REvli•ws OF GEOPHYSICS
ANDSPACEPHYSICS,VOL. 9, NO. 1, FESltUAIt¾1971
Distribution of Stressesin the Descending Lithosphere
from a Global Survey of Focal-MechanismSolutions
of Mantle Earthquakes
BRYAN
Earth
ISACKS
Sciences Laboratories
National Oceanographic
and AtmosphericAdministration
Lamont-DohertyGeological
Observatoryo[ ColumbiaUniversity
Palisa'des,New York
P•,T•,I•
10964
MOLNAI•
La•nont-DohertyGeo•og•ca•
ObservatoryoJColumbiaUniversity
Palisades,New York 1096•
A region-by-region analysis of 204 reliable focal-mechanismsolutions for deep and
intermediate-depth earthquakes strongly supports the idea that portions of the lithospherethat descendinto the mantle are slablike stressguidesthat align the earthquakegenerating stressesparallel to the inclined seismiczones. At intermediate depths extensional stressesparallel to the dip of the zone are predominant in zones characterized
either by gapsin the seismicityas a function of depth or by an absenceof deep earthquakes.Compressionalstressesparallel to the dip of the zone are prevalent everywhere
the zone exists below about 300 kin. These results indicate that the lithosphere sinks
into the asthenosphere
under its own weight but encountersresistanceto its downward
motionbelow about 300 km. Additionalresults'indicatecontortionsand disruptions
of the descending
slabs; however,stressesattributable to simplebendingof the plates
do not seem to be important in the generationof subcrustalearthquakes.This summary, intendedto be comprehensive,
includesnearly all solutionsobtainablefrom the
World-Wide StandardizedSeismographNetwork (WWSSN) for the period 1962through
part of 1968plusa selectionof reliablesolutionsof pre-1962events,and it includesdata
from nearly every region in the world where earthquakesoccurin the mantle. The
double-couple
or sheardislocationmodel of the sourcemechanismis adequatefor all
the data.
INTRODUCTION
The idea that deep and intermediate-depthearthquakesmark the location
of a descendingtongue of lithosphereprovidesa simple explanationfor the
peculiarorientationof subcrustalearthquakemechanisms;
the eventsare not
part of a deepmegathrust
fault but insteadoccurinsidethe descending
lithospherein response
to compressional
or extensional
stresses
alignedparallel to
the slablikegeometryof the lithosphere.This explanation[Oliver a•d Isacks,
1967; Elsasset,1967; Isacks et al., 1968, 1969] suggests
that focal-mechanism
data may thereforeyield informationaboutthe forcesappliedto the lithosphere
103
104
ISACKS AND MOLNAR
and about the contortionsand disruptions of its descendingparts. In our previous paper [Isacks and Molnar, 1969] we used focal-mechanismdata to support
the ideasthat at intermediatedepthsthe lithospheresinks into the asthenosphere
underits (•wnweightand at gi:eatdepthsencounters
strongeror densermaterial.
Our evidencecame from regionswhere the planar two-dimensionalstructure of
the arc is clear and well defined.In this paper we compile and summarizereliable
focal-mechanismsolutionsfor deep and intermediate-depth earthquakes (Figure
1) in nearly every region in the world where subcrustalearthquakesare known
to occur. We present further evidence for the gravitational sinking of the lithosphereand, in addition, showthat even in regionswhere the distribution of hypocentersand inferred stressesis complex,the idea of a stressedslab of lithosphere
still providesthe simplestinterpretationof the results.
The standard classificationsof shallow,intermediate,and deep loci [•utenberg a•d Richter, 1954] also correspondto the main types of focal-mechanism
solutionsin island arcs and thus appear lo be of major tectonic significance.
The boundary of 60-70 km between shallow and intermediate depths approximately dividesthe earthquakesthat occurin the shallowzone of thrust faulting
between the convergingplates of lithosphere from the earthquakesthat occur
inside the underthrust and descendingplates. An overlapping of both types,
although limited to few events, shows that the boundary is somewhat diffuse
and perhaps slightly variable in depth from region to region. The boundary of
approximately 300 km between intermediate and deep loci marks the most
outstanding feature of the distribution of the stress orientations inside the
descendinglithosphere; the stress parallel •o the dip of the zone tends to be
compressionalbelow 300 km and extensional or variable above 300 km.
Our purposeis to place emphasison individual solutionsthat are reliable
and well determined rather than to extract average features from a larger num-
ber of poorly determinedsolutions.We thereforeincludeonly solutionswhose
quality we are able to determineclearly (Figure 1, Table 1, and appendix).
One hundred seventy-nineare basedon data from the World-Wide Standardized
Seismograph
Network (WWSSN) and includenearly all the deepor intermediatedepth earthquakesthat occurredfrom 1962 throughpart of 19•8 and that were
large enoughthat a reliable solutioncould be obtained.The additional solutions
t•oreventsthat occurredprior to 1962 are relatively few in number (25) because
of the scarcity of reliable data for that period [Ritsema, 1964; Stevens and
Itodgson, 1968] and becausein certain casesthe data from which an assessment
of quality couldbe madewasnot availableto us.
