Table Of Content

1.01 Deep Earth Seismology: An Introduction and Overview AMDziewonski,HarvardUniversity,Cambridge,MA,USA BARomanowicz,UniversityofCalifornia,Berkeley,CA,USA;Colle`gedeFrance,Paris,France ã2015ElsevierB.V.Allrightsreserved. 1.01.1 DevelopmentsfromtheLateNineteenthCenturyuntiltheEarly1950s 2 1.01.2 Developmentsfrom1950stotheEarly1980s 4 1.01.3 From1980toPresent:TheEraofTomographyandBroadbandDigitalSeismicNetworks 9 1.01.4 CurrentIssuesinGlobalTomography 14 1.01.4.1 ResolvingPowerofDatasetsUsedforConstructingModels 16 1.01.4.2 TheoreticalAssumptions 17 1.01.4.3 RobustFeaturesofCurrentGlobalMantleModelsandTheirImplications 19 1.01.4.4 StabilityofthePlanetary-ScaleHeterogeneities 23 1.01.4.5 TheNeedforConsiderationofMoreCompleteModelingofMantleFlow 24 References 25 ApplicationsofseismologytothestudyoftheEarth’sinterior inthemeasurementsofnormal-modeandlong-periodsurface areonlyalittleover100yearsold.Itstoolsindeterminingthe waves. Two chapters are devoted to the computation of syn- propertiesofinaccessibleEartharethemostpowerfulamong thetic seismograms in the presence of lateral heterogeneity, all geophysical methods. The principal reasons are the avail- suitable for the case of body waves (see Chapters 1.05 and abilityofnatural(earthquakes)orcontrolled(explosionsand 1.06).Significantprogresshasrecentlybeenmadeinthecom- vibrators) sources of elastic waves and their relatively low putationofsyntheticseismogramsina3-DEarthusingnumer- attenuationwithdistance.Seismologicalmethodsspansome ical methods. A review is given in Chapter 1.07. With the six orders of magnitude in frequency, and the depth of an deploymentofdenseregionalarraysofbroadbandseismome- investigatedstructuremayrangefromafewmetersinengineer- ters,anotherareaofrapidprogresshasbeenthatoftheadap- ingapplicationstothecenteroftheEarth.Progressinseismol- tation of methodologies first developed in exploration ogy has been achieved through developments on several seismology to the case of fine structure imaging of the crust fronts: theory, instrumentation, and its deployment, as well and upper mantle at larger scale. These approaches are ascomputationalresources. describedinChapter1.08forpassive-sourceapplicationsand Even though the studies of earthquakes and the Earth’s inChapter1.15forthecaseofactivesources.Therealizationof structure are closely related, the two subjects are often dis- theimportanceofanisotropyintheEarthhasledtotheoretical cussed separately. This volume is devoted to the Earth’s and methodological developments (see Chapters 1.09 and structure and Volume 4 to studies of earthquakes. Neverthe- 1.18). Note that the issue of anisotropy is also discussed in less,therelationshipisintimate.Forexample,itispossibleto Chapter 1.19 in the context of the inversion of surface-wave formulate an inverse problem in which earthquake locations data. Inverse methods, in particular in the context of global are sought simultaneously withthe parameters ofthe Earth’s andregionaltomography,arediscussedinChapter1.10. structure, including three-dimensional (3-D) models (see InthesecondpartofVolume1,reviewsofthestatusofour Chapter1.10). knowledge on the structure of the Earth’s shallow layers are Inthepast25years,importantprogresshasbeenmadeon presented, starting with a global review of the Earth’s crustal several fronts: (1) the development of broadband digital structure(seeChapter1.11).Duringthelastdecade,therehas instrumentation (see Chapter 1.02), which has allowed the beenrapiddevelopmentinusingtheEarth’snoiseasasource construction of digital seismic databases of unprecedented ofthesignal. Areviewofthesedevelopments is presented in quality at both the global and the regional scales; (2) the Chapter 1.12. Two chapters discuss regional structure in the development of powerful data analysis tools, made possible oceans:Chapter1.13formid-oceanridgesandChapter1.14 byevermoreefficientcomputertechnology;and(3)theoreti- forhot-spotswells.Chapter1.18presentsresultsofstudying cal progress in the forward and inverse computation of the anisotropy in subduction zones with particular attention effects ofstronglateral heterogeneity onseismic-wave propa- devotedtotheflow-inducedpreferentialorientationofolivine gation. The combination of these factors has led to much crystals. Finally, two chapters are devoted to the results of improvedimagesofstructureattheglobalandregionalscale, regional experiments: upper-mantle studies using data from oftenhelpedbytheinclusionofconstraintsfromothertypesof portable broadband experiments (see Chapter 1.16) and data,primarilyfromthefieldsofmineralphysicsandgeody- crustal studies, specifically in Europe, from high-resolution namics.Thisvolumeisthusdividedintofourparts.Thefirst long-rangeactive-sourceexperiments(seeChapter1.17). part principally covers seismic instrumentation, theoretical The third part of this volume concerns the Earth’s deep developments,andseismicdataanalysistechniques.Chapter structure, divided into its main units: the upper mantle (see 1.03 discusses the state of the art in the computation of the Chapter1.19);thetransitionzoneandupper-mantlediscon- Earth’snormalmodes,whileChapter1.04describesprogress tinuities (see Chapter 1.21); regional tomography of TreatiseonGeophysics,SecondEdition http://dx.doi.org/10.1016/B978-0-444-53802-4.00001-4 1 2 DeepEarthSeismology:AnIntroductionandOverview subducted slabs, with particular attention given their stagna- tionatthebottomofthetransitionzone(Chapter1.20);the lowermantle;andthehighlycomplexD00regionatthebaseof the mantle (Chapter 1.22) as well as the Earth’s core (see Chapter1.23).Chapter1.24isdevotedtothesubjectofscat- tering in the Earth and Chapter 1.25 to that of attenuation. Finally,thefourthpartofthisvolumecomprisestwochapters, inwhichconstraintsontheEarth’sstructurefromfieldsother thanseismology,mineralphysics(seeChapter1.26)andgeo- dynamics(seeChapter1.27),arediscussed. Thisvolumeaddressesvariousaspectsof‘structuralseismol- Figure1 Thehistoricalfirstrecordingofateleseismicevent:an ogy’anditsapplicationstootherfieldsofEarthsciences.Notall earthquakeinJapanrecordedinPotsdamonatiltmeterdesignedbyvon thesubjectsarecoveredincomparabledetail,eventhoughthe Rebeur-Paschwitz.Theearlyseismogramshaddifficultywithdamping completeness of the coverage was the initial objective of the thependulummotionandmadephaseidentificationdifficult. editors.Comparedtothe2007editionofVolume1ofTreatise ReproducedfromvonRebeur-Paschwitz(1895)Horizontalpendal- on Geophysics, this edition contains four new (Chapters 1.02, BeobachtungenaufderKaiserlichenUniversitats-Sternwartezu 1.12, 1.18, and 1.20). Most of the other chapters have been Strassburg1892–1894.GerlandsBeitra¨gezurGeophysik2:211–536. significantly updated, except Chapters 1.14, 1.15, and 1.16, whicharereprinted‘asis’fromthe2007edition. 1000 In what follows, we briefly describe the developments in 1 seismology from the end of the nineteenth century until the Tg=90s present,withthemainemphasisonthedevelopmentofinstru- on Ts=15s mdreivnetnatisocinenacned.Aitnsadcecpoluonytmoefntth,ebehciastuosreysoefissmeiosmloogylogisyacadnatbae- nificati 100 found,amongothers,inAgnewetal.(2002).Wealsopresent ag Slope=3 M -1 our point of view, which some may consider controversial, specifically on current issues in global tomography and inter- 10 pretationofthe3-Dmodelsnotdiscussedinanyofthechapters. 0.001 0.01 0.1 1 10 We justify bringing these issues forward because of our belief Frequency (Hz) that interpretation of tomographic results in terms of mantle dynamicsdoesnotmatchtherobustnessofmodelsbuiltusing Figure2 Plotoftheground-motion(amplitude)responseofaWorld- WideStandardizedSeismographNetwork(WWSSN)stationwitha datathathavegoodresolutionatalldepthsinthemantle. seismographfreeperiod(T)of15sandgalvanometerwithafreeperiod s (T)of90s.Thesegmentbetweenthesetwoperiodshasaflatvelocity g response,characteristicofbroadbandseismometers.Theresponsein 1.01.1 DevelopmentsfromtheLateNineteenth moderninstrumentsisshapedelectronically;atypicalFDSNstationhasa CenturyuntiltheEarly1950s flatvelocityresponsefrom5Hzto360s.ReproducedfromWielandt E(2002)Seismicsensorsandtheircalibration.In:BormannIP(ed.) Thetheoreticalbeginningsofseismologymaybetracedtothe IASPEINewManualofSeismologicalObservatoryPractice,vol.I, eighteenth- and nineteenth-century studies of elasticity and pp.1–46.Potsdam:GeoForschungsZentrumPotsdam,Chapter1.06. propagation of elastic waves in solids. Lord Kelvin provided thefirstnumericalestimateoftheperiodofthefundamental vibrational mode ( S ) in 1863, but the development of the seismograph with controlled damping was built by Wiechert 0 2 proper theory for a homogeneous sphere had to wait nearly in 1904. Soon afterward, Galitzin (1914) developed an 50 years (Love, 1911). Lord Rayleigh solved the problem of electromagneticseismographsystem,wherethemotionofthe propagationofsurfacewavesinanelastichalf-spacein1877. seismometer’s pendulum generated an electric current Thisprecededthefirstmechanicalseismographs,whichwere by motion of a coil in the magnetic field. This current was, developedinthe1880s.Originally,theseismographshadvery in turn, carried to a galvanometer; the rotation of the low sensitivity and were used for the recording of local galvanometer’scoilinamagneticfieldwasrecordedonphoto- earthquakes.Thehistoryofglobalseismologybeginswiththe graphicpaperbyabeamoflightreflectedfromamirrorattached recordingofanearthquakeinJapanon19April1889byvon tothecoil.Theresponseofthesystemdependedonthesensi- Rebeur-Paschwitz. He associated a disturbance recorded on a tivityandfreeperiodoftheseismometerandofthegalvanom- tiltmeter,usedtostudytheEarth’s tides,withthereportsofa eterandtheirdamping.Whilethesystemwasmorecomplex,it greatearthquakeinJapan.Figure1showsacopyofthisrecord- allowed for much more flexibility in selecting a desired ingaspublishedinNature(vonRebeur-Paschwitz,1889,1895). response. Figure 2 shows the response of the seismograph– Theearlyseismographsweremechanicalpendulumswithno galvanometersystemandgivesanideaofthewayitcouldbe damping,otherthanfriction.Theirmagnifications(theratioof shaped by the choice of different free periods of the system’s theamplitudeonaseismogramtotheactualgroundmotion) components. With gradual improvements, the seismometer– wereverylow,andbecauseofthelackofdamping,therecords galvanometer system, recording on photographic paper, was were very oscillatory and it was difficult to distinguish commonly used during the following 60–70 years, when it the arrivals of different phases. An improved mechanical wasgraduallyreplacedbydigitalsystems(seeChapter1.02). DeepEarthSeismology:AnIntroductionandOverview 3 With the improvement of the recording systems technol- 14 ogy,phaseidentificationbecameeasier,anditwaspossibleto P identify P-arrivals (primary) corresponding to compressional 12 waves, S-arrivals (secondary) corresponding to shear waves, andL-arrivals,sometimescalled‘themainphase,’correspond- 1) −s 10 ingtosurfacewaves.Thesurfacewavescausedsomeconfusion m because therewasalso atransverse motion, not predicted by k Rayleigh. It was not until 1911 that Love showed that trans- city ( 8 S Seismic velocities vEearrstehl.y polarized surface waves can propagate in a layered Velo 6 