Physicsof theEarth andPlanetaryInteriors, 24 (1981) 15—29 15ElsevierScientific PublishingCompany,Amsterdam— Printedin TheNetherlands

SCATTERED PKKP: FURTHER EVIDENCE FOR SCATTERING AT A ROUGH CORE—MANTLEBOUNDARY

ANDRE C. CHANG’ andJOHNR. 2

1 TeledyneGeotech,Alexandria,VA 22314(U.S.A.)2 ResearchSchoolof Earth Sciences,AustralianNational University,Canberra, A.C. T. 2600 (Australia)

(ReceivedJanuary7, 1980; revisedandacceptedJuly 8, 1980)

Chang,A.C. andCleary,J.R.,1981. ScatteredPKKP: furtherevidencefor scatteringata roughcore—mantleboundary.Phys.Earth Planet.Inter., 24: 15—29.

PKKP signalsfrom NovayaZemlyarecordedatLASA atdistancesaround60°showconsistentanomaliesin bothslownessand azimuth.The observedanomalysuggeststhatthesignal is a BCbrancharrival,althoughthe arrivaltimecorrespondsto theDF branch.TheBCbranch,however,doesnot extendbackto this distance.Theazimuthofapproachis in therange229—245°,insteadof theexpected186°.Theseanomaliesareassociatedonly with PKKP;

analysisof thecorephasesPKIKP andP’P’ (BC) from thesameeventsshowthat theyarrived atLASA with theappropriateslownessesandazimuths.

ThePKKP signalscanbe interpretedas “scattered”PKKP;the scatteringoccurson undersidereflectionat thecore—mantleboundaryandis probablycausedby topographicirregularitieson theboundaryitself. Thecalculatedscatteringregionhasa surfaceprojectionat about60°S,134°E,which is outsidethediametralplanethroughsourceandreceiver,andabout21°from theexpectedPKKP reflectionpoint at 76°S,95°E.

Both the“direct” and “scattered”armsofthePKKP signalhavea PK pathcloseto that of the “C” endof theBC branch.Theunexpectedlylargeamplitudeof thearrival suggeststhat theremaybea focusingof energyatC,which would indicatea changein velocity gradientjustabovetheinner coreboundary.Theobservationsneverthe-lessrequire,on thescatteringinterpretation,lateralvariationsin thetopographyof thecore—mantleboundaryandaregionof relatively largetopographyresponsiblefor theanomalousPKKP observations.

1. Introduction Doornbos(1974)reportedobservationsof precursorsto PKKP beyondthe causticpoint B, in thedistance

Thetopic of seismicwavesscatteredon or nearthe range125—135°.TheseobservationsareimportantEarth’score—mantleboundary(CMB) hasbeena becauseBolt’s interpretationof the GHbranchcan-matterof considerableinterestin recentyears.The not explainprecursorsat suchdistances.conceptwasintroducedby Haddon(1972)and Thesepaperswere all relatedto observationsofClearyandHaddon(1972).Theydemonstratedthat forwardscattering,wherethe scatteringoccurredattheprecursorsto PKLKP observedin the distance entryto, or exit from,the core,andfrom the obser-range 125—140°canbe interpretedasseismicwaves vationsit is notclear whetherthe scatteringoccurredscatteredon or nearthe CMB in the form of “for- at theCMB or in theheterogeneouslowermantle.ward” scattering:PKJK(P).Subsequentpapers,e.g., The paperby ChangandCleary(1978),usingPKKPDoornbosandVlaar (1973)andHaddonandCleary dataof NovayaZemlyaexplosionsrecordedat LASA,(1974),supportedthe interpretation.Thesestudies showedthatback-scatteringalso probablyexists,incastdoubton the interpretationby Bolt (1964)and the form of PK(KP). Sincethe back-scatteringdoesothersthat the precursorswereproducedby a transi- not occurin the lowermantle,butat or nearthetionlayeror layersnearthe inner coreboundary. pointof undersidereflection at the CMB, thesedata

0031-9201/81/0000—0000/$o2.50© 1981 ElsevierScientific PublishingCompany

16

providedstrongsupportfor thehypothesisthat seis- PKKP~c,with scatteringtakingplaceat a consider-mic wave scatteringoccurson the CMB; it is impor- abledistanceoutsidethediametralplanethroughtantevidencethat topographicirregularitiesmay be sourceandreceiver.We demonstratethat this inter-presenton the surfaceof the boundary. pretationagreeswith theobservedslowness,azimuth,

In the samepaper,we remarkedthatalthough andtime. If the interpretationis correct,then the ob-PJU(Psignalsfrom NovayaZemlyawere detectedat servations,takenin conjunctionwith thoseof pre-the expectedarrival time, their slownesseswere ob- cursorsto PKKPby ChangandCleary(1978)andservedto be 2.5 sdeg.~insteadof the expected1 .3 s Doornbos(1980),provideadditionalevidencefor thedeg.’. The observedazimuthsare alsoin the range ‘ existenceof topographicirregularitieson the CMB.229—245 insteadof theexpected186 . To ourknowledge,therehavebeenno previousobserva-tionsof directphasesarriving sofar off the diametral 2. Dataplane.On the otherhand,suchbehavioris wellknown for precursorsto PKP,andis usuallytakenas Six NovayaZemlyaevents,numberedNZ1 toevidencein supportof their interpretationas scat- NZ6,were usedin this study.Publishedeventparam-teredwaves(see,for example,DoornbosandVlaar, etersof theseeventsare listedin TableI. The event1973;HaddonandCleary,1974). parameters,particularly origintimes, differ slightly

In this paperwe presentresultsof furtherinvesti- from one reportinginstitutionto another.Notethatgationinto theanomalousPKKP observations.Using theorigin timespublishedby NEIS andISC are abouta newdata-processingtechniquecalledAZ1VEL, we 2—3 searlierthanthe exacthour.ThesedifferencesshowthatPKKP for all six eventsexaminedare are probablythe result of errorcausedby usingtheanomalousbut consistent.Furthermore,wepresent Jeffreys—Bullentravel-timetablesin the locationpro-our interpretationthat this PKKP isa scattered cess.Differencesamongeventparametersof thismag-

TABLE I

Eventparametersof six NovayaZemlyaexplosions

Event Date Source Origin time Lat. Long. mb BAZ1No. (°N) (°E) (°)

NZ1 27 Sept.71 NEIS 05:59:55.2 73.4 55.1 6.1 59.52 6.1ISC 05:59:55.4 73.39 54.901 59.52 6.2BUS 06:00:00.0 73.5 54.4 7.1 59.38 6.3

NZ2 28Aug. 72 NEIS 05:59:56.5 73.3 55.1 6.3 59.62 6.2ISC 05:59:56.8 73.39 54.65 59.50 6.3BCIS 06:00:00.0 73.3 53.6 6.3 59.51 6.6

NZ3 12 Sept.73 NEIS 06:59:54.3 73.3 55.2 6.8 59.63 6.1ISC 06:59:54.6 73.32 54.97 6.8 59.50 6.2BCIS 17:00:00.0 73.3 54.0 59.54 6.5

