Slab Pull and the Seismotectonics of Subducting Lithosphere

REVIEWS OF GEOPHYSICS, VOL. 25, NO. 1, PAGES 55-69, FEBRUARY 1987

Slab Pull and the Seismotectonicsof Subducting Lithosphere

WILLIAM SPENCE

National Earthquake Information Center, U.S. Geolo•ticalSurvey, Denver, Colorado

This synthesislinks many seismic and tectonic processesat subduction zones, including great subduction earthquakes, to the sinking of subducted plate. Earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge pusJaforce. Interactions between the forces that drive and resist plate motions cause spatially and temporally localized stressesthat lead to characteristic earthquake activity, providing details on how subduction occurs. Compression is localized acrossa lockedinterface thrust zone, because both the ridgepush and the slabpull forcesare resisted there. The slab pull force increaseswith increasing plate age; thus becausethe slab pull force tends to bend subducted plate downward and decreasethe force acting normal to the interface thrust zone, the characteristicmaximum earthquake at a given interface thrust zone is inverselyrelated to the age of the subductedplate. The 1960 Chile earthquake (M w 9.5), the largestearthquake to occur in historic times, began its rupture at an interface bounding oceanic plate < 30 m.y. old. However, this rupture initiation was associatedwith the locally oldest subductinglithosphere (weakest coupling); the rupture propagated southward along an interface bounding progressivelyyounger oceanic lithosphere,terminating near the subductingChile Rise. Prior to a great subduction earthquake, the sinking subductedslab will cause increasedtension at depths of 50-200 km, with greatest tension near the shallow zone resisting plate subduction.Plate sinking not only leads to compressionalstresses at a locked interface thrust zone but may load compressionalstresses at plate depths of 260-350 km, provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle. This explains K. Mogi's observations of M •_ 7 thrustearthquakes at depthsof 260-350km, immediatelydowndip and within 3 yearsprior to five great, shallow earthquakes of northern Japan. The slab pull model explains the lower layer of double seismic zones as due to tension from the deeper, sinking plate and the upper layer as due to localized in-plate compression, as plate motion is resisted by the bounding mantle. Just downdip of the interface thrust zone,there occurs an aseismic20ø-50 ø dip increaseof subductedplate. This slabbend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change. Resistanceto subductionprovided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone.

INTRODUCTION Ridge Push and Slab Pull Although the slab pull force is recognizedas dominant in Horizontal density contrasts, resulting from cooling and moving tectonic plates, mechanisms by which this force is transmitted through locked subduction zones have received thickening oceanic lithosphere, produce the ridge push (sliding little attention. This is becausemany investigators believe that plate) force which contributes significantly to plate motions most of the slab pull force is balanced by friction between the [Hales, 1969; Andersonet al., 1976; Lister, 1975; Hager, 1978; plate and the surroundingmantle. Reasonsfor the persistence Solomon et al., 1980; Hager and O'Connell, 1981]. The nega- of this view include (1) the perception that great, interface tive buoyancy of subducted oceanic lithosphere (the slab pull thrust earthquakesat subductionzones primarily are caused force) is established as the major driving force for plate mo- by compression from the oceanic side, (2) results from glacial tions [Elsasset, 1969; McKenzie, 1969; Isacks and Molnar, rebound studies indicating that mantle viscosity is high 1971' Smith and Toks6z, 1972; Forsyth and Uyeda, 1975; Schu- enough to inhibit deep plate motions from influencing shallow bert et al., 1975; Solomon et al., 1975; Vlaar and Wortel, 1976; tectonics, and (3) calculations of vertical forces acting at Richter, 1977; Chapple and Tullis, 1977' Carlson, 1981, 1983]. trenches, based on observed trench topographies, indicating Both a trench suction force and back arc spreading have much smaller forcesthan are predicted from thermal modeling been suggestedas mechanismsfor the trenchward movement of subducted plate. In this paper a synthesisof earthquake and of the leading edges of overriding plates [Forsyth and Uyeda, 1975' Carlson, 1983' Chase, 1978' Karig, 1974], but these tectonic data weakens the main objections to accepting the mechanisms are shown to be related to the sinking and sea- dominance of the slab pull force in driving the earthquake cycle. Also, this synthesis provides the conceptual framework ward propagation of subducted plate [Garfunkel et al., 1986]. for reinterpretation of several important dynamic properties of Hot spot push also may contribute to driving plate motions subduction zones. [Morgan, 1972; Chase, 1978]. Forsyth and Uyeda [1975] determined that the slab pull FORCES THAT MOVE LITHOSPHERIC PLATES force, Fse, is 10 times more important than the ridge push Plate-driving forces largely are due to within-plate density force, F ae, in moving oceanic lithospherebut also suggested that most of the slab pull force is balanced by viscous dissi- contrasts. These forces largely determine how plates move at pation in the mantle rather than a large component being subduction zones and consequently determine how stresses balanced near shallow subduction zones. A linear inversion of accumulate there. plate velocities relative to specificsubduction zone geometries by Carlson [1983] indicatesthat the slab pull force is about 3 This paper is not subjectto U.S. copyright.Published in 1987 by times more significant than the ridge push force in determin- the American Geophysical Union. ing plate velocities. Paper number 6R0645. The slab pull force cannot initiate subduction. However, as

55 56 SPENCE: SLAB PULL

TABLE 1. Great Normal-Faulting Earthquakes, 1930 to the Present

Date Location M o, dyn cm Depth Reference

Jan. 15, 1931 Oaxaca 3.5 X 1027 shallow Singh et al. 1985] March 2, 1933 Sanriku 4.3 X 1028 shallow Kanamori [1971] Nov. 4, 1963 Banda Sea 3.1 X 1028 100 km Osada and Abe [1981] May 26, 1964 SouthSandwich Islands 6.2 X 1027 120 km Abe [1972a] May 31, 1970 CentralPeru 1.0 X 1028 60 km Abe [1972b] June22, 1977 Tonga 2.3 x 1028 65 km Silver and Jordan [1983] Aug. 19, 1977 East Sundaarc 2.4-4.0 x 1028 shallow Spence[1986]