GJeuftferenybse-rBgullen Progress in the first decade of the twentieth century was rapid.Someclassicalproblemssuchascomputationandinver- 4 sion of travel times for the velocity structure were solved by B C D E F G 0 (Benndorf,1905,1906;Herglotz,1907;Knott,1899;Wiechert, 0 1000 2000 3000 4000 5000 6000 1907;Zo€ppritz,1907)independentlydevelopedequationsfor Depth (km) theamplitudeofreflectedandtransmittedwavesatthebound- Figure3 ComparisonoftheseismicvelocitymodelsofGutenbergand arybetweentwoelasticmedia. Jeffreys,bothbuiltinthe1930s.Theprincipaldifferencebetweenthe As regards the Earth’s structure, there was a paper by modelsisthepresenceofthelow-velocityzoneintheGutenbergmodel Oldham (1906), in which he proposed the existence of the andthestructureneartheinner–outercoreboundarywherethelow- Earth’score,althoughtherehasbeensomeconfusioniniden- velocityzoneintheJeffreysmodeliserroneousandthevelocityincrease tification of phases: what he thought to be a delayed S-wave intheinnercoreislargerthanintheGutenbergmodel.Withthe wasactuallyanSS(e.g.,Schweitzer,2007).Gutenberg(1913) exceptionofthetransitionzone(400–650kmdepth),themodernmodels properlyidentifiedreflectionsfromthecore–mantleboundary arenotverydifferent.ReproducedfromAndersonDL(1963)Recent (CMB)anddeterminedtheradiusofthecorequiteaccurately, evidenceconcerningthestructureandcompositionoftheEarth’smantle. andJeffreys(1926)showedthatthecoreisliquid.Mohorovicˇic´ PhysicsandChemistryoftheEarth6:1–129. (1910) discovered the boundary between the crust and the upper mantle, thus beginning the era of studies of the crust andlithosphere,whichgreatlyacceleratedafterWorldWarII. andGutenbergwastheexistenceofalow-velocityzoneinthe Thefirstglobalseismographicnetworks(GSNs)wereestab- depthrange100–200kmintheuppermantle.Therewerevery lishedintheearlyyearsofthetwentiethcentury.Thefirstone hotdebatesonthisissue;itcannowbeexplainedbythefact wasdeployedbyJohnMilneinvariouscountriesoftheBritish thattheyuseddatafromtectonicallydifferentregions;thereis CommonwealthwiththesupportoftheBritishAssociationfor apronouncedlow-velocityzoneinthewesternUnitedStates, theAdvancementofScienceandeventuallyconsistedof30sta- butnotundertheEurasianshieldregions. tions(Adamsetal.,2002).TheJesuitNetworkwasestablished With internal reflections and conversions of waves at the soon afterward, with a particularly large number of instru- boundaries,seismologistsdevelopedasystemofphaseidentifi- mentsintheUnitedStates,butalsoincludingstationsonall cationthatreflectsacombinationofthetypesofwaves(PorS), continents(UdiasandStauder,2002).Withaglobalcoverage theregioninwhichtheypropagate(KandIfortheP-wavesin sufficienttolocatelargeearthquakes,informalbulletinswere theouterandinnercore,respectively;PKIKPdesignatesaphase published using the location method developed by Geiger thattravelsasPinthemantle,Pintheoutercore,andPinthe (1910, 1912), which (with many modifications) is still used innercore)andtheboundaryatwhichtheywerereflected(cfor today. In 1922, the International Seismological Summary CMB and i for the inner–outer coreboundary).A shear wave (ISS), with international governance, was established under reflectedoncefromthefreesurfaceatthemidpointofitspathis Professor Turner of the University of Oxford with the charge designatedbySS;highermultiplereflections,likeSSSorSSSSS, toproduce‘definitiveglobalcatalogs’from1918onward. canbeobservedbysamplingalargevolumeoftheEarthalong TheslowprogressinunravelingtheEarth’sstructureculmi- theirpaths.Forearthquakeswithafinitefocaldepth,theP-and nated in the 1930s with the discovery of the inner core by S-wavestravelingupwardfromthesourcehavedesignationofp Lehmann (1936) and the compressional velocity, shear or s; following reflection at the surface, they represent the velocity, and density models by Gutenberg (1913), Jeffreys so-calleddepthphases(e.g.,pPandsP);thetravel-timediffer- (1926),andBullen(1940).TheGutenbergandJeffreysvelocity encebetweenthearrivalofpPandPstronglydependsonfocal models are shown in Figure 3; except for the details of the depth. upper-mantle structure, these models are very similar to the Figure 4 shows examples of various seismic phases, and modern ones. The low-velocity zone above the inner–outer Figure5isthegraphicrepresentationofthetraveltimesasa core boundary in the model of Jeffreys illustrates the some- functionofdistancecomputedbyJeffreysandBullen(1940) timesunfortunatetendencyofseismologiststointroducephys- forthemodelofJeffreys(1939).Itisremarkablethatthissetof icallyimplausiblefeaturesinthemodelinordertomatchthe tables, including predictions for different focal depths, was data;Jeffreysneededtoadda2sdelaytomatchtheinner-core calculated using a manual mechanical calculator! The data travel times and accomplished it by inserting this feature, usedbyJeffreyswereextractedfromtheISS,theprecursorof whichisimpossibletoreconcilewiththechemicalandphys- the International Seismological Centre (ISC), which (with icalpropertiesofmaterialsinthisdepthrange(Birch,1952). internationalfinancialsupportandgovernance)resumedthe TheotherimportantdifferencebetweenthemodelsofJeffreys ISSrolein1964andcontinuesuntiltoday. 4 DeepEarthSeismology:AnIntroductionandOverview ScS P S P P pP S sP SKS pPcP PP SKP SP Solid SKKS inner core SS sPS PPP Fluid outer core SKKP PPS PKIKP Solid mantle PKJKP SSS P rays pPcPSKKP SKPPKP S rays Figure4 Examplesofseismicraysandtheirnomenclature.Themostcommonlyidentifiedphasesusedinearthquakelocationarethefirstarriving phases:PandPKIKP.ReproducedfromSteinSandWysessionM(2003)AnIntroductiontoSeismology,EarthquakesandEarthStructure.Oxford: Blackwell,ISBN:0865420785. Bullen(1949)dividedtheEarthintoanumberofconcen- surface waves (Rayleigh and Love) in a layered medium tricshells,designatedbylettersfromAtoF;inthisdivision,the with an arbitrary number of layers over a half-space. It lower mantle was designated by the letter D”. when Bullen involved multiplication of matrices, one for each layer, recognized that the deepest 150km of the lower mantle had changing the wave number for a fixed frequency such as an anomalously flat velocity gradient, he divided the region to matchtheboundaryconditions(vanishingofstresses)at DintoD0andD00.Morerecently,andnotentirelycorrectly,D00 the free surface. Because of the enormous amount of calcu- cametosignifythestructureinthedeepest300km,orso,of lations to be performed, it required application of an elec- the lower mantle, which is characterized by a still-growing tronic computer, and its application opened yet a new era collection of structural and compositional complexities (see in seismology. The Haskell’s matrix method has been Chapter1.22). adapted to other problems in seismology, such as calcula- It was recognized relatively early that the dispersion of tionofsyntheticseismogramsusingthe‘reflectivitymethod’ surface waves was different in the continents than in the (FuchsandMu¨ller,1971).Electroniccomputerswereatfirst oceans, with an indication that the oceanic crust was signifi- very expensive and rare, and it wasnotuntilthe1960s that cantlythinner.Computingthedispersionofsurfacewaveswas they became generally available at universities (Haskell algebraicallyandnumericallydifficult;thecorrectformulasfor worked at the Air Force Cambridge Research Laboratories). the dispersion of Rayleigh waves in a layer over a half-space Surface-wavedispersionbegantobestudiedintensivelyin wereformulatedbyStoneley(1928),andthecaseofthetwo the1950sprincipallyattheLamontGeologicalObservatoryof layersoverahalf-spacecouldbesolvedonlyforaveryspecific Columbia University, primarily by Ewing and Press, who setofparameters. observed mantle waves in the 1952 Kamchatka earthquake, identifying arrivals from R6 to R15 (see Chapter 1.04) and measuringtheirgroupvelocitiesuptoaperiodof500s(Ewing 1.01.2 Developmentsfrom1950stotheEarly1980s andPress,1954).Regionalmeasurementsofsurface-wavedis- persionwereinitiatedbyPress(1956).AmonographbyEwing It must have been frustrating for seismologists not to be et al. (1957) summarizes the state of the knowledge on able to use information about the Earth’s structure con- seismic-wave propagation in layered media at that time. tained in the most prominent features of the seismograms: Ewing and Press also developed a true long-period seismo- the dispersed surface waves. This changed when Haskell graph,whichwascapableofrecordingmantlewavesevenfor (1953) adapted to the case of elastic media the method moderately sized earthquakes. This instrument was deployed first proposed by Thomson (1950) in the acoustics case. at10globallydistributedInternationalGeophysicalYearnet- The approach made it possible to compute dispersion of workstationsoperatedbyLamont. DeepEarthSeismology:AnIntroductionandOverview 5 similarfreeoscillationperiodsforthegravestmodesbutdiffer atshorterperiodsby1–2%. When the greatest instrumentally recorded earthquake of 20May1960occurredinChile,seismologistshadallthetools (theory,computers,andinstrumentation)neededtomeasure and interpret its recordings. Three back-to-back papers in a 1961 issue of the Journal of Geophysical Research reported the firstcorrectmeasurements offreeoscillationperiods: Benioff etal.,Nessetal.(1961),andAlsopetal.(1961).Theobserva- tions were made on strainmeters in Isabella, N˜an˜a, and Ogdensburg; seismographs at Pasadena; and a gravimeter at the University of California, Los Angeles (UCLA). All three studies agreed in mode identification and found that their periods were very close to those predicted by the existing Earthmodels(Pekerisetal.,1961a).Moredetailedstudiesof thespectrarevealedthattheyaresplit;theeffectoftheEarth’s rotationwasshowntoexplainthiseffect(BackusandGilbert, 1961; Pekeris et al., 1961b). Thus, normal-mode seismology wasborn.Progressinthetheory,particularlyconsideringthe effect of lateral heterogeneities and mode coupling, would extend over decades to come. First attempts at inversion of normal-modedatawerenotparticularlysuccessful;thedensity model of Landisman et al. (1965) was flat throughout the lowermantle,implyingeitherastrongradialheterogeneityor animmenselysuperadiabatictemperaturegradient. BackusandGilbert(1967),BackusandGilbert(1968),and BackusandGilbert(1970)providedtheformalbackgroundfor consideration of geophysical inverse problems, and even though their own experiments with inversion of normal- modeperiodsforthevelocitiesanddensity,usingasubsetof normal-mode data, were discouraging (two very different modelswerefoundfittingthedatanearlyexactly;Backusand Gilbert, 1968), the idea of resolving kernels and trade-offs becamepartofthegeophysicalterminology. Figure5 Traveltimesofseismicphasesforthesurfacefocusas Becauseseismicmethodswereconsideredessentialindis- computedbyJeffreysandBullen(1940).ReproducedfromJeffreys criminating between earthquakes and nuclear explosions HandBullenKE(1940)SeismologicalTables,p.50.London:British AssociationfortheAdvancementofScience. (Berkner et al., 1959), an intensive observational program, calledVelaUniform,wasinitiated.Oneofitscomponentsof great significance to studies of the Earth’s interior was the Itisnotoftenthatamistakeleadstofavorableresults,but World-Wide Standardized Seismograph Network (WWSSN), thiswasthecasewiththefreeoscillationsoftheEarth.Benioff consisting of a set of three-component short-period and (1958)reportedanoscillationwithaperiodof57minseenin three-component long-period seismographs, with identical the record of the 1952 Kamchatka earthquake. Even though responses,exceptformagnification,whichdependedonlocal thisobservationwaseventuallyattributedtoanartifactinthe noiselevels.Atitspeak,theWWSSNconsistedof125stations functioningoftheinstrument,itstimulatedthetheoreticaland (Figure6),withdistributionlimitedbygeographyandpolitics: computational research needed to calculate eigenfrequencies therewerenostationsintheSovietUnion,China,andpoorly forarealisticEarthmodel.Someofthecalculationspreceded developedareasinAfricaandSouthAmerica.Thenovelaspect Benioff’sreport(Jobert,1956,1957),buttheeffortsofPekeris ofWWSSNwasitsstandardizedresponseandcentralizedsys- and Jarosch(1958) andTakeyuchi (1959)wereclearlymoti- tem of distribution of copies of seismograms. Individual sta- vatedtoexplaintheobservedperiod.Thesecalculations,using tions were sending the original seismograms to a central thevariationalapproachandtheJeffreys–BullenEarthmodel, location, where they were microfilmed, using a very high- predicted the period of S to be 52min and that of T resolution camera, and then returned to the stations. 