3'OURNALOF GEOPHYSICALRESEARCH VOL. 75, NO. 18, SEPTEMBER15, 1968

Seismologyand the New Global Tectonics

BRYAN ISACKS AND JACK OLIVER

Lamont GeologicalObservatory, Columbia University, Palisades,New York 10964

LYNN R. SYKES u

Earth Sciences Laboratories, ESSA Lamont GeologicalObservatory, Columbia University,Palisades, New York 10964

A comprehensivestudy of the observationsof seismologyprovides widely based strong support for the new global tectonicswhich is founded on the hypthesesof continental drift, sea-floorspreading, transform faults, and underthrustingof the lithosphereat island arcs. Althoughfurther developmentswill be requiredto explain certainpart of the seismological data, at presentwithin the entire field of seismologythere appear to be no seriousobstacles to the new tectonics.Seismic phenomena are generallyexplained as the result of interactions and other processesat or near the edgesof a few large mobile platesof lithospherethat spread apart at the ocean ridges where new surficial materials arise, slide past one another along the large strike-slip faults, and convergeat the island arcs and arc-like structureswhere surficial materials descend.Study of world seismicity showsthat most earthquakesare confinedto nar- row continuousbelts that bound large stable areas. In the zones of divergenceand strike-slip motion, the activity is moderate and shallow and consistent with the transform fault hy- pothesis; in the zones of convergence,activity is normally at shallow depths and includes intermediate and deep shocksthat grossly define the present configuration of the down-going slabs of lithosphere.Seismic data on focal mechanismsgive the relative direction of motion of adjoining plates of lithosphere throughout the active belts. The focal mechanismsof about a hundred widely distributed shocks give relative motions that agree remarkably well with Le Pichon's simplified model in which relative motions of six large, rigid blocks of lithosphere covering the entire earth were determined from magnetic and topographic data associatedwith the zones of divergence.In the zonesof convergencethe seismicdata provide the only geophysicalinformation on such movements. Two principal types of mechanismsare found for shallow earthquakesin island arcs: The extremely active zone of seismicity under the inner margin of the ocean trench is characterized by a predominance of thrust faulting, which is interpreted as the relative motion of two converging plates of lithosphere; a less active zone in the trench and on the outer wall of the trench is characterizedby normal faulting and is thought to be a surficial manifestation of the abrupt bending of the down-goingslab of lithosphere. Graben-like structuresalong the outer walls of trenchesmay provide a mechanismfor including and transporting sedimentsto depth in quantities that may be very significantpetrologically. Large volumes of sediments beneath the inner slopes of many trenches may correspond,at least in part, to sediments scrapedfrom the crust and deformed in the thrusting. Simple underthrustingtypical of the main zone of shallow earthquakesin island arcs does not, in general, persist at great depth. The most striking regularity in the mechanisms of intermediate and deep earthquakesin several arcs is the tendency of the compressionalaxis to parallel the local dip of the seismiczone. These events appear to reflect stressesin the rela- tively strong slab of down-goinglithosphere, whereas shearing deformations parallel to the motion of the slab are presumably accommodatedby flow or creep in the adjoining ductile parts of the mantle. Several different methodsyield average rates of underthrustingas high as 5 to 15 cm/yr for some of the more active arcs. These rates suggestthat temperatureslow enoughto permit dehydrationof hydrousminerals and henceshear fracture may persisteven to depthsof 700 km. The thicknessof the seismiczone in a part of the Tonga arc where very precisehypocentral locations are availableis lessthan about 20 km for a wide rangeof depths. Lateral variations in thickness of the lithosphere seem to occur, and in some areas the litho- spheremay not include a significantthickness of the uppermostmantle.

Lamont Geological Observatory Contribution 1234. Order of authors determined by lot. 5855 5856 ISACKS, OLIVER, AND SYKES The lengths of the deep seismic zones appear to be a measure of the amount of under thrusting during about the last 10 m.y. Hence, these lengths constitute another 'yardstick' for investigationsof global tectonics.The presenceof volcanism,the generation of many tsunamis (seismic sea waves), and the frequency of occurrenceof large earthquakes also seem to be related to underthrusting or rates of underthrusting in island arcs. Many island arcs exhibit a secondarymaximum in activity which varies considerablyin depth among the various arcs. These depths appear, however, to correlate with the rate of underthrusting,and the deep maxima appear to be located near the leading (bottom) part of the down-goingslab. In some casesthe down-goingplates appear to be contorted, possibly becausethey are encounteringa more resistant layer in the mantle. The interaction of plates of lithosphereappears to be more complex when all the plates involved are continents or pieces of continents than when at least one plate is an oceanic plate. The new global tectonicssuggests new approachesto a variety of topics in seismologyincluding earthquake prediction, the dete.ctionand accurate location of seismicevents, and the generalproblem of earth structure.

INTRODUCTION [1968], who pursuedthis conceptfurther by This paper relates observationsfrom the investigatingthe relative motion in plan of field of seismologyand allied disciplines to large blocksof lithosphere. what is here termed the 'new global tectonics.' Figure I is a block diagramillustrating some This term is used to refer in a general way to of the principalpoints of the mobilelithosphere current conceptsof large-scaletectonic move- hypothesis.In a relativelyundisturbed section, ments and processeswithin the earth, concepts three fiat-lying layers are distinguished:(1) that are basedon the hypothesesof continental the lithosphere,which generally includesthe drift [Wegener, 1966], sea-floor spreading crust and uppermost mantle, has significant [Hess, 1962; Dietz, 1961], and transform strength, and is of the order of 100 km in faults [Wilson, 1965a] and that include var- thickness; (2) the asthenosphere,which is a ious refinements and developments of these layer of effectivelyno strengthon the appro- ideas.A comprehensiveview of the relationship priate time scale and which extends from the between seismologyand the new global tec- base of the lithosphereto a depth of several tonics is attempted, but there is emphasison hundredkilometers; and (3) the mesosphere, data from earthquake seismology,as opposed whichmay have strengthand whichmakes up to explosionseismology, and on a particular the lower remainingportion of the mantle and version of the sea-floor spreading hypothesis is relativelypassive, perhaps inert, at present, in which a mobile, near-surface layer of in tectonic processes.(Elsasset refers to the strength,the lithosphere,plays a key role. Two lithosphereas the tectosphereand definessome basic questionsare considered.First, do the other terms somewhat differently, but the observations of seismologysupport the new terminology of Daly [1940] is retained here. global tectonicsin some form? To summarize The term 'strength,'which has many defini- briefly, they do, in general,give remarkable tions and connotations,is usedhere, following supportto the new tectonics.Second, what new Daly, in a generalsense to denoteenduring approachesto the problemsof seismologyare resistanceto a shearingstress with a limiting suggestedby the new global tectonics?There value.) The boundariesbetween the layers are many; at the very least the new global may be gradationalwithin the earth. The tectonics is a highly stimulating influence on thenospherecorresponds more or less to the the field of seismology;very likely the effect low-velocitylayer of seismology;it strongly will be one of revolutionary proportions. attenuates seismicwaves, particularly high- The mobile lithosphere concept is based frequency shear waves. The lithosphereand partly on an earlier study [Oliver and Isacks, the mesospherehave relatively high seismic 1967], but, as presentedhere, it incorporates velocitiesand propagateseismic waves without ideas from Elsasset [1967], who independently great attenuation. developeda model with many similar features At the principalzones of tectonicactivity basedon entirely differentconsiderations, and within the earth (the oceanridges, the island ideas from Morgan [1968] and Le Pichon arc or island-arc-likestructures, and the major SEISMOLOGY AND NEW GLOBAL TECTONICS 5857

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M F' S 0 S P H E R E

Fig. 1. Block diagramillustrating schematically the configurationsand rolesof the litho- sphere,asthenosphere, and mesospherein a versionof the new globaltectonics in whichthe lithosphere,a layer of strength,plays a key role. Arrowson lithosphereindicate relative movementsoœ adjoining blocks. Arrows in asthenosphererepresent possible compensating flow in responseto downwardmovement of segmentsof lithosphere.One arc-to-arc transform fault appearsat left betweenoppositely facing zones of convergence(island arcs), two ridge-to- ridge transformfaults alongocean ridge at center,simple arc structureat right. strike-slip faults) the lithosphereis discon- settled,beneath an islandarc or a continental tinuous; elsewhereit is continuous.Thus, the margin that is currentlyactive. At an inactive lithosphere is composed of relatively thin margin the lithospherewould be unbrokenor blocks, some of enormoussize, which in the healed. The left side of the diagram showstwo first approximationmay be consideredinfinitely island-arcstructures, back to back, with the rigid laterally. The major tectonicfeatures are lithosphereplunging in a differentdirection in the result of relative movement and interaction each case and with a transform fault between of theseblocks, which spreadapart at the rifts, the structures. Whereas the real earth must slidepast oneanother at largestrike-slip faults, be more complicated,particularly at this back- and are underthrust at island arcs and similar to-back structure, this figure represents,in structures•. Morgan [1968] and Le Pichon a generalway, a part of the Pacificbasin in- [1968] have demonstratedin a generalway and cluding the New Hebrides, Fiji, Tonga, the with remarkable success that such movement East Pacific rise, and western South America. is self-consistent on a worldwide scale and that The counterflowcorresponding to movement the movementsagree with the pattern of sea- of the lithosphereinto the deepermantle takes floor spreadingrates determinedfrom magnetic place in the asthenosphere,as indicatedsche- anomalies at sea and with the orientation of matically by the appropriate arrows in the oceanic fracture zones. McKenzie and Parker figure.To what extent,if any, there is flow of [1967] used the mobile lithosphereconcept to the adjoiningupper part of the asthenosphere explain focal mechanismsof earthquakes,vol- in the same direction as the overlying litho- canism, and other tectonic features in the sphereis an important but open question,par- northern Pacific. tially dependenton the definitionof the bound- Figure i also demonstratesthese concepts ary. A key point of this model is that the in block diagram form. Near the center of the pattern of flowin the asthenospheremay largely figure the lithospherehas been pulled apart, be controlledby the configurationsand motions leaving a pattern of oceanridges and trans- of the surfaceplates of lithosphereand not by form faults on the surface and a thin litho- a geometricalfit of convectioncells of simple sphere thickening toward the flanks beneath shape into an idealizedmodel of the earth. the ridge as the new surface material cools It is temptingto think that the basicdriving and gains strength. To the right of the dia- mechanismsfor this processis gravitationalin- gram the lithospherehas been thrust, or has stability resulting from surface cooling and 5858 ISACKS, OLIVER, AND SYKES hence a relatively high density of near-surface sible to begin consideringthe data, but in this mantle materials. Thus, convective circulation process it soon becomes evident that much in the upper mantle might occur as thin blocks more detailed information on the earth is of lithosphere of large horizontal dimensions available and that the hypothesisand the earth slide laterally over large distancesas they de- model can be developed much further. These scend; a compensatingreturn flow takes place developmentsare presented later in the text in the asthenosphere.The processin the real as the relevant data are discussed. earth must be more complexthan this simple This paragraph gives a brief review of some model, however. The reader is referred to of the developmentsleading to the new global Elsasset[1967] for a discussionof many points tectonics. A number of contributions vital to relating to this problem. the development of the current position on Alternatively, the surfaceconfiguration might this topic are cited, but the review is not in- be taken as the complicatedresponse of the tended to be comprehensive. The literature strong lithosphere to relatively simple con- bearing on this topic is voluminous,is wide- vection patterns within the asthenosphere. spread in space and time, and differs in degree Thus, the basic question whether the litho- of relevance,so that a thorough documentation sphere or the asthenospheremay be thought of its developmentis a job for a historian,not of as the active element,with the other being a scientist.The hypothesisof continentaldrift passive, is not yet resolved. Probably, how- had a substantial impact on the field of geol- ever, there has been a progressivethinning of ogy when it was proposedin 1929 by Wegener the convective zone with time, deeper parts [1966], but until recently it had not received of the mantle having also been involved dur- general acceptance, largely because no satis- ing early geologictime. factory mechanism had been proposedto ex- Figure 2, adapted from Le Pichon [1968] plain the movement,without substantialchange with additions, shows the plan of blocks of of form, of the continents through the oceanic lithosphere as chosen by Le Pichon for the crust and upper mantle. When many new data spherical earth and indicates how their move- became available, particularly in the fields of ments are being accommodatedon a worldwide marine geology and geophysics,Hess [1962] scale. The remarkably detailed fit between this and Dietz [1961] proposed that the sea floor scheme,based on a very small number of rigid was spreading apart at the ocean ridges so blocks of lithosphere (six) and the data of a that new 'crust' was being generated there number of fields, is very impressive.The num- while older 'crust' was disappearing into the ber and configuration of the blocks of litho- mantle at the sites of the ocean trenches. The sphere is surely larger than six at present and driving mechanism for this spreading was almost certainly the pattern has changedwithin thought to be convectionwithin the mantle. geologictime, but the present pattern must, The remarkable successwith which the hy- in general, be representative of at least the pothesisof sea-floor spreadingaccommodated Quaternary and late Tertiary. The duration such diversegeologic observations as the linear of the current episodeof sea-floor spreading magnetic anomalies of the ocean [Vine and is not known. Some evidence suggeststhat it Matthews, 1963; Pitman and Heirtzler, 1966], beganin the Mesozoicand has continuedrather the topography of the ocean floor [Menard, steadily to the present. Other evidence [Ewing 1965], the distribution and configurationof and Ewing, 1967] indicates that the most re- continental margins and various other land cent episodeof spreadingbegan about 10 m.y. patterns [Wilson, 1965a; Bullard, 1964; Bul- ago. This suggestionis consideredhere because lard e• al., 1965], and certain aspectsof deep- it opensnew possibilitiesfor explainingcertain sea sediments [Ewing and Ewing, 1967] raised seismological observations, particularly the this hypothesisto a level of great importance configurationof the deep earthquake zones. and still greater promise.The contributionsof Other explanationsfor such evidenceare also seismologyto this developmenthave been sub- considered,however. stantial, not only in the form of generalinfor- With this one very simple version of the mation on earth structure but also in the form new global tectonicsas backgroundit is pos- of certain studies that bear especiallyon this SEISMOLOGY AND NEW GLOBAL TECTONICS 5859