NZ4 29 Aug. 74 NEIS 09:59:55.5 73.4 55.1 6.4 59.52 6.1ISC 09:59:55.8 73.41 54.93 6.4 59.50 6.2BCIS 10:00:00.0 73.5 54.2 59.36 6.4

NZ5 27Oct. 74 NEIS 06:59:57.4 70.8 54.2 6.9 61.98 7.2ISC 06:59:57.6 70.80 53.92 6.9 61.95 7.3BCIS 07:00:00.0 70.8 53.0 61.87 7.6

NZ6 02 Nov. 74 NEIS 04:59:56.7 70.8 54.1 6.7 61.97 7.3ISC 04:59:56.9 70.81 53.91 6.4 62.24 7.4BCIS 05:00:06.7 70.5 51.5 62.02 8.3

17

nitude arecommonamongthe publishinginstitu- tions of side lobe patterns,andarraybeamcalibra-tions.In this studywe useexacthoursasthe correct tion for travel-timeanomalies.Sincediscussionsoforigin timesof the events(cf. Rodean,1979). thesefactorsappearonly briefly andsporadicallyin

Thedistancesfrom NovayaZemlyato LASA are publications,theywill besummarizedhere.about59.6°for eventsNZ1 to NZ4,andabout62° A totalof313beamsareplacedin a hexagonalfor eventsNZ5 and NZ6.UsingJordanandAnder- patternwith ahorizontalspacingof 0.00433skm’son’s(1974)model Bi, eventparametersof the anda verticalspacingof 0.00375 skm~(cf. Figs.PKKP-DFsignalare T= 31 m 13.6 s, p = 1 .29 s 4—11).This beamset coversthe slownessrangedeg.1,0 = 186°for eventsNZ1—NZ4, andT= 31 m radially from 0.0 at the centerof the patternto 4.4 s9.7s,p = 1.34sdeg.1,0 = 187°for eventsNZ5 and deg.’ at theperimeter,andthe entireazimuthrangeNZ6. measuredclockwisefrom north(thedirectionalong

the verticalaxis from thecenterto thetop of the

3 Aziveigrams pattern).Sincebeamsare hexagonallyarranged,themaximumpossibleerrorin signalparameterestimates

TheAZ1VEL processis a signal-processingscheme will occur at the centerof an equilateraltriangles~iithdevelopedby oneof theauthors(ACC). It enablesus sidesof 0.00433,i.e. themaximumerror is 0.0025sto determinethesignalslownessp, azimuth0, ampli- km’ with this beamspacing.The beamspacingistudeA andarrival time T for a continuoustime win- approximatelyhalfof the LASA arraybeamresolu-dow specifiedby theuser.Theprocessutilizesa set tion.of predeterminedarraybeams,313 beamsin total, Theresolutionof an arraybeamis a function ofcoveringthe slownessarea0—4.4sdeg.1(approxi- arraydiameteranddistribution of instruments,andmately from infinity to 25 km s’ in velocity). After alsoof the signalfrequency.Sincemostof the tele-thesebeamsare formed,the processwill searchfor seismicPandcore phasesignalsreceivedat LASA arethe maximumamplitudesof eachbeamin a moving 1 Hz signals,it is adequateto usesignalsof that fre-time window of 1 s,andprint thesemaximumam- quencyin the following discussion.Seismometersplitudesin a slownessandazimuthfield, knownas an in LASA areunevenlydistributed.Thirteensubarraysaziveigram.By usingthe AZIVEL process,onecan are arrangedwitha heavyconcentrationof seismome-determinetheincomingsignalparametersT, p. 0 tersat the arraycenters.Eachsubarrayconsistsof 16andA. A similarmethodcalledBEAMAN was devel- evenlyspacedseismometers(Fig. 1).This designwasopedat NORSARby King et al. (1976). In the aimedat reducingside lobe peaks,but due to theseis-BEAMAN processa window of slownessandazi- mometerconcentrationthemain lobe is somewhatmuth is specified.This is not requiredin AZWEL broaderthanfor anarraywith the four centralsub-sincethe processscrutinizesthe entireslownessspace arraysremoved.Figures2 and3 showarray-beamin all azimuths.Currentlytherearetwo setsof arraybeamsavailable,one for the corephasecoverageand

anotherfor the entirecorephaseandPphaserange 107 106 W

(from 0 to 10 sdeg.1). ____________________ _________

At thebeginningof the processthe desiredbeam 47’N - _____________________ _________

set is readfrom atape.The processwill print the /‘ ~ 2

index,azimuth,velocity,slow- ( 4..~~ness,inversevelocity andX andYcomponentsof ~. ,/ ~ ~ 14)

the inversevelocity.Whenasignalis identifiedin 45,,~...4 ?~X.~‘

the azivelgram,onerefersthe row—columnnumber Ito thebeamdirectory andobtainssignalparameters. ~

Theaccuracyof the AZIVEL processdependson I I I SUB-ARRAY AD

four basicfactors:the spacingof arraybeams,the i::. ~. ~

arraybeamresolution,spatialandtemporalvaria- Fig. 1. LASA arrayand subarrayinstrumentlayout.

18

!A’~A Rrr) ~3 ‘~‘1P~P2~V’‘T~AX =0.0)10FP?0)’~NCY= 1_00

0.04 ***********#****)‘*4**44***************4*****s**************************9887777777

***********4.)****1999 RR~)883999**************s**************ss********************

0.02 ****‘***********4*9Q)~)’999 8t188909***************************s***************s****4*4*4* *4*4*)’ *4*4*4*9 C1999~9 99P 9399 9~** 4*4*44*44*4*44*4 *4*4*4* *44*4*4*4*4*44*4*4*4

33 33 3333 3333 44

*******************4****qp7~5c’~)4333 22222222223333445566778899**************ss**s**

~

***4********)’****t***qp7lc~L43 37211100000300011 122334556789***s*s*****s**.*******

‘ 0.0 ***********************9’6 32211000000000000 1122314145678***********************************:~*********9~’79’~)132211 100000000030111223is45679***********************

**#*********)‘**4**ff*)’178771~L44l4332222111311222233l4l456679s****s*****s*s**********

*******4**”** ~******~*99°°P99777666 5555 5U4445555~67789***************************************4**4**********9999c19777c99 c6555555666677899*********************a*

—0.02 * * 4* * *4 * * * * * * * * * *4 * * * *4 * * * * * * * * * * * * * * * * * * * * * * * 999888888998999 * * * * * * * * * * * * * * * * * * *

**4******,****** * *************** *44*4* * 4*4*4*44*44*49998888838999 *****************

4*4*4*4*44* *********)‘**** 4*4*4*444*4* *4*44*4*4 ***********************************99Q9*******************~*********************************************************

938899999 ***********************************************************************97’7199)39q*****4******.-*********************** ***********************************777777999 9********°*********************-4****************4************s*********

—0.04 ~ 94*4 ******************** *******$***************************************

