing theparametersof QNA1 to vary accordingto uncer-

1—2.25, the factor y in Eq. (9) rangesfrom 0.4to tainty estimatesin Table 1. (b) Plot of the velocity models

0.8, allowing the velocity ratio to deviateslightly usedin the modellingaswell as the Q model,QNA1.

0. Gudmundssonetal. /Physicsof theEarth andPlanetaryInteriors84 (1994)207—226 219

from that of a Poisson solid. We will simply may be crudely approximatedas a linear combi-assumethat y = 0.7 and is constantalong each nation of the individual path integralsin Eq. (11),path. This is roughly equivalentto V~/V~= 31/2 where each is given a weight according to itsand QP/QS = 2. Thus, we take y outsideof the relativepeakamplitudeintegralandapproximate n ds

ds 0.7~~ w1f —~ / ~ w1 (12)6t* 0.7~t~= 0.7 1 (11) 1=1 path1Q~V5 i=1sp ~ hQV

pat S S where w1 (i = 1, 2, .. . , n) is the weight given to

We useEq. (11) to model the datain Fig. 6(a) in the ith phase.We estimatethe relativeweightsofterms of a Q5 model in the uppermantle.The themain triplicationphasesfrom syntheticreflec-approximationmade in Eq. (11) is recoverable tivity seismogramsfor theshear-velocitystructurefrom the resultingmodel. derivedby Kennettet al. (1994). This summation

As was discussedabove,our spectralanalysis approximationof the combined effects of moreextendsover time windows which include multi- than one arrival is subject to error where twopie phasesin the triplication region. Thus, the arrivalsinterfere,whichcanproducescallopinginintegral along the ray path in Eq. (11) actually the spectrum (Ganley and Kanasewich, 1980).involves multiple paths. However, owing to the Richardsand Menke (1983) showedhow an mi-exponential form of the function C, which de- tial pulsecansuffer broadeningasa resultof thescribesthe attenuationeffect, the observed~ constructive interference of forward scattered

Table 1Locations,origin times, magnitudes,epicentraldistancesandmeasuredspectralslopesof the eventsusedin the study

Event Year Date Time Latitude Longitude depth Magnitude Distance ~

(km) (deg) a (~)

1 1989 12.12 11:02:57.2 —4.390 135.351 33.0 4.9 15.6 0.172 1990 10.02 13:12:14.2 —5.263 151.271 10.0 5.3 21.9 1.323 1990 04.04 19:51:49.0 —4.738 151.645 31.2 4.9 22.7 1.614 1990 13.04 22:46:55.3 —6.638 130.513 17.7 5.2 13.5 0.245 1990 25.04 15:32:21.7 —7.077 150.112 23.0 5.4 19.9 0.666 1990 21.05 13:24:36.6 —8.137 109.043 28.0 5.5 27.2 1.277 1990 15.06 15:04:50.9 —6.422 131.240 62.0 5.0 15.2 0.258 1990 23.07 10:57:28.2 —4.711 145.578 30.0 5.0 18.7 0.669 1990 29.09 11:16:10.9 —8.701 122.385 33.0 5.0 16.1 0.02