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z 5860 ISACKS, OLIVER, AND SYKES hypothesis.Two specific examples are $ykes's hypotheses on globaltectonics, for example, [1967]evidence on seismicitypatterns and theexpanding earth and the contracting earth focalmechanisms to support the transformhypotheses, have been far lesssatisfactory in faulthypothesis of Wilson [1965a] and Oliver explainingseismological phenomena. and Isacks's[1967] discoveryof anomalous Certainlythe mostimportant factor is that zonesthat appear to correspondto underthrust the new global tectonics seem capable of draw- lithospherein the mantle beneath island arcs. ing togetherthe observationsof seisinology Thereare many important seismological facts andobservations of a hostof otherfields, such that areso apparentthat theyare commonlyas geomagnetism, marine geology, geochemistry, acceptedwithout much concern as to their gravity,and various branches of landgeology, origin;they fall into placeremarkably well undera singleunifying concept. Such a step underthe new globaltectonics. For example, is of utmostimportance to the earthsciences the generalpattern of seismicity,which con- and will surelymark the beginningof a new sists of a number of continuous narrow active era. beltsdividing the earth'ssurface into a hum- In the remainderof thispaper, the relation- ber of stableblocks, is in accordwith this shipbetween the newglobal tectonics and the concept.In part this agreementis by design, field of seismologyis discussedfor a variety for the blockswere chosen to someextent on of topicsranging from seismicity to tsunamis, thisbasis, but datafrom other fields were used from earth structureto earthquakepredic- as well.That the endresult is internallycon- tion.In eachcase what the authorsjudge to sistentis significant.Zones predicted by the be representative,reliable evidence from the theoryto be tensional,such as oceanrifts, are fieldof seismologyis presented. This judgment sites of only shallowearthquakes (the thin is basedon the qualityof raw data and their shallowlithosphere is being pulled apart; earth- analysis,not on the relationof the resultsto quakescannot occur in the asthenosphere),the new globaltectonics. Reasonable specula- and the generallevel of seismicactivity and tion is presentedwhere it seemsproper. The the sizeof the largestearthquakes are lower organizationof the paperis basednot on the there than in the more activecompressional classical divisions of seismologybut on the features.In the compressionalfeatures (the principaleffects predicted by the newglobal arcs)large, deep earthquakes occur and ac- tectonicsand relevant to seismology.As a re- tivity is high as the lithosphereplunges into sult of the remarkablecapacity of the new the deepermantle eventually to be absorbed.global tectonics for unification,an obvious Deepearthquakes can occur only where former divisionof materialamong the sectionswas crustaland uppermostmantle materials are not completelyachieved, however. nowfound in themantle. Where one block of The firsttwo sectionspresent seismological the lithosphereis movingpast another along evidencethat the worldwiderift systemand the surfaceat the zonesof largestrike-slip island arcs are the sourcesand sinks, respec- faulting,seismic activity is shallow,but occa- tively,for surficialmaterial. The third section sionalrather large shallow earthquakes are ob- on compatibilityof movementson a worldwide served.Some zones combine thrusting and scaleis closelyrelated to the firsttwo sections. strike-slip motion. The generalpattern of earth- quakefocal mechanisms isin remarkable agree- Fig.3. (Opposite)Summary mapof slip vec- mentwith the pattern predicted by themove- torsderived from earthquake mechanism studies. ments of the lithospheredetermined in other Arrowsindicate horizontal component of direction ways and providesmuch additionalinforma- of relativemotion of blockon whicharrow is tionon this process. The depth of thedeepest drawn to adjoiningblock. Crests of worldrift system are denoted by double lines; island arcs, earthquakes(about 700 km) hasbeen rea- andarc-like features, bybold single lines; major sonablywell known, but unexplained, formany transformfaults, by thinsingle lines. Both slip years.The mobilelithosphere hypothesis offers, vectorsare shownfor an earthquakesnear the at thiswriting, several possible alternatives to westernend of the Azores-Gibraltarridge since • rational choice between the two could not be explainthis observation. Manysimilar points made. Compare withdirections computed byLe areraised in theremainder of thispaper. Other Pichon(Figure 2). SEISMOLOGY AND NEW GLOBAL TECTONICS 5861

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i.' 5862 ISACKS, OLIVER, AND SYKES Evidence from seismologyon the structure of that resemblemajor fault zoneson the conti- the mantle in terms of a lithosphere, an nents. The apparent displacementsalong these asthenosphere,and a mesosphereis so volumi- fracture zoneshave been explainedin at least nous and well known that the section on this three different ways, including simple offset of topic, the fourth section, presents primarily the ridge by strike-slip faulting IVacquiet, additional evidence of particular relevance to 1962], in situ developmentof the ridge crests the new global tectonics.The fifth section,on at separate locations accompaniedby normal the impact of the new global tectonics on faulting along fracture zones [Talwani et al., seismology,is lessdocumented by data than the 1965b], and transformfaulting [Wilson, 1965a]. previous sectionspartly becausethe impact of Trans/orm [aults. Although the concept of the new global tectonics is quite recent. The simple offset tacitly assumesthe conservation natural lag.in pursuingthis aspectis suchthat of surface area, the growth or the destruction there has beento date relatively little emphasis of surface area is basic to the definition of the in this particular field. This section is, then, transform fault. In this hypothesis the active somewhatspeculative and, hopefully, provoca- portion (BC in Figure 4) of a strike-slip fault tive. along which large horizontal displacementhas Few scientificpapers are completelyobjective occurredends abruptly at the erestof a growing and impartial; this one is not. It clearly favors ocean ridge. The horizontal displacementalong the new global tectonicswith a strong prefer- the fault is transformed(or absorbed)by sea- ence for the mobile lithosphereversion of this floor growth on the ridge; the growingridge is, subject.In the final section,however, we report in turn, terminated by the fault. Two separate an earnest effort to uncover reliable informa- segmentsof ridge erest can be joined (Figure 4) tion from the field of seismologythat might by a strike-slip fault of this type; these faults provide a caseagainst the new global tectonics. are called transform faults of the ridge-ridge There appearsto be no suchevidence. This does type. not mean, however, that many of the data Wilson [1965a] recognizedthat the senseof could not be explained equally well by other hypotheses(although probably not so well by any other single hypothesis) or that further developmentor modificationof the new global tectonicswill not be required to explain some of the observationsof seismology.It merely means that, at present, in the field of seismol- ogy, there cannot readily be found a major obstacleto the new global tectonics. MID-0CEAN RIDGES--TIlE SOURCES 'vx Displacements along [racture zones. Recent studies of earthquakes have revealed several important facts about the nature of displace- ments on the oceanfloor [Sykes, 1967, 1968]. The recognitionof the worldwide extent of the mid-ocean ridge system (Figures 2 and 3) [Ewing and Heezen, 1956] led to a great in- terest in the significanceof this major feature to global tectonics. Although the ridge system Fig. 4. An idealized model of sea-floor spread- ing and transform faulting of the ridge-ridge type. appearsto be a continuousfeature on a large Hatching indicates new surface area created dur- scale,the crest of the ridge is actually discon- ing a given period of sea-floor spreading along the tinuous in a number of places (Figures 1, 2, active ridge crests BF and CE. Present seismicity and 3). These discontinuitiescorrelate with the (indicated by crosses) is confined to ridge crests and to segment BC of the fracture zone AD. intersectionsof the ridge and the major fracture Arrows denote sense of shear motion along active zones--longlinear zonesof rough topography segment BC. SEISMOLOGY AND NEW GLOBAL TECTONICS 5863 shear displacementalong transform faults of fracture zonesof the East Pacific rise (a branch the ridge-ridgetype would be exactly opposite of the mid-oceanridge system)is illustratedin that required for a simple offset of the two Figure 1. segmentsof ridge crest.He alsopointed out that Sykesalso showedthat earthquakeslocated seismicactivity along transform faults should on the ridge crests (segmentsBF and CE in be confinedto the regionbetween the two ridge Figure 4) but not locatedon fracture zonesare crests (segmentBC in Figure 4). If the crestal characterizedby a predominanceof normal zonesare being displacedby simpleoffset, how- faulting. Normal faulting on oceanridges had ever, seismic activity should be present along long beensuspected because of the existenceof the entire length of the fracture zone. a rift valley near the crest of large portionsof Earthquake mechanisms. The first motions the ridge system [Ewing and Heezen, 1956]. of seismicwaves from earthquakesoffer a means More than fifty mechanismsolutions (Figure 3) for ascertainingthe senseand type of displace- have now been obtained for the world rift system ments on fracture zones. First-motion studies [Sykes,1968; Tobin and Sykes,1968; Banghat are often called 'fault-plane solutions'or 'focal- and Sykes,1968]; they continueto confirmthe mechanism solutions.' Although many earth pattern of transformfaulting and normal fault- scientistshave been disappointedby the large ing describedby Sykes. Nearly the same tec- uncertainties involved in many first-motion tonic phenomenonis observedfor each of the studies,investigations of focal mechanismswere major oceans. vastly upgraded by the installation of the Seismicity. The distribution of earthquakes World-Wide Standardized Seismograph Net- is anotherkey pieceof seismicevidence for the work [Murphy, 1966]. Reliable calibration, hypothesisof transformfaulting. Nearly all the availability of data, high sensitivity, use of earthquakeson the mid-ocean ridges are con- seismographsof both long and short periods, fined either to the ridge crestsor to.the parts of and greater geographicalcoverage are someof fracture zones that lie between ridge crests the more important characteristicsof this net- [Sykes, 1967]. Seismicactivity along a fracture work, which commenced operation in 1962. zone ends abruptly (Figure 4) when the frac- Various studiesusing data from these stations ture zone encountersa ridge; only a few earth- have confirmedthat a double couple (or a shear quakeshave been detectedfrom the outer parts dislocation) is an appropriate model for the (segmentsAB and CD) of most fracture zones. radiation field of earthquakes [Stauder, 1967; If the transform fault theory is correct, the Isacks and Sykes, 1968]. Hence, the first mo- areas of sea floor that are now bounded by the tions observedat seismographstations around outer inactive parts of fracture zoneswere once the world may be usedto determinethe orienta- located between two ridge crests; these blocks tion and the sense of the shear motion at the of sea floor moved beyond either crest as sourcesof earthquakes in various tectonic re- spreadingprogressed. Thus, the age of deforma- gions. Additional backgroundinformation on tion becomes older as the distance from an earthquake mechanismswill be introduced in active crest increases. later sectionsas further clarification is required. Earthquake swarms. The occurrence of Mechanismsalong world ri)et system. Sykes earthquake swarmsalong the world rift system [1967] examinedthe focalmechanisms of seven- suggeststhat the crestalzone probably is char- teen earthquakesalong various parts of the acterized by submarine volcanic eruptions world rift system. In his study all the earth- [Sykes et al., 1968]. Earthquake swarmsare a quakeslocated on fracture zoneswere charac- distinctive sequenceof shocks highly grouped terized by a predominanceof strike-slipmotion. in space and time with no one outstanding In each case the shear motion was in the correct principal event. Although thesesequences some- sensefor transform faulting (Figure 4), but it times occur in nonvolcanicregions, most of the was consistentlyopposite in senseto that ex- world's earthquake swarms are concentratedin pectedfor simpleoffset. This is an instancein areas of present volcanismor geologicallyrecent the earth sciencesin which a yes-or-no answer volcanism [Richter, 1958; Minikami, 1960]. could be suppliedby data analysis.The sense Large swarmsoften occurbefore volcanicerup- of motion (left lateral) along one of the major tions or accompanyingthem; smaller swarms 5864: ISACKS, OLIVER, AND SYKES may be indicative of magmatic activity that excellentagreement with evidenceof spreading failed to reach the surface as an eruption. frommagnetic anomalies, ages of rocks,and the From the seismographrecords at Palisades, distributionof sediments[Vine, 1966;Heirtzler New York, Sykesei al. [1968] recognizedmore et al., 1968; Wilson,1963; Burckle et al., 1967]. than twenty swarms of earthquakesoccurring The world rift systemmust be recognizedas during the past 10 years. These swarms com- oneof the majortectonic features of the world. monly lasted a few hours or a few days. Al- It is characterizednearly everywhereby exten- though many of the larger earthquakesalong sional tectonics,sea-floor growth at its crest, the world rift system occur on fracture zones and transform faulting on its fracture zones. and are characterizedby strike-slip faulting, The focal depthsand the maximummagni- nearly all the swarmsare restrictedto the ridge tudesof earthquakes,the narrownessof seismic crests (segmentsBF and CE in Figure 4) and zones,and the propagationof S• wavesalong seem to be characterized by normal faulting. oceanridges and transform faults will be de- Swarms are commonly (but not always) asso- scribed in the sectionson worldwide compara- ciated with volcanic eruptions on islands or bility of movementsand on additionalevidence on or near the crest of the world rift system. for the existenceof the lithosphere. From a simulation of magnetic anomalies Implications[or continentaldrift. The simi- Matthews and Bath [1967] and Vine and larity of the earthquake mechanismsalong Morgan [1967] estimate that most of the new nearly the entire length of the ridge system surfacematerial along the world rift systemis suggeststhat transformfaulting and spreading injected within a few kilometersof the axis of have been occurringin these regionsfor ex- the ridge. In Iceland, where the rift may be tended,but as yet unspecified,periods of time. seenand studied in detail, postglacialvolcanism The distributionof magnetic anomalies,paleo- is confined largely to the median rift that magneticinvestigations, and the shapesof con- crossesthe island [Bodvarsson and Walker, tinental blocksthat supposedlywere split apart 1964]. The rift apparently marks the landward by spreadingfurnish a more completehistory continuation of the crest of the mid-Afiantic of the processesof sea-floorspreading and trans- ridge (Figures2 and 3). form faulting.A questionof particularinterest The lack of weatheringin rock samples,the is: Have the various segmentsof ridge grown youngages measured by radioactiveand paleon- in place; i.e., has the en echelonpattern of tologic dating of rocks and core materials, and ridgesand fracturezones prevailed throughout the generalabsence of sedimentas revealedby an episodeof sea-floorspreading? bottom photographsand by reflectionprofiling Both the Gulf of Aden and the Gulf of all attest to the youthful character of the Californiaare thoughtto have openedby con- crestal zones of the mid-ocean ridge [Ewing tinental drift duringthe last 25 m.y. [Hamilton, et al., 1964; Burckle ei al., 1967; van Andel 1961; Laughton,1966]. If drift occurredin and Bowin, 1968; Dymond and De#eyes,1968]. theseareas, the displacementsare at mosta few Thus, the occurrenceof earthquakeswarms is hundred kilometers. If continental drift can be compatiblewith the hypothesisthat new surface confirmedfor these features, inferencesabout materials are being eraplacedmagmatically near drift on an ocean-widescale are placed on a the axes of the ocean ridges.The large earth- much firmer basis. quake swarms(and perhapssome of the smaller Figure5 showsthe distributionof structural swarms) may be indicative of eruptionsor mag- features,earthquake epicenters, and earthquake matic processesin progressnear the ridgecrests. mechanismsfor the Gulf of Aden [Sykes,1968]. Nonetheless,more work is neededto ascertain Nearly all the epicentersare confinedeither to if a causal relationshipexists betweenthe two northeast-strikingfracture zones or to the ridge phenomena. that extends from a branch of the mid-ocean Synthesiso[ data [or ridges. Seismological ridge (the Carlsbergridge) near 9øN, 57øE to evidence of various types seemsto provide a the westernpart of the Gulf of Adennear 12øN, definitiveargument for the hypothesesof trans- 43øE. This ridge coincideswith the roughcen- form faulting and sea-floor spreading on the tral zonein Figure 5. As in other parts of the mid-ocean ridge system. These data are in world rift systemthe earthquakesoccurring on SEISMOLOGY AND NEW GLOBAL TECTONICS 5565