I I I I I I—0.04 —0.02 0 0.02 0.04

-~ uxFig. 2. LASA arraybeamresponsepatternwith 13 subarraysin A, B, C, D rings. Numbersrepresentbeamamplitudelossesindecibels.Asterisksshowareaof beamlossesgreaterthan 10dB. UX andUY arethe E—W andN—Scomponentsof inversevelo-city, in unitsof s km~.

responsepatternsof 1 Hz signalscomputedusing dB. Comparingthesetwo figuresone finds that theinstrumentsin 13 subarraysof ABCD rings andwith main lobehasabetterresolutionwithout B-ring sub-9 subarrays(ACD rings),respectively.Numbersin arrays.Notethat thediameterof the zero-lossareaisthesefiguresshowbeamlossesin decibels,and approximatelythe samein bothcases,about0.012asterisksrepresentthe areaof lossesgreaterthan10 s~~

1,which is approximatelyequivalentto three

19

Lk5~ ~C) ~~J’•i~X 3.913P~F0~F5CY =

0.04 99Q 9Q97 97 **** 4’~*******************************99877777777899***********9B76646666

9Q999Q0**** *****************t********************99.99998899*******s***9877666c556999999 ***** **************)‘******** *******************999**************99776666568

33 ~ 9 * ** * * * * ** * * * * ** * ) * * ** )‘ * ** * * * ** * ** * * ** * * * * * ** * * * ** * * * * ** * * * * * ** * * * *99877766666

9 ‘~* * * * ** * * * ~ * * * * ** * * ** * * ** * * * * ** * * * * ** ** * * * ** ** * ** * * * ** * * * * * ** ** * * * ** 99887777******************~ ************* *****************************************q9399 9************* ******** ** *********************** ********************************999

0.02 ******************~***********~*****************************s******************

****************t************999777777777993Q9 ******************************************************** *******Q997fS365c5566S~577q399*******************************

A **.~************************O87555)1LL333 3333334L455667789***************************************‘**)‘***********906 55)13332222222233 3114566789**************************

= 0.0 ***********4~’*************p~)133211OQ503Q93O0311233U573**************************

I ***************)‘***~**~‘**~55)1Ll332221 111 1122233145679***************************W **********************)‘***q47665)1L)333222222223331455699****************$**********

* * * * * * * * * * * * * * * * * * * ** * * * * * * 03 766 ~c 11)1 333333333 ~4556789 ** * * * ** * * * * * ** * * * * * * ** * ** * *

*********** ******4*****44******q0937~ 5666 5555�61567889******** ********************

* *4*4 * *4*4*4*)’ *4* )‘* *4* ********* *** *************** ***** *** **** * *******************

—0.02 ~

*********** *********************************** ******** ***************************

099888 900 * * * * * t * ** ** * * * * * * * * * ** * * * * * ** * * * ** ** * * * * ** * ** ** * ** ** * * *** * ** * * * * ** * * * * ** * *77773999**************4****************************************s***************9975”7778899**********************************************s******s**************99966668777399565356977804*4*44*44* * *4*0 q9*** ********** ************* *********************9q988 8

—0.04 656653673 0***********

I I I I I I I—0.04 —0.02 0.0 0.02 0.04

-~ uxFig. 3. LASA arraybeampatternwith 9 subarraysin A, C, andD rings.

beamspacings.Thereforeit is notsurprisingto see mometerscontributingto eacharraybeam.two or threeequal-amplitudebeamsin azivelgrams. Theoutputof the AZIVEL process,aswe seeinSince thebeamresolutionis slightly betterwithout Figs.4—11, is a seriesof computerprintoutsat theB-ring subarrays,we haveeliminatedthoseinstru- rateof oneazivelgramper datasecond.Aziveigramsmentsfrom theAZWEL process.Thereare also four areindexedby row andcolumnnumberson twoinoperativeseismometers,leavingatotal of 140 seis- sides.Whena peakis identifiedin theazivelgram,the

20

TIME 6 31 12SPACING OF SEAMSDELTA DX 0.0C433 SEC/KMDELTA DY — 3.00375 SEC/K~.CENTEF — 13,13

COLUPN NO — 1 2 3 4 5 6 7 8 9 10 11 12 13 IA 15 16 17 18 19 20 21 22 23 24 25 NON NO

NZ1PKKP 4 ‘4 3 2 2 1 1 3

2.lsec/deg 3 4 2 1 1 1 1 1

238M 3 3 4 5 5 4 2 2 2 2 2 2 2 5

06 3112

~ 199 ~~ ci ft

14II1 A 3 2 2 3 3 3 1 2 4 5 5 4 3 ‘4 5 8~° 39.77

4 3 2 2 2 2 2 1 2 3 5 5 4 3 4 4 4 902 L A/i P

o 14.99 3 3 3 1 2~ 3 2 3 3 4 4 4 4 3 3 4 4 4 10

~ 11.40 ~ /~j’lt\I\i~,A1f 2 3 44 3 2 2 2 4 4 4 4 3 3 3 3 4 4 3 3 11

~ C4

1.t9AA 2 4 4 3 2 3 4 5 4 3 3 2 4 4 5 4 3 3 1224.70

1 2 4 4 3 1 2 2 4 3 3 3 5 5 5 5 4 3 2 13C.,

44.80 1 1 5 6 4 3 2 1 2 3 5 5 5 5 5 4 3 2 144

19.20 ~ 3 2 3 7 8 7 4 2 2 3 4 5 5 5 4 2 2 2 2 15

~L~4 ~ 5 4 6 9/”~)7 4 1 4 5 5 5 4 3 3 3 3 3 16

17.12 ~ (~_~,/‘ 6 2 3 5 4 4 4 4 4 4 4 3 17

___________ I 5 6 7 8 5 3 2 4 4 3 ‘4 4 566 4 10

63105 63120 5 6 6 4 3 2 3 3 2 3 4 5(7 7)6 19

4 6 6 5 3 3 2 2 1 2 44 666 20

6 7 5 5 3 3 2 3 2 2 44 5 5 21

7 5 4 3 3 3 3 3 3 3 22

6 4 4 3 3 3 3 23

26

25

Fig. 4. Aziveigramsof PKKP arrivalat LASA from eventNZ1 (seeTable I). Thesubarrayrecordsectionon theleft showsthearrival time correspondingto theazivelgram.Theaverageslownessandazimuthof the amplitudepeakon theazivelgram(encircledarea)aregiven,togetherwith thearrival time,at thetop of thefigure.Thesecondarypeak(encircledby dashes)is discussedinthetext.

slownessandazimuthof that signalcanbe obtained thenproceedto computeAziveigramsfor thissimu-from the beamdirectory. latedsignalarrival.Fortunately,side lobesdid not