0 2 0 2 43.5min.NeitherwascloseenoughtoBenioff’s‘observation.’ Aseismologistcouldrequestcopiesofseismogramsforapar- The modern approach was developed by Alterman et al. ticular date and receive them as either microfilm chips or (1959), who recast the system of three second-order partial photographicenlargementstotheoriginalsize.Severallarger differentialequationsintoasystemofsixfirst-orderdifferential institutions had a blanket order on all the records. This data equations, thus removing the need for differentiation of the accessibility represented major progress with respect to the elastic constants, and allowed the use of standard numerical earlierprocedures,whereonehadtorequestcopiesofseismo- methodstoobtainthesolution.TestsusingtheGutenbergand grams from individual stations, which greatly limited and the Jeffreys–Bullen models showed that they predict very delayed the research. WWSSN functioned for 20–25 years, 6 DeepEarthSeismology:AnIntroductionandOverview Figure6 MapofthestationsofWWSSNestablishedintheearly1960s,followingrecommendationsofBerkneretal.(1959).CourtesyofUSGeological Survey. slowly declining in the quality and number of stations; it ceased functioning in the late 1980s when data from new digitalstationsbecameavailable. Anotherdevelopmentofthe1960swastheintroductionto seismologyofdigitalrecording,greatlyfacilitatingresearchand the development of massive, computerized data-processing methods. One such facility, the large aperture seismic array (LASA), shown in Figure 7, was built in the mid-1960s in Montana.Itconsistedofsix‘rings,’foratotalof21subarrays, each with 25 short-period seismometers emplaced in bore- holes,toimprovethesignal-to-noiseratio.Thedataweretele- meteredinrealtimetoacentrallocationinBillings,Montana, where they were processed for detection of a signal. Major scientific discoveries were made with this tool, particularly when weak signals were involved, for example, observations ofreflectionsfromtheinnercore.Inpracticalterms,thearray did not meet the expectations; the site response across this 200km aperture array varied so much that the signals had Figure7 Configurationofthelargeapertureseismicarray(LASA) limited coherency and signal enhancement by stacking was andanexpandedviewoftwoofitssubarrays.ReproducedfromStein not as effective as expected. A somewhat smaller array was SandWysessionM(2003)AnIntroductiontoSeismology,Earthquakes installedafewyearslaterinNorway(NORSAR);elementsof andEarthStructure.Oxford:Blackwell,ISBN:0865420785. this array are still active and have been upgraded recently to modern, high-dynamic-range band–band response. Most of thearraysoftheInternationalMonitoringSystem(IMS)used Oneoftheimportantresultsobtainedfromtheanalysisof forseismicdiscriminationpurposeshaveanapertureofonly array data was the detection of upper-mantle discontinuities severalkilometers;becauseonthatscale,thecoherencyat1Hz (Johnson, 1967), confirming the result predicted by experi- frequencycanbeachieved. mental petrology that there should be two discontinuities at DeepEarthSeismology:AnIntroductionandOverview 7 pressuresandtemperaturescorrespondingtodepthsofabout turnedouttobeanartifactofcouplingbetweentoroidaland 400and650km,respectively(Birch,1952). spheroidal (Russakoff et al., 1998) modes, but the requisite Surface-wavestudiesblossomedinthe1960s.Atfirst,mea- theory to consider this effect was not available until 1984. surements of dispersion involved rather simple ‘analog’ GilbertandDziewonski(1975)presentedtwomodelsbased methods,suchasthepeak-and-troughapproachtomeasuring on measurements of eigenfrequencies of 1064 modes and phaseandgroupvelocities.Someveryimportantresultswere mass and moment of inertia for a total of 1066 data. They obtained in this way, such as the Canadian Shield study of derived two models 1066A and 1066B, with the first being Brune and Dorman (1963). Digital analysis, however, was smooththroughthetransitionzoneandthelatterincluding soontotakeover.Manualdigitizationofanaloguerecordings, the400and660kmdiscontinuities;bothmodelsfitthedata WWSSNdatainparticular,becameeasier,andwithincreasing equallywell. availabilityofcomputersanddecreasingcostofcomputations, Atthe1971GeneralAssemblyoftheInternationalUnion varioustechniquesweredeveloped,forthemostpartinvolving ofGeodesyandGeophysics(IUGG)inMoscow,theneedfor applicationsoftheFouriertransform.Withthedevelopmentof a reference Earth model was stated, and a Standard Earth thefastFouriertransform(FFT)algorithm(CooleyandTukey, Model Committee formed under the chairmanship of Keith 1965),thecostoftime-seriesanalysisdeclineddramatically;a Bullen. The Committee appointed several subcommittees, reviewbyDziewonskiandHales(1972)summarizesthestate includingonefortheradiusoftheCMB:therewerediscrep- of the art at the beginning of the 1970s. Some of these ancies on the order of 10km at the time. The value recom- methods,suchasthemultiplefiltrationtechniquetomeasure mended by the subcommittee was 3484km (Dziewonski group velocity dispersion, residual dispersion measurements, and Haddon, 1974), which – within 1km – withstood the and time-variable filtration, are still in use today. The 1960s trial of time. Hales et al. (1974) proposed that the seismic have also seen the first studies of intrinsic attenuation by velocitiesanddensityinthestandardEarthmodelshouldbe Anderson and Archambeau (1964), who developed partial describedbyalow-orderpolynomial,withdiscontinuitiesat derivatives (kernels) for Q from mantle-wave attenuation. the appropriate depths. Dziewonski et al. (1975) con- Also,thefirststudiesoflateralheterogeneity wereconducted structed such a model, named parametric Earth model usingthe‘purepath’approach(Tokso€zandAnderson,1966). (PEM), which satisfied the normal-mode, travel-time, and Seismic experiments with controlled sources were con- surface-wave data. The novelty of this model was that, in a ductedinamulti-institutionalmode.Oneofthelargestexper- single inversion, different structures were obtained for the imentswas‘EarlyRise’carriedoutintheJulyof1966.Aseries continental and oceanic crust and upper mantle. The of38explosionsinLakeSuperiorofupto5tonsofoutdated normal-mode periods predicted by these two models torpedoeswereusedasthesourceofsignalsrecordedbyhun- (PEM-O and PEM-C) averaged in 2/3 and 1/3 proportion dreds of seismometers – deployed by 12 governmental and were constrained to match the observed periods and tele- academicgroups–spreadingradiallyinalldirections.Signals seismic travel times but separate data sets for continental wererecordedasfaras2500km,reachingteleseismicdistances and oceanic surface-wave dispersion. The differences and providing a detailed profile of P velocity under a conti- betweenthesetwomodelsceasedatthe400kmdiscontinu- nental upper mantle (Green and Hales, 1968). A detailed ity, at which depth they became identical with the average review of crustal and upper-mantle studies with controlled Earth model, PEM-A. sourcesisprovidedinChapter1.11. ThedrawbackofthePEMandallthepreviousmodelswas With a large new data set, particularly measurements of that they did not consider the physical dispersion due to previously unreported periods of long-period overtones pro- anelasticattenuation.Forasignalpropagatinginanattenuat- videdbytheanalysisoffreeoscillationsgeneratedbythe1964 ing medium to be causal, the wave with higher frequencies Alaskan earthquake and recorded at WWSSN stations must propagate with higher velocities. Thus, waves with a (DziewonskiandGilbert,1972,1973),studiesof1-Dstructure frequencyof1Hzwillpropagatemore rapidlythanwavesat enteredanewera.Theresolutionofthisdatasetwassufficient a frequency of 1mHz. In order to reconcile the seismic data toconstrainthedensityprofileinthemantleandthecore;this thatspan3.5ordersofmagnitude,itisnecessarytoconsider turned out to be quite consistent with the behavior, in the the frequency dependence of elastic parameters. This was lower mantle and outer core, of a homogeneous material pointed out by Liu et al. (1976). The Preliminary Reference under adiabatic compression. Jordan and Anderson (1974) Earth Model (PREM) constructed by Dziewonski and were the first to combine the normal-mode and travel-time Anderson(1981),followingtheideaofparametricrepresenta- data,includingdifferentialtravel-timedata. tion,consideredthefrequencydependenceusingtheassump- Numerous additional overtone data were obtained by tionthatQisconstantinthebandfrom0.3mHzto1Hz. Mendiguren (1973) and Gilbert and Dziewonski (1975) by ThisnecessitatedobtainingtheradialprofilesofQmandQk; introducingphaseequalizationtechniques,suchas‘stacking’ fortunately,therewerenewmeasurementsavailableofnormal- and‘stripping.’Thesemethodsrequiretheknowledgeofthe modeandsurface-waveQ(SailorandDziewonski,1978)such sourcemechanismtopredicttheproperphaseforeachseis- that a formal inversion for Q could be conducted simulta- mogram to be considered; this in itself turned out to be a neously with the inversion for the velocities and density. It challenginginverseproblem.DziewonskiandGilbert(1974) wasrecognizedearlierthattoexplaintheobservedattenuation derivedthe spectrumofall sixcomponentsof themoment- of radial modes, which contain a very high percentage of rate tensor as a function of time for two deep earthquakes compressional energy (97.5% for S ), it was necessary to 0 0 (Brazil, 1963; Colombia, 1970). For both events, they introduce a finite bulk attenuation; Anderson and Hart detected a precursive isotropic component. Eventually, this (1978)preferredtoplaceitintheinnercore,andSailorand 8 DeepEarthSeismology:AnIntroductionandOverview Figure8 ThePreliminaryReferenceEarthModel(PREM)ofDziewonskiandAnderson(1981).Inadditiontothedistributionofseismicvelocitiesand density,PREMcontainsalsothedistributionofattenuationoftheshearandcompressionalenergy.FromthewebsiteofEdGarnero. Dziewonski (1978) thought that Qk is finite in the upper Twodigitalseismographicnetworkswereestablishedinthe