10 1990 18.10 03:06:52.3 —7.348 129.279 48.0 5.3 14.1 0.2111 1990 07.12 20:31:03.2 —6.787 132.260 33.0 5.2 13.3 0.2312 1990 16.12 20:19:48.1 —6.002 142.167 33.0 5.4 15.9 0.2313 1991 17.05 06:37:47.0 —9.941 119.780 21.0 5.2 17.1 0.2014 1991 26.09 09:14:50.4 —9.283 158.620 26.0 5.2 25.7 1.5915 1991 27.09 23:01:25.6 —3.359 137.625 64.0 5.5 17.6 0.1716 1991 27.12 17:14:30.7 —9.093 157.840 33.0 5.2 25.1 1.5017 1991 28.12 03:40:31.4 —6.424 150.840 24.0 5.4 20.9 1.8418 1991 28.12 11:05:20.3 —6.325 150.189 33.0 5.4 20.5 1.8419 1992 21.01 07:57:21.7 —9.369 122.821 33.0 5.1 15.4 0.2020 1992 24.01 12:09:24.1 —9.402 148.795 33.0 5.4 17.5 0.2621 1992 14.02 12:02:03.0 —4.382 146.376 42.0 5.2 19.6 0.8922 1992 05.03 00:19:16.4 —6.569 130.443 60.0 5.1 [5.1 0.0623 1992 11.09 22:24:52.2 4.153 126.464 46.0 5.4 22.3 1.3323 1992 11.09 22:24:52.2 4.153 126.464 46.0 5.4 23.7 1.0223 1992 11.09 22:24:52.2 4.153 126.464 46.0 5.4 25.6 0.7523 1992 11.09 22:24:52.2 4.153~ 126.464 46.0 5.4 26.4 1.15a Correctedto a commondepthof 33 km. Eventlocationsare from monthlyPDE listings.

220 0. Gudmundssonet al. /Physicsof theEarth andPlanetaryInteriors84 (1994) 207—226

waves in a heterogeneousmedium. This they These layers are a lithosphere layer from themodelled with an apparentscattering Q. The surfaceto a depth of 210 km, an asthenospherescatteringprocessis, however,not a lossyprocess layer betweendepthsof 210 and410 km, and aandin their simulations(in a simplified scattering transition-zonelayer betweendepthsof 410 andmedium) the coda following the first arrival is 660 kin. The top of the lower mantleis assumedenhancedin high frequencies.As our time win- to havea Q~ valueof 1300,which we takefromdows include a significant portion of the coda, the model of Der et al. (1986) at 0.3 Hz.the net effect of scatteringon apparentattenua- This procedureis clearly not unique.Further-tion is minimized. It shouldbe noted that Aki more, considerablescatter and uncertainty areandChouet(1975)describedlocal codaobserva- evidentin the data,particularly beyond20°.How-lions as a time-modulatedtime seriesfrom which ever, this simple parametrizationis sufficient tothey inferred intrinsic Q in the shallow litho- model the resolvablebehaviourin our data, as issphere.No such simple time modulation can be evidentin Fig. 6(a). The uncertaintiespresentedfitted to the codatrains in Figs. 3 and4, andthey in Fig. 6(a) imply that the scatterin the datais inremainrelativelyconstantin amplitudewith time, largepart due to poor spectralresolution. ThisAccording to Kennett (1983) and Richardsand effectis compoundedby the fact thatwe computeMenke (1983), the scatteringcontributionto at- theFourierspectraoverlongtimewindows,whichtenuationmaybe accountedfor by contain codawith largely incoherentphase(the

uncertaintyof the amplitude spectrumof a time1 = 1 + 1 (13~ serieswith an incoherentphasespectrumequals

Qobserved Qintrinsic Qscattering “ ‘ the expectationof the amplitude spectrum(e.g.Aki and Richards, 1981)). Interference effects,

Eq. (12) is the ultimate expressionthat we use unmodelledfeaturesof the sourcespectrum,andto model our measurementsof &~. It is a linear path differences between P and S waves mayrelationshipbetweenour dataandthe inverseQ contribute to the scatter. The geometryof ourstructureof the uppermantle.Weusetheexpres- experimentis by no meansideal for one-dimen-sion in a forwardsense.We usethe shear-velocity sionalinterpretation(Fig. 2), and lateralhetero-model derivedby Kennettet al. (1994)to gener- geneitymay contributeto the scatter.It shouldbeatethe ray paths,use it for the c5, which occurs noted however, that analysesof recordings ofin the integrandof Eq. (12), and useit to gener- Event 23 in Table 1 in the Molucca Sea withatethe syntheticswhich definetheweights in Eq. portablebroadbandCMG-3ESPinstrumentsde-(12). We chooseto divide the uppermantle into pioyed around WRA fall within error boundsthreelayers,which coincidewith the major fea- insidethe overall trendin the datain Fig. 6(a).tures of the velocity model (shown in Fig. 6(b)), The solid curve in Fig. 6(a)correspondsto theand assumea constant Q value within each. modelgiven in Fig. 6(b) andTable2. The dashed

Table 2The QNA1 model giving Q, and as functionsof depthin four-layer stack

Layer Depthrange Description Q~no. (km)

1 0—210 Lithosphere 1400±400 28002 210—410 Asthenosphere 100 ± 20 2003 410—660 Transitionzone 600 ±300 12004 > 660 Lowermantle 1300

It is assumedthat 2Q5.Uncertaintiesdo not take theuncertaintyof this assumptioninto account.