Generallyspeaking,signalside-lobesdo notpose causeanyproblemsin this study,but theconfirma-severemisidentificationproblems.However,detec- tion of slownessandazimuthanomaliesin PKKPwastion andidentificationof signalside-lobesbecomesan notpossiblewithout signalsimulation.importantissuewith the studyof scatteredseismic The signalanalysisrequiresthreethings: subarraywavesor signalprecursors.It isimportantto point waveforms,azivelgrams,andsimulatedazivelgrams.outthat side.lobepatternschangespatiallyandtem- Signalsare checkedfor clarity andconsistencywithporally,beforeandafter thearrival of the truesignal subarraywaveformplots,andcheckedagainstside(Changet al., 1976).We foundthatthe bestwayto lobeswith simulatedazivelgrams.Whenthesignalismonitor sidelobesis by seismicsignalsimulationwith confirmed,azivelgramsprovidetheneededsignalthe AZ1VEL process.In thesimulationprocessthe parameters.locationof the main signalis assigned.Thesimula- LASA arraybeamshavebeencalibratedfor 183tor will usethe instrumentdelaysassociatedwiththat teleseismicPlocations(ChiburisandAhner, 1973).beamlocation,andimplanta desiredsignalin the Travel-timeanomaliesfor PKIKP are publishedbydatabase(with or withoutnoise).Theprocesswifi EngdahlandFelix (1971).Theirpaperalso shows

T107 = 01 17 -

074471’S 07 77557DOLTS ‘I X=.~’.’3 540/65

441 5,~177 SEC/5l.CNIXR — 13,11

COLrlPS 10 1 2 I. 7 0 P 9 1’ 11 12 13 144 15 16 ‘7 18 19 22 21 22 23 24 25 NO~ NO

NZ2PKKP 2 1 1 1 1 1 3

2.5sec/deg I 2 2 2 1 1 1 1 1 1 44

23501 1 2 2 2 2 1 1 1 1 1 1 1 5

06 31131 1 2 2 2 2 1 1 2 2 2 1 1 N

RU ~ .P”.~’ AAAA~A 5 2 2 2 2 1 1 2 3 2 1 1 1 7~ 7.10 / V~V Vl\jIj 4~IV 1 0 1 1 2 2 1 1 1 2 3 3 2 1 1 2 N

~ 2o.90”h/”””~j~J\~/1/\t’\/\ 1 1 1 1 1 1 1 1 2 3 2 2 1 2 2 2 9

C2 ~ j~’4AJ~.~IV91A 1 I 1 1 1 1 1 2 2 3 2 2 2 1 1 2 2 2 10

o C3 ~ ~ 1 5 1 ‘ 7 1 2 3 2 3 2 2 1 1 2 1 9 1 2 11

~ 1 1 2 2 2 23 3 2 2 2 1 2 2 2 2 1 1 1212.06 ~ 1 2 1 1 2 2 2 2 1 2 2 1 2 2 1 1 13

27.06 ~ ~\L~J\.1’~A.tV’11 1 2 3 2 1 1 1 1 1 3 3 3 2 2 2 I 1 ~14

15

D4 ~1015 I I ,23?2’22122?222

63105 631201 3 ‘4 3 1 1 1 1 1 1 2 2 2 2 20

3 5 3 1 1 1 2 1 1 1 2 2 21

3 3 3 1 1 1 2 2 1 1 22

3 2 2 I 1 2 2 23

Fig. 5. Azivelgramof PKKP arrivalatLASA from event NZ2. SeeFig. 4 captionfor details. 24

25

0010:06 CF 65M720L72 ‘JO = . ‘07 2CZK~.D7.L1’10.’377 500K5.,C09OF — 33.13

CCL’~N.C~ 1 2 ‘ 17 .5 ‘4 0 11 12 13 114 15 16 17 19 19 20 21 22 23 24 25 000NO

NZ3PKKP 33’ 2 1 1 3

2.5sec/deg 2 3 3 , i 1 1 2

2350 2 1 3 44 2 7 1 1 2 3 3 5

013111 — 2 2 3 4 44 3 2 2 2 3 44 7 2 6

2 2 1 1 3 3 2 2 2 3 ‘4 44 3 2 2 7

~ ~ :111111223432233 :~ 29.91 I~’~-”\ \J,~v’~’JV~-1\j’~ 3 3 2 2 2 2 2 3 3 2 2 2 2 1 2 2 2 10

~ ~ 11

03 Mll\ JVI\t~—N 11.19 V V V 1 1 7 44 3 1 2 2 2 1 1 1 1 2 2 2 2 2 13

25.43 ~‘\0~/\1\1V’\~.’\~r’\ 2 2 .5 7 7 5 1’ 3 3 2 2 1 2 3 3 3 2 2 iN

~ ~ 44. 2 ‘. 71,. 6 .5 44 2 2 2 3 44 4 3 3 2 2 15

~ 44’47ll~l352733G444432 46N 03 IVI \I\116

14.36 1V

1 YVIJ 9 2 1. 1 2 2 3 3 2 3 5 (~ 6) 4 18

04 I1t1IAI1 I\A

16.18 IVVVVVVV 0 0 7 ‘ 6 3 1 2 2 2 1 3 5\�,15 19

I I ‘4 .4 .4 2 1 1 2 2 1 2 1 4 5 20

73105 73120 4 3 2 1 1 1 1 1 2 2 3 21

5 3 2 1 1 1 2 2 22

3 3 2 1 2 2 23

Fig. 6. Aziveigramof PKKP arrivalatLASA from eventNZ3. SeeFig. 4 captionfor details.

TIll! — 10 31 17 0SPACING OP OE&PSDELTA DX 0.18433 SEC/NIlDELTA YE 0.4J,~375 SEC/KI4.CENTEO 13.13

COLUMN 80 — 1 2 3 5 5 0 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 26 25 009 NO

NZ4PKKP ‘4 4’ 4 4 3 3 1 3

2.5sec/deg 2 4 4 s 4 3 3 2 1 5

2400 3 2 3 44 4 4 4 2 2 1 1 I 5

10 31123 3 1 2 4 4 3 2 2 2 2 1 1 6

2 3 2 2 2 3 3 2 3 3 3 2 2 1 1 7

IOU 2 2 2 2 1 2 2 2 3 4 5 3 2 1 1 2 8~ 9.68

6 4 3 2 2 2 2 2 2 3 4 4 3 2 1 2 2 9~ Cl~ 23.61

5 5 5 3 2 3 2 2 2 2 3 3 4 3 2 I 1 1 10~ C2~ 10.06

3 5 6 5 4 2 3 3 2 2 2 2 3 3 2 2 1 1 iici 033 5 6 6 5 2 2 2 2 2 3 1 3 3 3 2 2 1 12~ 6.97

2 2 5 7 7 6 4 2 2 2 3 3 2 3 3 3 3 2 I 13~ C4 2 5 7 9®11 9 5 3 4 3 2 3 3 3 2 2 1 16

17.302 3 5 9 9 8 6 3 3 3 3 3 2 3 3 3 3 2 14

~ 0124.82

2 2 5 8 11 11 9 6 3 2 2 2 3 3 3 2 2 2 2 15

.05ri 03 2 5 8 10 11 10 7 3 ‘4 4 2 3 4 4 3 3 2 17

10.373 6 8 9 9 8 5 2 4 4 2 3 4 5 9 II 18

0412.724 6 7 7 7 5 4 2 3 3 2 3 5/’69 19

103105 103120 35554432333 4~~6 20

1 8 5 4 3 4 ‘4 1 2 3 3 ‘4 5 21

5 ‘4 3 3 3 3 1 3 4 3 22

5 4 3 3 2 2 3 23

Fig. 7. Aziveigram of PKKP arrivalatLASA from eventNZ4. SeeFig.4 caption for details. 24