initially subducted lithosphere enters the upper mantle, it is tion indicate that at shallow subduction zones the maximum apparent that slab sinking, due to the slab pull force, becomes tensional stressesof slab pull origin are significantly greater increasingly dominant in subduction processes.The motions than the maximum compressional stresses of ridge push of plates and subducted lithospheres are important compo- origin. nents of global patterns of mantle flow [Hager et al., 1983; SLAB PULL AS A CAUSE OF SUBDUCTION Garfunkel et al., 1986]. gONE EARTHQUAKES Earthquake Evidencefor Relative Size of In order to providea frameworkfor the developmentof this Slab Pull and Ridge Push Forces paper, the tectonicmodel advocatedhere is summarized.Sub- Although modeling of plate motions implies that the slab sequentsections provide observationalsupport for the model pull force is the dominant force at subduction zones, earth- or deal with model refinements. quake data have not been directly used to confirm this con- How can tensional stressesarising from slab pull forceslead clusion. The episodic nature of subduction is implicit in the to thrust-faultingearthquakes at an interfacethrust zone when widely accepted seismicgap hypothesis [Fedotov, 1965; Mogi, such earthquakes imply that compression acts across this 1968; Kelleher et al., 1973; McCann et al., 1979; Sykes and interface? Consider the analogy of pulling a massive,rigid box Quirtmeyer, 1981]. In the time interval between repeating across a rough floor. While the body forces within the box great earthquakes at a seismicgap, plate motion there is large- would be tensional, a strong box would not deform signifi- ly blocked, and stressesaccumulate. An examination of the cantly but would tend to translate,with compressionalstresses sizes of earthquakes downdip and updip of interface thrust localized at the interface between the box and floor. In the zones, outside of the times of great interface thrust earth- caseof a subductingoceanic lithosphere, the slab pull force is quakes, should indicate the plate strains there and the relative resisted by the overriding plate, causing compressionat the size of the slab pull and ridge push forces acting at shallow interface thrust zone. subduction zones. Prior to a great subductionearthquake, the subductedplate The four great normal-faulting earthquakes at depths great- slowly sinks,increasing extensional stresses at depthsof 50 to er than 50 km in Table 1 occurred downdip of the correspond- 200 kin, with the greateststresses at shallowdepths. Although ing interface thrust zones. Also, the great Oaxaca earthquake slab pull stressesare guided updip, these stressesare dimin- probably is below the primary interface thrust zone. Although ished by forces resisting subduction, leaving 5-10% of the shallow-dipping lithosphereexists downdip of the central Peru grossslab pull force to be supportedat and near locked inter- and Oaxaca events (Figure 1), such normal-faulting earth- face thrust zones, consistent with the views of Shimazaki quakes require extensional tectonics. The downdip tension [1974], Davies [1980], Reynersand Coles [1982], and Spence axes for these five great earthquakes are consistentwith their [1985]. Finally, stressesoriginating with the ridge push and being causedby slab pull forces. slab pull forces exceed the strength of the locked interface The great, normal-faulting Sanriku and East Sunda arc thrust zone, and a great thrust earthquake occurs,with the earthquakes (Table 1) occurred near oceanictrenches. Old and maximum compressivestress, a•, oriented with the relative deeply subducted lithosphere exists downdip of these earth- plate motion. Postseismically,because the slab pull force ex- quakes, and the correspondinglarge slab pull forces are inter- ceeds the ridge push force at interface thrust zones, some ex- preted to have pulled the oceanic plate away from the overrid- tension propagates seaward from the rupture zone, causing ing plate, partially decoupling the interface thrust zone. These plate bending and occasional normal-faulting earthquakes two earthquakes are interpreted as due to slab pull stresses near the trench. This regionally and temporally localized ten- that have been transmitted through partially decoupled inter- sion facilitates a trenchward diffusion of the oceanward hori- face thrust zones to add to bending stressesat the trench zontal plate, acting under ridge-push forces. Also post- zones [Spence, 1986]. seismically,an independent strain pulse moves downdip from While earthquakes due to compression sometimes occur in the rupture zone, returning the downdip plate to a less ex- the oceanic lithosphere at the trench vicinity and seaward tended state. After the rupture of the great earthquake reheals, [Christensen and Ruff, 1983; Hanks, 1971; Mendiguren, 1971; the processrepeats. McAdoo et al., 1978], these earthquakes are unusual, and none have been observed with the size of the normal-faulting MANTLE VISCOSITY AND SUBDUCTION earthquakes of Table 1. The largest known such thrust event is The slab pull force can act strongly at shallow depths, pro- the 1981 M s 7.2 earthquakelocated seawardof the 1985 Val- vided that the viscosity of the surrounding mantle is less than paraiso interface thrust earthquake [Christensen and Ruff, about 2-3 x 1020 Pa s (2-3 x 1022 P) [McKenzie, 1969; 1983, 1985; Nishenko, 1985]. The earthquake data of this sec- Hager and O'Connell, 1981; Hager et al., 1983]. Direct esti- SPENCE: SLAB PULL 57

DISTANCE FROM TRENCH, KM $o I00 I$0 200 250 $00 35O

I I I I I I •rz-----q ßß "x.'"'•x-• E. SUNDA ARC 2 50

C. PERU

Q' I00 S. NEW ß LU HEBRIDES ARC6 NE SHUMAGIN GAP, JAPANß••. ALEUTIAN Xx ALASKAI 150 ARC•,• ARC4

Fig. la. Dips of well-locatedsubducting plates, at depthsof 0-150 km. Sectionsare perpendicularto arcs.The dip of typical plate increasesfrom about 10ø at the interfacethrust zone (ITZ) to 30ø-70ø in the upper mantle. This slab bend occursalong about 40 km of plate length,just downdip from the interfacethrust zone. Sourcesfor plate dips are (1) Hauksson[1985], (2) Spence[1986], (3) C. Langer and W. Spence(unpublished manuscript, 1984), (4) Engdahlet al. [1984], (5) Hasegawaet al. [1978], and (6) Coudertet al. [1981].

mates of mantle viscosity, based on lithospheric adjustment earthquake cycle. These values are somewhat smaller than the rates after great earthquakes,are 5 x 10TM Pa s [Nur and top layer viscositiesin the mantle models of Cathles [1975] Mavko, 1974], 6 x 10•8 Pa s [Spence,1977], and 1 x 10•9 Pa and Yokokura[1981]. Cathlesobtained a viscosityof 4 x 10•9 s [Thatcher et al., 1980]. These estimates are determined Pa s betweendepths of 64 and 128 km and 102• Pa s for the within earthquake zones and within a small fraction of the rest of the mantle, from consideration of glacial rebound data.