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o o 5866 ISACKS, OLIVER, AND SYKES

fracture zones are mostly restricted to the re- patterns of the island arcs. The closeassociation gionsbetween ridge crests.Mechanism solutions of the major ocean deeps with these arcs is for events 22 and 23 (numbers after Sykes obviousand suggestsexceptional subsidence in [1968]) indicatetransform faulting of the ridge- thesezones, but other facts are equallystriking. ridge type. Nearly all the world's earthquakesin the deep If the opening of the Gulf of Aden was ac- and intermediate range, most of the world's complishedthrough a simple process of sea- shallowearthquakes, and the largestdepartures floor spreading and transform faulting, the from isostatic equilibrium are associatedwith fracture zonesshould join points in Arabia and island arcs or arc-like structures,as shownby in Africa that were together before the drifting Gutenbergand Richter [1954]. Volcanoes,sea- commenced.Also, the fracture zonesshould not level changes,folding, faulting, and other forms continueinto the two continentalplates. Laugh- of geologicevidence also demonstratethe high ton [1966] has shown,in fact, that thesefaults level of tectonic activity of these features. A do not continue inland. In addition, his pre- conceptof global tectonicsin which the arcs do Miocene reconstruction,in which the two sides not play an important role is unthinkable. If of the Gulf of Aden are moved together parallel crustalmaterial is to descendinto the mantle, to the fracture zones,juxtaposes a large num- the island arcs are suspectas sites of the sinks. ber of older structural features on the two sides The asymmetrical structure of the arcs and of the gulf. The en echelon arrangement of the associatedpattern of earthquakeoccurrence segmentsof ridge is alsomirrored in the stepped in the mantle led many investigators(e.g., shape of the continentalmargins of Arabia and Vening Meinesz [1954], Benio# V1954],Hess Africa. Hence, the present en echelonpattern [1962], Dietz [1961]) to postulate that the seemsto have prevailedsince the initial breakup structures are the result of compressivestresses of these two blocks about 5 to 25 m.y. ago. normal to the arc and are the sites of vertical A similar pattern of en echelonridges is pres- movements in various convective schemes. Al- ent in the Gulf of California (Figure 6). Earth- though such ideas were supportedby the in- quake mechanismsfrom this region are in- vestigationsof focal mechanismsof earthquakes dicative of a series of northwesterly striking madeby Honda et al. [1956] and by the gravity transform faults with right-lateral displacement studies of Vening Meinsz [1930] and Hess [Sykes, 1968]. These transform faults, which [1938], later analysesby Hodgson [1957] for are arranged en echelon to the San Andreas focalmechanisms and by Talwani et al. [1959] fault, connect individual segmentsof growing and Worzel [1965] for gravity led to different ridges in the Gulf of California. Hence, sea- conclusions. This section reviews the data and floor spreadingand transformfaulting alsowere showsthat there is strongsupport for the com- responsiblefor the displacementof Baja Cali- pressive nature of island arcs and for their role fornia relative to the mainland of Mexico. If as sites where surface material moves down- these two blocksare reconstructedby horizontal ward into the mantle. In particular, a variety displacementsparallel to the northwesterly of evidencesupports the model of the arc shown striking fracture zones,the peninsula of Baja in Figure 1. In this model the leadingedge of California is placedin the indentationor 'hitch' the lithosphereunderthrusts the arc and moves of the mainland of Mexico near 21øN, 106øW. downwardinto the mantle as a coherentbody. Thus, the two pieces appear to fit together in The proposedpredominance of strike-slipfault- this reconstruction.Wilson [1965a] has pointed ing in island arcs [Hodgson, 1957] is not in out that the steppedshape of the fracture-zone- agreementwith this modelbut appears,in view ridge pattern in the equatorial Atlantic is of recentand vastly improvedseismic data, to mirrored in the steppedshape of the coastlines be based on unreliable determinations of focal and the continental margins of Africa and mechanisms[Hodgson and Stevens,1964]. The Brazil. extensional features of structures based on gravity and seismicdata appear to be surficial ISIsAND ARcs--T• S•NKS and canbe reconciledwith, and in fact are pre- Almost anyone who glances casually at a dictedby, the new hypothesis. map of the world is intriguedby the organized High-Q and high-velocityzones in the mantle SEISMOLOGY AND NEW GLOBAL TECTONICS 5867

/ Faults

•0 ø II Troughsstriking NE 0 Earthquake Epicenters •,•'• EarthquakeMechanisms_

0 o i'

"{: 25 ø % .i "..,• o •"'

PacificOcean •;,•? , •t-. _

115ø IiOø Io5ø

Fig. 6. Structural features of the Gulf of California [after Sykes, 1968]. Relocated epi- centers of earthquakes for the period 1954 to 1962. Seismicity and focal mechanismssupport the hypothesis of spreading by ocean-ridge-transform-fault mechanism. beneathisland arcs. The grossstructure of an generatedby deep earthquakesin the seismic idealized island arc as shown in Figure i is zone and propagatedalong two different kinds basedon the resultsof Oliver and Isacks [1967]. of paths, one along the seismiczone and one Their study was primarily concernedwith the through an aseismicpart of the mantle, demon- Fiji-Tonga area. Comparison of seismicwaves strated the existence of an anomalous zone in 5868 ISACKS, OLIVER, AND SYKES the upper mantle. The anomalouszone was esti- each parameter on temperature is reasonable. mated to be about 100 km thick and to be This point is discussedfurther in another sec- boundedon the upper surfaceby the seismic tion. zone. Thus, the zone dips beneath the Tonga Bending o[ lithospherebeneath an island arc. are at about 45ø and extendsto depthsof al- The evidencesupporting the model in which most 700 km. The zone is anomalous in that the lithosphereplunges beneath the island arc attenuation of seismic waves is low and seismic is varied. To explorethis point further, consider velocities are high relative to those of the first the configurationof the upper part of the mantle at comparabledepths elsewhere. Recent lithosphere in the vicinity of an island arc studies of the Japanese are [Wadati et al., (Figure 7a). Seismic refraction studies of a 1967; Utsu, 1967] have confirmedthe existence number of island arcs have been made. In- of sucha structurefor that region.Similar zones variably they show the surface of the mantle, appear to be associatedwith other island arcs which is shallowbeneath the deep ocean,deep- [Oliver andIsacks, 1967; Cleary,1967; Molnar ening beneaththe trench, as suggestedby Fig- and Oliver, 1968]. ure 7a. Although someauthors suggest that the The presenceof a high-velocityslab beneath mantle merely deepens slightly beneath the an island are introducesa significantazimuthal islandsof the arc and shoalsagain behind the variation in the travel times of seismic waves. arc, evidencefor such a structure is incomplete. Such variations with respect to sourceanom- Mantle velocities beneath the islands, where alies are shownby Herrin and Taggatt [1966], determined, are low, and there is no case for Sykes [1966], Cleary [1967] and are indicated which the data could not be interpreted as sug- by the data of Carder et al. [1967] all for the gestedin Figure 7a (see, e.g., Badgley [1965] case of the Longshotnuclear explosion.With and Offriceret al. [1959]). In fact, the difficulty respect to station anomalies such variations are experiencedin documentingthe model in which shown by Oliver and Isacks [1967], Utsu the mantle is merely warped beneaththe islands [1967], Cleary and Hales [1966], and Herrin is evidenceagainst this model.The main crustal [1966] from data from earthquakes. These layer as determined from seismic refraction effects must therefore be taken into account as studies seems to parallel the surface of the sourcesof systematicerrors in the locationsof dipping mantle beneath the seaward slope of earthquakesand the constructionof travel-time the trench. In some interpretationsthe crustal curves.The large anomaly of Q associatedwith layer thins beneath the trench; in others it the slab must play a very important role in the thickensor remains constant.Perhaps these are Q structureof the mantle, especiallyfor studies real variations from trench to trench, but the based on body waves from deep earthquakes. data are not always definitive. Studies in which this effect is ignored [e.g., Thinning of the crusthas been interpreted by Teng, 1968] must be reassessedon this basis. Worzel [1965] and others as an indication of Oliver and Isacks associated the anomalous extension,and there is considerableevidence in zone with the layer of low attenuation near the the structure of the sediments on the seaward surfaceto the east of Tonga. In their interpre- slopes of many trenches supporting the hy- tation of the data, they correlatedlow attenua- pothesisof extension (see, e.g., Ludwig et al. tion with strength to arrive at the structure of [1966]). Figure 8, one of Ludwig's sections Figure I in which the lithosphere, a layer of acrossthe Japan trench, demonstratesthis point strength, descendsinto the mantle. This con- dramatically. Several graben-like structures are figuration suggeststhe mobility of the litho- seen on the seaward slope of the trench. Al- sphereimplied in Figure I and describedin the though such evidence for extensionhas been introduction. Based on current estimates of cited as an argument against sea-floorspread- lithospherevelocities and other parameters,the ing and convectionon the basisthat down-going down-goingslab would be much coolerthan its currents at the sites of the ocean deepswould surroundingsfor a long time interval. Although causecompression normal to the arcs,the argu- there is little evidencesupporting a direct rela- ment loses its force when the role of the litho- tion between low attenuation and strength, an sphereis recognized.All the evidencefor exten- indirect relation based on the dependenceof sion relatesonly to the sedimentsand crust,i.e., SEISMOLOGY AND NEW GLOBAL TECTONICS 5869

550km 300 250 200 150 I00 5O 0 50 I00 km LOCAL TENSION

CRUST S S CRUS LITHO-

SPHERE $ P HERE 0

ß ß ß ß ß

ß ß ß ß

ASTHENOSPHERE ,ß , •$THENOSPHERE

Fig. 7a.

50 km :500 250 200 150 I00 50 0 50 IOOkm LOCAL TENSION S • CRUST S S CRUST,

L I T H 0 S P HERE $ P H E LOCAL (• TENSION "••'""

ASTHENOSPHERE ASTHENOSPHERE 15( km

Fig. 7b. Figure 7 showsvertical sectionsthrough an island arc indicatinghypothetical structures and other features.Both sectionsshow down-goingslab of lithosphere,seismic zone near surface of slab and in adjacentcrust, tensional features beneath ocean deep where slab bends abruptly and surfaceis free. (In both sections,S indicatesseismic activity.) (a) A gap in mantle portion of lithosphere beneath island arc and circulation in mantle associatedwith crustal material of the slab and with adjoining mantle [Holmes, 1965]. (b) The overridinglithosphere in contact with the down-goingslab and bent upward as a result of overthrusting.The relation of the bending to the volcanoesfollows Gunn [ 1947]. No vertical exaggeration. the upper few kilometersof the lithosphere.For activity beneaththe seawardslope of the trench the modelspictured in Figure 7 in which a thick is, in general, infrequent and apparently of strong layer bendssharply as it passesbeneath shallow depth. The focal mechanismsthat have the trench, extensionalstresses are predicted been determined for such shocks indeed indi- near the surface on the convex side of the bend cate extension as predicted, i.e. normal to the even though the principal stressdeeper in the trench, the axis of bending (Stauder [1968] lithospheremay be compressional.Earthquake and T. Fitch and P. Davis, personalcommuni- 5870 ISACKS, OLIVER, AND SYKES

M d

', 5120m

App ro xi mat e xxx-!O

HorizontalI , , , , I , Scale, , , I 0 50 I00 km Fig. 8. Seismicreflection profile acrossthe Japan trench extendingeasterly along 35øN from point M near Japanto point N [after Ludwig et al., 1966].Vertical scalerepresents two- way reflectiontime in seconds(i.e., I sec ----I km of penetrationfor a velocity of 2' km/sec). Note block faulting along seawardslope of trench demonstratingextension in crustand inclu- sion of sediments in basement rocks. Also note shoaling of oceanic basement on approaching trench as suggestedby work of Gunn [1937]. Vertical exaggeration~25:1.

cartons).Stauder demonstratesthis point very tion of underformed sediments depends on well in a paper on focalmechanisms of shocks the ratio of rate of sediment accumulation of the Aleutian arc. to rate and continuity of underthrusting, The extensional features also suggest a and such data must be evaluated for each area mechanismfor includingand transportingsome with these factors in mind. The South Chile sedimentswithin the down-goingrock layers. trench, for example,has a large sedimentation As implied by Figure 7a, sedimentsin the rate but no associateddeep earthquakes,sug- graben-likefeatures may be carried down to gestinglittle or no recent thrusting. The results some depth in quantitiesthat may be very of Scholl et al. must be consideredin this light. significantpetrologically, as suggestedby Coats The Itikurangi trench (east of northern New [1962]. Probablynot all the sedimentscarried Zealand), another exampleof a partially filled into the trenchby motionof the seafloor or by trench, is also thought to be in a zone of low normalprocesses of sedimentationare absorbed convergencerate (see Le Pichon [1968] and in the mantle,however. There are largevolumes Figure 2). of low-densitymaterial beneaththe inner slope Underthrusting beneath island arcs. The on the island side of most trenches [Talwani shallow earthquakesmentioned above that indi- and Hayes, 1967] that may correspondto sedi- cate extensionnormal to the are occur relatively ment scrapedfrom the crust and deformedin infrequently and appear always to be located the thrusting.Unfortunately, the structureof beneath or seaward of the trench axis. The these low-densitybodies is not well explored, earthquakesthat accountfor most of the seismic and,in fact,the very difficultyof exploringthem activity at shallowdepths in island ares are lo- may be an indicationof their contortednature, cated beneath the landward slope of the trench which results from great deformation. and form a slab-like zone that dips beneath the The above argumentsapply to trenchesthat island are [Fedotov et al., 1963, 1964; Sykes, are relatively free of fiat-lying sediments,such 1966; Hamilton, 1968; Mitronovas et al., 1968]. as the Japan or Tonga trenches.The occurrence This point is illustrated in Figure 9, which of substantial quantities of fiat-lying unde- showsa vertical sectionthrough the Tonga are. formed sediments in some other trenches has Note that, for a wide range of depths, foei are been cited as evidence against underthrusting confined to a zone 20 km or less in thickness. in island arcs [Scholl e• al., 1968]. Accumu]a- In the focal mechanisms of the shallow SEISMOLOGY AND NEW GLOBAL TECTONICS 5871

Volcanic Coralline Arc Arc 1.1..%1:. ß. Water 0 ß•:. :::• , • Depth, km 5 '":' "::.....:h ]0 I I :(:'":':':•:':::"" 0 + 200