25

TIlE = 7 31 12 CSF410110 OF 50455DELTA OX ~.gc433 SEC/NIDOLTS DY — . 0375 D!C/K!1.CE!4TEO - 13.13

COLONS NO • 1 2 3 6 5 11 7 8 9 10 11 12 13 15 10 16 17 iN 19 20 21 22 23 24 25 0044 00

NZ5PKKP 8 8 7 6 5 4 2 3

2.3sec/deg 4 6 7 7 7 5 A 3 3 2 4

2370 ‘4 2 4 5 6 7 6 5 5 5 5 ‘1 3 5

073112 4 3 3 3 5 6 6 5 5 6 6 5 44 2 6

3 3 3 3 4 3 4 5 6 6 7 7 5 3 2 7

RU 3 2 3 3 3 3 3 3 5 5 6 7 5 8 2 2 85 4 2 3 3 3 3 3 4 4 5 6 5 5 3 2 2 9~ 16.66

8 5 ‘4 2 3 4 ‘4 4 3 3 3 5 5 5 4 3 2 2 10CI~

~., 24.786 6 6 4 3 3 ‘4 4 3 2 3 4 4 4 4 4 3 3 2 11~ 02

13.585 6 6 5 4 3 3 3 3 2 3 4 3 3 3 3 3 3 12

ci C33 5 6 7 7 7 6 4 3 3 3 3 2 2 2 3 3 3 2 13

~ 6.50L 04 3 6 8 10 10 9 7 5 4 3 2 2 1 1 2 2 1 14421.29

03 1 6 9 12~11 7 3 2 2 2 2 3 3 2 3 2 161 5 710121211 N 5 3 2 2 2 2 I 1 1 15~ Dl31.18D 02in 21.12 3 7 11’ 12 12 11 9 5 2 7 2 3 3 4 5 5 5 174 7 10 Il 11 9 6 2 3 3 2 3 4 6 7 7 IN20.06114 ~ rVP\jS...~_\f\Jt.f\ 4 6 8 N 7 5 3 1 2 2 2 3 5 7(8 1924.07

________ _________ 3 3 4 4 3 2 1 1 1 2 4 6 1 20

73105 73120 11222221 245 21

2 2 1 2 3 3 3 2 2 2 22

2 2 2 3 3 2 2 23

Fig. 8 Azivelgramof PKKP arrivalat LASA from eventNZ5. SeeFig. 4 captionfor details. 244

25

71221 = 3’ ‘2S’l.CCC.OF r’ CDL? I’’0— CC.5KM,3001T1’l’.777 JJ,:/K..:0 “‘“05= 13.13

00L’44’8 044 = - 2 ‘4 = 44 7 0 .5 1” 11 12 13 1’. 15 16 ‘7 18 19 27 21 22 23 24 25 004 NO

NZ6PKKP ‘4 7 0 5 3 2 1 3

2.lsecldeg 0 .5 7 -, .5 44 2 1 2 1 4

2370 0 ‘4 7 6 2 2 3 44 3 2 5

053112 = 1 2 0’ 5 5 4 3 3 5 6 5 3 2 6

2 ‘ 21 3 3 ‘4 2 3 4 .5 .5 5 3 3 7

HO ~ 2 1 7 2 1 21 1 1 2 44 5 6 .5 4 3 4 N~ 16.62 ~ A ~ ~r~’t..9,J\Ai -. 1 ‘4 2 2 1 1 2 2 44 5 5 44 14 3 3 9

~ 22.21~ ‘4’ 7 3 ‘42122,1’ 2 7 I ‘4 4 3 3 3 10

~C2 =“=“

~ 14.79 (‘.lV’~’54/ 2’3442 1 1 2 2 1 2 2 2 2 3 ‘4 44 3 3 II

~ 12.33 ~ ~ 11 1 1 2 3 4 A 3 12

~ 04 ~ 1 11 - ‘4 .4 2 . 2 1 1 2 4 4 ‘4 3 1327~59 ~

1A ,A=t’-~!~~:1 44 .5 ‘3 ‘3 5 3 2 1 1 1 2 3 4 3 14

~ 20.42 ~ ~VVVV\L\ 44 .5 ‘4 1 11 ‘4 6 ‘4 1 1 1 1 1 1 2 2 2 15

~~ ~ ?07(~2l,9l44’112734444

04 ~ .44 •1 ‘ ‘~ ‘ 3 5 0 3 2 2 ~ 5 6 6 1N13.99 I ~ , 0 3 2 2 3 1 25 7/7~ 19

53105 53,120 ‘4 = 344422221136(9 2P

3 I 2 2 1 1 1 1 ‘ 7 212 7 1 1 2 2 1 1 1 1 27

2 2 2 2 2 2 2 23Fig. 9. Azivelgramof PKKP arrivalatLASA from eventNZ6. SeeFig. 4 caption for details. 24

25

0033173 OF BEAMS

DECK OX = 0.001477 27506”DOlT) OX ‘4 0.00300 S00761.CSNTFB 13,13

COL7PS ‘0 — 1 2 3 ‘4 .5 44 7 7 9 10 11 12 13 14 15 16 17 18 19 20 21 72 23 24 25 548 80

NZ5 PKiKP1.7 sec/deg 2

16° 10 6 ‘4 9 7 7 9 3

071715 12 7 ‘1 .5 ‘4 4

43.76 144101: 6 : 91012111110111:121714,6121. ~ : 8

~ ~ /9J\J\J~XJ’\J.J\J\J\ 13 14 12 9 9 12 13 18 18 19 17 14 ‘11 .5 6 1 N

~ 1.213141311:8111211417171614410:6:3:4:57

I. J\f\j1 1f’V”\.J J1v4~\jJ’v\J~\jv~.5. 44 7 3 8 5 4 7 8 11 12 11 11 9 8 4 6 7 9 12

30.74 ~ ~

~&63 8 9101110 7 6 6 9 12 12 9 N ‘4

~2. 17 ~ ~ 3 3 7 7 7 9 11 17 12 8 6 8 10 13 12 11 18

9 911 12 9 6 5 6 ~11 9131112 19

_______ ________ 66777:6:6:5:4:3:3:35360

071705 071725 74

Fig. 10. Azivelgramof PKIKP arrivalat LASA from eventNZ5. SeeFig. 4 captionfor details. 25