oF .:,•,M America12øN (15), C Aleutian(55), L Antilles(85) Andama " S Chile 40øS (25) C Chile 30 øS (40) lOO o Alaska (20) (25) Scotia (130) 200 Sumatra 5 øS (50) New Zealand (85) N Chile 20 øS (40) E New Hebrides (70) 20 ø -•400 Solomons (125)

Kermadec 33' S (lOO) Dip New Britain (100) 4 0 ø 6OO Java(125) Philippine (50) Mariana (160) 60 ø

80 ø

Fig. lb. Dips of Benioff-Wadatizones below the slab bend zone' dip angle is referredto plate depth of 100 km (adaptedfrom Uyedaand Kanarnori[1979]). Dashedand dottedlines indicate zones with few or no earthquakes.Ages of platesat trenchesare shownby numbersin parentheses[Schlater et al., 1981]. Plate dips are positivelycorrelated with age. 58 SPENCE: SLAB PULL

TRENCH Schubert et al. [1975] and Fitch [1977] also calculated the additional, large body forcesdue to the phase change olivine to /• spinel and a significant body force due to the deeper r'RP /,,,, / 'wSB . spinel to postspinelphase change. The depth of the olivine to /• spineltransition may vary significantlybetween slabs, as this depth is a function of the internal temperature distribution in a slab, which in turn depends on slab age, slab thickness,and subduction rate [Sung and Burns, 1976a, b; Liu, 1983; Ruble, 1984]. Thus wide variations exist between slabs in the contri- bution to the slab pull force from the olivine to/• spinel phase change. The insets in Figures 2 and 3 show forces acting at a unit volume within the subducted slab that are due to that unit volume's excess mass. The primary vertical force (Fsp) is brokenint ø two components,one parallelto the dip • of the Fig. 2. Diagram of driving and resistingforces acting on subduct- subductedplate (Fll) and the other normal to theouter sur- ing plate. The slabpull force,Fsv, shownby insetforce diagram, acts faces of the subducted plate (FN). Similar forces pertain throughoutthe entirelength of subductedslab. Forces opposing plate throughout the plate having excessmass. The downdip T or P motions include the viscousresistance to downdip plate motion pro- stress axes of many mantle earthquakes [lsacks and Molnar, videdby the mantle,Fst R, the cornerflow that resiststhe platefalling to the vertical,Fcv, the momentleading to internal deformationand 1971] imply that the subducted slab is a stressguide at least torquingat the slab bend,Mss, the resistanceto plate motion at the down to the 650-kmdiscontinuity. F ll sumsthroughout the interface thrust zone, Frz, the forces leading to formation of the length of subducted plate and promotes downdip plate trench and outer rise, F r, and the resistanceof the attached oceanic motion. FN acts to move the entire plate trenchward [Tovish plate to trenchwardmotion, F o. Dashed lines indicatezone of slab et al., 1978; Yokokura, 1981; Carlson and Melia, !984; Gaf- bend. funkel et al., 1986]. The slab pull and ridge push forces act together to subduct oceaniclithosphere. Because subduction occurs, Fsp + Fap ex- Yokokura obtainedviscosities of about 10z9 Pa s for the top ceeds the sum of resisting forces. Corner flow mechanisms 100km of the uppermantle, 102 z Pa s for the restof the upper [Jischke, 1975; Tovish et al., 1978; McAdoo, 1982] preserve mantle, and about 1022 Pa s for below about 650 km, from plate dips below 100 km depth (Figure lb). The seawardprop- consideration of plate dips and plate torques due to the nega- agation of trenches [Carlson and Melia, 1984] combined with tive buoyancy of subducted plates. The depths of most the fairly constantdip of deeply subductedplate impliesthat normal-faulting earthquakes in the upper mantle are within the entire subduction system slowly propagates seaward [Gar- the low-viscosity,top layer of Cathles [1975] and Yokokura funkel et al., 1986], i.e., F• > Fcv (Figure 2). The rate of sea- [1981]. ward advance of plate is positively correlated with plate age The viscosityof 102• Pa s beneath the top layer of the [Garfunkelet al., 1986], and related increasesin Fcv and Msa upper mantle seemsto be too high to permit observeddeep may explain the lack of strong correlation of subduction ve- plate motions. During a subductioncycle, variations of pres- locity with plate age [Ruff and Kanamori, 1980]. sure within the subducted plate and correspondingpressure variations in the immediately surrounding mantle may lead to Slab Bend temporarily lowered mantle viscosity[Brady, 1976] and more Subducting oceanic lithosphere dips about 8ø-15ø beneath easily permit downdip movementof subductingplate. the accretionarywedge, until a depth of 20-40 km above the top of the subductedplate is reached [Pennington,1983, 1984; SLAB PULL FORCE AND FORCE BALANCES Davies and House, 1979; Ruff and Kanamori, 1983b] (Figure The slab pull force is proportional to the excessmass of the la). Then the plate dip steepensto 300-70ø (Figure lb), form- subducted oceanic lithosphere in relation to the mass of the ing the slab bend. The six slab bend zones shown in Figure la warmer, displacedmantle. This excessmass is a function of the are for subducting plates whose positions are well resolved subducted plate's volume, and its positive density contrasts from detailed seismicitystudies; the slab bend zone is indicat- with the surrounding mantle. ed in Figures 2 and 3. The slab bend occurs within about 40 McKenzie [1969] calculated the density contrast times km of plate length, which for an average convergencerate of 6 volume of a deeply subducted plate, based only on temper- cm/yr occursover a period of about 600,000 years.The degree ature considerations,and found that the slab pull stressacting of bending at slab end zones is much greater than at trench- at shallow regions should be 100-200 MPa (1-2 kbars), if the outer rise systems.However, large earthquakes generally are mantle provided no resistance.Schubert et al. [1975] calcu- absent at the slab bends, implying that the rheology through- lated this stressto be 65 MPa (0.65 kbar). Similar values for out slab bends is largely anelastic. the slab pull stressare obtained by Fitch 1-1977],Molnar and Because of the beam strength of oceanic plate, the plate Gray [1979], Davies [1980], and Wortel [1986]. Becausesub- resistsbeing bent downward and consequentlyremains cou- ducted plates have widely different ages and penetration pled to the overriding plate at the interface thrust zone. A depths,there existwide variationsin the magnitudeof the slab great interface thrust earthquake nucleatesjust updip of the pull force. For example,Davies [1983] calculatesthe effective slab bend and then propagatesupdip and laterally [Kelleher slab pull force acting at the Marianas trench to be nearly 15 and Savino, 1975; Dewey and Spence,1979; Davies et al., 1981; times greater than at the Aleutian trench. Lay et al., 1982; Dmowska and Li, 1982]. Downdip from the SPENCE: SLAB PULL 59