Dislance from Niumate, Tonga, km -400 -200 0 + 200 o -o

200

/ / , E

/ 400 55O /

6 ' 600

6OO km

, • - •50 - •00 - 250 km

Fig. 9. Vertical section oriented perpendicularto the Tonga arc. Circles represent earth- quakesprojected from within 0 to 150 km north of the section; triangles correspondto events projected from within 0 to 150 km south of the section.All shocksoccurred during 1965while the Lamont network .of stations in Tonga and Fiji was in operation. Locations are based on data from these stations and from more distant stations. [No microearthquakesfrom a sample of ?50 events originated from within the hatched region near the station at [Niumate, Tonga (i.e., for $-P times less than 6.5 sec). A vertical exaggerationof about 13:1 was used for the insert showing the topography [after Raitt et al., 1955]; the horizontal and vertical scalesare equal in the crosssection depicting earthquake locations. Lower insert shows enlargement of southern half of section for depths between 500 and 625 km. [Note small thickness (less than •20 km) of seismiczone for wide range of depths. 5872 ISACKS, OLIVER, AND SYKES shocksalong the slab-like zone of the Tonga- that of the deep seismiczone below it, appears Kermadee are, Isacks and Sykes [1968] find to be confinedmainly to the crust and to be consistent evidence for underthrusting of the secondaryto the activity alongthe main seismic seaward block beneath the landward block. zone. The Niigata earthquake of June 16, Abundant evidence for a similar process for 1964, appearsto be locatedin sucha secondary various island arcs of the North Pacific is zone of the North Itonshu arc. The mecha- found by Stauder [1962, 1968], Udias and nism of this earthquake [Hirasawa, 1965] indi- Stauder [1964], Stauder and Bollinger [1964, cates that the axis of maximum eompressive 1966a, b], Aki [1966], and Ichikawa [1966]. stress is more nearly horizontal than vertical Critical evaluation of focal mechanism data by and trends perpendicularto the strike of the Adams [1963], Hodgson and Stevens [1964], North ttonshu are. It is interesting that this Stauder [1964], and Ritsema [1964] showsthat stress is also perpendicularto the trend of the generalization that strike-slip faulting is Neogene folding in North ttonshu [Matsuda predominant in island arcs is based on unre- et al., 1967]. These resultsmight indicatesome liable data and possiblesystematic errors in the compressive deformation of the overriding analyses.The recent data, greatly improved in plates in the modelsof Figure 7. quality and quantity, indicate that, in fact, dip- Deep earthquakes:the down-goingslab. The slip mechanismsare predominant in island arcs. shallow seismic zone indicated by the major The thrust fault mechanisms characteristic of seismic activity is continuous with the deep shallow earthquakesin island arcs thus appear zone, which normally dips beneath the island to reflect directly the relative movementsof the are at about 45 ø. The thickness of the seismic convergingplates of lithosphereand the down- zone is not well known in most cases,but it ward motion of the oceanic plate. The com- appearsto be lessthan about 100 km and some patibility of these motions as determined by evidencesuggests that it may, at least in some focal mechanismdata with the worldwide pat- areas, be less than 20 km. Figure 9 illustrates tern of plate movementsis discussedlater and this point for a sectionthrough the Tonga arc. is shown to be excellent. Although the surfaceapproximating the distri- Considerableevidence for underthrusting in bution of hypocentersmay be describedroughly the main shallow seismic zone exists in other as above, it is clear that significantvariations kinds of observations. Geodetic and geologic from this simplepicture exist and are important. studies of the Alaskan earthquake of 1964 For example, the over-all dips may vary from [Parkin, 1966; PlaCket,1965] stronglysupport at least 30ø to 70ø, and locally the variation the concept of underthrusting. Geologic evi- may be greater, as suggestedby the data in dence also indicates the repeated occurrenceof Figure 9. For the Tonga-Kermadee arc, the such thrusting in this arc during recent time number of deep eventsis large and the zone can [Plafker and Rubin, 1967]. Data from other be definedin somedetail [Sykes, 1966]. Sykes arcs on crustal movements are voluminous and was able to show,as a result of a marked curva- have not all been examined in light of the ture of the northern part of the Tonga are, a hypothesesof the new global tectonics.In fact, clear correlation between the configurationof in many arcs the principal zone of underthrust- the deep seismiczone and surface features of ing would outcrop beneath the sen and im- the arc, thereby demonstrating the intimate portant data would be largely obscured.One relationshipbetween the deep and the shallow important point can be made. It is well known processes. [Richter, 1958] that vertical movements in For most arcs, however,the number of deep island arcs are of primary importance.This con- events, particularly since the World-Wide trasts with the predominantlyhorizontal move- Standardized SeismographNetwork has been ment in such zonesas California, where strike- in operation,is relatively small; therefore,the slip faulting predominates. deep zonescannot be definedas preciselyas one Other shallow activity in island arcs. In might desire.Nevertheless, sufficient information some island arcs there is appreciable shallow is availableon the pattern of seismicactivity so seismicactivity landward of the principal seis- that the conceptof the mobile lithospherecan mic zone. This activity, which is distinct from be tested in general, and it must be assumed SEISMOLOGY AND NEW GLOBAL TECTONICS 5873 that subsequentdetailed studiesof other island than the compressionalaxes, these axes are not arcs may reveal contortionsin the seismiczone randomlyoriented. The axis of tensiontends to comparablewith the contortionsalready found be perpendicularto the seismiczone; the null in Tonga-Fiji. axis, parallel to the strike of the zone. These Focal mechanisms. The simple underthrust- generalizationsare shownschematically in Fig- ing typical of the shallow earthquakesof the ure 11. The slip planes and slip directionsare principalzones does not, in general,persist at thus systematicallynonparallel to the seismic great depths.For shocksdeeper than about 100 zones; the orientationsare therefore difficult to km the orientation of the focal mechanisms reconcilewith a simple shearingparallel to the varies considerablybut exhibitscertain clear-cut seismiczone as suggestedby the common con- regularities.To understandthese regularities,it cept of the zone as a large thrust fault. Sugi- is important to recall what is determined in a mura and Uyeda [1967] soughtto reconcilethe focal mechanism solution. The double-couple observationswith that concept by postulating solution, which appears to be the best repre- a reorientation of crystalline slip planes per- sentation of most earthquakes,comprises two pendicularto the axis of maximumcompressive orthogonalnodal planes, either of which may stress,such that in the caseof horizontalcom- be taken as the slip plane of the equivalent pressionthe slipplanes would tend to be vertical. shear dislocation.Bisecting these nodal planes are the axis of compression,P, in the quadrants STRIKE OF of dilatational first motions and the axis of ARC tension, T, in the quadrants of compressional first motions. The axis formed by the intersec- tion of the nodal planes is the null, or B, axis parallel to which no relative motiontakes place. / /,•' // X x x• If one nodal plane is chosenas the slip plane, X//Xo .// the pole of the other nodal plane is the direction / o o• of relative motion of the slip vector. It is im- PLUNGE / o portant to realize that the primary information OF given by a double-couplesolution is the orien- SEISMIC oE5 o, tation of the two possibleslip planes and slip ZONE øxI vectors.The interpretationof the double-couple mechanisms in terms of stress in the source region requiresan assumptionabout the failure x x process.The P, T, and B axescorrespond to the maximum, minimum, and intermediateaxes of compressivestress in the medium only if the shear dislocation is assumed to form parallel Fig. 10. Orientations of the axes of stress as given by the double-couplefocal mechanismsolu- to a plane of maximumshear stress in the me- tions of deep and intermediateearthquakes in the dium, i.e., a plane that is parallel to the axis of Tonga arc, the Izu-Bonin arc, and the North intermediate stress and that forms a 45 ø angle ttonshu arc. Open circles are axes of compression, to the axes of maximum and minimum stress. P; solid circlesare axes of tension, T; and crosses are null axes, B, all plotted on the lower hemi- Patterns of focal mechanismsfor deep earth- sphere of an equal-area projection. The data, se- quakes. The most striking regularity in the lected from available literature as the most reli- orientation of the double-couple focal mecha- able solutions, are taken from Isacks and Sykes nisms of deep and intermediateearthquakes is [1968], Honda et al. [1956], Ritsema [1965], and Hirasawa [1966]. The data for each of the the tendencyof the P axesto parallel the local three arcs are plotted relative to the strike of the dip of the seismiczone. Figure 10 illustrates arc (Tonga arc, N 20øE; Izu-Bonin, N 15øW; this point for the three zones (Tonga, Izu- North ttonshu arc, N 20øE). The dips of the zones Bonin, and North Itonshu) for whichreliable vary between about 30ø and 60ø, as indicated by data are most numerous.This figure also shows the dashed lines in the figure. Note the tendency of the P axes to parallel the dip of the seismic that, althoughthe orientationof the axes of zone and the weaker tendency for the T axes to tension and the null axes tend to be less stable be perpendicular to the zone. 5874 ISACKS, OLIVER, AND SYKES ,,/ TRENCH•AXIS 0 •

200 KM o/ 6ooI y / 2ooI,• • Fig. 11. Vertical sectionsperpendicular to the strike of an island arc showingschematically typical orientations of double-couplefocal mechanisms.The horizontal scale is the same as the vertical scale. The axis of compressionis represented by a converging pair of arrows; the axis of tension is representedby a diverging pair; the null axis is perpendicular to the section. In the circular blowups, the sense of motion is shown for both of the two possible slip planes.The featuresshown in the main part of the figure are based on results from the Tonga arc and the arcs of the North Pacific. The insert showsthe orientation of a focal mechanism that could indicate extension instead of compressionparallel to the dip of the zone.

Alternatively,Isacks and Sykes [1968] show would, therefore, be expected, especially near that, if it is assumedthat the slip planesform parts of the zone with complex structure. at angleswith respectto the axis of maximum The important feature of the interpretation compressivestress that are not significantly presented here is that the deep earthquake different from 45ø, then a very simpleinterpre- mechanisms reflect stresses in the relatively tation can be made on the basis of the model strong slab of lithosphere and do not directly of Figure 1. In this interpretationthe axis of accommodatethe shearing motions parallel to maximumcompressive stress is parallel to the the motion of the slab as is implied by the dip of the seismiczone (i.e., parallel to the simple fault-zonemodel. The shearingdeforma- presumedmotion of the slab in the mantle), tions parallel to the motion of the slab are pre- and the axis of least compressivestress is per- sumablyaccommodated by flow or creepin the pendicularto the zoneor parallelto the thin adjoining ductile parts of the mantle. dimension of the slab. Do the stressesin the slab vary with depth? The tendencyfor the compressireaxes to be In particular, the axis of least compressive more stable than the other two axes can be stress,the T axis, may be parallel to the dip interpretedto indicatethat the differencebe- of the zone if the material at greater depths tween the intermediate and the least principal were sinking and pulling shallowerparts of the stressesis less than the differencebetween the slab [Elsasset,1967]. In the Tonga, Aleutian, greatestand the intermediateprincipal stresses. and Japanesearcs, the focal mechanismsindicate In general,the stressstate may be quitevari- that the slab is under compressionparallel to its able owingto contortionsof the slab, as sug- dip at all depths greater than about 75 to 100 gestedby Figure9. Possiblylarge variability km. In these arcs, therefore, any extensionin in the orientations of the deep mechanisms the slab must be shallower than 75 to 100 kin. SEISMOLOGY AND NEW GLOBAL TECTONICS 5875 Very limited evidencefrom the Kermadec ures7a and 11 and supportedby the data shown [Isacksand Sykes,1968], New Zealand(North in Figure 9. Although catastrophic phase Island) [Adams,1963], SouthAmerican (A. R. changesmay be ruled out as direct sourcesof Ritsema,personal communication), and Sund• seismic waves on the basis of the radiation (T. Fitch, personalcommunication) arcs sug- pattern of the waves, the possibility remains gests,however, that mechanismsindicating ex- that the stressesresponsible or partly respon- tensionof the slab, as shownin the insert of sible for shear failure may result from some- Figure 11, may existat intermediatedepths in what slowerphase changes. some arcs. Further work is required to dis- Seismic activity versus depth. Frequencies tinguish such mechanismsfrom the under- of earthquakesversus depth for several island thrustingtype of mechanismcharacteristic of arcs are shown in Figure 12. There are two earthquakesat shallowdepths or from complex main resultsemerging from theseanalyses. (1) mechanismsrelated to changesin structure or In all island arcs studied the activity decreases contortions of the slab. in the upper 100 to 200 km approximately Processo• deep earthquakes. The idea that exponentially as a function of depth with a deepearthquakes occur in downgoingslabs of decay constantof about 100 km [Sykes, 1966]. lithospherehas importantimplications for the (2) At greater depths the seismicactivity in problem of identifying the physicalprocess re- many (but not all) island arcs increasesrela- sponsiblefor suddenshear failure in the en- tive to the exponentialdecay extrapolatedfrom vironment of the upper mantle. That deep shallowerdepths, and the seismicactivity shows earthquakes are essentially sudden shearing a fairly well-definedmaximum in some depth movements and not explosive or implosive range in the upper mantle. The variation of changesin volume is now extensivelydocu- seismicactivity with depth is thus grosslycor- mented (seeIsacks and Sykes [1968] for refer- related with the variation of seismic focal ences).Anomalous temperatures and composi- mechanismswith depth and supports the gen- tion might be expectedto be associatedwith eralization that deep earthquake mechanisms the down-goingslab, either or both ot• which have a different relationship to the zone than may accountfor the existenceof earthquakesat shallow mechanisms do. In this correlation the great depths. earthquakesthat define the shallow exponential Several investigators[Raleigh and Paterson, decay in seismicactivity are characterizedby 1965; Raleigh, 1967] concludedthat dehydra- the underthrusting type mechanisms,whereas tion of hydrous minerals can releaseenough the deeper earthquakesappear to be related to water to permit shear fracture at temperatures the stressesin the down-goingslab. between about 300ø and 1000øC. Although There is an approximate correlation of the Griggs [1966] and Griggs and Baker [1968], decreasein seismicactivity versus depth with assumingnormal thermal gradients,suggested a similar general decreasein seismicvelocities, that these reactions would not take place for Q, and viscosity in the upper 150 km. These depthsgreater than about 100 km, rates of effectsmay be related to a decreasein the differ- underthrustingas high as 5 to 15 cm/yr sug- ence between the temperature and local melting gest that temperatureslow enoughto permit temperature.Thus, the decreasein activity with these reactions to occur may exist even to depth may correspondto an increase in the depthsof 700 km. Certainlya re-evaluationof ratio of the amount of deformation by ductile theseprocesses is in order. flow to that by suddenshear failure. An impli- The lowest temperatures,the largest tem- cation of this interpretation is that the ex- perature gradients,and the largest composi- ponential decay constant of 100 km may tional anomalies would probably be most roughlyindicate the thicknessof the overthrust marked near the upper part of the slab, i.e. plate of lithosphere.This interpretation is illu- the part correspondingto the crust and upper- strated in Figure 7b, in which the overriding most mantle in the surficial lithosphere.Thus, plate of lithosphere is in 'contact' with the the seismicactivity associatedwith theseanom- down-goingplate along the seismic zone. As aliesmight be expectedto concentratenear the shown in a later section,the assumptionthat upper part of the slab,as is suggestedin Fig- the depth distribution of shallow earthquakes 5876 ISACKS, OLIVER, AND SYKES