24

CMI = 7 39 35 0SPICES 07 024480TELl’ 75 = 0.07170 SOC/KMPICA 00 = 1.502153FC/7~. CE 81443= 13.43

70L014 NO ‘ 1 2 8 I’ 5 6 7 3 IF 11 12 13 14’ 15 16 17 18 19 20 21 22 23 25 25 4DM ND

NZ5 P’P’ — BC 1

2.9 sec/deg 2

185° 22 16 35 53 69 77 96 3

~ ~ 7 ~2 ::2 ~ 8 ~:,~:�~:: : 2 1 6 6

46 72 52 90 ‘43 3’ 31 36 244 61142630

251.25 ~ ~‘\~v’l1~f’i/’\js~,./s‘441 ‘SC 33=’ 344 42 .41 502 ‘~6 41 20 ‘E 24 37 145 50 9

~94 88 ~ :: ~ : ~ ~ :~~::‘4~:: 4:3 s::, ~~:59:7:48i46:i36

~ ..4__5,J’\/=\f4J’l1j’\J~\I1 \J”\j~~_\,/J\..\f\J_=~13 2° 32 25 29 33 35 31 79 44 56 73 78 ‘0 5° 52 43 33 22 13

72 ‘4, 1::: 3136::3 27 0444 ‘4~~: ~ ~ ‘4:36:32:, 5:129

298,88 ~ ~f\flJ\vfli\.~J9v\‘41 13 ‘42 01 86 51 72 70 69 59 45 ‘40 40 07 444 45 39 ‘7

46 I 62 :. 5:259 ‘4~::~::~~ : ~ ~. ~: : ‘4:’4~::~~

222.04 r\j\JS.vJo\/8~.4=~Jl~/4J \f’\/~’~’v~’’v==’J\ ‘‘4 s2 93 ~7 05 9710103775032 54 76 21

I I 52 ~ 60 71 92 8-4 80 ‘C ‘4~ 22

073925 D7~945 48 50 50 69 71 ~ 49 23

Fig. 11. Azivelgram of P’P’ arrivalat LASA from eventNZ5. SeeFig. 4 caption for details.

that this set of anomaliesis the properone to useas 2.5 s deg.’ (0.0225skm~)will covera slownesstravel-timecorrectionsfor all corephasesin all azi- rangeof 1.83—3.17sdeg.~.The resultsfor PKKPmuths.Thiswill bediscussedin thenext section. signalparametersobtainedfrom aziveigramanalysis

aresummarizedin Table II.

TheobservedPKKP signalparametersin the table4. Theargumentfor scattering show a slownessrangeof 2.1—2.9 sdeg.’~and an

azimuthrangeof 229—245°.Travel timesarealsoSix aziveigramsandassociatedsubarraybeam consistent,from 31 m 11 sto 31 m 13 s. According

waveformsof PKKP from NovayaZemlyaeventsare to theoreticalcalculationsusingmodelBi , only thegiven in Figs.4—9. In eachfigure, the cursorline DF branchof the PKKPis possibleat this distance,acrossthewaveformplot showsthe time of the azivel- withp = 1.3 sdeg.~,0 = 186°andT= 31 m 12 s.gram, andtheslownessandazimuthshowncorre- Theobservedsignalsagreewith theoreticalTbutspondto thesignalparametersof the peakof the bothp and0 areanomalous.Furthermore,theob-azivelgramin the encircledarea.Numbersprintedin servedslownessof 2.5 sdeg.’ suggeststhis signalisazivelgramsare signalamplitudesin units of milli- a BC brancharrival insteadof theexpectedDF.microns.In theseareasthe encircledareasgenerally Theoreticallythe BC branchis a recedingbranchincludetwo beampositions.Thiskind of ambiguity which terminatesat the distanceof 66°.Thetravelisin accordancewith thebeamresolution.Recalling time of PKKPBC at 66°is 31 m 29 s, whichclearlythat theradiusof the zerobeamloss areain Fig. 3 is doesnotagreewith the observedtime. The observed0.006skm~,thesizeof the main lobecenteredat signal is also notthe GHbrancharrival proposedby

25

TABLE II

Parametersof core phasearrivalsatLASA from NovayaZemlya

Event Phase Slowness(s deg.’~) Azimuth (s deg.1) Traveltime

(miii., s)Average Range Average Range

NZ1 PKKP 2.7 2.5—2.9 238 235—250 31 12NZ2 PKKP 2.5 2.5—2.6 235 229—240 31 13NZ3 PKKP 2.5 2.5—2.6 235 229—240 31 11NZ4 PKKP 2.5 2.5 240 240 31 12NZ5 PKKP 2.3 2.1—2.5 237 233—240 31 12NZ6 PKKP 2.7 2.5—2.9 237 229—245 31 12

NZ1 PKiKP l.7(l.2)* 16 (6) 17 16(14)NZ5 PKiKP 1.7 (1.3) 16 (7) 17 15 (16)NZ6 PKiKP 0.5 (1.3) 30 (7) 17 16 (16)

NZ1 P’P’—BC 2.5 (2.7) 180 (186) 39 40(43)NZ2 P’P’—BC 2.6 (2.7) 141 (186) 39 40(43)NZ3 P’P’—BC 2.6 (2.7) 191 (186) 39 40(43)NZ4 P’P’—BC 2.9 (2.7) 185 (186) 39 40 (43)NZ5 P’P’—BC 2.9 (2.7) 185 (187) 39 35 (43)NZ6 P’P’—BC 2.9 (2.7) 185 (187) 39 35 (38)

* Thenumbersin parenthesesareslownesses,azimuthsandtravel timescalculatedfrom modelBi of JordanandAnderson(1974).

Bolt (1964)because,accordingto Qamar’s(1973) brancharrival with anextraordinarypath outsidetheKOR5 model,this branchterminatesat a distanceof diametralplane.68°with T= 31 m 20 s. Theobservedsignalis also Beforewe concludethat the signalis a scatterednot a seismicwavescatteredon or nearthe inner core wave from outsidethediametralplane,it mustbeboundary(ICB),becausescatteredwavesof this type shownthat theanomalousparametersare notcausedwill arrivelaterthanthe traveltime at theendof the by a heterogeneouscrustanduppermantle.Travel-branch(ChangandCleary,1978). timeanomaliesare knownto exist at LASA, butthe

Summarizingtheseresults,we founda consistent arrayhasbeencalibratedfor Pwavesandcorephases.signalfrom NovayaZemlyato LASA with T corre- - Masseet al. (1974)usedthe travel-timecorrectionsspondingto PKKPDFbut a slownesscloseto that of for LASA publishedby EngdahlandFelix(1971),forPKKPBC.The azimuthof arrival deviatesfrom the observationsof PKIIKP, PcPandPKiKP signalsfromdiametralplaneby about50°.No arrival was ob- the NZ5 event.AlthoughPcPandPKiKP arrive fromservedwith a slownessandazimuthappropriatefor a the front azimuthof 6°andPKIIKP arrivesfrom theDF branchsignal. This in itself is notsurprising,since reverseazimuthof 186°,the sameset of correctionsPKKP is expectedtobe of very low amplitude(cf. were foundto be adequate.Wealso usethis set ofEngdahl,1968),andasfar aswe are awarehasnot travel-timecorrectionsby EngdahlandFelix for allbeenobservedin this range.PKKPDFhasnotbeen core phasesin theAZ1VEL process.In orderto showdetectedat LASA from eventsin eastKazakhstan that the anomalousparametersof theseobserved(z~~-‘ 83°),althougha strongPKKPBC signalis often PKKP arereal, PKiKPand ~‘~‘BC for the six eventspresent(ChangandCleary,1978).Amplitudesof the wereexaminedwith azivelgrams.Fromthe six events,BC brancharrivalsare muchlargerandtheyare gen- we foundonly threePKiKP,becausetheamplitudeserallywell reported.Thehighamplitudesof BC of this phaseare rathersmall. It is also interestingbrancharrivals, thematchingslowness,andthe azi- that thereare manyunidentifiedarrivalsin the timemuthaldeflection,suggestthat the signalis a BC window from 16 m 30sto 18 m 30 s. Thesearrivals