OUTER TRENCH ACCRETIONARY RISE WEDGE CRUST VOLCANIC ARC

BASIN

LITHOSPHERE

TRENCH • RIDGE PUSH ROLLBACK '•'•,

IOO (1000'(;) MELT

200 (1400'G) /

3OO (1500*G) DUETO EXCESS MASS

OLIVINE (1600'G) 400 •'SPINEL

500

Fig. 3. Schematicdiagram of the role of slabpull forcein establishingsteady state conditions at a subductionzone. The greatestplate bending is at the slabbend zone, just downdipof the interfacethrust zone. Unlabeled arrows within the plateshow extension (open arrows) and compression (arrows meeting) subducted; the body force at a givendepth point is the sumof platepush from above, plate pull frombeneath, and resistive forces (Figure 2). The lithosphericthickness is drawnfor 60-m.y.-old lithosphere at the trench [Watts et al.,1980]. Back arc basins tend to beabove old subdueting plate [Molnarand Atwater, 1978; Ruff and Kanamori, 1980; England and Wortel, 1980; Carlson, 1983; Carlson and Melia, 1984; Garfunkelet al., 1986].Temperatures of oceanic asthenosphere and the boundary of theolivine to fl spinelphase change are from Schubertet al. [1975]. slab bend, earthquakesgenerally occur within the subducted lated with lithospheric age [Veith, 1974; Jarrard, 1986] plate. (Figure lb), implying that the slab pull force is a major deter- Severalinvestigators [Rogers, 1982; Pennington,1982, 1984; miner of plate dip. In this paper the slab bend is interpretedto Ruff and Kanarnori,1983b-I suggest that the slab bend is as- be a steady state pivot for subducted slab that is below the sociated with various dehydration reactions and the phase bend,reflecting the summedslab pull forceof the deeperplate. changein the subductedoceanic crust from "wet" gabbro to The aseismic slab bend occurs at the zone of the crustal "dry" eclogite[Ahrens and Schubert,1975]. Crustal hydration gabbro to eclogitephase change, because that rheology and may be enhancedby the migrationof water through normal plastic behavior in the remaining bending lithosphere[e.g., faults that developed near oceanic trenches and that have McAdoo et al., 1978] permit the slab pull force to most easily moved downdip with the subductingplate. bend the plate to its mantle equilibriumposition. The volume decrease associated with this crustal phase changein dry mineralsis about 17%. Assumingtotal conver- Trench Depth Discrepancy sion within a 5- to 7-km-thick crust implies that the subduct- Trenchesand outer risesare due to plate bending [Hanks, ing crust would withdraw from the interfacezone by about 1 1979; Chappleand Forsyth, 1979] and are dynamicallysus- kin, decouplingthe interfacethrust zone [Ruff and Kanarnori, tainedby downdipmass transfer beneath the trenches[Davies• 1983b-I.The decouplingis aided by crustalthinning due to this 1983; Chase and McNutt, 1982]. Attempts to reconcile the bending. Forearc basins,on the inner trench slope, are posi- calculatedexcess mass of a subductedplate with actual trench tioned above the slab bend [Rogers, 1982], and thus the topographyfind that by taking reasonablevalues of FTz and downdip edge of a forearc basin should correspondto the FMR,the calculatedforce due to slabpull is 3-10 + timesgreat- downdip edge of the interfacethrust zone. er than that expressedin trench topography [Chase and The dip angle of deeply subductedplate is positivelycorre- McNutt, 1982; Hager et al., 1983; Davies, 1983]. Because 60 SPENCE: SLAB PULL

12 8.5 95 C. Chile OS.Chile

8.2 8.2 10 NE Japan 9.0 Peru• Ki •s Kamchatka

8.1 C. America 8.8 9.1 • Colombia 8 7.1 Aleutians -' Java 8.1 Kermadec

7.8 •6 in •New Zealand a• ß

Q nQS 7.9 72 New Hebri 7.5 Carribbea •Sc 7.0

160 140 120 100 80 60 40 20 0 Age, m.y. Fig. 4. Maximumcharacteristic earthquakes M w,plotted as functions of subductionrate and age of subductedplate (adaptedfrom Ruff andKanamori [1983b]). Included in lowerleft-hand corner are oldestplates, which show weakest couplingand are associatedwith marginalbasin formation. Included in upperright-hand corner are youngestplates, whichshow strongest coupling and are associatedwith seawardadvance of overridingplate.