0. Gudmundssonetal. /Physicsof theEarth and PlanetaryInteriors 84 (1994)207—226 221

curvesrepresentthe rangeof predicteddataval- scatterin the datain Fig. 6(a) is largeand300 is aues allowing for the uncertaintiespresentedin probable uncertainty bound (Q5 = 600 ±300).Table 2 and discussedbelow. The low valuesof This value is compatiblewith the results of An-

out to an epicentraldistanceof 18°leadto a dersonandGiven (1982)andDer et al. (1986)forhigh estimateof the average Q5 in the litho- this depth range.We will hereafterrefer to thesphere,Q~= 1400±400. Thisis ahighvaluewhen model presentedin Fig. 6(b) and Table 2 ascomparedwith, for example,the global model of model QNA1. It would perhapsbe morenaturalAndersonandGiven(1982)or the Eurasianshield to quoteerror in 1/Q, but theseerror estimatesmodel of Der et al. (1986). It should be noted are not formal. The above uncertaintiesdo notthat a constantQ in the lithosphereresults in a include the uncertaintyof the valueof y (0.7) ingraduallyincreasing~ with distance.The data Eq. (10).in Fig. 6(a) show no sign of such a slope,which In the above,we havebuilt a simplemodel forcan be taken to indicate that Q increaseswith the qualityfactor, Q, in the uppermantle,assum-depth in the lithosphere.If we acceptthis as a ing that attenuationis independentof frequencyresolvablefeatureof our data, that placescon- and occursprimarily in shear,and ignoring po-straintson the mechanismof dissipationthrough tential effectsof scatteringand sourceradiation.the pressureand temperaturederivative of Q. Theseassumptionsdeservefurther discussion.This featureimplies that a reductionin 1/Q asaresult of a pressureincreaseis more importantthan an increasein 1/Q as a result of rising 6. Discussiontemperaturein the lithospherewith depth.

Owing to the low-velocity zoneunderthe lid, The broadbandrecords from the Indonesianour data have a poor depth resolution in the and New Guinea seismic zonesto the Warra-asthenosphere.The discontinuity in the t~t~(i~) mungasite, with propagationthrough the uppercurve in Fig. 6(a) is broughtaboutby the defocus- mantle, show the merit of maintaining a largeing effect of the bottomof the lid, which diverts frequency-bandwidthbridging the gap betweenrays away from the top of the asthenosphere. the short-periodandlong-periodfrequencybandsThis sharpincreasein 5t~at 18—20°,by about commonlyconsideredin global seismology.The1.5 s, can be modelled by a layer of 200 km intermediatefrequencycontent (0.2—0.5 Hz) ofthicknesswith Q5 = 100 and = 200,butequally the S arrivals beyond 18°allows for a higherwell by a thinner layerwith lower Q values.The resolution in the modelling of velocity structurevalue for Q5 (100, assuminga thicknessof 200 in the uppermantle than long-periodmodellingkm) canbetradedoff with the Q of the transition (0.03—0.1Hz) (Kennettet al., 1994).Most studieszoneto somedegree.A rough uncertaintyesti- of the uppermantle using S body waves havemate for Q5 of 20 can be establishedby visual usedonly the long-periodtransverse(SH)compo-inspectionof the deteriorationin data fit (see nent (e.g. Grand and Helmberger, 1984). Thedashedcurvesin Fig. 6(a)).Our estimateof Q5 = radial (SV) componentsat Warramungaare of100±20 is similar, but somewhatlowerthan the uniformly good quality (Figs. 3 and 4), withoutestimatesof Andersonand Given (1982)and Der contaminationfrom coupling to P waves,becauseet al. (1986)for Q5 in the asthenosphere. the surfaceP-wavevelocity is high (more than 6