I00

6O

;50

2O

-__'X--••,••-.•L TONGA- FIJI-KERMADEC - 2 _ , _ KURIL-KAMCHATKA /', AN 0.6 _- - ' - 0.3 - ALASKAi v -

0.2 - I EXlCO -

0.1 CARIBBEANI .06 _ I I I I I I I I I •1 I I I I I I I I I I I 0 I00 200 500 400 500 600 700 Depth, km Fig. 12. Number of earthquakesper 25-km depth intervals as function of depth for several island arcs.Except for Japan, data are from Sykes [1966]. Data for Japan expressedas percent- age of events per 50-km depth intervals [Katsumata, 1967]. Since the various curves were not normalized for the sample lengths and for the lower limit of detectability in each area, only the relative shapesand not the absolute levels of the various curves should be comparedwith one another. The number of earthquakes per unit depth within the upper 200 km of all these island arcs is approximately proportional to exp (--Z/100), where Z is the depth in kilometers.Peaks in activity below 200 km appear to fluctuateboth in amplitude and in depth among the various arcs. of the mid-oceanrift system yields a measure rectly beneath and behind the arc as shown in of the thickness of the lithosphere is not an Figure 7b. Oliver and Isacks [1967] and Molnar unreasonable one. and Oliver [1968] showthat high-frequency$• Several lines of evidence do not, however, does not propagate acrossthe concaveside of support the existenceof thick lithosphere di- island arcs, which probably indicatesthat the SEISMOLOGY AND NEW GLOBAL TECTONICS 5877 uppermost mantle there has low Q values. This result is in agreementwith the low P, velocities generally found beneath islands of many arcs. Also, the active volcanismand high heat flow eharacetristie of the concave side of island arcs o - • • • o • o o • • - [Uyeda and Horai, 1964; Sclater et al., 1968] oF suggestthat the lithospheremay be thin there. These data are qualitatively fitted by the model shown in Figure 7a. One implication of this figure is that at least part of the shallowearth- quake zone might not result from the contact 400- between two pieces of lithosphere but might instead indicate an embrittled and weakened zone formed by the downwardmoving crustal 600 materials [Raleigh and Paterson, 1965; Griggs, 1967]. In this case the exponential decay in activity might reflect changesin the properties 8OO of the earthquake zone as a function of depth. Fig. 13. Depth range of maxima in the seismic Thus, both models in Figure 7 must be re- activity (numbers of earthquakes) as a function tained for the present. of depth in island arcs and arc-like structuresfor which data are sufi%iently numerous. The data Although in some arcs such as the Aleutians are from Gutenberg and Richter [1954], Katsu- or Middle America the exponential decay in mata [1967], Sykes [1966], and listings of earth- activity appears to be the only feature present quakes located by the USCGS in the preliminary in curves of activity versus depth, most arcs determination of epicenters (PDE). The numbers exhibit a more or less well-defined maximum in at the bottom of the figure give the rate (in centimeters per year) of convergence for the arc activity in the mantle, as illustrated in Figure as plotted in Figure 2. Note that the maxima 12. The approximate ranges of depth of these occur over a wide range of depths and that the maxima are shown in Figure 13 for several depths appear to correlate, in general, with the island arcs. The main point of this figure is to calculated slip rate. show that the depths of these maxima vary considerablyamong the various ares and do not cared no earthquakeswith a depth greater than appearto be associatedwith any particularlevel 690 km during the period 1961-1967. These of depth in the mantle, contrary to general depths are near the region of the mantle in opinion. As shown in Figure 13, the depths of which gradients in the variation of seismic the deep maxima are approximatelycorrelated velocitiesmay be high [Johnson,1967]. Ander- with the rates of convergencein the arcs as cal- son [1967a] arguesthat this region corresponds culated by Le Piehon. As will be shown later to a phasechange in the material. These depths (see Figure 16), the correlationis considerably may therefore be in some way related to the better betweenthe rate of convergenceand the boundary of the mesosphereas shown in Fig- lengthof the zonemeasured along the dip of the ures I and 14 and as discussed in the next sec- zone. Thus, the simplestexplanation, one direct tion. The second feature is the absence of consequenceof the model of Figure 1, is that maxima around 300 km. Thus, in a worldwide the deep maxima are near the leading parts of compositeplot of activity versusdepth, a mini- the down-goingslabs. mum in activity near this depth generally ap- Two features of the distributions shown in pears. Figures 12 and 13 may be related to certain Downward movement of lithosphere in the levels of depth in the mantle. Although the mantle: some hypotheses. Although the con- length of the seismiczone measuredalong the cept is so new that it is difScult to make dip of the zone exceeds1000 km for several definitive statements, a brief speculative dis- cases,no earthquakeswith depths greater than cussionis in order to emphasizethe importance 720 km have ever been documented. The U.S. of these resultsin global tectonics.Figure 14, Coast and Geodetic Survey (USCGS) has lo- four hypothetical and very schematiccross sec- 5878 ISACKS, OLIVER, AND SYKES tions of an island arc, illustratessome points that should be considered. Figure 14a showsa case in which the litho- sphere has descendedinto the mantle beneath an island arc. In this model the lithospherehas not been appreciably modified with regard to its potential for earthquakesand the length of the submergedportion, l, and hencethe depths of the deepest earthquakes are dependent on Fig. 14a. Length l is a measureof the amount the rate of movementdown dip and the dura- of underthrustingduring the most recent period of tion of the current cycle of sea-floorspreading sea-floor spreading. and underthrusting. In Figure 14b the leadingedge of the descend- ing lithosphere has encounteredsignificant re- sistance to further descent and has become distorted. In this situation, the depth of the deepestearthquakes in such a zone dependson the depth to the mesosphere.Sykes' [1966] analysisof the relocationsof earthquakesin the Tonga arc (see also Figure 9) revealsthe pres- Fig. 14b. Lithosphere is deformed along its enceof contortionsof the lower part of the seis- lower edge as it encountersa more resistant layer mic zone which might indicate a phenomenon (the mesosphere). similar to that pictured in Figure 14b. This model has the interesting consequencethat a cycle of sea-floor spreading might be termi- nated or sharply modified by the bottomingof the lithosphereat certainpoints. Figure 14c indicates schematicallythat the depths of the deepest earthquakesmight de- pend on modification of the lithosphereby its environment. In this model the depth of the deepestshocks depends on the rate of descent Fig. 14c. Length of seismiczone is the product and the rate of modificationsor absorptionof of rate of underthrusting and time constant for the lithosphere. assimilation of slab by upper mantle. In Figure 14d the lower portion of the des- cendingpart of the lithosphereis not connected with the upper portion, possiblybecause it has pulled away as a result of a large density con- trast between the sinking part of lithosphere and the surroundingmantle. Anotherpossibility is that the lower piece represents a previous episode of movement, so that the break be- tween the pieces then representsa period of //////////// quiescencein the surface movements. For ex- Fig. 14d. A piece (or pieces) of the lithosphere ample, the Spanish deep earthquake of 1954 becomes detached either by gravitational sinking [Hodgson and Cock, 1956] and the very deep or by forcesin the asthenosphere. earthquakesbeneath the North Island of New Zealand [Adams, 1963] might indicate isolated piecesof lithosphere.A variant of Figure 14d Figure 14 shows four possible configurations of is the case in which movements of the ductile an underthrust plate of lithosphere in island arcs. Solid areas indicates lithosphere; white area, material of the asthenosphere,movements that asthenosphere;hatched area, mesosphere. could be quite different from the movements SEISMOLOGY AND NEW GLOBAL TECTONICS 5879 of the surficiallithospheric plates, could deform for extension and compressionalfeatures of the slabs and possibly break off pieces. For island arcs. The results suggestthat there are example,the marked contortionsof the deep two basic types of focal mechanisms.The first seismiczone of Tonga may be explainedby type is apparently confinedto shallow depths such deformation.Thus, althoughthe evidence and directly accommodates,and thereforeindi- is at presentonly suggestive,such evidence is cates the direction of, the movementsbetween importantbecause of the implicationsof the the plates of lithosphere.The secondtype indi- hypotheseswith regard to the dynamicsof cates stress and deformation within a plate of the systemwithin the asthenosphere.Various lithosphereand includes,besides the deep and combinations of the effects illustrated in Fig- intermediate earthquake mechanisms,the nor- ure 14 may alsobe considered. mal-faulting mechanisms at shallow depths Lateral terminationsof island arcs. The dis- beneath the axis of the trench and, possibly, cussionsabove are based on, and apply largely the shallow earthquake mechanisms located to, the structureof an islandarc taken in a landward of the underthrust zone. The deep vertical section normal to the strike of an arc. earthquake zonesmay provide the most direct The three-dimensionalconfiguration of the arc source of information on the movement of mate- must also be considered.The plate model of rial in the asthenosphereand on the basic tectonicsprovides, in a simple way, for the questionof the relative importanceof the litho- termination of an island arc by the abrupt or spheric and asthenosphericmotions in driving gradualtransition to a transformfault, by a the convective system. The global pattern of decreasein the rate of convergenceto zero, or motions between the plates, derived in part by somecombination of these.In the first case from the shallowfocal mechanismin island arcs, the relative movement that is predominantly providesa severetest of the hypothesisof plate normal to the zone of deformation changesto movementsin generaland providesin particular relative movementthat is predominantlyparal- key evidencefor the conclusionthat island arcs lel to the zone. In the second case the pole are the major zonesof convergenceand down- governingthe relative motion betweenthe ward movements of the lithospheric plates. platesmay be locatedalong the strikeof the This evidence,including observationsof direc- feature.Isacks and Sykes [1968] describewhat tions as well as rates of movement,is discussed may be a particularlysimple case of the first in the next section. possibility.The northernend of the Tongaarc COMPATIBILITY OF MOVEMENTS ON A appearsto endin a transformfault that strikes WORLDWIDE SCALE approximatelynormal to the arc. In this case evidenceis also found for a scissorstype of In this sectiondeformations along the world faulting in which the downgoingPacific plate rift system and along island arcs and major tears away from the part of the plate remain- mountain belts are examined for their internal ing at the surface. consistencyand for their global compatibility. Summaryo• data on islandarcs. The litho- The major finding is that these displacements spheremodel of an island arc thus gives a can be approximatedrather precisely by the remarkably simple accountof diverseand im- interactions and the relative movements of portant observedfeatures of islandarcs. The large plates of lithosphere,much of the defor- existence and distribution of earthquakes in mation being concentratedalong the edgesof the mantle beneath island arcs, the anomalous the platesand relativelylittle deformationbeing transmissionproperties of deep seismiczones, within the individual plates themselves.It has and the correspondencesin the variationsof long been recognizedthat recent deformations seismicactivity and the orientationsof the focal of the earth's surface are concentrated in nar- mechanismsas functions of depth are all in row belts. These belts, which largely coincide agreementwith the conceptof a cooler,rela- with the major seismic zones of the world, tively strongslab movingthrough a relatively include the world rift system,island arcs, and ductile asthenosphere.The bend in the slab re- such island-arc-like features as active mountain quired by this movementprovides a simple belts and active continental margins. These means of reconcilingthe conflictingevidence major tectonic features do not end abruptly; 5880 ISACKS, OLIVER, AND SYKES they appearto be linked togetherinto a global data were taken from Stauder [1962, 1968], tectonic scheme. Stauder and Udias [1963], Stauder and Boll- Continuity of seismicbelts and distributionof inger [1964, 1966a, b], Harding and Rinehart seismicactivity. Figure 15, a compilationof [1966], Ichikawa [1966], Sykes [1967, 1968], about 29,000 earthquake epicenters for the Banghat and Sykes [1968], Isacks and Sykes world as reportedby the U.S. Coast and Geo- [1968], and Tobin and Sykes [1968]. Although detic Surveyfor the period 1961 to 1967 [Bara- no attempt was made to ensure that the collec- zangi and Dotman, 1968], showsthat most of tion of mechanismsolutions represented all the the world's seismicactivity is concentratedin reliableprevious work, nonetheless,the data are rather narrow belts and that thesebelts may be thought to be representative; no attempt was regardedas continuous.Thus, if globaltectonics made to select the data by criteria other than can be modeledby the interactionof a few large their reliability. In some cases such as the plates of lithosphere, this model can account aftershocksof the great Alaska earthquake of for most of the world's seismicactivity as effects 1964 and the aftershocks of the large :Rat at or near the edgesof the plates. Figure 15 Islands earthquake of 1965 [Stauder and Boll- also shows that the earthquakes occur much inger, 1966b; Stauder,1968], only representa- more frequently, in general, in the zones of tive solutionswere included,since the number of convergence,the arcs and arc-like features,than solutions for these regions was too large to in the zones of divergence,the ocean ridges. depict clearly in Figure 3. Solutions cited as Along the ocean ridges, where the less compli- reliable by Ritsema [1964] as well as those of cated processesof tectonics are apparently Honda et al. [1956], for example,were not used occurring, the zones are narrow; on the con- becausethey pertain to subcrustalshocks. tinents, where the processesare apparently From each mechanism solution used one of more complex, the zones are broad, and dis- two possibleslip vectors was chosenas indica- tinctive features are not easily resolved. I)eep tive of the relative motions of the two interact- earthquake zones,indicated in Figure 15 only ing blocks of lithosphere.For each slip vector by the width of epicentralregions behind arcs, the arrow depicts the relative motion of the correspond to the zones of underthrusting. block on which it is drawn with respectto the Thus, all the major features of the map of block on the other side of the tectonic feature. seismic epicenters are in general accord with Since the double-couplemodel (or shear dis- the new global tectonics.No other hypothesis location) appears to be an excellentapproxi- has ever begun to accountso well for the dis- mation to the radiation field of earthquakes,it tribution of seismicactivity, which must rank is not possibleto choosefrom seismicdata alone as one of the primary observationsof seismol- one of the two possibleslip vectors as the actual ogy. The details of the configurationof the motion vector (or alternatively to chooseone seismicbelts of Figure 15 are discussedfurther of two possiblenodal planes as the fault plane). in other sectionsof this paper. Nevertheless,the choiceof one vector is not Slip vectors. Figure 3 illustrates the distri- arbitrary but is justified either by the orienta- bution of these major tectonic features and tion of the vectors with respect to known tec- summarizesazimuths of motion as indicatedby tonic featuressuch as fracture zonesor by the the slip vectorsdetermined from various studies consistencyof a set of vectorsin a givenregion. of the focal mechanisms of shallow-focus earth- For earthquakeslocated on such major trans- quakes.I)eep and intermediateearthquakes as form faults as the oceanicfracture zones,the well as shallow earthquakeswith normal fault- San Andreas fault, and the Queen Charlotte ing mechanismsnear trencheswere not repre- Islandsfault (Figure 3), one of the slip vectors sented in this figure, since these mechanisms is very nearly parallel to the transform fault on are not thought to involve the relative displace- which the earthquake was located. Observed ments of two large blocksof lithosphere.Earth- surfacebreakage and geodeticmeasurements in quake mechanismswere included in Figure 3 someearthquakes, the alignmentof epicenters only when, by careful examination of the first- alongthe strike of major transformfaults, the motion plots, we could verify that the slip lineartry of fracture zones, and petrological vectors were reasonablywell determined.These evidence for intense shearing stressesin the SEISMOLOGY AND NEW GLOBAL TECTONICS 5881 5882 ISACKS, OLIVER, AND SYKES vicinity of fracture zones constitute strong evi- ruled out in the new global tectonics,rapid ex- dencefor making a rational choicebetween the pansion of the earth is not required to explain two possibleslip vectors [Sykes, 1967]. The the large amountsof new materialsadded at the choiceof the other possibleslip vector (or nodal crests of the world rift system.This approxi- plane) for many of the oceanicfracture zones mate equality of surfacearea is, however,prob- would indicatestrike-slip motion nearly parallel ably maintained for periodslonger than thou- to the ridge axis. On the contrary, earthquakes sandsto millionsof years,but minor imbalances along the ridge crest but not on fracture zones very likely could be maintained for shorter do not containa large strike-slipcomponent but periods as strains within the plates of litho- are characterizedby a predominanceof normal sphere. More exact knowledgeof these im- faulting. balances could be of direct interest to the Evidence •or motions o• lithospheric plates. problem of earthquake prediction. One of the most obviousfeatures in Figure 3 is Figure 3 suggeststhat nearly all the east- that the slip vectors are consistentwith the west spreading along the East Pacific rise and hypothesisthat surface area is being created the mid-Atlantic ridge is taken up either by along the world rift system and is being de- the island arcs of the western Pacific or by the stroyed in island arcs. Along the mid-Atlantic arc-like features bordering the west coasts of ridge, for example, slip vectors for more than Central and South America. Much of the north- ten events are nearly parallel to one another south spreading in the Indian Ocean is ab- and are parallel to their neighboringfracture sorbedin the Alpide zone,which stretchesfrom zones within the limits of uncertainty in either the Azores-Gibraltar ridge acrossthe Mediter- the mechanism solutions (about 20 ø) or the ranean to southern Asia and then to Indonesia. strikes of the fracture zones. Relative motions in the southwest Pacific. Morgan [1968] and Le Pichon [1968] showed Le Pichon's computed directions of motion in that the distribution of fracture zones and the the Tonga and Kermadec arcs of the south- observed directions and rates of spreading on west Pacific agree very closelywith mechanisms ocean ridges as determined from geomagnetic we obtained from a specialstudy of that region data could be explainedby the relative motions [Isacks and Sykes, 1968]. Mechanismssouth of of a few large plates of lithosphere.They deter- New Zealand along the Macquarie ridge [Sykes, mined the poles of rotation that describethe 1967; Banghat and Sykes,1968] indicatea com- relative motion of adjacent plates on the globe. bination of thrust faulting and right-lateral Our evidence from earthquake mechanisms strike-slip motion. These data suggestthat the and from the worldwide distribution of seismic pole of rotation for these two large blocks is activity is in remarkable agreementwith their located about 10 ø farther south than estimated hypothesis.Although their data are mostly from by Le Pichon [1968]. Althoughthe Pacificplate ridgesand transformfaults, earthquakemecha- is being underthrust in the Tonga and Kerma- nisms give the relative motions along island dec arcs and in northern New Zealand,this plate arcs as well as along ridges and transform is apparently being overthrust along the Mac- faults. quatie ridge (see also Summerhayes[1967]). Le Pichon used data from ocean ridges to In this interpretation the Alpine fault is a infer the direction of motion in island arcs. IIis right-lateral transform fault of the arc-arc type predicted movements (Figure 2), which are that connectstwo zones of thrusting with op- based on the assumption of conservation of posing dips. Computed slip vectors for this surface area and no deformation within the regionalso indicate a componentof thrust fault- platesof lithosphere,compare very closelywith ing either along the Alpine fault itself or in mechanism solutions in a number of arcs. This other parts of the South Island of New Zea- agreementis a strong argument for the hy- land. Wellman's [1955] studies of Quaternary pothesisthat the amountof surfacearea that is deformation, which indicate a thrusting com- destroyedin island arcs is approximatelyequal ponent as well as a right-lateralstrike-slip com- to the amount of new area that is created along ponent of motion along the Alpine and asso- the world rift system. Thus, although modest ciated faults, seemto be in generalaccord with expansionor contractionof the earth is not this concept. SEISMOLOGY AND NEW GLOBAL TECTONICS 5553 Likewise,the Philippine fault appearsto con- larger strike-slipcomponent along the Aleu- nect a zoneof underthrustingof the Pacificfloor tian arc as the longitudebecomes more westerly. near the Philippine trench with a region of ]Displacementsin the Kurile, Kamchatka,and overthrustingwest of the island of Luzon near Japanesearcs are nearly pure dip-slip and the Manila trench. Also, the existenceof a deep representunderthrusting of the Pacific plate seismiczone in the New Hebrides arc that dips beneath the arcs. toward the Pacific rather than away from the The systemof great east-westfracture zones Pacific as in the Tonga arc [Sykes, 1966] is in the northeastern Pacific (Figure 2) appar- understandableif the Pacificplate is beingover- ently was formedmore than 10 m.y. ago.Since thrust in the New Hebrides and underthrust in that time the directionsof spreadingchanged Tonga (Figure 1). The ends of these two arcs from east-west to their present northwest- appear to be joined togther by one or more southeastpattern [Vine, 1966]. The more west- transform faults that pass close to Fiji, but erly strike of the slip vectorsalong the Juan additional complicationsappear to exist in this de Fuca and Gorda ridges is consistentwith area. the hypothesisthat the area to the east of North Pacific. The uniformity in the slip di- these ridges representsa small separateplate rections and the distribution of major faults [Morgan, 1968; McKenzie and Parker, 1967] along the margins of the North Pacific (with that was underthrust beneath the coast of perhaps some systematicdeparture in the slip Washingtonand Oregonto form the volcanoes directionsoff the coast of Washington and Ore- of the Cascaderange. A few earthquakesthat gon) indicatethat only two blocksare involved have been detected in western Oregon and in the major tectonics[Tobin and Sykes, 1968; Washington are apparently of a subcrustal Morgan, 1968; McKenzie and Parker, 1967]. In origin [Neumann, 1959; Tobin and Sykes, this scheme,the San Andreas fault, the Queen 1968]. The tectonicsof this block appearsto Charlotte Islands fault, and a series of north- be quite complex.Internal deformationwithin westerly striking faults in the Gulf of Cali- this block, as indicated by the presenceof fornia are interpreted as major transform earthquakes[Tobin and Sykes,1968], and the faults [Wilson, 1965a, b]. The observedrates small number of subcrustal events previously of displacementalong the San Andreas as de- mentionedsuggest that the tectonicregime in termined geodetically [Whirten, 1955, 1956] this regionis beingreadjusted. are very similar to the rates determined from Depths o/ earthquakes,volcanism, and their the seismicity by means of a dislocation model correlation with high rates o/ underthrusting. [Brune, 1968]. These rates are in close agree- The depths of the deepest earthquakes,the ment with the rates inferred from magnetic presenceof volcanism,and the occurrenceof anomaliesfor the region of growing ridges at tsunamis(seismic sea waves) seemto be related the northwestern end of the San Andreas fault closelyto the present rates of underthrustingin [Vine and Wilson, 1965; Vine, 1966]. Esti- island arcs. In the series of arcs that stretches mates of the total amount of offset along the from Tonga to the Macquarie ridge the depths San Andreas [Hamilton and Meyers, 1966] of the deepest earthquakes generally decrease are comparableto the amount of offset needed from about 690 km in the north lo less than to close the Gulf of California and to the width 100 km in the south [Hamilton and Evison, of the zonesof northeasterlystriking magnetic 1968]. The rates of underthrusting computed anomaliesoff the coast of Oregon and Wash- by Le Pichonalso decrease from north to south. ington [Vine and Wilson, 1965]. Volcanic activity, which is prominent north of Thus, the present tectonics of much of the the South Island of New Zealand,dies out when North Pacific can be related to the motion of the deepest part of the seismiczone shoalsto the Pacific plate relative to North America depths less than about 100 to 200 km. Simi- and northeastern Asia. The slip vectors strike larly, the depth of deepest activity in the northwesterly along the west coast of the Aleutian arc and the number of volcanoes United States and Canada, represent nearly (Figure 2) appear to decreasefrom east to pure dip-slip motion in southern Alaska and west as the rate of underthrusting decreases. the easternAleutians, and have an increasingly In this arc the rate of underthrustingdecreases 5884 ISACKS, OLIVER, AND SYKES becausethe slip vectors become more nearly in Hawaii [Gutenberg and Richter, 1954]. parallel to the arc. Volcanism and the depth Other generatingmechanisms, such as volcanic of seismicactivity increasespectacularly as the eruptions and submarineslumping, cannot be slip vectors change from a predominanceof excludedas the causativeagents for at least strike-slip motion in the western Aleutians to some tsunamis. largely dip-slip motion in Kamchatka, the Gutenberg [1939] argued that the epicen- Kurile Islands,and Japan. Most of the world's ters of severalearthquakes generating tsunamis active volcanoesare located either along the were located on land. Although the epicenter, world rift systemor in regionsthat contain which representsthe point of initial rupture, intermediate-depthearthquakes. Hence, the may be locatedinland, the actual zoneof rup- latter volcanoesare presumablylocated in the ture in great earthquakesis now known to regionswhere the lithospherehas been under- extend for severalhundred kilometers. In many thrust to depthsof at least 100 km. of the great earthquakesassociated with island Tsunamis. Although some prominent sets- arcs and with active continentalmargins at mologists(e.g., Gutenberg[1939]) have argued least a portion of the zoneof rupture is located that some of the largest tsunamis (seismic in water-coveredareas. Hence, the locationof sea waves) are generatedby submarineslides, the epicenteritself is not an argumentfor the many other geophysicistsmaintained that sud- absenceof significantvertical displacementsin den dip-slip motion along faults during large nearby submarine areas. Utsu [1967] has earthquakesis the principal generatingmecha- shownthat, as a result of the anomalouszones nism for most of the world'smore widespread in the mantle beneathisland arcs, locations of tsunamis. L. Bailey (personal communication), shallow shocks based on teleseismicdata are followinga lecture by Sykes, suggestedthat usually landwardof the actual location.Both tsunami generationmight correlatewith areas improved locationsbased on a better knowl- of large dip-slip motion. This suggestionmay edge of seismicwave travel times in anoma- haveconsiderablemerit. lous regions such as island arcs and rapid Figure 2 showsthe epicentersof earthquakes determinationsof focal mechanismsmay be for which tsunamiswere detectedat distances of substantialvalue in tsunamiwarning sys- of 1000 km or greater [Heck, 1947; Gutenberg rems. and Richter, 1954; U.S. Coast and Geodetic Length o• seismiczones in island arcs. The Survey, 1935-1965]. The distancecriterion was systematicchanges in the seismiczones in the used to eliminatewaves of more local origin, Aleutiansand in the southwestPacific suggest some of which actually may be related to that the lengthsof zonesof deep earthquakes seichesor to submarine slides. Although this might be a measureof the amount of under- compilationundoubtedly is not completeeven thrustingduring the last severalmillion years. for the presentcentury, the conclusionsdrawn Usingthe mapsof deepand intermediate-depth here should not be seriouslyaffected by the earthquakesprepared by Gutenbergand Richter choiceof data. [1954], Oliver and Isacks [1968] estimated Since most of the earthquakes generating the area of these zones and divided the total tsunamisare locatedin regionsthat are char- area by the length of the world rift system acterizedby a high rate of dip-slip motion (about two great circles) and by 10 m.y., (Figure 2), the two phenomenaappear to be which is the duration of the latest cycle of c,•usallyrelated in many cases.Although most spreadingbased on data from ocean-floorsedi- of the world's largest earthquakesoccur in ments and from magnetics[Ewing and Ewing, these areas and are characterizedby a large 1967; Vine, 1966]. They obtainedan average componentof thrust faulting,large earthquakes rate of spreadingfor the entire rift systemof along major strike-slipfaults near the coasts 1.3 cm/yr for the half-velocity.This value is of southeastAlaska and British Columbiahave reasonablefor the averagevelocity of spread- not generatedsea waves that were observable ing alongthe world rift system. at distancesgreater than about 100 km. An The hypothesisthat the lengthsof deep earthquakelocated off the coast of California seismiczones are a measureof the amount of in 1927, however,generated a wave detected underthrustingduring the past 10 m.y. is SEISMOLOGY AND NEW GLOBAL TECTONICS 5885 examinedin greater detail in Figure 16 and in underthrusting.Since the calculatedslip rates Table 1. Figure 16 illustrates the lengths of and some of the measured lengths may be the seismic zones in various arcs and the cor- uncertainby 20% or more, the correlationbe- respondingrates of underthrustingas calculated tween the two variablesis, in fact, surprisingly by Le Pichon [1968] from observedvelocities good. of sea-floorspreading and the orientation of Althoughsix points fall well above a line of fracture zonesalong the world rift system.In unit slope,which representsan age of 10 m.y., nearly all casesthe regionswith the deepest all but one of the lengths are within a factor earthquakes (and hence the longest seismic of 2 of the lengths predicted from the hy- zonesas measuredalong the zonesand perpen- pothesisthat these zones representmaterials dicular to the arcs) correspondto the areas underthrust during the last 10 m.y. Three of with the greatest rates of underthrusting; re- the more discrepantpoints, which are denoted gions with only shallow- and intermediate- by crosseson the figure,represent a small num- depth events are typified by lower rates of ber of deep earthquakes that are located in