entire subduction systemstend to propagate seaward, it is curred in zones where the age of the oceanic lithosphereis implied that slab bend zonestrack this seawardpropagation. < 60 m.y.B.P. Theseobservations imply that seismiccoupling The resistancesto bending and seaward propagation of the is maximizedby the buoyancyof young oceaniclithosphere slab bend may be an important resistanceto the slab pull and seawardadvance of back arc plate. High seismiccoupling force acting at shallow subductionzones, and this neglected leadsto large asperities,whose ruptures cause great subduc- resistancemay partly explain the trench depth discrepancy. tion earthquakes[ Lay et al., 1982; Ruff and Kanarnori, Another factor which could help resolvethe trench depth dis- 1983a]. crepancyis the episodicapplication of bendingmoment at the The vertical force acting on very old (> 80 m.y.B.P.) and trench and outer rise, following decouplingearthquakes. As a densesubducted plate will minimizecoupling at an interface ruptured interface thrust zone reheals, the slab pull force thrust zone [Ruff and Kanarnori,1980; Davies,1983; Carlson acting updip of the interfacewould be blocked,and thus the et al., 1983; Spence,1986], resultingin the absenceof great slab pull forcewould act at the trenchand outer rise only for interface thrust earthquakes and sometimesa fairly steady a small fraction of an earthquake cycle. subductionprocess, as may be occurring at the Marianas. Low couplingcan allow stressesof slabpull origin to migrate updipand directlyadd to bendingstresses beneath an oceanic SEISMIC COUPLING trench, causing great normal-faultingearthquakes such as Ruff and Kanarnori [1980] and darrard [1986] show that those that occurred at Sanriku in 1933 and at the East Sunda the force acting normal to an interfacethrust zone is related to arc in 1977. the age of the subducting lithosphere and by implication is The stresses that are loaded at interface thrust zones are related to the size of the ridge push and slab pull forces.Other obtainablefrom studiesof great earthquakes.Stress drops of factors that determine seismiccoupling are the rate of seaward 1-6 MPa (10-60 bars) are representativefor great interface advance of plate in the back arc zone, the load of the accre- thrust earthquakes[Lay et al., 1982]; the total stresses(dy- tionary wedge, the resistanceof the plate to downward bend- namic stresses)inferred from the rupturing of asperitiesare ing, the width and asperity content of the interface thrust not significantlydifferent from the staticresults of Lay et al. zone, and possibly the quantity and composition of subducted [1982] (G. Choy, personalcommunication, 1986), supporting sediments.As is implied in Figure 4, coupling due to the ridge the assumptionof Kanamori [1977] of total stressdrops for push force and resistanceto bending of oceaniclithosphere are greatearthquakes. The stressdrop for the great 1977 Sumba more than offset by the decoupling due to the corresponding earthquake,which faulted through the brittle oceaniclitho- slab pull force. sphere,was about 20 MPa [Spence,1986]. The stressesgreater Decoupling earthquakeswith sizesgreater than M w 8.7 or than 60 MPa determined at the Shumagin gap [House and M 0 1029are shown in theupper right-hand corner of Figure4, Boatwright,1980] are atypical.Generally, stresses of subduc- and these earthquakes indicate that factors that maximize seis- tion earthquakesare considerablyless than those available mic coupling. They occur where the back arc plate is advanc- from the total slab pull force of 50-200 MPa (0.5-2.0 kbars) ing seaward and, except for the Kamchatka event, have oc- and imply that much of the slab pull force is diminishedby SPENCE: SLAB PULL 61 resistingforces that act more deeply than the thrust zone and bend this plate downward. This southward lesseningof the trench. slab pull force for the 1960 Chile zone is consistent with The repeat times for great, interface thrust earthquakes vary Herron's [1981] observations that as one goes from the from 35 to 150+ years [Sykesand Quittrneyer,1981]. Vari- Mocha block to the Chile rise, trench depth shallows and the ations in strengths of the plate-driving and -resisting forces at trench free-air gravity anomaly lessens;it is also consistent a given seismicgap zone determine the magnitude and repeat with the observationof Kadinsky-Cadeand lsacks [1983] that the interface zone south of the Mocha block is more horizon- time of earthquakes for that zone. Given enough time, even , plate less than 30 m.y. old will sink sufficiently to strongly tal and has greater downdip extent (until decoupledat the slab load a locked interface thrust zone. bend) than that north of the Mocha block. It is concluded that the 1960 foreshocks and main shock THE GREAT 1960 CHILE EARTHQUAKE nucleated at the zone of strongest slab pull and weakest cou- The May 22, 1960, Chile earthquake is the greatest known pling, at the Mocha block. The main shockrupture then prop- earthquakein recordedhistory (M n, 9.5; M o 2 x 1030dyn cm agated southward into the zone of weaker slab pull force but [Kanamori, 1977]). The main shock rupture was about 900 strongercoupling. It is observedthat rupture of the Mocha km long, 60-200 km wide, and had an average displacement block independentlyhas a much faster repeat time than the of 20-25 m along a low-angle thrust fault [-Plafkerand Savable, more southern zone [Nishenko, 1985]. Periodic ruptures of the 1970; Kanamori and Cipar, 1974]. The area of rupture is indi- entire 1960 zone may be triggered by a subsetof Mocha block cated in Figure 5 by the primary earthquakes and vertical ruptures. deformation associatedwith this earthquake series.The main SLAB PULL AND TENSION IN THE UPPER MANTLE shock produced a great, ocean-wide tsunami, resulting from uplift of the seafloor [Plafker and Sava•Ie,1970]. As assumption in this paper is the dominance of tensional In the 23 hours prior to the main shock, there were numer- stressin subducted plate at depths of 50-200 km. However, ousforeshocks (including two of M 7•--7•)x x at the northern end there exist significantstress variations at intermediate depths of the Mocha block. The main shock nucleated near these in subducted plates [Isacks and Molnar, 1971; Isacks and foreshocksand propagated southward [Kanarnori and Cipar, Barazangi, 1977; Vassiliou et al., 1984; Fujita and Kanarnori, 1974] to landward of the trench-rise-trenchtriple junction. 1981]. Fujita and Kanarnori [1981] find that as a global Figure 5 shows the largest known events (M > 5.6) of this average, only 65% of intermediate-depth earthquakes have earthquake series,for the years 1960-1962. Aftershocksat the downdip stressaxes. In the depth range of 50-200 km, devi- interface thrust zone are concentrated at the Mocha block. ations from downdip tensionalfocal mechanismsoccur at the South of the Valdivia fracture zone, large aftershocksare re- upper layer of double seismiczones and at certain geographic markably lacking at the interface thrust zone but are con- regions [Fujita and Kanarnori, 1981], particularly for slabs centrated near the Chile trench. The trench activity began whose seismicityis nearly continuous to over 600 km depth about 8 hours after the main shock and continued for at least [Isacks and Molnar, 1971; Vassiliouet al., 1984]. 1« years' none of thesehave magnitudegreater than 7.0. B. Isacks (cited by Sykes [1971]) found normal-faulting focal Plate Motion Oblique to Island Arcs mechanisms for many of these earthquakes near the Chile At the Aleutian arc, for example, a westwardcomponent of trench. relative plate motion is due to the strongridge push and slab The magnetic anomalies and implied ages of the oceanic pull forcesarising from very old lithosphereon the west side plate shown in Figure 5 are from Herron [1981] and Herron of the Pacific plate [Gordonet al., 1978]. Becauseof the physi- et al. [1981]. The age of the Mocha block at the Chile trench cal integrity of plate subductedat the Aleutian arc, the oblique is about 25 m.y.B.P. [Herron, 1981]. North of the Mocha motion of the Pacific plate is transmitted to intermediate fracture zone, the age of the subductingplate is about 32 m.y. depths. This causesplate distortion, as is shown by earth- B.P. South of the Valdivia fracture zone, the age of the Nazca quake focal mechanismsthat indicate strike-slipdeformation plate at the trench is about 20 m.y.B.P., and the subducting or lateral extension [Stauder, 1968, 1972]. plate becomesprogressively younger to the south, approach- ing the intersection of the Chile rise with the Chile trench Double Seismic Zones [Herton, 1981; Herron et al., 1981]. The phenomenonof double seismiczones, where two layers Approximationsof Fse acting along the rupture zone of the of seismicityseparated by about 35 km occur at the depth 1960 Chile earthquake are given in Figure 6. These estimates range of 60-190 km, has been critically reviewedby Fujita and assumethat the subducting plate is an effective stressguide Kanarnori [1981]. They conclude that the zone near the slab's and do not considerforces resisting Fs•,; thus the shapeof the upper surfaceis dominated by downdip compression,that the resulting curve is more significant than its absolute value. zone deeper in the plate is dominated by downdip tension, Figure 6 shows that the slab pull force is greatest at and just and that double zones are defined best in old and "rapidly" south of the Mocha block. As one looks south from the subductingslabs. The popular explanation of double seismic Mocha block, the progressivelyyounger oceanic lithosphere zonesis that they are due to unbendingof plate that originally would lead to both a smaller slab pull force and a larger was bent at the top 50 km of plate, especiallyat the trench coupling, until encountering extremely young oceanic litho- and outer rise [Engdahl and Scholz, 1977; Isacks and Bara- sphere at the southern end of the 1960 Chile rupture, which zangi, 1977; Kawakatsu, 1986]. Sleep [1979] and Fujita and may not be strong enough to sustainhigh stresses.Moreover, Kanarnori [1981] argue that plate unbending is an unlikely near the Chile rise the plate interface may be decoupled,be- explanation for double seismiczones, particularly since such causethe low resistanceto plate bending of the Chile rise will zones are not a general feature of subduction zones, all of permit the slab pull force of locally subductedplate to easily which have zones of bending. Moreover, bending stresses 62 SPENCE: SLAB PULL