The gradualdecreaseof 5t~between21°and km ~1)~ As a result, the SV phasescan also be25°is in part causedby a progressivelysteeper usedfor structuralstudies,whichoffers the possi-angleof propagationthroughthe attenuativeas- bility of a comparisonof SH andSV wave struc-thenospherewith increasingdistance.The level tures and direct anisotropy studies for theof 6t~ in this distancerange and beyond is upper-mantletransition(Tong et al., 1994). Theotherwisecontrolledby attenuationin the transi- frequencycontentof the S arrivals, in particular,tion zone.Our estimateof Q5 = 600 for the tran- placesstrongconstraintson possibleQ structuressition zoneis clearly notwell constrained,as the within the uppermantle.There is efficient high-

222 0. Gudmundssonetal. /Physicsof theEarth andPlanetaryInteriors 84 (1994)207—226

frequencypropagationof SV andSR in the litho- spectralratio. Goingbackto Eqs.(6) and (7), wesphere(down to 210 km), and the comparative write

absenceof deeper-mantlearrivals in the short-periodbandcanbeexplainedby a frequencyshift C(v) = exp(—irSv ‘ t*) : — [log (R )}to intermediateperiodsas a result of attenuation dv e ~P

below210 km. It shouldbenotedthat thisshift inthe dominant frequencyof the S waves is not = ~. — ~ + ~ — t*) (14)associatedwith eventsize or event location. ~‘ dv S p

We have quantified this frequency shift intermsof the logarithmicslopeof the S/Pspectral Assuming the samevelocity- and quality-factorratio and modelled those data in terms of a ratios as in Eq. (11), we simplify tomodel of shearattenuationin the uppermantle. 1 d dt’~The resulting model is compatible with results 5t’ = — — — [loge(Rs )] = 0.7 ‘t’ 1 + —from the Eurasianshield(Der et al., 1986) in the i~dv t~’ di-’asthenosphereand transition zone,but requires (15)either a high Q

5 value or a low QP/QS ratio inthelithosphere.This inferenceis subjectto possi- According to Table3, the dispersionin the tele-ble effects related to dispersionin attenuation, seismic t’ for shallow sourcesis aboutdt’/dv =

sourceradiation,andscattering. — 1 s2 for the dispersivemodels(Andersonand

It is commonly found and generallyaccepted Given, 1982; Der et al., 1986) between1 and0.1that Q increasesslightly with frequency,andpos- Hz, and t~’= 2 s. Theset’~valuesaredominatedsibly stronglyso aboveabout1 Hz (Minster,1981). by the low Q value in the asthenosphere.Insert-The dispersiveQ modelsof AndersonandGiven ing thesenumbersinto the bracketedquantityin(1982) and Der et al. (1986) display this be- Eq. (15) implies a 25% underestimateof t’~’ byhaviour. Such dispersiondoesnot merely affect the spectralratio method at around3 s periodthe spectralratio estimateby inducing nonlinear and an equivalentoverestimateof Q. Althoughbehaviour, it also contributesto the slopeof the we hesitateto take the dispersionin Q in the

Table 3A comparisonof Q models by meansof the parametert~’calculatedfor a shallow (0 km) and deep (660 km) sourceand anepicentraldistanceof 35°(note the inferred dispersionin Q, which primarily occurs in the lower mantle, and the generalagreementof the QNA1 modelwith the appropriatefrequencyrangein modelABM andEurasia)