mantle;unfortunately,theradialmodesdonothavethereq- mid-1970s.OnewastheInternationalDeploymentofAcceler- uisite depth resolution. Figure8 showsthe seismicvelocities ometers(IDA;Agnewetal.,1976,1986),consistingof18glob- anddensityasafunctionofradius;theattenuationinPREMis allydistributedLaCoste–Romberggravimeterswithafeedback discussedinChapter1.25.AnothernovelaspectofPREMwas systemthatalloweddigitizationofthesignal.Itwasdesignedto its radial anisotropy between the Moho and 220km depth. recordverylong-periodwaves,includingthegravestmodesof This feature, at first suspected to be an artifact of the nonli- freeoscillationsoftheEarth:onesamplewastakenevery20s nearityoftheinverseproblem,hasbeenconfirmedbyglobal (laterchangedto10s).Onlytheverticalcomponentofacceler- tomographic studies (e.g., Ekstro€m and Dziewonski, 1998). ation was recorded and the word length was 12 bits; the ThePREMmodelremainstothisdayawidelyused1Drefer- dynamicrangewas,therefore,ratherlimitedbutstillconsider- ence model for seismological studies based on long period ablygreaterthanthatofanaloguerecordings.Thesensitivitywas data. For P wave studies, especially those concerning core set such that the scale was saturated for the first surface-wave sensitive phases, model AK135 (Kennett et al., 1995), which arrivalsforeventswithmagnitude7.0,orso,dependingonthe wasbuilttoconstrainaglobaldatasetoftraveltimesfromthe station’s distance from the source and radiation pattern. The ISCbulletins,isoftenpreferredasa1Dreferencemodel.Itisto IDA network was operated by the Scripps Institution of benoted,however,thatthismodelincludesa36kmcontinen- Oceanography,andthecentrallycollecteddatawerefreelydis- talcrust,andcannotbeusedforwaveformmodelingoutsideof tributed to the academic community. This later became the continentalareas. future standard in global seismology. An early illustration of The1970shavealsoseenthebeginningofseismictomog- thepowerofsuchaglobalarraywastheanalysisofthesplitting raphy;twostudiespublishedsimultaneously(Akietal.,1977; ofthegravestmodesoffreeoscillationsgeneratedbythe1977 Dziewonski et al., 1977) addressed the problem on different Sumbawaearthquake(Bulandetal.,1979;M only8.4). w scales: regional and global. Aki et al. solved for 3-D velocity Theothernetworkconsistedoriginallyofnineinstallations structure under the NORSAR array, while Dziewonski et al. calledSeismicResearchObservatories(SROs)andfiveAbbre- obtained a very low-resolution model of 3-D velocity viated Seismic Research Observatories (ASROs). The SROs perturbations in the entire mantle and showed significant used borehole instruments, with significantly suppressed correlationbetweenvelocityanomaliesinthelowermostman- wind-generated noise levels, particularly on horizontal com- tleandthegravestharmonicsofthegravityfield.Thestudyof ponents. The ASROs were placed in underground tunnels or Dziewonskietal.(1977)wasmotivatedbyapaperbyJulian mine shafts and the seismographs were protected from the and Sengupta (1973) who noticed that travel times for rays effectsofchangingpressureandtemperature.Theinstrumen- bottoming in the same region of the mantle tend to show tationisdescribedbyPetersonetal.(1976).Thisnetworkwas similar residuals. They interpreted this result qualitatively as designedformonitoringtheNuclearTestBanTreatyandhigh the evidence of lateral velocity variations; no modeling was sensitivity was the main objective. In order to increase the presentedinthatpaper.Thefirstcontinental-scale3-Dmodel dynamic range, the signal was sharply band-pass-filtered, so oftheuppermantle,underNorthAmerica,waspublishedby thatatverylongperiods(>200s),theresponsetoacceleration Romanowicz(1979). was falling as o(cid:2)3,while itwas flat for theIDA instruments. DeepEarthSeismology:AnIntroductionandOverview 9 Even so, the SRO and ASRO stations were able to produce from 1976 till present, and this catalog is available online usefulmantle-waverecordsforeventswithmagnitudegreater (http://www.globalcmt.org). than about 6.5. Later, the network was augmented by Mastersetal.(1982)measuredcenterfrequenciesofspec- 10WWSSNstations,withtheanalogueoutputamplifiedand tral peaks of the fundamental spheroidal mode from hun- digitizedat16-bitanalogtodigitalconverters.Theentiresys- dreds of IDA records and discovered that there are spatially tem was called Global Digital Seismographic Network distinctpatternsinthefrequencyshiftswhenplottedatloca- (GDSN). There was no general data distribution system, but tions of the poles of the great circles corresponding to the data for selected dates were available upon request from the pathsbetweenthesourceandthereceiver.Byfittingspherical AlbuquerqueSeismologicalLaboratory. harmonics (even degrees only, because of the symmetry), Until then, a seismic station typically comprised a set of theseauthorsrealizedthatthepatternisdominatedbyspher- seismometers with either ‘long-period’ or ‘short-period’ ical harmonics of degree 2, an observation also made from responses,orsometimes,aswasthecasefortheWWSSN,one great-circling surface waves by Souriau and Souriau (1983). ofeach.Thissetupwasdesignedatthetimeofanaloguerecord- Figure9showsthepatternoftheshiftsofspectralpeaksand ingtoavoidthemicroseismicnoisepeakaround6–7speriod, zero line of the best-fitting spherical harmonics of degree 2 which would have made it impossible to digitize all but the forfourgroupsof0S‘modeswithdifferentrangesofdegree‘. largestearthquakesignals.Withdigitalrecording,andthepos- Note that the modes with the lowest ‘ show a different sibility of filtering out the microseismic noise by post- pattern than the remaining three groups. Our current under- processing, this traditional instrument design became standing of this fact is that the low-‘ modes sample deeper unnecessary. structure (lower mantle) than the higher-‘ groups that pre- Averyimportantdevelopmentinseismicinstrumentation dominantlysampletheuppermantle.Theauthorsperformed took place in Germany in the mid-1970s. An array of a new aparametersearch,inwhichtheychangedtheradiiofashell kindofinstrumentswithdigitalrecordingwasdeployednear inwhichtheanomalyislocated.Thebestvariancereduction Gra¨fenberg(HarjesandSeidl,1978).Itusedanovelfeedback wasfortheanomalyplacedinthetransitionzone.Thelasting seismograph (Wielandt and Streckeisen, 1982). The system importance of this chapter is that it demonstrated that het- was rapidly recognized for its linearity and large dynamic erogeneity of very large wavelength and sizeable amplitude range within a wide band of frequencies – hence the name ((cid:3)1.5%)exists intheEarth’sinterior. ‘broadband.’ The Gra¨fenberg array’s central station had been Followingthedevelopmentofawaveform-fittingtechnique colocatedwiththeSROboreholestationGRFO,andthecom- thatallowedthesimultaneousmeasurementofphasevelocity parisons were very favorable for the broadband instruments, and attenuation along a great-circle path (Dziewonski and which were capable of reproducing different narrowband Steim,1982),WoodhouseandDziewonski(1984)developed responsesusingasingledatastream.Thistypeofinstrumenta- an approach to the interpretation of waveforms that could tionbecamethepatternforfuturedevelopments(e.g.,Chapter extract both the even and the odd harmonic coefficients of 1.02). lateral heterogeneity as a function of depth. This method Inadditiontothedevelopmentsininstrumentation,thelate involves the ‘path average approximation,’ sometimes called 1970s saw important theoretical developments, related to the PAVA.Theseismogramsarerepresentedasasumofallnormal asymptoticpropertiesandcouplingofthenormalmodes.Exam- modes (spheroidal and toroidal) up to a certain frequency plesofsuchdevelopmentsarepapersbyWoodhouseandDahlen o . For a given great circle, each mode is assumed to be max (1978),Jordan(1978),andWoodhouseandGirnius(1982). affected by the average deviation from reference structure along the great-circle path (which is sensitive only to even- order harmonics) and along the minor-circle path (sensitive tobothevenandoddharmonics).Theeffectofthegreat-circle 1.01.3 From1980toPresent:TheEraofTomography pathcanbemodeledbyashiftineigenfrequencyofthemode; andBroadbandDigitalSeismicNetworks theeffectoftheminor-arcstructureismodeledbyafictitious shiftoftheepicentraldistanceforthatmode;thisshiftdepends Thedatafrombothglobalnetworksofthe1970sledtoresults onbothevenandoddpartsofthestructure.Woodhouseand thatdemonstratedtheneedfordevelopmentofaglobalnet- Dziewonski (1984) processed about 2000 mantle-wave seis- work that would better satisfy the needs of basic research in mogramsfrom theGDSNand IDA networksand obtained a seismology; three studies are noteworthy. A robust method, modeloftheuppermantle(Moho–670km),M84C,usingas which uses entire segments of (digital) seismograms, was basis functions spherical harmonics up to degree 8 for hori- developedtoobtainreliablemechanismsofearthquakeswith zontal variations and Legendre polynomials as a function of magnitude 5.0 and above (Dziewonski et al., 1981; Ekstro€m radiusuptodegree3.Figure10showsamapofshearvelocity etal.,2005).Inaddition,themethodrefinesthelocationofthe anomaliesatadepthof100km;therewasnoaprioriinfor- source,whichforaneventoffinitesizeneednotbeidentical mationusedonthelocationoftheplateboundaries.Correc- with the hypocenter determined from the first arrivals ofthe tions were made for crustal thickness, recognizing only the P-waves.ThistopicisdiscussedatlengthinChapter4.16.The continental and oceanic structure. An experimental model, reason why the subject is brought up here is that in most M84A,obtainedwithoutapplyingcrustalcorrections,showed aspectsofusingwaveformanalysisforthepurposeofdrawing that not taking crustal thickness into account may result in inferencesabouttheEarth’sstructure,itisnecessarytoknow mapping artificial anomalies at depths as large as 300km. thesourcemechanism.Theso-called‘centroidmomenttensor’ ModelM84Chadastrongdegree-2anomalyinthetransition method has now been applied to over 40000 earthquakes zone,confirmingtheresultsofMastersetal.(1982). 10 DeepEarthSeismology:AnIntroductionandOverview Figure9 Mapsoftheobservedfrequencyshiftsofthefundamentalspheroidalmodesforfourrangesoftheordernumbersasreportedby Mastersetal.(1982).Thefrequencyshiftsareplottedatthepolesoftheindividualgreat-circlepaths.Itindicatesthepresenceofverylargewavelength– velocityanomaliesintheEarth’sinterior;thepreferredlocationofthesourceoftheanomalyshowninthefigureisthetransitionzone.However, thefrequencyshiftsforthelargestwavelengthpanel(topleft;modes S and S)showadifferentpatternthanintheotherthreepanels(modesfrom 0 8 0 9 S to S );thelow-‘datahavethegreatestsensitivityinthelowermantle.ModifiedfromMastersG,JordanTH,SilverPG,andGilbertF(1982) 0 27 0 34 AsphericalEarthstructurefromfundamentalspheroidalmodedata.Nature298:609–613. Figure10 Shearvelocityanomaliesatadepthof100kminthemodelM84CofWoodhouseandDziewonski(1984).Thescalerangeis(cid:3)5%and theresolvinghalf-wavelengthis2500km.Plateboundariesareshownasthinyellowlines.Exceptforthecorrectionforcrustalthickness,therewasno otheraprioriinformationincludedintheinversion.Thus,theresultdemonstratesthatthewaveforminversionapproachisabletodistinguishtheslow velocitiesunderthemid-oceanridgesandancientcratons,forexample.