82 79 76 73 70

2.5 cmlyr

4B Fig. 5. Map showingforeshocks (solid circles), aftershocks (asterisks), the M w 9.5 1960Chile earthquake (circled star), associatedvertical displacementin meters[Plafker and Savage,1970], age of the oceaniclithosphere [Herron, 1981; Herronet al., 1981],and volcanos (solid triangles). Earthquakes with M > • arecircled. Foreshocks, the main shock, and manyaftershocks are concentratednear the Mochablock, defined by the zonebetween the landwardextensions of the Mocha and Valdivia fracturezones. The rupture of the lockedinterface thrust zone allowedslab pull forcesto migrate updip,producing high strains and normal-faultingearthquakes near the Chiletrench. Earthquake locations were done on computerby the staffat the InternationalSeismological Summary, and earthquakemagnitudes are from the U.S. Coast and GeodeticSurvey. Just north of the triplejunction, earthquakes and volcanismoccur downdip, implying the existence of plateto depthsof at least125 km. The apparentease with whichthe Chile rise is subducted[Herron et al., 1981;Cande and Leslie,1986] suggeststhat rather than ridge pushforces of the rise causingits own subduction,this subduction primarilyis due to slabpull forces.Absolute plate velocitiesare from Minsteret al. [1974]. SPENCE'SLAB PULL 63

E --30m • 1( \ \&---- -&7- 20•' Plate Age-/

. -

lO,• •&. (TRIPLE

- o 37 39 41 43 45 47

Fig.6. Ageof Nazca plate and estimates ofFsv acting at rupture zone of the 1960 Chile earthquake. Fsvestimates are takenparallel to thestrike of thecoast and are based on the relationships of McKenzie [1969] and Davies [1983]. The dottedline suggests slab pull force due to platesubducted beneath the triple junction.

should be concentrated at the slab bend zone, but the lack of great earthquakes.In modelingthese geodetic data, Thatcher earthquake activity there implies that the stressesdue to that andRundle [1979, 1984] have considered a precursory, slowly bending (as well as residual stressesfrom bending at the acting slip downdip from the interfacethrust zone. trench and outer rise) are dissipatedlocally at the slab bend. The precursorynormal-faulting earthquakes occurring just Possiblesagging of slab betweenzones of relatively high vis- downdip of locked interface thrust zones are direct evidence cositiesis an alternativefor theorigin of doubleseismic zones for precursorytension downdip of great subductionearth- [Sleep, 1979], but this hypothesisis inconsistentwith double q.uakes. These data are consistent with the influence of the zonesbeing best defined in rapidlysubducting plates. In the slabpull forcein loadingstresses at a lockedinterface thrust slab pull model the tensionalearthquakes directly reflect slab zoneand supportthe hypothesisthat the slab pull forceis an Pull forces,whereas the compressionalearthquakes reflect import&ntcontributor to stressescausing great subduction local resistanceto rapid downdipplate motion.The causeof zone earthquakes. thiscompression is like that desdrib6dearlier for slabpull stressescontributing to compressionalstresses at an interface thrust zone. STRESS-GUIDE EFFECTS Many intermediate-depthearthquakes that fail by other The downdiptension of typicalearthquakes in the depth than downdip tension can be explained in ways such as that range of 50-200 km is interpretedhere as due to the summed for the Aleutianarc or by their pr.esence in the upperlayer of slabpull force of thedeeper plate. In a deeplysubducted plate, double seismiczones. This leavesa predominatinggroup of oftenthere is a hiatusof earthquakesbeginning at depthsof intermediate-depthearthquakes that fail by downdiptension, 300+ 50 km and extending50 to 200 km downdip[lsacks as is originallysuggested by lsacksand Molnar [1971]. andMolnar, 1971; Abe and Kanamori, 1979; McGarr, 1977; Giardiniand Woodhouse,1984; Va•siliou et al., 1984].With PrecursorsDowndip of LockedInterface very few exceptions,earthquakes deeper than the hiatusshow Thrust Zones comp?essionalfailure, and a relative maximum in seismic The repeattime for large, shallowearthquakes along the energyrelease exists at depthsof 500-650 km. Middle AmericaTrench, offshoreof Mexico, is 35-40 years [Gonzalezet al., 1984;Nishenko and Singh,1984]. Analysisof Mantle Resistance to Slab Pull a completecatalog of smallerearthquakes in this zone showed The usual explanation for the hiatus is that it reflectslow- an onset of normal-faulting earthquakes directly downdip ered deviatoricstresses [lsacks and Molnar, 1971; Davies, from the interfacethrust zone,beginning about 15 yearsprior 1980;Vassiliou et al., 1984;WOrtel, 1986], although a local- to Severallarge, shallow earthquakes [Gonzalez et ai., 1984; ized weakeningof subductedplate du• to the exothermicoli- McNally et al., 1986].Other examplesof largenormal-faulting vineto/? spinel phase change may •be an alternate. explanation earthquakesdowndip from interfacethrust zones that preced- [Ruble,1984; Wortel, 1986]. Compressional stresses at depths ed largegap-filling earthquakes have been reported for Chile beginningat 250-350 km may be causedby increasedresist- [Malgrange and Madariaga, 1983] and Peru [Dewey and anceto slabpenetration due to a viscosityincrease near these Spence,1979; Beckand Ruff, 1984]. Suchnormal-faulting ac- depthsand to phaseor chemicalchanges in the hostmantle, tivity, beginningat the final • of the repeattime and con- beginningat depthsof about400 km and 650 km [lsacksand tinuingto near the time of the main shock,would be expected Molnar,1971; Christensen and Yuen, 1984; Anderson and Bass, as the slab pull force extends the subductedplate and in- 1986].Vassiliou etal. [1984] took the 650-km disco.ptinUity to creasestensional stresses at the plate'sshallow regions. be a strongresister to platepenetration, without regard to the Inversionsof geodeticdata from areas affectedby great originsof platemotion• and sucdessfully modeled much of the earthquakesoccurring at Japanesesubduction zones suggest stressand seismicity of subduCtedplates. slip at depthsdowndip from the interfacethrust zones,before In additionto the resultsof Vassiliouet al., the slabpull 64 SPENCE' SLAB PULL