Ninety-five of the solutionsare new and have not beenpublishedpreviously.
The solutionsfor the New Guinea-New Britain-Solomons,New Hebrides, and
South Sandwichregionsare the first to be reportedthat rely upon WWSS•N data.
We also report numerousnew data for the Peru-Chile and the Kurile-Japan
regions.
We summarizeand'discussthe main resultso• this study on • global basis
in the first two sections,
and in'..'
the final sectionpresentthe data and its analysis
in detail for eachregion.The region-by-region
analyses,besidesprovidingthe
basis for our generalizationsand inferences,yield information about the extent
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114
ISACKS AND MOLNAR
and configuration
of the parts of lithosphericplatesthat have descended
into
the mantle during the past 10 m.y. or more.
DOUBLE-COUPLE
MODEL
OF THE
SOURCE
MECI•ANISM
The double-couple
model [Honda, 1962] is adequatefor all the solutionswe
obtainedor examined.Alternate models,suchas a suddenvolumechange,if
present
in combination
with a double-couple
component,
wouldcausethe nodal
planesto be nonorthogonal
and would affectthe directionsof first motionsof
shear waves. We find no evidencefor such effects [see also Ritsema, 1964],
althoughwe cannot,in general,excludeminor contributions
from such components [Randall, 1968].
We adhereto the followingstandardterminologyfor the parametersof the
double-couple
solutions:The two orthogonalaxesthat bisectthe quadrantsof
dilatationalandcompressional
firstmotions
are the compressional•
or P, axis
and the tensional,or T, axis,respectively.
A third axis,perpendicular
to boththe
P and T axesand coincidentwith the intersectionof the nodal planes,is the B, or
null, axis.In the dislocationmodelequivalentto the doublecouple,one of the
nodalplanesis the fault plane,andthe poleof the othernodalplaneis parallelto
the slip vector.
The correspondence
of the P, B, and T axesand the principal axesof maximum, intermediate,and minimum compresslye
stressis an assumption,justified
basically by the results-(describedin the next section),that for deep and inter-
mediate-depthearthquakesthe P, B, and T axesare the mostcloselygroupedof
the double-coupleparametersand are most nearly parallel to the planar geometry of the seismiczones[Isackset al., 1968, 1969;McKenzie, 1969a;Isacks and
Molnar, 1969]. Hence, the physicalmechanismsof mantle earthquakesmay be
suchthat the shearing
or faultingformsa• an angleof about45ø to the axisof
maximum compressirestress.If the angle betweenthe P axis and the axis of
maximumcompressirestresswere as much as 15ø, however,detectionof suchan
effect [Isacks et al., 1969']would be difficultwith the presentresolutionof our
data. Thus, in terms of a Coulomb-Mohr-typeprocess,we can only suggestthat
the effectivecoefficient
of internalfrictionis probablylessthan 1 and may be 0.
STRESS
Parallelism
WITHIN
DESCENDING
between Stress Axes and Inclined
LITHOSPHERIC
PLATES
Seismic Zones
The parallelismbetweenthe axesof compression
or tensionof the doublecouplesolutionsand the seismiczonesis the primary evidencethat the earthquakes in the mantle occur inside descendinglithosphericplates in responseto
stresses
within the plates.The nonparallelism
betweenthe nodalplanes(oneof
whichis the fault plane) and the seismiczonesshowsthat the solutionscannotbe
interpretedas simpleshearingparallel to the zones.We demonstrated
theserelationshipsin our previouspaper (Figure 1 of Isacks and Molnar [1969]) by
plottingthe axesandpolesof the solutionson a commonprojectionorientedrelative to the local strike and dip of the well-definedinclinedseismiczones.We do
STRESSES IN THE DESCENDING
LITHOSPHERE
115
not usethis methodof presentationfurther in this paperbecause(1) the precision
with which the orientation of a zone can be determinedvaries greatly from region
to region,and (2) a major causeof variationsamongsolutionsfor a regionmay
be variations in the structure of the zone rather than some inherent feature of the
mechanism.
Thus, furtherquantitativestudyof the closeness
of groupingof the
axesrequiresdetailedresolution
of the distributionof hypocenters
nearthe events
with we!l-determinedsolutions.In this paper we seek instead a broad global
surveyof the outstanding
featuresthat canbe discerned
with presentlyavailable
information.
The parallelismbetweenthe stressaxesandthe seismiczonesis demonstrated
individuallyfor eachregionin Figures7 through29. In most caseseither the P
or the T axis is parallel to the zone.Often either the P or the T axis may group
stronglyabouta commondirection,whilethe othertwo axesinterchange
posi:
tions or form a girdle about the stably oriented axis. This feature suggeststhat
the intermediateprincipal stressis nearly equal in magnitudeto one of the other
principal stresses[Balakina, 1962] In some.regionsall three stressaxes interchangepositions.Althoughthe causeof sucha marked spatial variability in the
distribution of stressesmay be obscure,the idea of a stressedplate doesoffer a
generalexplanationfor this otherwisepuzzlingresult.