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1.01 Deep Earth Seismology: An Introduction and Overview AMDziewonski,HarvardUniversity,Cambridge,MA,USA BARomanowicz,UniversityofCalifornia,Berkeley,CA,USA;Colle`gedeFrance,Paris,France ã2015ElsevierB.V.Allrightsreserved. 1.01.1 DevelopmentsfromtheLateNineteenthCenturyuntiltheEarly1950s 2 1.01.2 Developmentsfrom1950stotheEarly1980s 4 1.01.3 From1980toPresent:TheEraofTomographyandBroadbandDigitalSeismicNetworks 9 1.01.4 CurrentIssuesinGlobalTomography 14 1.01.4.1 ResolvingPowerofDatasetsUsedforConstructingModels 16 1.01.4.2 TheoreticalAssumptions 17 1.01.4.3 RobustFeaturesofCurrentGlobalMantleModelsandTheirImplications 19 1.01.4.4 StabilityofthePlanetary-ScaleHeterogeneities 23 1.01.4.5 TheNeedforConsiderationofMoreCompleteModelingofMantleFlow 24 References 25 ApplicationsofseismologytothestudyoftheEarth’sinterior inthemeasurementsofnormal-modeandlong-periodsurface areonlyalittleover100yearsold.Itstoolsindeterminingthe waves. Two chapters are devoted to the computation of syn- propertiesofinaccessibleEartharethemostpowerfulamong thetic seismograms in the presence of lateral heterogeneity, all geophysical methods. The principal reasons are the avail- suitable for the case of body waves (see Chapters 1.05 and abilityofnatural(earthquakes)orcontrolled(explosionsand 1.06).Significantprogresshasrecentlybeenmadeinthecom- vibrators) sources of elastic waves and their relatively low putationofsyntheticseismogramsina3-DEarthusingnumer- attenuationwithdistance.Seismologicalmethodsspansome ical methods. A review is given in Chapter 1.07. With the six orders of magnitude in frequency, and the depth of an deploymentofdenseregionalarraysofbroadbandseismome- investigatedstructuremayrangefromafewmetersinengineer- ters,anotherareaofrapidprogresshasbeenthatoftheadap- ingapplicationstothecenteroftheEarth.Progressinseismol- tation of methodologies first developed in exploration ogy has been achieved through developments on several seismology to the case of fine structure imaging of the crust fronts: theory, instrumentation, and its deployment, as well and upper mantle at larger scale. These approaches are ascomputationalresources. describedinChapter1.08forpassive-sourceapplicationsand Even though the studies of earthquakes and the Earth’s inChapter1.15forthecaseofactivesources.Therealizationof structure are closely related, the two subjects are often dis- theimportanceofanisotropyintheEarthhasledtotheoretical cussed separately. This volume is devoted to the Earth’s and methodological developments (see Chapters 1.09 and structure and Volume 4 to studies of earthquakes. Neverthe- 1.18). Note that the issue of anisotropy is also discussed in less,therelationshipisintimate.Forexample,itispossibleto Chapter 1.19 in the context of the inversion of surface-wave formulate an inverse problem in which earthquake locations data. Inverse methods, in particular in the context of global are sought simultaneously withthe parameters ofthe Earth’s andregionaltomography,arediscussedinChapter1.10. structure, including three-dimensional (3-D) models (see InthesecondpartofVolume1,reviewsofthestatusofour Chapter1.10). knowledge on the structure of the Earth’s shallow layers are Inthepast25years,importantprogresshasbeenmadeon presented, starting with a global review of the Earth’s crustal several fronts: (1) the development of broadband digital structure(seeChapter1.11).Duringthelastdecade,therehas instrumentation (see Chapter 1.02), which has allowed the beenrapiddevelopmentinusingtheEarth’snoiseasasource construction of digital seismic databases of unprecedented ofthesignal. Areviewofthesedevelopments is presented in quality at both the global and the regional scales; (2) the Chapter 1.12. Two chapters discuss regional structure in the development of powerful data analysis tools, made possible oceans:Chapter1.13formid-oceanridgesandChapter1.14 byevermoreefficientcomputertechnology;and(3)theoreti- forhot-spotswells.Chapter1.18presentsresultsofstudying cal progress in the forward and inverse computation of the anisotropy in subduction zones with particular attention effects ofstronglateral heterogeneity onseismic-wave propa- devotedtotheflow-inducedpreferentialorientationofolivine gation. The combination of these factors has led to much crystals. Finally, two chapters are devoted to the results of improvedimagesofstructureattheglobalandregionalscale, regional experiments: upper-mantle studies using data from oftenhelpedbytheinclusionofconstraintsfromothertypesof portable broadband experiments (see Chapter 1.16) and data,primarilyfromthefieldsofmineralphysicsandgeody- crustal studies, specifically in Europe, from high-resolution namics.Thisvolumeisthusdividedintofourparts.Thefirst long-rangeactive-sourceexperiments(seeChapter1.17). part principally covers seismic instrumentation, theoretical The third part of this volume concerns the Earth’s deep developments,andseismicdataanalysistechniques.Chapter structure, divided into its main units: the upper mantle (see 1.03 discusses the state of the art in the computation of the Chapter1.19);thetransitionzoneandupper-mantlediscon- Earth’snormalmodes,whileChapter1.04describesprogress tinuities (see Chapter 1.21); regional tomography of TreatiseonGeophysics,SecondEdition http://dx.doi.org/10.1016/B978-0-444-53802-4.00001-4 1 2 DeepEarthSeismology:AnIntroductionandOverview subducted slabs, with particular attention given their stagna- tionatthebottomofthetransitionzone(Chapter1.20);the lowermantle;andthehighlycomplexD00regionatthebaseof the mantle (Chapter 1.22) as well as the Earth’s core (see Chapter1.23).Chapter1.24isdevotedtothesubjectofscat- tering in the Earth and Chapter 1.25 to that of attenuation. Finally,thefourthpartofthisvolumecomprisestwochapters, inwhichconstraintsontheEarth’sstructurefromfieldsother thanseismology,mineralphysics(seeChapter1.26)andgeo- dynamics(seeChapter1.27),arediscussed. Thisvolumeaddressesvariousaspectsof‘structuralseismol- Figure1 Thehistoricalfirstrecordingofateleseismicevent:an ogy’anditsapplicationstootherfieldsofEarthsciences.Notall earthquakeinJapanrecordedinPotsdamonatiltmeterdesignedbyvon thesubjectsarecoveredincomparabledetail,eventhoughthe Rebeur-Paschwitz.Theearlyseismogramshaddifficultywithdamping completeness of the coverage was the initial objective of the thependulummotionandmadephaseidentificationdifficult. editors.Comparedtothe2007editionofVolume1ofTreatise ReproducedfromvonRebeur-Paschwitz(1895)Horizontalpendal- on Geophysics, this edition contains four new (Chapters 1.02, BeobachtungenaufderKaiserlichenUniversitats-Sternwartezu 1.12, 1.18, and 1.20). Most of the other chapters have been Strassburg1892–1894.GerlandsBeitra¨gezurGeophysik2:211–536. significantly updated, except Chapters 1.14, 1.15, and 1.16, whicharereprinted‘asis’fromthe2007edition. 1000 In what follows, we briefly describe the developments in 1 seismology from the end of the nineteenth century until the Tg=90s present,withthemainemphasisonthedevelopmentofinstru- on Ts=15s mdreivnetnatisocinenacned.Aitnsadcecpoluonytmoefntth,ebehciastuosreysoefissmeiosmloogylogisyacadnatbae- nificati 100 found,amongothers,inAgnewetal.(2002).Wealsopresent ag Slope=3 M -1 our point of view, which some may consider controversial, specifically on current issues in global tomography and inter- 10 pretationofthe3-Dmodelsnotdiscussedinanyofthechapters. 0.001 0.01 0.1 1 10 We justify bringing these issues forward because of our belief Frequency (Hz) that interpretation of tomographic results in terms of mantle dynamicsdoesnotmatchtherobustnessofmodelsbuiltusing Figure2 Plotoftheground-motion(amplitude)responseofaWorld- WideStandardizedSeismographNetwork(WWSSN)stationwitha datathathavegoodresolutionatalldepthsinthemantle. seismographfreeperiod(T)of15sandgalvanometerwithafreeperiod s (T)of90s.Thesegmentbetweenthesetwoperiodshasaflatvelocity g response,characteristicofbroadbandseismometers.Theresponsein 1.01.1 DevelopmentsfromtheLateNineteenth moderninstrumentsisshapedelectronically;atypicalFDSNstationhasa CenturyuntiltheEarly1950s flatvelocityresponsefrom5Hzto360s.ReproducedfromWielandt E(2002)Seismicsensorsandtheircalibration.In:BormannIP(ed.) Thetheoreticalbeginningsofseismologymaybetracedtothe IASPEINewManualofSeismologicalObservatoryPractice,vol.I, eighteenth- and nineteenth-century studies of elasticity and pp.1–46.Potsdam:GeoForschungsZentrumPotsdam,Chapter1.06. propagation of elastic waves in solids. Lord Kelvin provided thefirstnumericalestimateoftheperiodofthefundamental vibrational mode ( S ) in 1863, but the development of the seismograph with controlled damping was built by Wiechert 0 2 proper theory for a homogeneous sphere had to wait nearly in 1904. Soon afterward, Galitzin (1914) developed an 50 years (Love, 1911). Lord Rayleigh solved the problem of electromagneticseismographsystem,wherethemotionofthe propagationofsurfacewavesinanelastichalf-spacein1877. seismometer’s pendulum generated an electric current Thisprecededthefirstmechanicalseismographs,whichwere by motion of a coil in the magnetic field. This current was, developedinthe1880s.Originally,theseismographshadvery in turn, carried to a galvanometer; the rotation of the low sensitivity and were used for the recording of local galvanometer’scoilinamagneticfieldwasrecordedonphoto- earthquakes.Thehistoryofglobalseismologybeginswiththe graphicpaperbyabeamoflightreflectedfromamirrorattached recordingofanearthquakeinJapanon19April1889byvon tothecoil.Theresponseofthesystemdependedonthesensi- Rebeur-Paschwitz. He associated a disturbance recorded on a tivityandfreeperiodoftheseismometerandofthegalvanom- tiltmeter,usedtostudytheEarth’s tides,withthereportsofa eterandtheirdamping.Whilethesystemwasmorecomplex,it greatearthquakeinJapan.Figure1showsacopyofthisrecord- allowed for much more flexibility in selecting a desired ingaspublishedinNature(vonRebeur-Paschwitz,1889,1895). response. Figure 2 shows the response of the seismograph– Theearlyseismographsweremechanicalpendulumswithno galvanometersystemandgivesanideaofthewayitcouldbe damping,otherthanfriction.Theirmagnifications(theratioof shaped by the choice of different free periods of the system’s theamplitudeonaseismogramtotheactualgroundmotion) components. With gradual improvements, the seismometer– wereverylow,andbecauseofthelackofdamping,therecords galvanometer system, recording on photographic paper, was were very oscillatory and it was difficult to distinguish commonly used during the following 60–70 years, when it the arrivals of different phases. An improved mechanical wasgraduallyreplacedbydigitalsystems(seeChapter1.02). DeepEarthSeismology:AnIntroductionandOverview 3 With the improvement of the recording systems technol- 14 ogy,phaseidentificationbecameeasier,anditwaspossibleto P identify P-arrivals (primary) corresponding to compressional 12 waves, S-arrivals (secondary) corresponding to shear waves, andL-arrivals,sometimescalled‘themainphase,’correspond- 1) −s 10 ingtosurfacewaves.Thesurfacewavescausedsomeconfusion m because therewasalso atransverse motion, not predicted by k Rayleigh. It was not until 1911 that Love showed that trans- city ( 8 S Seismic velocities vEearrstehl.y polarized surface waves can propagate in a layered Velo 6 