26

will bestudiedin futurework. Signalparametersfor — — —

~~BC are veryconsistent,with largeamplitudesob- / \

served.Resultsof azivelgramanalysesof PKiKP and / B~/PPBC are shownin TableII. Also, in orderto showexamplesof thesesignals,azivelgramsandwaveforms / a

of PKIKPandP’P’BC aregiven in Figs. 10 and 11, / B’respectively.Wechosethis eventasthe example / / “..•

becauseit wasthe oneusedby Masseet al. (1974). ‘~

Fluctuationsofp and0 of PKIKP are large,butthey I —

arewithin the boundsof the arraybeamresolution — — S..-.

(andthe signal-to-noiseratio is small). Fluctuations _~. A

inp and0 of P’P’BC are verysmall. Furthermore, c—ø—-------—’ b

bothsignalsshowthat theyare arriving in thediametralplane.Notethat theanglesof emergenceofPKKPand~~BC are nearlyidentical,4 and8°,respectively.Theangleof emergenceof PKIKP is also4°,but from the front azimuth.Thisshowsthat if thetravel-timecorrectionsaregood for bothPP~cand Fig. 12.Sketchshowingthe Earth sectionedthroughandat

PKIKP theymust be good forPKKP also,andsothe right anglesto thediametralplanethroughNovayaZemlya

observedanomalousray parametersarenot causedby (A) andLASA (C). The curveAB’C (dash—dot)representsthepathof a PKKP arrival scatteredat B outof thediame-

impropercorrections. tralplane.B is the surfaceprojectionof B’. andthegreatcir-

Of theevents,NZ1 —NZ4 are from onelocation; clearcsAB andBChave angulardistancec anda respectively.

NZ5 andNZ6 from another.Thetwo locationslie on Note thepossibility of a mirror imageof AB’C in thelower

a diametralplanethroughLASA with anazimuthof hemisphere(seetext).

6°to the array,andhavea distanceseparationofabout2°.The observedT, p andU of NZS andNZ6PKKP arrivalsare,however,virtually identicaltothoseof NZ1—NZ4.As will be seen,our interpreta- c = 143°(usingmodelBi of JordanandAnderson,tion, that the observedPKKP signalshavefollowed 1974).The arcb = 60°is the distancebetweena pathof BC type afterhaving beenscatteredon un- NovayaZemlyaandLASA. Then,sincea, b andc aredersidereflectionat theCMB outsidethe diametral all greatcircle distances,the angleACB canbeob-plane,providesan explanationof this remarkable tamedby simple sphericaltrigonometry.phenomenon. TheangleACB turnsoutto be 140°,correspond-

ing to anazimuthof approachat LASA of 226°,which is verycloseto theobservedvalues.Also,

5. Themechanismof scattering accordingto model B 1, the slownesscorrespondingto a PKBC of 143°is 2.3 sdeg.~,whichis againvery

Figure 12 isa sketchshowingthe Earthsectioned similar to thatobserved.Thelocationof B is aboutacrossandat right anglesto the diametralplane 60°S,l34°E,about21°from the expectedPKKPthroughNovayaZemlyaandLASA. Point A is the reflectionpointat 76°S,95°E.location of the NovayaZemlyaevents,andpointC Onceit is establishedthat theobservedPKKPis athe locationof LASA. TheassumedPKKPpathis scatteredBC branch,it is not necessaryto adhereshownby dash—dotlines,with scatteringtaking place rigidly to theassumptionthat thetwo PK legsareon the CMB at thereflectionpoint B’. PointB is the equal.UsingmodelBi, we find thatPK combinationssurfaceprojectionof B’. Assumethat thegreatcircle of 144 and142°,145 and141°,etc.,will also satisfyarcsc anda areequal,but that B is off the diametral the observedtraveltimes,sothatsomefocusingofplaneby anunknownamount.Thenif bothc anda scatteredenergycanoccur.The questionof the am-arePKBC, the observedarrival time is satisfiedif a = plitudesof the arrivalswill nowbeexamined.

27

6. The amplitude problem F I I 44

PKKP TRAVEL TIMES

Doornbos(1980,fig. 7) haspointedoutthat the Model

amplitude(relativeto PcP)of PKKP from the NovayaZemlyaeventcorrespondingto NZ5 in TableI ismuchtoohighto be eitherPKKPDFor PKXPBC in - C’

the diametralplane.He examinesour suggestion h1~

(ChangandCleary, 1978)that the arrival is adeflectedBC branch,andconcludesthat “scatteringbya slightly roughCMB is clearly insufficientto pro-ducetheamplitudes”.He also examinesthe possibil-ity that the NovayaZemlyaPKKP andits precursorsare producedby asymmetricreflectionat a dippingCMB. Althoughsucha modelresultsin amplitudeswhich arecloseto thoseobserved,henotesthat “Theimpliedmodel of CMB is, however,embarrassing,sincethe dip is about7°andin order to build upthe 29~ -amplitude,this dip mustextendover manywave- B

lengths;as a consequence,radialperturbationsofCMB would exceedseveralkm; this hasnotbeenob-servedfrom our data”.

The difficulty is that the amplitudeof PKKP isat I I I I I I 44

leastaslargeaswould be expected(accordingto the 20 40 60 80 00 120 140 lBS 100

standardmodels)if it werea PKKP arrival in the DISTANCE

diametralplaneat a distancegreaterthanabout70°, Fig. 13. PKKP branchesfrom model B 1 of JordanandAnder-o . . son (1974).Thesymbols(~)indicatethearrival timesand

ratherthanat 60 . A scatteringor asymmetricreflec-slownessesobservedatLASA from the two NovayaZemlya

tion modelthereforerequiresthat virtually all the sites.C’ is apoint on theBC curvewith approximatelytheenergywhich would normally appearasPKKPBC sametime and slownessastheseobservations.

at 70°or moreis deflectedto LASA at 60°.Since theangleof deflectionat the presumedreflectionpointis about15°,it is difficult to envisage,at leastin thelight of presentknowledge,a plausibleCMB topog- NovayaZemlyasites,althoughdiffering by about2°raphywhich would producesuchan effect, in distancefrom LASA, give PK,KP arrivalshaving the