Model Period(s) t~’ Reference

0 km depth 660 km depth

ABM 1 1.4 0.7 AndersonandGiven(1982)ABM 10 2.5 1.5 AndersonandGiven(1982)ABM [00 4.0 2.7 AndersonandGiven(1982)ABM 1000 3.7 2.9 AndersonandGiven(1982)Eurasia 1 1.2 0.7 Deretal.(1986)Eurasia 3 1.8 1.1 Deretal. (1986)Eurasia 10 2.1 1.4 Deretal. (1986)Eurasia 30 2.5 1.6 Deretal. (1986)Eurasia 100 2.9 1.9 Deretal. (1986)QBS 100—3000 4.3 2.8 Sailor andDziewonski(1978)SL1 20—3000 3.7 2.8 AndersonandHart (1978a)SL8 20—3000 4.4 3.1 AndersonandHart (1978b)QNA1 2—5 1.9 1.1 This study

0. Gudmundssonetal. /PhysicsoftheEarth andPlanetaryInteriors 84 (1994)207—226 223

abovemodelsfor granted,the abovecomputationis a clear indication of a potentially significant ~ spectralratio slope= 0.5 secondseffect of dispersionon the Q structureinferred ~from a spectralratio method.The QNA1 model ~mayoverestimateQ in the uppermantleby 25%. ~ 0.8

Themost relevantlaboratorymeasurementsof ~ 0.7 - - - SIPinternal friction at the top of the mantle are ~ 0.6 —

thoseof Jacksonet al. (1992),who measuredthe ~ 0.5 - — —

relaxationof the shearmodulusin aheat-treated ~ 0.4 - -.

dunite sampleover the seismic-frequencybandat cn

temperaturesin the range 0—1000°Cand pres-suresup to 300 MPa(pressurescorrespondingto ~the top 10 km of the Earth). They found a very ~ 0 2slight pressurederivativeof the internal friction ~(a(1/Q

5)/0P) in their pressurerange,but a sig- 0 0 02 04 ‘ 06 08 1 0

nificant temperatureand frequencydependence. . .

Somequestionsremainwith regardto the appli-cability of their results to the Earth, related to frequency[Hz]possibleeffectsof microcrackingand volatiles. it Fig. 7. Logarithmsof amplitude—velocityspectrafor P and Sis nevertheless,difficult to reconcile their esti- wavesconstructedasdescribedin text (drawn as fine lines).

The corner frequencyof the P-wave source-spectrumwasmate of Q5 = 1000 at room temperaturewith our assumedto be 50%larger than that for S waves.The solid,

in situ estimateof Q~= 1400±400 in the litho- thick curve representsthe spectralratio, S/P. It hasbeen

sphere.Allowing for the dispersionin Q asmea- fitted by the straightdashedline by linear regressionbetween

suredby Jacksonet al. (1992) (a power-lawin- 0.2 and 1.0 Hz. The slope of the line is —0.5

creasein Q with frequency,with anexponentof (d(log~(S/P))/dv= —0.5).

about 0.2) reducesour value of 1400 to 1100usingEq.(15) above.Assumingthat temperatureincreaseslinearly with depth in the lithospherefrom 0°Cto 800°C,the apparentinternal friction frequencyfor magnitude5—5.5 as 0.18 Hz forfor our paths out to z~= 18°would be about shearwavesaccordingto Aki (1967) and add on

= 250, if we use the resultspresentedin Fig. 50% for P waves(0.27 Hz). We further assume8(b) of Jacksonet al. (1992)(log10(1/Q5)= —3 + that the displacement—amplitudespectrumdrops(T/800)). This prediction probablyyields an un- by a decadein value over a decadein frequencyderestimate,becauseof the incompletely con- centredon thecornerfrequencybeforeleadingtoverged temporal evolution of Q measurements a second-order,power-law decay.Fig. 7 showswith exposuretime foundby Jacksonet al. (1992). the logarithm of the P and S wave amplitude—Nevertheless,we havehalf an orderof magnitude velocity spectraandthe spectralratio in this caseto makeup. over the frequencyrangeof 0—1 Hz. Theslopeof