GJeuftferenybse-rBgullen Progress in the first decade of the twentieth century was rapid.Someclassicalproblemssuchascomputationandinver- 4 sion of travel times for the velocity structure were solved by B C D E F G 0 (Benndorf,1905,1906;Herglotz,1907;Knott,1899;Wiechert, 0 1000 2000 3000 4000 5000 6000 1907;Zo€ppritz,1907)independentlydevelopedequationsfor Depth (km) theamplitudeofreflectedandtransmittedwavesatthebound- Figure3 ComparisonoftheseismicvelocitymodelsofGutenbergand arybetweentwoelasticmedia. Jeffreys,bothbuiltinthe1930s.Theprincipaldifferencebetweenthe As regards the Earth’s structure, there was a paper by modelsisthepresenceofthelow-velocityzoneintheGutenbergmodel Oldham (1906), in which he proposed the existence of the andthestructureneartheinner–outercoreboundarywherethelow- Earth’score,althoughtherehasbeensomeconfusioniniden- velocityzoneintheJeffreysmodeliserroneousandthevelocityincrease tification of phases: what he thought to be a delayed S-wave intheinnercoreislargerthanintheGutenbergmodel.Withthe wasactuallyanSS(e.g.,Schweitzer,2007).Gutenberg(1913) exceptionofthetransitionzone(400–650kmdepth),themodernmodels properlyidentifiedreflectionsfromthecore–mantleboundary arenotverydifferent.ReproducedfromAndersonDL(1963)Recent (CMB)anddeterminedtheradiusofthecorequiteaccurately, evidenceconcerningthestructureandcompositionoftheEarth’smantle. andJeffreys(1926)showedthatthecoreisliquid.Mohorovicˇic´ PhysicsandChemistryoftheEarth6:1–129. (1910) discovered the boundary between the crust and the upper mantle, thus beginning the era of studies of the crust andlithosphere,whichgreatlyacceleratedafterWorldWarII. andGutenbergwastheexistenceofalow-velocityzoneinthe Thefirstglobalseismographicnetworks(GSNs)wereestab- depthrange100–200kmintheuppermantle.Therewerevery lishedintheearlyyearsofthetwentiethcentury.Thefirstone hotdebatesonthisissue;itcannowbeexplainedbythefact wasdeployedbyJohnMilneinvariouscountriesoftheBritish thattheyuseddatafromtectonicallydifferentregions;thereis CommonwealthwiththesupportoftheBritishAssociationfor apronouncedlow-velocityzoneinthewesternUnitedStates, theAdvancementofScienceandeventuallyconsistedof30sta- butnotundertheEurasianshieldregions. tions(Adamsetal.,2002).TheJesuitNetworkwasestablished With internal reflections and conversions of waves at the soon afterward, with a particularly large number of instru- boundaries,seismologistsdevelopedasystemofphaseidentifi- mentsintheUnitedStates,butalsoincludingstationsonall cationthatreflectsacombinationofthetypesofwaves(PorS), continents(UdiasandStauder,2002).Withaglobalcoverage theregioninwhichtheypropagate(KandIfortheP-wavesin sufficienttolocatelargeearthquakes,informalbulletinswere theouterandinnercore,respectively;PKIKPdesignatesaphase published using the location method developed by Geiger thattravelsasPinthemantle,Pintheoutercore,andPinthe (1910, 1912), which (with many modifications) is still used innercore)andtheboundaryatwhichtheywerereflected(cfor today. In 1922, the International Seismological Summary CMB and i for the inner–outer coreboundary).A shear wave (ISS), with international governance, was established under reflectedoncefromthefreesurfaceatthemidpointofitspathis Professor Turner of the University of Oxford with the charge designatedbySS;highermultiplereflections,likeSSSorSSSSS, toproduce‘definitiveglobalcatalogs’from1918onward. canbeobservedbysamplingalargevolumeoftheEarthalong TheslowprogressinunravelingtheEarth’sstructureculmi- theirpaths.Forearthquakeswithafinitefocaldepth,theP-and nated in the 1930s with the discovery of the inner core by S-wavestravelingupwardfromthesourcehavedesignationofp Lehmann (1936) and the compressional velocity, shear or s; following reflection at the surface, they represent the velocity, and density models by Gutenberg (1913), Jeffreys so-calleddepthphases(e.g.,pPandsP);thetravel-timediffer- (1926),andBullen(1940).TheGutenbergandJeffreysvelocity encebetweenthearrivalofpPandPstronglydependsonfocal models are shown in Figure 3; except for the details of the depth. upper-mantle structure, these models are very similar to the Figure 4 shows examples of various seismic phases, and modern ones. The low-velocity zone above the inner–outer Figure5isthegraphicrepresentationofthetraveltimesasa core boundary in the model of Jeffreys illustrates the some- functionofdistancecomputedbyJeffreysandBullen(1940) timesunfortunatetendencyofseismologiststointroducephys- forthemodelofJeffreys(1939).Itisremarkablethatthissetof icallyimplausiblefeaturesinthemodelinordertomatchthe tables, including predictions for different focal depths, was data;Jeffreysneededtoadda2sdelaytomatchtheinner-core calculated using a manual mechanical calculator! The data travel times and accomplished it by inserting this feature, usedbyJeffreyswereextractedfromtheISS,theprecursorof whichisimpossibletoreconcilewiththechemicalandphys- the International Seismological Centre (ISC), which (with icalpropertiesofmaterialsinthisdepthrange(Birch,1952). internationalfinancialsupportandgovernance)resumedthe TheotherimportantdifferencebetweenthemodelsofJeffreys ISSrolein1964andcontinuesuntiltoday. 4 DeepEarthSeismology:AnIntroductionandOverview ScS P S P P pP S sP SKS pPcP PP SKP SP Solid SKKS inner core SS sPS PPP Fluid outer core SKKP PPS PKIKP Solid mantle PKJKP SSS P rays pPcPSKKP SKPPKP S rays Figure4 Examplesofseismicraysandtheirnomenclature.Themostcommonlyidentifiedphasesusedinearthquakelocationarethefirstarriving phases:PandPKIKP.ReproducedfromSteinSandWysessionM(2003)AnIntroductiontoSeismology,EarthquakesandEarthStructure.Oxford: Blackwell,ISBN:0865420785. Bullen(1949)dividedtheEarthintoanumberofconcen- surface waves (Rayleigh and Love) in a layered medium tricshells,designatedbylettersfromAtoF;inthisdivision,the with an arbitrary number of layers over a half-space. It lower mantle was designated by the letter D”. when Bullen involved multiplication of matrices, one for each layer, recognized that the deepest 150km of the lower mantle had changing the wave number for a fixed frequency such as an anomalously flat velocity gradient, he divided the region to matchtheboundaryconditions(vanishingofstresses)at DintoD0andD00.Morerecently,andnotentirelycorrectly,D00 the free surface. Because of the enormous amount of calcu- cametosignifythestructureinthedeepest300km,orso,of lations to be performed, it required application of an elec- the lower mantle, which is characterized by a still-growing tronic computer, and its application opened yet a new era collection of structural and compositional complexities (see in seismology. The Haskell’s matrix method has been Chapter1.22). adapted to other problems in seismology, such as calcula- It was recognized relatively early that the dispersion of tionofsyntheticseismogramsusingthe‘reflectivitymethod’ surface waves was different in the continents than in the (FuchsandMu¨ller,1971).Electroniccomputerswereatfirst oceans, with an indication that the oceanic crust was signifi- very expensive and rare, and it wasnotuntilthe1960s that cantlythinner.Computingthedispersionofsurfacewaveswas they became generally available at universities (Haskell algebraicallyandnumericallydifficult;thecorrectformulasfor worked at the Air Force Cambridge Research Laboratories). the dispersion of Rayleigh waves in a layer over a half-space Surface-wavedispersionbegantobestudiedintensivelyin wereformulatedbyStoneley(1928),andthecaseofthetwo the1950sprincipallyattheLamontGeologicalObservatoryof layersoverahalf-spacecouldbesolvedonlyforaveryspecific Columbia University, primarily by Ewing and Press, who setofparameters. observed mantle waves in the 1952 Kamchatka earthquake, identifying arrivals from R6 to R15 (see Chapter 1.04) and measuringtheirgroupvelocitiesuptoaperiodof500s(Ewing 1.01.2 Developmentsfrom1950stotheEarly1980s andPress,1954).Regionalmeasurementsofsurface-wavedis- persionwereinitiatedbyPress(1956).AmonographbyEwing It must have been frustrating for seismologists not to be et al. (1957) summarizes the state of the knowledge on able to use information about the Earth’s structure con- seismic-wave propagation in layered media at that time. tained in the most prominent features of the seismograms: Ewing and Press also developed a true long-period seismo- the dispersed surface waves. This changed when Haskell graph,whichwascapableofrecordingmantlewavesevenfor (1953) adapted to the case of elastic media the method moderately sized earthquakes. This instrument was deployed first proposed by Thomson (1950) in the acoustics case. at10globallydistributedInternationalGeophysicalYearnet- The approach made it possible to compute dispersion of workstationsoperatedbyLamont. DeepEarthSeismology:AnIntroductionandOverview 5 similarfreeoscillationperiodsforthegravestmodesbutdiffer atshorterperiodsby1–2%. When the greatest instrumentally recorded earthquake of 20May1960occurredinChile,seismologistshadallthetools (theory,computers,andinstrumentation)neededtomeasure and interpret its recordings. Three back-to-back papers in a 1961 issue of the Journal of Geophysical Research reported the firstcorrectmeasurements offreeoscillationperiods: Benioff etal.,Nessetal.(1961),andAlsopetal.(1961).Theobserva- tions were made on strainmeters in Isabella, N˜an˜a, and Ogdensburg; seismographs at Pasadena; and a gravimeter at the University of California, Los Angeles (UCLA). All three studies agreed in mode identification and found that their periods were very close to those predicted by the existing Earthmodels(Pekerisetal.,1961a).Moredetailedstudiesof thespectrarevealedthattheyaresplit;theeffectoftheEarth’s rotationwasshowntoexplainthiseffect(BackusandGilbert, 1961; Pekeris et al., 1961b). Thus, normal-mode seismology wasborn.Progressinthetheory,particularlyconsideringthe effect of lateral heterogeneities and mode coupling, would extend over decades to come. First attempts at inversion of normal-modedatawerenotparticularlysuccessful;thedensity model of Landisman et al. (1965) was flat throughout the lowermantle,implyingeitherastrongradialheterogeneityor animmenselysuperadiabatictemperaturegradient. BackusandGilbert(1967),BackusandGilbert(1968),and BackusandGilbert(1970)providedtheformalbackgroundfor consideration of geophysical inverse problems, and even though their own experiments with inversion of normal- modeperiodsforthevelocitiesanddensity,usingasubsetof normal-mode data, were discouraging (two very different modelswerefoundfittingthedatanearlyexactly;Backusand Gilbert, 1968), the idea of resolving kernels and trade-offs becamepartofthegeophysicalterminology. Figure5 Traveltimesofseismicphasesforthesurfacefocusas Becauseseismicmethodswereconsideredessentialindis- computedbyJeffreysandBullen(1940).ReproducedfromJeffreys criminating between earthquakes and nuclear explosions HandBullenKE(1940)SeismologicalTables,p.50.London:British AssociationfortheAdvancementofScience. (Berkner et al., 1959), an intensive observational program, calledVelaUniform,wasinitiated.Oneofitscomponentsof great significance to studies of the Earth’s interior was the Itisnotoftenthatamistakeleadstofavorableresults,but World-Wide Standardized Seismograph Network (WWSSN), thiswasthecasewiththefreeoscillationsoftheEarth.Benioff consisting of a set of three-component short-period and (1958)reportedanoscillationwithaperiodof57minseenin three-component long-period seismographs, with identical the record of the 1952 Kamchatka earthquake. Even though responses,exceptformagnification,whichdependedonlocal thisobservationwaseventuallyattributedtoanartifactinthe noiselevels.Atitspeak,theWWSSNconsistedof125stations functioningoftheinstrument,itstimulatedthetheoreticaland (Figure6),withdistributionlimitedbygeographyandpolitics: computational research needed to calculate eigenfrequencies therewerenostationsintheSovietUnion,China,andpoorly forarealisticEarthmodel.Someofthecalculationspreceded developedareasinAfricaandSouthAmerica.Thenovelaspect Benioff’sreport(Jobert,1956,1957),buttheeffortsofPekeris ofWWSSNwasitsstandardizedresponseandcentralizedsys- and Jarosch(1958) andTakeyuchi (1959)wereclearlymoti- tem of distribution of copies of seismograms. Individual sta- vatedtoexplaintheobservedperiod.Thesecalculations,using tions were sending the original seismograms to a central thevariationalapproachandtheJeffreys–BullenEarthmodel, location, where they were microfilmed, using a very high- predicted the period of S to be 52min and that of T resolution camera, and then returned to the stations. 