We showedin the previoussectionthat theob- sametravel-timeandslowness.In termsof our model,servedPKKP parametersareconsistentwith a model this implies that the arrivalsfrom bothsitesare ofof scatteringat a reflectionpoint on theCMB, such PKc’(PKc’) type,anddiffer only in the scatteringthat the directandscatteredlegsof PK(KP) both angleat t1~eCMB reflection point. As may beseencover anangulardistanceof about 143°.In Fig. 13, from Fig.13, it implies also thatPK(KP) arrivalsatthe point C’, at a distanceof 74°on thePKKPBC LASA correspondingto non-scatteredPKKP pathscurve,correspondsto the (non-scattered)path below C’ on thePKKPBC curve arenot of higham-PKKPC, forwhich both legsPKc, subtendan angleof plitude. A reasonableinferencefrom this isthat143°.Thenfor all scatteredpathsfor which bothPK PKc’ terminatesat a high-amplitudecuspon thelegs subtenda143°angle,it follows thatalthoughthe CMB, andsimilarly thatwavesbackscatterednearepicentraldistanceandazimuthwill vary accordingto the cusparerefocusedat thePK~’distanceon thethe scatteringangleat thereflection point, the time Earth’ssurface,thusgiving rise to theobservedandslownessof thearrival remainsthe sameasthat PK(KP)phaseat LASA. It is of interest,therefore,of C’. to enquirewhetherhigh-amplitudearrivals,associ-

We returnnow to the questionwhy thetwo atedwith the inferredPKc’ cuspat theCMB, have

28

beenobservedneartheC endsof PKKPBC orPKPBC. tenceof a regionof relativelylargetopographyon theIt shouldberemarkedthat the positionsof neither CMB is also requiredby theinterpretation.

PKKPC nor PKPC are well determined,but varycon-siderablyfrom modelto model.PKKPc in model Biis, asindicatedin Fig. 13,at a distanceof about66°, 7. Conclusionswhereasin model 1066Bof Gilbert andDziewonski(1975) it occursat 76°.Thepositionsof PKPC for We havearguedthat the anomalousslownessesandthe two modelsare at about159 and153°,respec- azimuthsof PKKPsignalsat LASA from Novayatively. Zemlyaeventscanbeinterpretedasaneffect of back-

Wearenot awareof anysystematicstudyof scatteringfrom theundersideof the CMB, thussup-PKKPBC amplitudesfor distanceslessthan80°.Nor portinga similar interpretationof precursorsto PKKPis muchamplitudedataavailablefor PKPBC,partly observedat LASA from the sameevents(Changandbecauseof confusionaboutthe possiblepresenceof a Cleary, 1978).Although theLASA observationsseemGH branchin the sameregionof the time—distance at presentto beuniqueasfar asthe amplitudesof thefield,aspostulatedby Bolt (1964).Buchbinder arrivalsare concerned,the smallerprecursorsto(1971,figs. 7, 10, 17) hasplottedthe amplitudesof PKKP found at NORSARby Doornbos(1980),andPKParrivals,which heidentifies asbelongingto the his accompanyingtheoreticalanalysisof the effect ofGHbranch,thoughin all probabilitytheyarePKPBC a slightly roughCMB, also give strongsupportto thissignals.Theamplitudesincreasefairly rapidly to a interpretation.As Doornbospointsout, the largepeakatabout154°.AnderssenandCleary(1980), variability in the amplitudesof the PKKP precursorfrom a studyof ISC bulletindata,haveinferredthat observationsis indicativeof considerablelateralvan-thePKPBC branchterminatesat about 153°,aspre- ation in thetopographyof the CMB. The “scattereddictedby model 1066B.This would suggestthat PKKP” datawhich wehaveexaminedheresuggestPKKPc is at about76°,reasonablycloseto thevalue thepresenceof a regionof relativelylargetopographyof 74°indicatedby our simplecalculations.Of course on theCMB, eventhoughthe possibilityexiststhatit isthe positionof the cuspon the CMB, ratherthan theamplitudesof thephasehavebeenenhancedbyat thesurface,which is the crucial factor.Although focusingof PK at theCMB nearthe endof the BCmoredirect evidenceis desirable,theavailabledata branch.are thusnotinconsistentwith thepresenceof a high- The chiefaim of our previouspaper(Changandamplitudecuspin the vicinity of C’ in Fig.13,which Cleary,1978)wasto establishtheidentityof the ob-in turn would imply anincreasein velocity gradient servedPKKP precursorsasscatteredsignals.Whilewejust abovetheinner coreboundary. notedthat theazimuthsof approachof the precur-

In spiteof this possibility,it remainstruethat the sors were similar to thatof thePKKP signal,wedid

PKKP datarequire,on our interpretation,a regionof notexaminethe implicationsof this featurein anystrongscatteringon the CMB. In this connectionit detail.Weare at presentstudyingthe precursorsagainshouldbenotedthat, for reasonsof symmetry,scat- usingtheAZ1VEL program,andintendto interpretteredsignalsof thesametypewould beexpectedto theresultsin the light of whatwe havelearnedfromarriveat LASA from an azimuthof about 130°(i.e., the presentstudy.from thelowerhemispherein Fig. 12).ThePKKPazivelgramsdo, in fact, displayminorpeaksat aboutthis azimuth(encircledby dashedlinesin the figures) Acknowledgementsfor all eventsexceptNZ2. The slownesses(~3.5 sdeg.~)correspondingto thesepeaksare,however, We thankDrs.D.J.Doornbos,R.R.Blandford,I.S.greaterthanthoseexpectedon ourmodel,sowe are SacksandK.J. Muirheadfor their commentson theunableto identify them asPK(KP). It seemssafe to original manuscript.Thestudywas commencedwhileconcludefrom this absenceof symmetrythat, oneof us(JRC)wasvisiting TeledyneGeotech,andalthoughtheamplitudesof thescatteredsignalsmay waspartly supportedby anH.O. WoodFundgranthavebeenenhancedby focusingeffects,the exis- from theCarnegieInstitutionof Washington.

29

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Doornbos,D.J.,1974.Seismicwavescatteringnearcaustics: Masse,R.P.,Flinn, E.A., Seggelke,R.M. andEngdahl,E.R.,observationof PKKP precursors.Nature(London),247: 1974.PKIIKP andtheaveragevelocity of theinner core.352—353. Geophys.Res.Lett., 1: 39—42.

Doornbos,D.J, 1980.Theeffect of a roughcore—mantle Qamar,A., 1973.Revisedvelocity in theEarth’score.Bull.boundaryon PKKP. Phys.EarthPlanet.Inter., 21: 351— Seismol.Soc.Am., 63: 1073—1106.358. Rodean,H.C., 1979. ISCOrigin Times for Announcedand

Doornbos,D.J.andVlaar,N.J., 1973. Regionsof seismic PresumedUndergroundNuclearExplosionsatSeveralwavescatteringin theEarth’smantleandprecursorsto Test Sites.Rep.UCRL-52882,LawrenceLivermore Lab-PKP.Nature(London), Phys.Sci., 243: 58—61. oratory,Livermore,CA.