If effects other than intrinsic attenuationare the spectralratio is measuredbetween0.2 andto help usreconcilethis difference, their contri- 1.0 Hz, andis foundto be about —0.5. Thus,thebution to the apparentvalue of 1/Q~must be contributionof a sourceeffect suchas thiswouldgreater than that to 1/Q5. As discussedabove, be to add a value of 0.15 (0.5/v) to 6t~.Thethe corner frequencyfor P wavesradiatedfrom measuredlevel of 3t~out to 18°is about thisan earthquakesourceis perhapsexpectedto be level, and could thus be attributed to a sourcehigher thanthat for Swaves(Hanks, 1981).Thus, effect. However, this would force us to infer anthe sourcespectrumfor P risesfaster or decays infinite Q in the lid. It shouldbe noted that themore slowly than that for S waves.This effect is spectrain Fig. 5 argueagainstthe importanceofdemonstratedin Fig. 7. We estimatethe corner a sourceeffect. Also, no systematicdeviationsare

224 0. Gudmundssonetal. /Physicsof theEarth andPlanetaryInteriors 84 (1994)207—226

observedin the estimateof ~ with magnitude, value for Q in the lid comesfrom the study ofA systematicsourceeffectof the order estimated Molnar and Oliver (1969), who found evidenceabovewould havea moderateeffecton our mea- for unusuallyefficient short-periodS,~propaga-surementsof 8t~beyond18°.It shouldbe noted tion through the Australian craton to Newthat the argumentthat P waves contain more Guinea.Exposedrock in the TennantCreekareahigh frequenciesthando S waves(higher corner andto its northareprimarily Proterozoicgranitesfrequency),which Hanks(1981) so vigorouslyar- and metamorphicrocks and Phanerozoicsedi-gued, and that therefore inferences about Q ments,but Plumb (1979) suggestedthat mostofbasedon the S/Pspectralratio arebiased,is in the Precambriancratonin Australiamay be un-part circular. Measurementsof the corner fre- derlainby Archaeancontinentalcrust (andlitho-quencyare dependenton the assumedQ struc- sphere).If so,mostof the propagationpathsfromture. We have two inseparableeffects. Perhaps Flores through to Irian Jaya and to WRA areour well-constrainedmeasurementsof the rela- containedwithin very old lithospherewhich islive spectralslopeof SandP wavesin thevicinity presumablycold.The Australiancratonshowsupof the expectedcorner frequencyfor pathscon- stronglyas a fast anomalyin global,upper-man-tamedin the lid can assistin solvingthisdilemma. tie, surface-wave-velocityinversions, again sug-If therewere a significant source effect, the in- gestingcold conditions.The northernportion offerred Q5 in the lithospherewould be large (Q5 the cratonhasbeenfound to havean unusually> 3000). This is unrealistic. Thus, therecannot thick lid, 210 km (Bowman and Kennett, 1990),be a significantsourceeffect. an inferencewhich was recentlyextendedto the

Richardsand Menke (1983) argued that the entire craton on the basis of travel-time datascattering Q for P waves should be about the (Bowman and Kennett, 1993). The structureun-sameas for S waves.If so, wemay expect t~’> t~ der the lid appearsnotto deviatemuch from thefor scatteringby about 70%.Again, a scattering iasp9l global reference model (Bowman andeffect is expectedto shift ~ upwardsin value. Kennett, 1993; Kennett et al., 1994). The thickTo reverse this effect of scattering on t’~,we andhigh-Q lithosphereunderthe Australiancra-would haveto infer strongerscatteringin P than tonmay indicatethat it is unusuallycold,whereasin 5, i.e. strongerrelative heterogeneityin v~ normalvelocities and a somewhatlow Q in thethanin V~which is contraryto what is inferredat asthenosphereunderthe roots of the cratonmayintermediatelength scales(100—1000km) on the imply that lateral flow from a hot upwelling un-basisof stationcorrections(e.g.Doyle andHales, der the Coral Sea (and Tasman Sea)provides1967) andstatisticalanalysisof global traveltimes heatto the asthenosphere,which hasnotdiffused(Davieset al., 1992).Suchan effectwould ariseif into the lid.the lithospherewere impregnatedwith dry cracks Our model for Q5 in the uppermantleis most(Jackson,1991), which is unlikely. Although the simply comparedwith other Q modelsand rele-seismogramsat short rangeclearly contain coda vantbody-waveobservationsthroughthe teleseis-(Fig. 3), the velocity spectraof P andS wavesare mic ti’. It is somewhatunfortunatethat two dif-reasonablyflat (Fig. 5), which indicatesthat scat- ferent definitions of this parameterhave beentering attenuationin the interval 0.2—1.0 Hz is widely used in the literature.We haveusedthenot strong. We conclude that scattering and definitionsourceeffectsare probablynot affectingour mea- ds dtsurementsof 5t~significantly, andwe interpret t’