0 2 0 2 43.5min.NeitherwascloseenoughtoBenioff’s‘observation.’ Aseismologistcouldrequestcopiesofseismogramsforapar- The modern approach was developed by Alterman et al. ticular date and receive them as either microfilm chips or (1959), who recast the system of three second-order partial photographicenlargementstotheoriginalsize.Severallarger differentialequationsintoasystemofsixfirst-orderdifferential institutions had a blanket order on all the records. This data equations, thus removing the need for differentiation of the accessibility represented major progress with respect to the elastic constants, and allowed the use of standard numerical earlierprocedures,whereonehadtorequestcopiesofseismo- methodstoobtainthesolution.TestsusingtheGutenbergand grams from individual stations, which greatly limited and the Jeffreys–Bullen models showed that they predict very delayed the research. WWSSN functioned for 20–25 years, 6 DeepEarthSeismology:AnIntroductionandOverview Figure6 MapofthestationsofWWSSNestablishedintheearly1960s,followingrecommendationsofBerkneretal.(1959).CourtesyofUSGeological Survey. slowly declining in the quality and number of stations; it ceased functioning in the late 1980s when data from new digitalstationsbecameavailable. Anotherdevelopmentofthe1960swastheintroductionto seismologyofdigitalrecording,greatlyfacilitatingresearchand the development of massive, computerized data-processing methods. One such facility, the large aperture seismic array (LASA), shown in Figure 7, was built in the mid-1960s in Montana.Itconsistedofsix‘rings,’foratotalof21subarrays, each with 25 short-period seismometers emplaced in bore- holes,toimprovethesignal-to-noiseratio.Thedataweretele- meteredinrealtimetoacentrallocationinBillings,Montana, where they were processed for detection of a signal. Major scientific discoveries were made with this tool, particularly when weak signals were involved, for example, observations ofreflectionsfromtheinnercore.Inpracticalterms,thearray did not meet the expectations; the site response across this 200km aperture array varied so much that the signals had Figure7 Configurationofthelargeapertureseismicarray(LASA) limited coherency and signal enhancement by stacking was andanexpandedviewoftwoofitssubarrays.ReproducedfromStein not as effective as expected. A somewhat smaller array was SandWysessionM(2003)AnIntroductiontoSeismology,Earthquakes installedafewyearslaterinNorway(NORSAR);elementsof andEarthStructure.Oxford:Blackwell,ISBN:0865420785. this array are still active and have been upgraded recently to modern, high-dynamic-range band–band response. Most of thearraysoftheInternationalMonitoringSystem(IMS)used Oneoftheimportantresultsobtainedfromtheanalysisof forseismicdiscriminationpurposeshaveanapertureofonly array data was the detection of upper-mantle discontinuities severalkilometers;becauseonthatscale,thecoherencyat1Hz (Johnson, 1967), confirming the result predicted by experi- frequencycanbeachieved. mental petrology that there should be two discontinuities at DeepEarthSeismology:AnIntroductionandOverview 7 pressuresandtemperaturescorrespondingtodepthsofabout turnedouttobeanartifactofcouplingbetweentoroidaland 400and650km,respectively(Birch,1952). spheroidal (Russakoff et al., 1998) modes, but the requisite Surface-wavestudiesblossomedinthe1960s.Atfirst,mea- theory to consider this effect was not available until 1984. surements of dispersion involved rather simple ‘analog’ GilbertandDziewonski(1975)presentedtwomodelsbased methods,suchasthepeak-and-troughapproachtomeasuring on measurements of eigenfrequencies of 1064 modes and phaseandgroupvelocities.Someveryimportantresultswere mass and moment of inertia for a total of 1066 data. They obtained in this way, such as the Canadian Shield study of derived two models 1066A and 1066B, with the first being Brune and Dorman (1963). Digital analysis, however, was smooththroughthetransitionzoneandthelatterincluding soontotakeover.Manualdigitizationofanaloguerecordings, the400and660kmdiscontinuities;bothmodelsfitthedata WWSSNdatainparticular,becameeasier,andwithincreasing equallywell. availabilityofcomputersanddecreasingcostofcomputations, Atthe1971GeneralAssemblyoftheInternationalUnion varioustechniquesweredeveloped,forthemostpartinvolving ofGeodesyandGeophysics(IUGG)inMoscow,theneedfor applicationsoftheFouriertransform.Withthedevelopmentof a reference Earth model was stated, and a Standard Earth thefastFouriertransform(FFT)algorithm(CooleyandTukey, Model Committee formed under the chairmanship of Keith 1965),thecostoftime-seriesanalysisdeclineddramatically;a Bullen. The Committee appointed several subcommittees, reviewbyDziewonskiandHales(1972)summarizesthestate includingonefortheradiusoftheCMB:therewerediscrep- of the art at the beginning of the 1970s. Some of these ancies on the order of 10km at the time. The value recom- methods,suchasthemultiplefiltrationtechniquetomeasure mended by the subcommittee was 3484km (Dziewonski group velocity dispersion, residual dispersion measurements, and Haddon, 1974), which – within 1km – withstood the and time-variable filtration, are still in use today. The 1960s trial of time. Hales et al. (1974) proposed that the seismic have also seen the first studies of intrinsic attenuation by velocitiesanddensityinthestandardEarthmodelshouldbe Anderson and Archambeau (1964), who developed partial describedbyalow-orderpolynomial,withdiscontinuitiesat derivatives (kernels) for Q from mantle-wave attenuation. the appropriate depths. Dziewonski et al. (1975) con- Also,thefirststudiesoflateralheterogeneity wereconducted structed such a model, named parametric Earth model usingthe‘purepath’approach(Tokso€zandAnderson,1966). (PEM), which satisfied the normal-mode, travel-time, and Seismic experiments with controlled sources were con- surface-wave data. The novelty of this model was that, in a ductedinamulti-institutionalmode.Oneofthelargestexper- single inversion, different structures were obtained for the imentswas‘EarlyRise’carriedoutintheJulyof1966.Aseries continental and oceanic crust and upper mantle. The of38explosionsinLakeSuperiorofupto5tonsofoutdated normal-mode periods predicted by these two models torpedoeswereusedasthesourceofsignalsrecordedbyhun- (PEM-O and PEM-C) averaged in 2/3 and 1/3 proportion dreds of seismometers – deployed by 12 governmental and were constrained to match the observed periods and tele- academicgroups–spreadingradiallyinalldirections.Signals seismic travel times but separate data sets for continental wererecordedasfaras2500km,reachingteleseismicdistances and oceanic surface-wave dispersion. The differences and providing a detailed profile of P velocity under a conti- betweenthesetwomodelsceasedatthe400kmdiscontinu- nental upper mantle (Green and Hales, 1968). A detailed ity, at which depth they became identical with the average review of crustal and upper-mantle studies with controlled Earth model, PEM-A. sourcesisprovidedinChapter1.11. ThedrawbackofthePEMandallthepreviousmodelswas With a large new data set, particularly measurements of that they did not consider the physical dispersion due to previously unreported periods of long-period overtones pro- anelasticattenuation.Forasignalpropagatinginanattenuat- videdbytheanalysisoffreeoscillationsgeneratedbythe1964 ing medium to be causal, the wave with higher frequencies Alaskan earthquake and recorded at WWSSN stations must propagate with higher velocities. Thus, waves with a (DziewonskiandGilbert,1972,1973),studiesof1-Dstructure frequencyof1Hzwillpropagatemore rapidlythanwavesat enteredanewera.Theresolutionofthisdatasetwassufficient a frequency of 1mHz. In order to reconcile the seismic data toconstrainthedensityprofileinthemantleandthecore;this thatspan3.5ordersofmagnitude,itisnecessarytoconsider turned out to be quite consistent with the behavior, in the the frequency dependence of elastic parameters. This was lower mantle and outer core, of a homogeneous material pointed out by Liu et al. (1976). The Preliminary Reference under adiabatic compression. Jordan and Anderson (1974) Earth Model (PREM) constructed by Dziewonski and were the first to combine the normal-mode and travel-time Anderson(1981),followingtheideaofparametricrepresenta- data,includingdifferentialtravel-timedata. tion,consideredthefrequencydependenceusingtheassump- Numerous additional overtone data were obtained by tionthatQisconstantinthebandfrom0.3mHzto1Hz. Mendiguren (1973) and Gilbert and Dziewonski (1975) by ThisnecessitatedobtainingtheradialprofilesofQmandQk; introducingphaseequalizationtechniques,suchas‘stacking’ fortunately,therewerenewmeasurementsavailableofnormal- and‘stripping.’Thesemethodsrequiretheknowledgeofthe modeandsurface-waveQ(SailorandDziewonski,1978)such sourcemechanismtopredicttheproperphaseforeachseis- that a formal inversion for Q could be conducted simulta- mogram to be considered; this in itself turned out to be a neously with the inversion for the velocities and density. It challenginginverseproblem.DziewonskiandGilbert(1974) wasrecognizedearlierthattoexplaintheobservedattenuation derivedthe spectrumofall sixcomponentsof themoment- of radial modes, which contain a very high percentage of rate tensor as a function of time for two deep earthquakes compressional energy (97.5% for S ), it was necessary to 0 0 (Brazil, 1963; Colombia, 1970). For both events, they introduce a finite bulk attenuation; Anderson and Hart detected a precursive isotropic component. Eventually, this (1978)preferredtoplaceitintheinnercore,andSailorand 8 DeepEarthSeismology:AnIntroductionandOverview Figure8 ThePreliminaryReferenceEarthModel(PREM)ofDziewonskiandAnderson(1981).Inadditiontothedistributionofseismicvelocitiesand density,PREMcontainsalsothedistributionofattenuationoftheshearandcompressionalenergy.FromthewebsiteofEdGarnero. Dziewonski (1978) thought that Qk is finite in the upper Twodigitalseismographicnetworkswereestablishedinthe