1500 I I I I [ • I I I I I

Fiji X Japan-Manchuria

South America

Marianas + Ryukyu

_11ooo New Hebrides•l 12øS_ 17øS LLI Indonesia•l Philippines+ Z Tonga• Palau o Kurile- Kamchatka N X Deepest Events New Zeoland

X Spanish Deep Earthquake

,';15oo -- Kermadec New Zealand•l - North Island •I S. Alasko + E. Aleutians

- •ICentral America Andaman Arc Mexic outh Sandwich

ew Zealand-South Is. + Macquarie Ridge leutians near 170øE ALaska 0 5 I0 CALCULATED S LIP RAT E _L ARC (CM/YR) Fig. 16. Calculatedrates of underthrusting[Le Pichon, 1968], and length of seismiczone for various island arcs and arc-like features (solid circles), for several unusual deep events (crosses),and for SouthernChile (diamond).The solid line indicatesthe theoreticallocus of pointsfor uniformspreading over a 10-m.y.interval. 5886 ISACKS, OLIVER, AND SYKES

TABLE 1. Comparison of Estimated Slip Rates for Three Island Arcs Values are in centimeters per year.

Slip Rate* Assuming Slip Rate from Length SeismicZone Shallow-Focus Calculated Slip Rate, Measures Amount Seismicity, after Data from Ocean Ridges, Underthrust during Island Arc Brune [1968] after Le Pichon [1968] Last 10 m.y.