135 ø 140 ø 145" E earthquakesoccurring during this time-spaceinterval were related empirically in this way to the great shallow earthquakes. Take the shallow subduction zone to be locked and the subducted plate at 50 to 200 km depth to be under tension. Further plate sinking in the zone of lowest mantle viscosity will cause an increasein shallow tension. This episode of sinking plate will increase the stress in the deeper plate, because motion of deeper plate is resisted by the higher viscosity mantle there. The deeper compression, whose time history is describable by a Maxwell model, may cause the observed deeper compressional earthquakes (Figure 8a). Continued plate sinking would further increaseshallow tensional stresses, / I " eventually loading sufficient stressat the locked shallow zone /I • • . ..S•nriku- to lead to the observed subduction zone earthquakes. The greatest extension would be immediately downdip from the 4• • '• oki - moststrongly coupled zone (the controlling asperity) for each great, shallow earthquake, and correspondingly,the increased compressionwould be immediately downdip from each main shock epicenter, consistentwith the observed data. It is concluded that becausesubducted plate is a stressguide, temporal changes in tension in the upper part of subducted plate can induce stress changes (and occasional faulting) in the deeper subducted plate, if the period of plate sinking in the lowest Fig. 7. Three M > 7, h • 3• km thrustearthquakes that preced- viscositypart of the upper mantle is faster than the relaxation ed the great 1933 Sanriku normal-faultingearthquake [after Moji, 1973]. Other shallow earthquakesof M > 7.7, whoseoccurrences mi- time of the deeper, resistingmantle. grate southward through the 1933 earthquake zone, are also shown by hatchedareas, but thesewere thrust-faulting earthquakes. force may contribute to the stress regime of the entire plate SLAB PULL AND POSTEARTHQUAKESTRAIN PROPAGATION [Davies, 1980, 1983; Wortel, 1986]. Consider the stressesdue Trench, Normal-Faulting Earthquakes to the slab pull force, as one moves downdip within the sub- Following Decoupling,Interface ducted slab, beginningjust below the slab bend. At first the Thrust Earthquakes deviatoric stresseswould be large and tensional becauseof the excessmass beneath (Figure 3). Moving deeper, there would be a diminished slab pull force, owing to diminished excess Normal-faulting earthquakes at oceanic trenchesand outer mass beneath and an increased downdip push from the in- riseshave been observed for daysto manymonths following creased excessmass above. Below a transition depth of low decouplinginterface thrust earthquakes [Stauder, 1968; Sykes, deviatoric stress, the deviatoric stress becomes increasingly 1971; Spence,1977; Chappleand Forsyth,1979; Hanks, 1979; compressionalbecause of the weight of the plate above. Thus Christensenand Ruff, 1983]. These normal-faultingearth- the factors of slab pull (plate weight) acting in a stressguide quakesnearly alwayshave axes of least-compressivestress and resistanceto plate motion at the discontinuitiesbeginning that trend downdip, regardlessof the direction of local relative at about 400 km and 650 km combine to explain the observed plate motion.The largestknown such normal-faulting earth- intraplate earthquake activity of tensional failure in the depth quake(M s 7.5) wasimmediately trenchward of the epicenter range of 50-200 km, the low seismicity at 300 __+50 km, and of the 1965 Rat Island earthquake[Spence, 1977]. An excel- the compressionalfailure at greater depths. lent exampleof postearthquakenormal faulting was shown for the 1960Chile earthquake series (Figure 5). Suchearthquakes are explainableby a trenchwardmigrating, tensionalstrain Mogi's Thrust Earthquakesat Depths pulse (Figure 8b), whoseamplitude is a function of the local of 260-350 km, Precursory to Great, seismicslip at the interfacethrust zone and whosespeed is Shallow Earthquakes controlled by the effectiveviscosity of the mantle. When this tensionalstrain reachesthe trench and outer rise, it adds to Mogi [1973] noted that within 3 years prior to the oc- existing bending stressesand causes the observed normal- currenceof the five great (M s > 7.8) shallow earthquakesat faulting earthquakes. and near northern Japan during the interval 1930-1968, there A slab pull origin of this tensional strain is consistentwith occurredone or more large (M > 7) earthquakesat depthsof thefinding that the shapesof outerrise and trenchsystems are 260-350 km, directly downdip from the hypocenterof each bestexplained in the absenceof large,horizontal compres- pending great shallow earthquake. Where focal mechanisms sional stresses[Chapple and Forsyth, 1979; Caldwellet al., are known, these intermediate-depth earthquakes exhibit 1976; Parsonsand Molnar, 1976; Meloshand Raefsky,1980] thrust faulting. The great shallow earthquakesare typical butl•erhaps are due to bendingmoments originating at or interfacethrust events,except for the great, normal-faulting belowinterface thrust zones. Such a temporallyand regionally 1933 Sanriku earthquake. Figure 7 shows the large localizedtensional strain would help evena weakridge push intermediate-depthearthquakes that preceded the Sanriku force to produce a trenchwarddiffusion of oceanicplate earthquake. Mogi notes that all large intermediate-depth [Melosh, 1976]. SPENCE: SLAB PULL 65

COMPRESSION AT LOCKED ITZ TRENCH TRENCH

100- ß I RIDGE'PUSH SEAWARD COMPRESSION EXTENSIONAL SLAE PULSE PULL 200- TENSION o•_

300- THRUST EARTHQUAKES

OLIVINE OLIVINE

400- -SPINEL -SPINEL

500

600-

% % 700- % t

800-

a TWO YEARS PRE-EARTHQUAKE b ONE YEAR POST-EARTHQUAKE

Fig. 8. Schematicdiagram of temporaldeviatoric stresses (a) 2 yearsbefore decoupling interface thrust zone earth- quakeand (b) 1 yearafter decoupling interface thrust zone earthquake. Relative extensional stress is indicatedby widely spacedlines normal to the slabsurface; relative compressional stress is indicatedby closelyspaced lines. Greatest tensional stressesare just beneaththe interfacethrust zone and prior to the decouplingearthquake; greatest compressional stresses are at the interface thrust zone. Entrained mantle flow is not indicated.