mantle;unfortunately,theradialmodesdonothavethereq- mid-1970s.OnewastheInternationalDeploymentofAcceler- uisite depth resolution. Figure8 showsthe seismicvelocities ometers(IDA;Agnewetal.,1976,1986),consistingof18glob- anddensityasafunctionofradius;theattenuationinPREMis allydistributedLaCoste–Romberggravimeterswithafeedback discussedinChapter1.25.AnothernovelaspectofPREMwas systemthatalloweddigitizationofthesignal.Itwasdesignedto its radial anisotropy between the Moho and 220km depth. recordverylong-periodwaves,includingthegravestmodesof This feature, at first suspected to be an artifact of the nonli- freeoscillationsoftheEarth:onesamplewastakenevery20s nearityoftheinverseproblem,hasbeenconfirmedbyglobal (laterchangedto10s).Onlytheverticalcomponentofacceler- tomographic studies (e.g., Ekstro€m and Dziewonski, 1998). ation was recorded and the word length was 12 bits; the ThePREMmodelremainstothisdayawidelyused1Drefer- dynamicrangewas,therefore,ratherlimitedbutstillconsider- ence model for seismological studies based on long period ablygreaterthanthatofanaloguerecordings.Thesensitivitywas data. For P wave studies, especially those concerning core set such that the scale was saturated for the first surface-wave sensitive phases, model AK135 (Kennett et al., 1995), which arrivalsforeventswithmagnitude7.0,orso,dependingonthe wasbuilttoconstrainaglobaldatasetoftraveltimesfromthe station’s distance from the source and radiation pattern. The ISCbulletins,isoftenpreferredasa1Dreferencemodel.Itisto IDA network was operated by the Scripps Institution of benoted,however,thatthismodelincludesa36kmcontinen- Oceanography,andthecentrallycollecteddatawerefreelydis- talcrust,andcannotbeusedforwaveformmodelingoutsideof tributed to the academic community. This later became the continentalareas. future standard in global seismology. An early illustration of The1970shavealsoseenthebeginningofseismictomog- thepowerofsuchaglobalarraywastheanalysisofthesplitting raphy;twostudiespublishedsimultaneously(Akietal.,1977; ofthegravestmodesoffreeoscillationsgeneratedbythe1977 Dziewonski et al., 1977) addressed the problem on different Sumbawaearthquake(Bulandetal.,1979;M only8.4). w scales: regional and global. Aki et al. solved for 3-D velocity Theothernetworkconsistedoriginallyofnineinstallations structure under the NORSAR array, while Dziewonski et al. calledSeismicResearchObservatories(SROs)andfiveAbbre- obtained a very low-resolution model of 3-D velocity viated Seismic Research Observatories (ASROs). The SROs perturbations in the entire mantle and showed significant used borehole instruments, with significantly suppressed correlationbetweenvelocityanomaliesinthelowermostman- wind-generated noise levels, particularly on horizontal com- tleandthegravestharmonicsofthegravityfield.Thestudyof ponents. The ASROs were placed in underground tunnels or Dziewonskietal.(1977)wasmotivatedbyapaperbyJulian mine shafts and the seismographs were protected from the and Sengupta (1973) who noticed that travel times for rays effectsofchangingpressureandtemperature.Theinstrumen- bottoming in the same region of the mantle tend to show tationisdescribedbyPetersonetal.(1976).Thisnetworkwas similar residuals. They interpreted this result qualitatively as designedformonitoringtheNuclearTestBanTreatyandhigh the evidence of lateral velocity variations; no modeling was sensitivity was the main objective. In order to increase the presentedinthatpaper.Thefirstcontinental-scale3-Dmodel dynamic range, the signal was sharply band-pass-filtered, so oftheuppermantle,underNorthAmerica,waspublishedby thatatverylongperiods(>200s),theresponsetoacceleration Romanowicz(1979). was falling as o(cid:2)3,while itwas flat for theIDA instruments. DeepEarthSeismology:AnIntroductionandOverview 9 Even so, the SRO and ASRO stations were able to produce from 1976 till present, and this catalog is available online usefulmantle-waverecordsforeventswithmagnitudegreater (http://www.globalcmt.org). than about 6.5. Later, the network was augmented by Mastersetal.(1982)measuredcenterfrequenciesofspec- 10WWSSNstations,withtheanalogueoutputamplifiedand tral peaks of the fundamental spheroidal mode from hun- digitizedat16-bitanalogtodigitalconverters.Theentiresys- dreds of IDA records and discovered that there are spatially tem was called Global Digital Seismographic Network distinctpatternsinthefrequencyshiftswhenplottedatloca- (GDSN). There was no general data distribution system, but tions of the poles of the great circles corresponding to the data for selected dates were available upon request from the pathsbetweenthesourceandthereceiver.Byfittingspherical AlbuquerqueSeismologicalLaboratory. harmonics (even degrees only, because of the symmetry), Until then, a seismic station typically comprised a set of theseauthorsrealizedthatthepatternisdominatedbyspher- seismometers with either ‘long-period’ or ‘short-period’ ical harmonics of degree 2, an observation also made from responses,orsometimes,aswasthecasefortheWWSSN,one great-circling surface waves by Souriau and Souriau (1983). ofeach.Thissetupwasdesignedatthetimeofanaloguerecord- Figure9showsthepatternoftheshiftsofspectralpeaksand ingtoavoidthemicroseismicnoisepeakaround6–7speriod, zero line of the best-fitting spherical harmonics of degree 2 which would have made it impossible to digitize all but the forfourgroupsof0S‘modeswithdifferentrangesofdegree‘. largestearthquakesignals.Withdigitalrecording,andthepos- Note that the modes with the lowest ‘ show a different sibility of filtering out the microseismic noise by post- pattern than the remaining three groups. Our current under- processing, this traditional instrument design became standing of this fact is that the low-‘ modes sample deeper unnecessary. structure (lower mantle) than the higher-‘ groups that pre- Averyimportantdevelopmentinseismicinstrumentation dominantlysampletheuppermantle.Theauthorsperformed took place in Germany in the mid-1970s. An array of a new aparametersearch,inwhichtheychangedtheradiiofashell kindofinstrumentswithdigitalrecordingwasdeployednear inwhichtheanomalyislocated.Thebestvariancereduction Gra¨fenberg(HarjesandSeidl,1978).Itusedanovelfeedback wasfortheanomalyplacedinthetransitionzone.Thelasting seismograph (Wielandt and Streckeisen, 1982). The system importance of this chapter is that it demonstrated that het- was rapidly recognized for its linearity and large dynamic erogeneity of very large wavelength and sizeable amplitude range within a wide band of frequencies – hence the name ((cid:3)1.5%)exists intheEarth’sinterior. ‘broadband.’ The Gra¨fenberg array’s central station had been Followingthedevelopmentofawaveform-fittingtechnique colocatedwiththeSROboreholestationGRFO,andthecom- thatallowedthesimultaneousmeasurementofphasevelocity parisons were very favorable for the broadband instruments, and attenuation along a great-circle path (Dziewonski and which were capable of reproducing different narrowband Steim,1982),WoodhouseandDziewonski(1984)developed responsesusingasingledatastream.Thistypeofinstrumenta- an approach to the interpretation of waveforms that could tionbecamethepatternforfuturedevelopments(e.g.,Chapter extract both the even and the odd harmonic coefficients of 1.02). lateral heterogeneity as a function of depth. This method Inadditiontothedevelopmentsininstrumentation,thelate involves the ‘path average approximation,’ sometimes called 1970s saw important theoretical developments, related to the PAVA.Theseismogramsarerepresentedasasumofallnormal asymptoticpropertiesandcouplingofthenormalmodes.Exam- modes (spheroidal and toroidal) up to a certain frequency plesofsuchdevelopmentsarepapersbyWoodhouseandDahlen o . For a given great circle, each mode is assumed to be max (1978),Jordan(1978),andWoodhouseandGirnius(1982). affected by the average deviation from reference structure along the great-circle path (which is sensitive only to even- order harmonics) and along the minor-circle path (sensitive tobothevenandoddharmonics).Theeffectofthegreat-circle 1.01.3 From1980toPresent:TheEraofTomography pathcanbemodeledbyashiftineigenfrequencyofthemode; andBroadbandDigitalSeismicNetworks theeffectoftheminor-arcstructureismodeledbyafictitious shiftoftheepicentraldistanceforthatmode;thisshiftdepends Thedatafrombothglobalnetworksofthe1970sledtoresults onbothevenandoddpartsofthestructure.Woodhouseand thatdemonstratedtheneedfordevelopmentofaglobalnet- Dziewonski (1984) processed about 2000 mantle-wave seis- work that would better satisfy the needs of basic research in mogramsfrom theGDSNand IDA networksand obtained a seismology; three studies are noteworthy. A robust method, modeloftheuppermantle(Moho–670km),M84C,usingas which uses entire segments of (digital) seismograms, was basis functions spherical harmonics up to degree 8 for hori- developedtoobtainreliablemechanismsofearthquakeswith zontal variations and Legendre polynomials as a function of magnitude 5.0 and above (Dziewonski et al., 1981; Ekstro€m radiusuptodegree3.Figure10showsamapofshearvelocity etal.,2005).Inaddition,themethodrefinesthelocationofthe anomaliesatadepthof100km;therewasnoaprioriinfor- source,whichforaneventoffinitesizeneednotbeidentical mationusedonthelocationoftheplateboundaries.Correc- with the hypocenter determined from the first arrivals ofthe tions were made for crustal thickness, recognizing only the P-waves.ThistopicisdiscussedatlengthinChapter4.16.The continental and oceanic structure. An experimental model, reason why the subject is brought up here is that in most M84A,obtainedwithoutapplyingcrustalcorrections,showed aspectsofusingwaveformanalysisforthepurposeofdrawing that not taking crustal thickness into account may result in inferencesabouttheEarth’sstructure,itisnecessarytoknow mapping artificial anomalies at depths as large as 300km. thesourcemechanism.Theso-called‘centroidmomenttensor’ ModelM84Chadastrongdegree-2anomalyinthetransition method has now been applied to over 40000 earthquakes zone,confirmingtheresultsofMastersetal.(1982). 10 DeepEarthSeismology:AnIntroductionandOverview Figure9 Mapsoftheobservedfrequencyshiftsofthefundamentalspheroidalmodesforfourrangesoftheordernumbersasreportedby Mastersetal.(1982).Thefrequencyshiftsareplottedatthepolesoftheindividualgreat-circlepaths.Itindicatesthepresenceofverylargewavelength– velocityanomaliesintheEarth’sinterior;thepreferredlocationofthesourceoftheanomalyshowninthefigureisthetransitionzone.However, thefrequencyshiftsforthelargestwavelengthpanel(topleft;modes S and S)showadifferentpatternthanintheotherthreepanels(modesfrom 0 8 0 9 S to S );thelow-‘datahavethegreatestsensitivityinthelowermantle.ModifiedfromMastersG,JordanTH,SilverPG,andGilbertF(1982) 0 27 0 34 AsphericalEarthstructurefromfundamentalspheroidalmodedata.Nature298:609–613. Figure10 Shearvelocityanomaliesatadepthof100kminthemodelM84CofWoodhouseandDziewonski(1984).Thescalerangeis(cid:3)5%and theresolvinghalf-wavelengthis2500km.Plateboundariesareshownasthinyellowlines.Exceptforthecorrectionforcrustalthickness,therewasno otheraprioriinformationincludedintheinversion.Thus,theresultdemonstratesthatthewaveforminversionapproachisabletodistinguishtheslow velocitiesunderthemid-oceanridgesandancientcratons,forexample.

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