Tonga 5.2 7.6 8.4 Japan 15.7 8.8 16.3 Aleutians 3.8 6.1 4.0

* Correctedfor azimuth of slip vector using data of Le Pichon [1968]. Rates given for the three methods are the magnitudesof the total slip vectors and not the magnitudesof the dip-slip componentsused in Figure 16. unusual locations with respect to the more ac- of the normal oceaniclithosphere. Hence, this tive, planar zones of deep earthquakes.They value is not an unreasonablefirst approximation include the unusual deep Spanish earthquake to the time constant for assimilation of the of 1954, three deep earthquakesin New Zea- lithosphere in island arcs. It should be recog- land, and a few deep events under Fiji that nized, however, that the time constant for appear to fall between the deep zonesin the ocean ridges may be uncertain by more than Tonga and New Hebridesarcs. a factor of 2 because of scatter in heat flow Other estimates o[ slip rates in island arcs. data. At present, it does not seem possibleto Slip rates for Tonga, Japan, and the Aleutians ascertain which of the two alternative pro- (Table 1) were calculated (1) from the disloca- posals (Figure 14a or 14c) governsthe lengths tion theory usingdata on the past occurrenceof of seismic zones in island arcs. earthquakes[Brune, 1968], (2) from the ob- With the exception of the data for south- served spreadingvelocities along ocean ridges ern Chile (the diamond in Figure 16) there assumingthat all of the spreadingis absorbed are no points lying significantlybelow the 10- by underthrustingin island arcs [Le Pichon, m.y. isochron. A reduction in slope at the 1968], and (3) from the lengthsof the seismic higher spreadingrates might occur if the litho- zonesassuming these lengths are a measureof spherewere suddenlymodified at a given depth the amount of underthrusting during the last throughwarming or a phasechange that would 10 m.y. For each arc the three rates are within prevent deeper earthquakes from occurring. a factor of 2 of one another. These processesapparently have not led to The 10-m.y. isochron. Two hypothesesmay significantdecreases in the lengths of the se;_s- explain the correlationbetween the length of mic zones comparedwith those predicted by seismic zones and the computed rate of under- the 10-m.y. isochronin Figure 16. thrusting (Figure 16). One theory, which was Southern Chile. In southern Chile between mentionedearlier, assumesthat the presentseis- 46ø and 54øS, the absenceof observabledeep mic zones in island arcs were created during activity, the near-absenceof shallowseismicity, a recent episode of spreading which began and the presence of a sediment-filled trench about 10 m.y. ago (Figure 14a). In the other [Ewing, 1963; Hayes, 1966] are in obvious hypothesis,10 m.y. is regardedas the approxi- conflict with the predicted length of the seis- mate time constant for assimilation of the mic zone. Le Pichon'somission of two very lithosphereby the upper mantle (Figure 14c). active features,the West Chile ridge and the Since the zone of anomalouslyhigh heat flow Galapagosrift zone (Figure 15) in his analysis on ocean ridges appears to be confinedto re- may, however,explain this discrepancy.Wilson gionsless than about 10 m.y. old [McKenzie, [1965a] argued that since Antarctica is almost 1967; Le Pichon and Langseth,1968], 10 m.y. surroundedby spreadingridges, the coast of is a reasonable time constant for the creation South America south of its juncture with the SEISMOLOGY AND NEW GLOBAL TECTONICS 5887 West Chile ridge at 46øSshould not be seismi- or the typical islandarcs, occupies a very broad cally active. An alternativeexplanation for the region (Figure 15). Spreadingin the Indian near-absenceof shallowseismicity is that activ- Ocean is apparently being absorbedin several ity duringthe past70 yearsof instrumentalseis- subzoneswithin the Alpide belt. This conclu- mologyis not representativeof the long-term sion is supportedby the widespreaddistribu- activity. This explanationis, however,difficult tion of large shallowearthquakes and by the to apply to the hypotheticaldeep seismiczone relatively small number of intermediate-and that is predicted from Le Pichon's calcula- deep-focusearthquakes [Gutenberg and Rich- tions. Hence, we feel that either Wilson's pro- ter, 1954]. posal or some other explanationis neededto Although, in principle, it seemsreasonable account for the tectonics of southern Chile. to describe the tectonics of Eurasia by the Departuresfrom the 10-m.y. isochron. Sev- interaction of blocks of lithosphere,it is not eral factors could explain the six points that yet clear how successfulthis idea will be in fall well above the 10-m.y. isochronin Figure practice becauseof the large number of blocks 16: (1)Some piecesof lithospherebecame de- involved. The interaction of blocks of litho- tached from the main dipping zones of deep sphereappears to be much more complexwhen activity by active processes,such as gravita- all the blocksare continentsor piecesof con- tional settling of slabs of lithosphereor con- tinents than when at least one is an oceanic vection currents in the asthenosphere(Figure block. In addition to activity in the Alpide 14d). (2) The initiation of underthrustingand belt, earthquakesin East Africa, northern Si- spreadingwas not simultaneousin all regions. beria, and western North America (including (3) Someor M1 anomalousdeep events may be Alaska) are more diffusedin areal extent (Fig- related to previousepisodes of underthrusting. ure 15). In contrast, seismiczones associated (4) The computed slip rates are not correct. with oceanridges, island arcs, and many active (5) If the thermal time constant for assimi- continental margins appear to be extremely lation of the lithosphereis generallyabout 10 narrow and well defined. Several factors may m.y. (Figure 14c), the anomalouspoints may explain these differences:(1) The lithosphere represent pieces of lithosphere with anoma- may be more heterogeneousin someor all con- lously high time constantsso that they are tinental areas and, hence, may break in a not completelyassimilated. more complexfashion. (2) Old zonesof weak- Although some of the computed spreading ness in continental areas may be reactivated. rates may be in error (particularly the values (3) •Becauseof its relatively low density, it for Indonesia and South America may be in may not be possibleto underthrusta block of error becausethe magneticdata in the Indian continental lithosphere into the mantle to Ocean are poor and becausethe West Chile depths of several hundred kilometers. This ridge and the Galapagosrift zone (Figure 14) third point is supportedby the relatively large were not included in Le Pichon's analysis), it areas of older continentalrocks. is difficult to argue that these rates are greatly As much of the sea floor appears to be rela- in error for each of the six anomalouspoints. tively young, plates of oceanic lithosphere The possibleramifications of the various al- probably do not contain a large number of ternative proposalsshould be exciting enough zones of weakness,whereas the continental to encouragefurther study of these new ap- plates, which are older, seem to contain many proaches to the distribution of deep earth- zones of weakness.Except for a few earth- quakes. quakesalong the extensionsof fracture zones, Interaction of continental blocks of litho- the ocean basins are extremely quiet seismi- sphere. The Alpide belt, which comprises cally. Continents,however, exhibit a low level much of the Mediterraneanregion, the Middle of activity in many areas that do not appear East, and large parts of central and southern to be undergoingstrong deformation. This con- Asia, was not included in Figure 16 because trast in activity does not appear to be an it is very difficult to define the total amount artifact of the detection system but rather of underthrusting.Seismic actvity in the Alpide appearsto be related to the different charac- zone,unlike that alongeither the oceanridges ter of the oceanicand continentallithosphere. 5888 ISACKS, OLIVER, AND SYKES One exceptionto this rule is the near-absence African rift valleys; if spreadingfrom the east- of observable activity in Antarctica. Unlike ern flank of the East Pacific rise near Mexico the other continents,Antarctica is almost sur- is absorbedonly a few hundred kilometers rounded by ocean ridges that are probably from the rise in the Middle America trench, migrating outwardly [Wilson, 1965a]. Hence, it is difficult to imagine how materials on the antarctic plate may not be subjected to the western flank of the rise travel more stressesas large as the stressesoccurring in than 10,000 km before they are absorbed other continents. in the trenches of the western Pacific. A1- Summary o• evidence•or globalmovements. though some of the fracture zoneson ocean The concentrationof seismicactivity and the ridges may be as long as 1000 km, some consistencyof individual mechanismsolutions of these zones are less than 50 to 100 km for mid-oceanridges, for island arcs, and for apart. It is almost impossibleto believe that most of the world's active continentalmargins these long narrow strips reflect the shape of indicate that tectonic models involving a few independent convection cells. These and sev- plates are highly applicable for these areas. eral similar dilemmasmay be resolvedif a Since individual mechanism solutions in a layer of greater strength (the lithosphere) given regionare highly coherent,each solution overliesa regionof much lesserstrength (the may be regarded as an extremely pertinent asthenosphere). datum. It is not necessaryto resortto complex The configurationof the asthenosphereand statistical methodsfor analyzing the relative of the various pieces of lithospheremay in motions of these large blocks. For these re- someways be analogousto blocksof ice float- gions much of the scatter in many previous ing on water. Althoughthe surfacepattern of mechanismstudies appears to be largely a re- the ice may be very complex,the pattern of sult of poor seismicdata and errors in anal- convectionin the water below or in the air ysis.Although the conceptof interactingblocks abovemay be very simple,or it may be com- of lithospheremay not be as easily applied to plex and of a completelydifferent character the complexinteractions of continentalblocks than that of the motionsof the ice. This anal- in such areas as the Alpide belt, a consistent ogy may be relevantto the more complexnon- (but complex)tectonic pattern may yet emerge symmetricaltectonic pattern exhibitedby the when the pattern of seismicityis well defined earth's surface.A lithosphericmodel readily and when a sufficientlylarge number of high- accountsfor the asymmetricalstructure of is- quality mechanismsolutions is available.Thus, land arcs and for the symmetricalconfigura- the new global tectonicsamong other things tion of oceanridges. It alsoexplains the rather appearsto explainmost of the major seismic smoothvariations in rates of spreadingin a zonesof the world, the distributionand con- given ocean. figurationof the zonesof intermediateand deep In this model the ridgesand islandarcs are earthquakes,the focal mechanismsof earth- dynamicfeatures, and hencethey appear to quakesin many areas,and the distributionof move with respectto one another.Thus, a a large numberof the world'sactive volcanoes. regionof extensionaltectonism may be located between two spreading ridges. What is de- ADDiTiOnALEWDE•CS fOREX•STS•CS orL•T•O- mandedin this model, however, is that sur- srI-I•R•,ASTI-Im•osrI-I•R•, ANDM•sosrI-I•R• facearea be conservedon a worldwidebasis. Complex tectonicpatterns near the earth's To what extent the motionsof large plates of surface. A number of objectionshave been lithosphere are coupled to motions in the raised to models of sea-floorspreading that asthenosphereremains an unsoIvedproblem. involve simple symmetricalconvection cells ex- Depths o• earthquakesalong the world ri/t tending from great depths to the surface of system. The existenceof a layer of strength the earth. They include such statements as: near the earth's surface is compatiblewith the Since the Atlantic and Indian oceansare both observationthat all earthquakeson the ocean spreading,the tectonicsof Africa shouldbe of ridges are apparentlyof shallowfocus (i.e., a compressionalnature rather than of an ex- lessthan a few tensof kilometersdeep). Using tensional nature, as reported for the East a dislocationmodel, Brune [1968] estimated SEISMOLOGY AND NEW GLOBAL TECTONICS 5889 that the zone of earthquake generation for [1968] observed that ridges with spreading some oceanicfracture zonesappears to be less rates higher than about 3 ½m/yr are typified than 10 km in vertical extent. by fairly smooth relief, a thin crest, and the The San Andreas fault of California appears absenceof a median rift valley. Ridges with to be a major transformfault connectinggrow- lower spreadingrates are, however,½haracter- ing ocean ridges off the west coast of Oregon ized by high relief, a thick crust, and a well- and Washingtonwith spreadingridges in the developedmedian valley. van Andel and Bowin Gulf of California (Figure 3). Observedsets- [1968] suggestthat materials undergoingbrit- mi½ activity along the San Andreassystem is tie fracture may be thin at sites of faster confinedto the upper 20 km of the earth, and spreadingbecause higher temperaturesmight much of this activity is confinedto the upper occur at shallower depths. The crest of the 5 or 10 km [Press and Brace, 1966]. Most East Pacific rise is characterizedby heat flow estimatesof the depths of faulting in the great anomaliesthat are broader and apparently of San Franciscoearthquake of 1906,the Imperial greater amplitude than the anomaliesassoct- Valley earthquakeof 1940, and the Parkfield ated with ridge crestsin the Atlantic and In- earthquakeof 1966 are within the upper 10 to dian oceans [Le Pichon and Langseth, 1968]. 20 km of the earth [Kasahara, 1957; Byefly Thus, higher temperatures beneath the East and DeNoyer, 1958; Chinnery, 1961; McEvilly Pacificrise may suppressthe buildup of stresses et al., 1967; Eaton, 1967]. Since displacements large enough to generate observable seismic of at least 100 km (and probably 300 to 500 activity. km) probably have occurred along the San If the occurrenceof earthquakesand the Andreas fault [Hamilton and Meyers, 1966], presenceof a median rift valley correlate with it is almost certain that deformationby creep high strength and their absences(in spite of without observable earthquake activity must large deformations)correlate with low strength, be occurring at depths greater than 20 km. the lithosphere may be very thin or almost Thus, an upper layer of strength in which nonexistentnear the crest of the East Pacific earthquakesoccur associatedwith a zone of rise. The lithospheremust be present,however, low strength below seemsto be demandedfor within a few tens of kilometers or less of the California and for other parts of the world crest to account for the much higher seismic rift system. activity along fracture zones.It is possiblethat Variations in thickness o• the lithosphere seismicactivity along the crest of the East Pa- along the world ri•t system. Although earth- cific rise may occur mainly as small earth- quakes on the mid-Atlantic, mid-Indian, and quakes that either are not detected or are Arctic ridgesoccur along the ridge creststhem- rarely detected by the present network of selvesas well as along fracture zones,observ- seismographstations. A microearthquakestudy able seismicactivity along much of the East using either hydrophonesor ocean-bottomsets- Pacific rise is concentratedalmost exclusively mographscould furnish important information along fracture zones [Menard, 1966; Tobin about the mode of deformation at the crest and Sykes, 1968]. With the exceptionof the of thesesubmarine ridges. Gorda ridge off northern California, which ap- High-•requency $,, waves crossing ocean pears to be spreading relatively slowly, earth- ridges. Molnar and Oliver [1968] studied $• quakesare only rarely recordedfrom the crest propagation in the upper mantle for about of most of the East Pacific rise. Sinceevidence fifteen hundred paths; they did not observe from magneticanomalies indicates a relatively any high-frequencyS• waves for ocean paths fast rate of spreading (greater than 3 cm/yr that either originateat or crossan active ridge for the past 1 m.y.) along much of the East crest. Their observationsalso suggestthat the Pacific rise [Heirtzler ei al., 1968] and since uppermostmantle directlybeneath ridge crests earthquakesare presenton the boundingtrans- is not includedin the lithospherebut that the form faults, much of the crest of this rise ap- uppermost mantle must be included in the pears to be spreading by ductile flow with lithospherebeyond about 200 km of the crest little or no observableseismic activity. in order to explainthe propagationof high- Menard [1967] and van Andel and Bowin frequency$• from several events on some of 5890 ISACKS, OLIVER, AND SYKES the larger fracture zones.The distancefor any mic energy in earthquakes; island arcs and given ridge may dependon the spreadingrate, similar arcuate structures contribute more but the data were inadequate to confirm this than 90% of the world's energy for shallow assumption.The thinning or near-absenceof earthquakesand nearly all the energyfor deep lithospherein a zonenear the ridge crestwould earthquakes [Gutenberg and Richter, 1954]. be compatible with the magmatic emplace- The relatively small amount of seismicactivity ment of the lithosphere near the crests of on the ridge systemand its largely submarine ocean ridges and with a gradual thickeningof environmentprobably explain why the signifi- this layer as it cools and moves away from canoe and the worldwide nature of this tee- the crest. This thinning may also explain the tonic feature were not clearly recognizeduntil maintenanceof near-isostaticequilibrium over about 12 years ago. ocean ridges [Talwani et al., 1965a]. Although Richter [1958] lists more than 175 Thicknessof the lithospherefrom heat flow very large earthquakes(magnitude M greater anomalies. If the spreading rates inferred than or equal to 7.9) as originatingin arcs and from magnetic anomalies are accepted, the arc-like features, he reports only 5 events of calculated heat flow anomaly over ridges for this size for the world rift system. Whereas a model of a simple mantle convectioncurrent the largest event on the ridge system was of that extendsto the surface is not compatible magnitude 8.4, the largest known earthquakes with the observedheat flow anomaly [Langseth from island arcs were of magnitude8.9. This e• al., 1966; Bott, 1967; McKenzie, 1967]. difference in magnitude correspondsto about These observationsare, however, compatible 8 times as much energy in the larger shocks. with the computed heat flow for a simple Most (and perhaps all)of the earthquakeson model of cooling lithospheric plate approxi- the ridge system with magnitudesgreater than mately 50 km thick [McKenzie, 1967]. In this about 7 appear to have occurredalong major model the heat flow anomaly results from the transform faults. coolingof the lithosphereafter it is emplaced Thus, the maximum magnitudesof earth- magmatically near the axis of the ridge. Be- quakes during the last 70 years for various cause of the scatter in the heat flow data and tectonic features may be summarized as fol- because of the simplifying assumptionsused lows: island arcs, 8.9; major fracture zones, to obtain the estimateof 50 km, the value for 8.4; ridge crests in the Atlantic, Indian, and the thicknesseither may be uncertain or may Arctic oceans,about 7; most of the crest of vary by a factor of 2 or more. The occurrence the East Pacific rise (with the exceptionof the of earthquakes as deep as 60 km beneath Gorda ridge), few (and possibly no) events Hawaii [Eaton, 1962] may also be used as an larger than 5. The relative frequenciesof very estimate of the thicknessof the lithosphere. large earthquakesand possiblythe upper lim- Orowan [1966] tried to fit the observedheat its to the sizes of earthquakes appear to be flow pattern for ridges with models in which related to the area of contact between pieces the crust is stretchedon the ridge flanks and of lithosphere that move with respect to one the rate of spreadingis a function of distance another. On the crest of the East Pacific rise from the ridge. His model does not appear to the zone of contact between brittle materials be compatible either with the symmetry of is either absent or is confined to a thin sur- magnetic anomaliesclose to the ridge or with ficial layer; at the crests of other ridges the the narrow width of the seismiczones on most lithosphere appears to be somewhat thicker. ocean ridges. For fracture zones the thicknessof the litho- Maximum sizes o] earthquakes. The sizes sphere may be as great as a few tens of kilo- of the largest earthquakesalong ocean ridges meters; thus, the maximum magnitudesmay and along island arcs may be related to the be limited by the length of the fracture zone thickness of the lithosphere in each of these and by the thickness of the lithosphere.In tectonic provinces.The world rift system (in- island arcs the thicknessof the lithospheremay eluding California, southeastAlaska, and east be greater than that on the ridgesbecause the Africa) accountsfor lessthan 9% of the world's temperatures beneath