Strain Pulse Moving Downdip From downdip end of the main thrust zone. Sourcemodeling for a Decoupled Earthquake Zone two of the largestof theseearthquakes indicate stress drops Following a decoupling earthquake, the stretched downdip greater than 60 MPa, and theseearthquakes have been inter- slab responds like a stretched, damped spring after one end of preted as the breaking of two small but strongly loaded as- the spring has abruptly been released(Figure 8b). Becausethe perities•House and Boatwright,1980]. Within the subducting postearthquake,downdip propagating strain pulse is returning plate and below the slab bend, at depths between 45 and 120 the plate stresses toward lithostatic values, brittle fracture km, there are two discrete earthquake subzonesthat show would tend riot to occur unlessmantle resistanceto plate extensional or compressionaldeformation. The senseof defor- motion causes high compressional stressesnear the plate's mation within these two subzones reversed between 1979 and upper surface, similar to the mechanism by which slab pull 1981 I-Reynersand Coles, 1982; Hauksson et al., 1984], cor- can lead to compression at a locked interface thrust zone or at relative with a sudden change in surface tilt I-Beavanet al., the upper layer of double seismic zones. Evidence for such 1984]. Interpretation of the seismicityand tilt data by Beaven strain pulsesincludes earthquakes with downdip compression et al. [1984] and Haukssonet al. [1984] indicatesmajor stress axes following the decoupling earthquakes of 1960 in Chile changesdowndip from the main interface thrust zone. If these [Astiz and Kanamori, 1985], of 1965 at the Aleutian arc phenomenaare precursoryto the large earthquakethat some [Spence,1977] and of 1977 in Indonesia [Spence,1986]. Fur- investigatorsexpect to fill the Shumaginseismic gap, then it is ther evidencefor downdippropagating postearthquake strain implied that detailed seismicmonitoring for precursorsto is provided by geodetic data, where Savage [1983] inferred other interfacethrust earthquakesshould include the regions rapidslip to a depthof near100 km, well below the downdip just updip and downdip from the slab bend. Savageet al. end of the interfacethrust zone. The forcesresisting downdip [1986] suggestthat the Shumaginseismic gap is not storing plate movment will determine how much the tension in the significantstrain. In this casethe precedingobservations indi- upper 200 km of plate is diminishedby such a downdip prop- cate processesof largely decoupled subduction and indicate agation of strain. that the slab pull forceacting through the slab bend is impor- , tant in such processes. DISCUSSION The possible association of excitations of the Chandler The Shumagin seismicgap, Alaska, has been the subject of wobblewith great earthquakesis well known, but only about much study [Davies et al., 1981]. The updip part of the inter- 10% of the requiredexcitation power can be accountedfor by face thrust zone at the Shumagingap is seismicallyquiescent, the global static deformation field of great earthquakes and interface thrust earthquakes presently are only at the rDahlen, 1973; O'Connell and Dziewonski, 1976; Mansinha et 66 SPENCE' SLAB PULL al., 1979' Gross, 1986' Chao, 1985]. The plate motions de- resistancesto the slab pull force being delivered to an interface scribed in this paper, which precede and follow decoupling thrust zone. earthquakes, may provide much of the missing excitation A decoupling earthquake will release subducted plate that power for the Chandler wobble and changesin length of day. was extended by slab pull forces, and a subsequentstrain pulse will propagate downdip, returning the subductedplate SUMMARY AND CONCLUSIONS to a less stressedstate. Becausethe slab pull force typically is This synthesisof subduction seismotectonicsindicates that greater than the ridge push force, a postearthquake tensional gravity expressedthrough the negative buoyancy of subducted pulsewill propagatetrenchward. This tensionalpulse leads to lithosphere causes most earthquakes at shallow subduction accelerated plate bending and occasional normal-faulting zones and in subducted lithospheres. These earthquakes are earthquakes at the trench and easestrenchward diffusion of causedby interactions between the forcesthat drive and resist the oceanic plate by the ridge push force. Following rehealing plate motions. The slab pull force and the weaker ridge push of a decoupling earthquakes'srupture zone, the earthquake force cause localized compression at interface thrust zones, and subduction cycles will repeat. The slab pull model indi- becauseplate motion due to these forces is resisted there. The catesthat the zonesjust updip and downdipof the slab bend large slab pull force of very old plate tends to pull the sub- should be examined for precursors to great subduction zone ducting plate vertically away from the interface thrust zone earthquakes. and lead to low coupling, whereas the smaller slab pull force of younger plates allows the strength of subducting plate to Acknowledgments.The reviews by Stu Nishenko, Warren Hamil- ton, Mark Richards, Jim Dewey, and Richard Gross are gratefully resist bending and lead to strong coupling. The rupture of the acknowledged.Work was partially supportedby the U.S. Agencyfor great 1960 Chile earthquake began at an interface zone International Development agreementBOF-0000-P-IC-4051-00. bounding oceanic plate <30 m.y. old, which is locally the oldest oceanic plate (implied relatively weak coupling); the rupture then propagated southward along an interface bound- REFERENCES ing increasingly younger oceanic lithosphere, terminating near Abe, K., Focal processof the South Sandwich Islands earthquake of the subducting Chile rise. May 26, 1964, Phys. Earth Planet. Inter., 5, 110-122, 1972a. Subducted plate at mantle depths or 50-200 km shows per- Abe, K., Mechanisms and tectonic implications of the 1966 and 1970 vasive normal faulting and occasional great earthquakes, due Peru earthquakes,Phys. Earth Planet.Inter., 5, 367-379, 1972b. to pulling by the excessmass of deeper plate. In the slab pull Abe, K., and H. 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