1’ = f = f —our datain termsof intrinsicattenuation. path V ~ path

Our value for Q in the lithosphereis some- throughoutthis paper,whereasa multiplicationwhat high, andwe can argueagainstit being an factor of ~‘ is often included(Teng, 1968).artefactof the sourceor scattering,butwecannot A comparisonof the predicted t~’ at an epi-rule out the possibility that it is a good estimate central distanceof 35°for a shallow sourceandaof intrinsic Q. One argument to support our deepsourcefrom our regionalQNA1 model and

0. Gudmundssonet al. /Physicsof theEarth and PlanetaryInteriors84 (1994)207—226 225

otherpublishedmodelsis presentedin Table 3. difference in the tectonic regime of our sourceThe t~ for P wavescanbe estimatedby division regions and the WRA site. We argue that ourof t~ by a factor of aboutfour. The global ABM model, QNA1, is most directly relevant to themodel of Andersonand Given (1982) and the northern edge of the Australian craton. TheregionalEurasiamodel of Der et al. (1986) are QNA1 model is similar to the Eurasianshielddispersivein Q, whereasthe other models as- model of Der et al. (1986) at the appropriatesume a frequency-independentQ. The QBS period.Themostsignificant differencelies in themodel of SailorandDziewonski(1978) is a global valueinferred for Q in the lithosphere.Thismaymodel basedon normal-modedataonly. The SL1 be explainedby a significantthermoelasticcontri-andSL8 modelsof AndersonandHart (1978a,b) bution to attenuation,or it maybe a reflectionofincluded normal-mode,surface-wave,and body- differencesbetweenthe two shields.A high Q iswave dataavailableat the time. The sameholds not unexpectedin the Australianlithosphere,asfor the absorptionbandmodel of Andersonand it has been found to be unusually thick (andGiven (1982). The Eurasianmodel of Der et al. probably cold). A strong point of the QNA1(1986) is basedon long- and short-periodbody modelis the resolutionby whichwe canconstrainwaves.The dispersionin Q in the frequency-de- the top of the low-Q layer. It must coincidewithpendentmodelsandits relationto directobserva- the bottomof the lid.tion wasdiscussedby Der et al. We simply point Our dataandresultsdemonstratethe valueofout the close agreementof thevaluesfor QNA1 broadbandobservationsover the frequencybandwith those for the Eurasianmodel in the appro- 0.03—10Hz, whichbridgesthe gapbetweentradi-priate frequencyband and that both valuesfail tional short-periodandlong-periodrecording.Atmidway betweenthevaluesfor ABM at 1 and 10 siteswith low microseismicnoise characteristics,s. This may imply that the vertical integral of recordingof the intermediate-frequencybandcan1/Q5 does not vary much on a global scale or yield valuableobservationsof upper-mantleshearthat the ABM global model is biased toward phases,which otherwisego undetected.They inshields. turn supplyvaluableconstraintson Q, the veloc-

It shouldbe interestingto seemore regional, ity distribution, and the potential to studybody-wave Q models for the upper mantle, as anisotropy by comparingSV and SR waves inattenuationin the lower mantle appearsto be time.weakat the short-periodendof theseismicband,and more such models shouldbe feasible nowthat the population of ,broadband instruments Acknowledgementswhich spanthe short-periodbandhasgrowncon-siderably.

We thank Anton Hales, Roger Bowman, IanJackson,Phil Cummins and two anonymousre-

7 Conclusions viewers for constructive comments on themanuscript.

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