ridges are higher and earthquakesand for less than 6% of the seis- perhapsbecause some material is added to the SEISMOLOGY AND NEW GLOBAL TECTONICS 5891 bottom of the lithosphereas it moves away estimatesrepresent real differencesin thick- from a ridge crest. Since the dip of seismic hessor merely differencesin the relations zonesin islandarcs usuallyis not vertical,the tween thicknessand the various physical area of contact between plates of lithosphere rametersmeasured. may be increasedby this factor alone. For The asthenospherein the three-layeredmodel example,the zone of slippagein the great shownin Figure I alsoroughly coincides with Alaska earthquake of 1964 appears to dip the low-velocityzone for S waves [Gutenberg, northwesterlyat a shallowangle (about 10ø) 1959; Dotman ei al., 1960; Anderson,1966; and to extendfor about 200 km perpendicular Ibrahim and Nuttli, 1967], regionsof either to the Aleutian island arc [PlaCket, 1965; low velocity or nearly constant velocity for Savageand Hastie, 1966]. Since the dip is P waves[Lehmann, 1964a, b] and of low den- shallow,this zone doesnot extendmore than sity or nearly constantdensity [Pekeris, 1966], about 40 km below the surfaceat its deepest a regionof high attenuationfor seismicwaves, point. Thus, it is not unreasonablethat much particularlyS waves [Andersonand Archam- greateramounts of elasticenergy can be stored beau, 1964; Anderson, 1967b; Oliver and and releasedalong isIand arcs than along the Isacks, 1967], and a low-viscosityzone in the mid-oceanridges. upper mantle [McCo•nell, 1965]. Although Since single great earthquakes apparently these observationsyield very little information have not involved a rupture that extended about the actual mechanismsof dissipation, alongthe entirelength of the largerisland arcs they are consistentwith the hypothesisthat [Richter, 1958], someother factor appearsto the asthenosphereis a region of low strength limit the lengthof rupture and hencethe maxi- boundedbelow by the mesosphere,a region of mum sizes of earthquakes.Tear faults (i.e., greater strength.The simplestexplanation for transformfaults of the arc-arc type) within thesephenomena is that the closestapproach the upper thrust plate may divide island arcs to melting (or partial melting) occursin the into smaller subprovincesthat are not com- asthenosphere.That relative displacementsof pletelycoupled mechanically. the earth's surfacemay be modeledby a series Some properties o• the lithosphereand as- of moving plates is a strong argument for a thenosphere.Thus far we have defined the region of low strength (the asthenosphere)in lithosphereas a layer of high strengthand the the upper mantle. Nevertheless,the physical asthenosphereas a layer of low strength.Since properties and configurationof the astheno- the rheologicalproperties of the mantle are sphereand the lithospheremay vary from place not at all well understood,we have purposely to place.In fact, variationsof thesetypes seem usedthe term strengthin a generalsense with- to be requiredto accountfor many of the com- out beingmore specificabout the actualmecha- plexities of the outer few hundred kilometers nismsof deformation.Thus, it is not at all cer- of the earth. The evidence for such lateral rain that various techniquesfor measuringthe variations is now so strong as to demand re- thicknessand the presenceof the lithosphere evaluation of studiesof velocity and Q struc- necessarilyyield comparablevalues since these ture based on modelsthat consistonly of con- methods probably sample different physical centricspherical shells. parameters. It is encouraging,however, that estimatesof the thickness of the lithosphere COnFLiCTinGS•SMOLO6•CA• EWD•CE fromanalyses of heat flow anomalies, theab- A•DSoME PROblEMS senceof earthquakes (in spite of large defor- The new global tectonics, in one form or mation), the requirementsof isostasyand the another, has been remarkably successfulin ex- maintenanceof mountains [Daly, 1940], the plaining many grossfeatures and observations amplitudesand wavelengthsof gravity anoma- of geology,but its developmentmust be con- lies [McKenzie, 1967], the propagation of tinued in an effort to establisha theory that high-frequencyS• waves, and a transition in is effective throughout the earth sciencesat the types of earthquakemechanisms in island levels of increasingly greater detail. Many arcs are about 100 km or less.It is not clear, dif•cult problemsremain. A quantitativeunder- however,to what extent the variationsin these standing of the processesin the mantle that 5892 ISACKS, OLIVER, AND SYKES result in the observedsurface features is ur- does not readily fit into the new global tec- gently needed.The roles of the earth's initial tonics at presentdoes exist, however.For ex- heat,gravitational energy, radioactive heat, and ample,the locationsof the Spanishdeep earth- phase changesmust be understood.The me- quake and certain deepshocks in New Zealand chanicsof flow in the mantle is little known. and Fiji are not easily explainedby current The history of sea-floorspreading through geo- thinking, as has been pointed out above.The logictime and the relationbetween the vast patternsof minorseismicity, including an quantities of geologic observationsand the casionallarge earthquakein certaincontinental hypothesesmust be worked out. A vital un- areassuch as the St. LawrenceValley and the answeredquestion is why islandarcs and ocean Rocky Mountainsand broad regionalscatter- ridgeshave their particularcharacteristic pat- ing of epicentersas in east Africa, are not terns. These grand questionsare of concern simply explainedas yet, nor is the almostcorn- to seismologists,not only becausethere is gen- plete lack of earthquakesin the oceaniclitho- eral interestin the hypothesesbut alsobecause sphere,which presumablycan and doestrans- the solutionsmay very well be dependenton mir stressover large distances.The occurrence evidence from seismology.In the absenceof of large active strike-slip faults that trend all but the most preliminaryattempts to solve alongthe arc structure,such as the Alpine fault theseproblems, however, it is difficultto spec- in New Zealand,the Philippinefault, the North ulate on the direction and force of the evi- Anatolian fault in Turkey, and the Atacam• dence.At the present there appears to be no fault in Chile, ofters somediffculties. Tentative evidencefrom seismologythat cannoteventu- explanationshave been proposed for the Alpine ally be reconciledwith the newglobal tectonics and Philippinefaults; therefore,this matter in someform. Sometraditional views that once may be resolved.Perhaps related to this prob- would have appeared contradictory (such as, lem is the occurrencein island arcs of occasional for example, the assignmentof substantial earthquakeswith large strike-slipcomponents rigidity to the entire mantle over a long time along faults subnormalto the arc. The tec- interval becauseof efficient propagationof tonicsof the arc can hardly be as simpleas shear waves with periods of about 10 sec) that impliedby Figure 1. have long beendiscounted on the basisof in- A generalproblem involving seismology con- formation on glacial rebound, gravity, and, cerns contrastsbetween continentaland oce- more recently, creep along long strike-slip anic areas [MacDonald, 1964, 1966]. Some faults, such as the San Andreasfault. That studies[e.g., Brune and Dotman, 1963; Tok- deep earthquakesgenerate shear waves and sSz and Anderson,1966] showcorresponding radiate wavesin a pattern characteristicof a structuraldifferences to depthsof severalhun- sheardislocation can hardly be taken as evi- dred kilometers.Are suchdifferences contrary dencefor strengththroughout the mantle in to the new globaltectonics? Might the obser- that depthrange in light of a hypothesisthat vations be equally well satisfiedby other suggeststhat the mantle in the deep earth- modelsthat are compatiblewith requirements quakezones is muchdifferent from the mantle of the new hypotheses?A seriousdifficulty at comparabledepths elsewhere. may arisehere if they cannot.Comparable and The arc-likepatterns of the activezone is related problemsarise in other disciplines. but oneof the problemsof a subjectthat might Observationsuggests that heat flow per unit be termed 'lithosphere mechanics.' Others are area is about the same under the oceans as the shape of the deep zones,which sometimes under the continents.If it is assumedthat the appearto be near-planarafter turning a rather heat flux is largelydue to radioactivedecay sharp corner near the surface; the distribu- and that radioactivityis heavily concentrated tion of stressand strain in the lithosphere, in certain continentalrocks, lateral hetero- not merelyin activeseismic areas but through- geneityof the uppermantle is requiredand the out the world; the interactionof lithosphere mixing predictedby the new tectonicsmay and asthenosphere;and flow in the astheno- be so great as to destroysuch heterogeneity. sphere. Perhaps the amount of radioactivity in the Somespecific evidence from seismologythat earth has been highly overestimated,as has SEISMOLOGY AND NEW GLOBAL TECTONICS 5893 occasionallybeen suggested[Verhoogen, 1956], be carefully and thoroughlyconsidered for indi- and much of the heat lost at the surface is cationsof new directionsfor, and new attitudes transportedfrom the deep interior by convec- toward, seismologicalresearch. This section is tion. More accurate determination of deep largely speculativeand someof the points may structure by seismologicaltechniques is thus seem farfetched. If, however, a basic under- relevant to the problem of the amount of standing of global tectonicsis imminent, even radioactivity in the earth and its distribution. the most imaginative and wisest forecast of its Seismologyis alsovitally linked with heat flow effect on seisinologyis likely to be too conserva- in the island arcs, where low heat flow values tive. Nor is seismologyalone in this regard, for appear to correlatewith zonesof descending all branchesof geologyand geophysicsrelated lithospherebut anomaloushigh values appear to the earth's interior will be comparably over the deepseismic zones, and at the ocean affected.The assumptionthat a major advance ridges,where high heat flow may correspond in our understanding of global tectonics has with low strength and hence low seismicac- been achievedis tacit in the following. tivity. Seismicity is an important branch of seis- Other disciplinesare also involved. The new inologyand one that will be stronglyaffected by structuresbased mainly on seismologicalinfor- the impact of the new global tectonics.Such mation must be tested against gravity data. basic questions as why earthquakes occur The record in the ocean sedimentssurely pro- largely in narrow belts separatedby large stable vides the most completehistory of the ocean blocks, why these belts are continuous on a basins and hence must provide insight into worldwidescale, why they branch,why certain seismologicalprocesses. Perhaps the crucial details of their configurationare as they are, evidenceon the validity of new globaltectonics why intermediateand deep earthquakesoccur will come from cores of the entire sedimentary in someareas and not othersare in the process column of the ocean floor. of being answeredtoday. There is every indica- In petrologyan interestingquestion concerns tion that the relation between seismicactivity the origin of the belts of andesiticvolcanoes. and geologywill soonbe understoodto a much Coats [1962] proposed that ocean crust and greater extent than contemplated heretofore. sediments were thrust into the mantle under Improved accuracy in hypocenter location re- the Aleutian arc and subsequentlyerupted with sulting from a better knowledge of velocity mantle rock to account for the petrology of structure will facilitate this development. It those islands. Can all the andesires of the active will also assist in solving seismology'schief tectonic belts be explained in this manner? Are problemof a politicalnature, distinguishing be- there any young (lessthan 10 m.y.) andesiresof tween earthquakesand undergroundnuclear ex- the same type that are not associatedwith plosions.The self-consistentworldwide pattern active deep earthquake zones and arcs? Can of focal mechanisms will also be valuable here. this sort of information be used to identify Perhapslocation of new seismographstations ancient arcs ? in the proper relation to high-Q zoneswithin The important problemsarising from the new the earth will result in improved detection global tectonicsare many; some are crucial. capability. Important questions still remain. Evidence from seismology against the new For example,why have the earthquake belts global tectonicsappears, however, to lack force. and the associated tectonic belts assumed their present configuration? What can be learned EFFECTS OF NEW GLOBAL TECTONICS ON about palcoseismicityand its relation to mod- SEISMOLOGY ern seismicity?How can the pattern of minor That seismologyis providing abundant and seismicity,including the occasionalmajor earth- important information for testing the new quake outsidethe establishedseismic belts, be global tectonics is demonstrated in other sec- fit into the new global tectonics? tions of this paper and elsewhere.To date, most Closely related to the distribution of seismic of the seismologicalwork related to this topic events in time and space and to tectonics is has beenso directed.The counteringimpact of the focal mechanismof the earthquake.Data the new globaltectonics on seismologymust also of qualityand quantityfrom modernobserving 5894 ISACKS, OLIVER, AND SYKES stationscan provide information for determina- a basic understandingof the earthquake phe- tion of focal mechanismsthat severelytest the nomenonis established.The new globaltectonics new hypothesesand are crucial to their devel- offers great promise for such an achievement. opment. It appears that with only modest It has already provided a theory that predicts advances in technique properly documented over-all strain rates in tectonicareas throughout earthquakes will provide reliable detailed in- the world. It suggestsa means for predicting formation on tectonicactivity. Focal mechanism, the maximum size of an earthquake in a given stressdrop, slip, and orientationof prineipaI region. It provides a framework in which to stresses should be available from individual relate measurements of distortion, such as earthquakes,and singly or eumulatively these strains,tilts, and sea-levelchanges, with obser- data will be integrated with the tectonicpattern. vations on the mechanismof the earthquake. It Implicit in the new global tectonicsis a new has establishedthe continuityof active zonesand attitude toward mobility of the earth's strata. has shownthat apparently inactive segmentsof Measurementsof fault creep and other forms of otherwiseseismic belts must, indeed, be active, earth strain over recent short intervals of time either by subsequentearthquakes or by creep. are based on a variety of measuringtechniques Refinementsand further developmentsmust be that include geodeticsurveying, tide and strain anticipated. gaging, and measurementsof earthquake slip. Seismologyhas long been the principal source Various methods of field geologygive data ex- of information on the structure of the earth's tending over varying but greater intervals of interior and is likely to continue in that role, time. When all these data are reduced to veloci- with or without the new global tectonics.The ties that describethe motion of one point in the new hypotheseswill, however, certainly stimu- earth relative to another point located in an late radically new approachesto the exploration adjacent relatively underformed block, values of the earth's interior. A commonand powerful are obtained that are in the same range as the technique of seismologyinvolves the use of velocity valuesassociated with sea-floorspread- simplifiedearth modelsfor prediction of certain ing determinedthrough analysisof geomagnetic observedeffects. The new global tectonicsealls data. This range is from about I to perhaps10 for an entirely new kind of model. Layered cm/yr or more. These values are considerably modelsin which the shellsare spheriealIysym- higher than the values usually assumedin the metric are now outdated for many areas of the past by most earth scientistsconsidering de- earth. Models basedon the effect of spreading formation of this type; hence, new attitudes and growth of the lithosphereat the rifts and toward old problems must be anticipated. A underthrustingat the island ares must be tested prime examplein seismoIogy,cited above,is the against observation.The conventionaldivision associationof the length alongthe dip of a deep of the earth's surface into oceanic and conti- seismiczone with the amount of relatively re- nental areas, or oceanic, shield, and tectonic cent underthrustingin a region. areas,requires a new look when the lithosphere, With the new global tectonics, interrelation- with its lateral variations, is involved. New ships on a large, perhaps worldwide, scale can models in which the age and the thickness of be predictedand perhapsobserved. Thus, major the lithosphere,as well as other properties,are seismicactivity in one area could be related to taken into accountare required. that in an adjoining, or perhaps distant, area In the new tectonics,information on prop- associatedwith the same lithospheric unit or erties and parameters such as attenuation, units, for, althoughthe propagationtime for the strength, creep, viscosity,and temperature is effect may be long, stress may be transmitted eritieally needed.Further efforts must be made over large distancesthrough the lithosphere.Of to understandthe relation betweensuch prop- specialinterest is the new insight into the sub- erties and the seismicproperties that are meas- jeer of earthquake prediction, even prevention. ured in a more straightforward manner. At the Although an empirical method of prediction island ares, where cold exotic materials are could by chancebe found in the absenceof an descendinginto the mantle, nature is perform- understanding of the process, an effective ing the experimentof subjectingwhat are nor- method is much more likely to be achieved if mally near-surfacematerials to the higher tem- SEISMOLOGY AND NEW GLOBAL TECTONICS 5895 peratures and pressuresof the asthenosphere,an are shaping the surficial features of interest to experimentin many respectsmuch like the ones classical geology. Even if it is destined for being performed in various laboratories.As our discard at some time in the future, the new techniques for measuring the compositionof global tectonics is certain to have a healthy, the material and the dynamic and environ- stimulating,and unifying effect on all the earth mental parameters of this situation improve, sciences. this experimentshould yield important informa- Acknowledgments. We thank Professor L. A. tion on many topics. Of great interest to seis- Alsop and Drs. Neff Opdyke and Walter Pitman mologistsis the long-standingproblem of the for critically reading the manuscript. M. Bara- mechanismof deep earthquakes.The radiation zangi, J. Dorman, W. Elsasser, R. l:Iamilton, X. patterns of seismic waves from many events Le Pichon, P. Molnar, W. J. Morgan, and W. present a reasonablyconsistent pattern but one Stauder, S. J., furnished preprints of their papers prior to publication. W. Ludwig of Lamont pro- that cannot readily be explained in detail by vided an original tracing of the seismic reflection existing hypotheses. The variation of seismic profile of the Japan trench that was used in the activity with depth is another source of infor- paper. We appreciate and acknowledge discus- mation. Are the focal mechanisms of earth- sions with many colleaguesincluding J. Ewing, D. Griggs, D. McKenzie, and W. Pitman. Messrs. quakes at depth really as much like those in R. Laverty and J. Brock assistedin preparing the near-surface brittle materials as they seem? data for computation, and Mrs. Judith l:Iealey What is the role of water, of other interstitial assistedin editing and preparing the manuscript. fluid, of partial melting? Are phase changesin Computingfacilities were made available by the the exotic mantle materials important, perhaps NASA Goddard SpaceFlight Center, Institute for SpaceStudies, New York. This study waspartially not as sources of seismic waves but as con- supported under Department of Commerce Grant centrators of stress that leads ultimately to E-22-100-68G from the Environmental Science rupture? How are the seismic observationsof Services Administration, through the National focal mechanism,spatial and temporal distribu- Science Foundation grant NSF GA-827, and through the Air Force Cambridge ResearchLab- tion, energy, etc., related to the material of the oratories, Office of Aerospace Research, under mantle and what can we learn about that mate- contract AF19(628)-4082 as part of the Advanced rial? How are these data related to the general ResearchProjects Agency'sProject Vela-uniform. configuration of the seismic zone and of the i•EFERENCES geology of the island arc? These are some of the topics on which this experimentmay pro- Adams,R. D., Sourcecharacteristics of somedeep vide information. New Zealand earthquakes, New Zealand J. Geol. Geophys.,6, 209, 1983. 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