Hypothesesfor deep earthquakesof the type proposedby Sugimura and
Uyeda [1967] and Savage[1969] do not accountfor the resultsof the preceding
paragraph.In thesehypotheses
the shearingand faulting is constrainedto be
parallelto planesof pre-existing
weakness
in the material;thusthe polesof the
nodalplanes,or a• least oneset of poles,rather than the P or T axes,shouldbe
the mostcloselygroupedparameterof the solutionsand have the most consistent
relationshipto the orientationof the seismiczone.Savagerecognizes
this problem
and proposes
that the interchanging
of stressaxesis characteristic
only of more
contorted
partsof slabs.In ourstudy,however,
we canfindlittle evidence
to
supportthis; the interchanging
of axesoccursin zonessuchas thosein partsof
the Izu-Bonin,Tonga,and New Hebridesregionsthat are relativelyun½ontorted.
Ritsema [1964, 1966, 1970] reportsthat the polesof the nodal planesfor
earthquakesin the mantle tend to be horizontalor vertical and that the sense
of shearingacrossthe morehorizontalnodal plane reversesfrom intermediateto
greatdepths.To explaintheseresultsRitsemaproposes
a predominantly
horizontal motion of material through the gap of seismicitybetweendeep and inter-
mediate-depthearthquakes(see also Harrington [1963]). This reversal in
orientationis in agreemen•with our findingsexceptthat we interpret this as a
changefrom extension
to compression
insidean inclinedslabof lithosphere.
The
main differenceis that we find that the nodal planesor their polesdo not tend to
be horizontalor vertical. A histogramof the plungesof the poleslisted in Table 1
showsa nearly uniformdistributionof anglesbetween0ø (horizontal)and 90ø
(vertical).Thedifference
seems
to bea resultof Ritsema's
[1970,p. 504]rejection of solutionsin whichthe pattern of firs• motionsis dominatedby onepolarity.
His selectionwould thus be biased against solutionsthat dø not have a nearly
verticalnodalplane.
Thelackof a strong
tendency
forthenodalplanes
to beverticaland:
hori_
116
ISACKS
AND
MOLNAR
zontal also arguesagainstthe hypothesis(e.g.,Lliboutry [1969]) that the lithospherebreaksup and sinksvertically alongen echelonvertical faults.
A few solutionsfor intermediate-depthearthquakescan be interpretedas a
downwardcontinuationof the thrust faulting characteristicof shallowportions
of inclinedseismiczones.The depthsof theseeventsare lessthan about 100 km.
Althoughin somecaseseither or both the hypocentraldepth and the dip of the
seismiczoneare uncertain,further resultsof this type may give someindication
of the thicknessof the overthrustplate of lithosphere.
Predominance
and Variationso• Down,-DipStressOrientations
For brevity we refer to orientations of the stress axes in which the P or T
axis is parallel to the local dip of the seismiczone as 'down-dip compression'or
'down-dip extension,'respectively.The prevalenceof these orientationsis demonstrated by the fact that two-thirds of the 204 solutionslisted in Table I are of
one type or the other. Included are solutionsfrom nearly every region where
mantle earthquakesoccur,so that the generalizationis globalin scope.In regions
where the structureof the zoneis known best,the axestend to scatterwithin 25.0
or less of the direction of dip; it is not certain how much of the scatter is due to
complexitiesin the structureof the zonesor small systematicdifferencesbetween
the P, T, and B axes and the principal axes of stress in the material. In other
regions,where the orientationsof the zonesare not so well known, the available
evidence is still sufficient in many casesto concludethat the parameter most
nearly parallel to the dip of the zoneis either the P or the T axis. The remaining
third of the solutionsare for eventsin markedly contortedparts of zonesor in
zonesof undeterminedstructure, or are more or lesswell-establishedexceptions.
Figure 2 is a summaryof the variationsof down-dipstresstype as a function
of regionand depth.Also shownare the approximateconfigurations
of the deep
seismiczones,the maximum depths,and the locationsof marked gapsin seismic
activity as a function of depth. The data and interpretations upon which this
figure is basedare describedin detail in the regionalsectionsand are summarized
in the next-to-last column of Table 1. Not included in the figure are certain
complexregionsnear prominentjunctionsor endsof zones (discussedin the next
section) and a few regionswherethe structureis very poorly known. The figure
illustrates the following major resultsof this study:
1. There are remarkablesystematicdifferences
betweenintermediate-depth
and deepsolutions:
(a)
(b)
The deepsolutions
are very stronglydominatedby down-dipcompression.
The intermediate-depth solutionsare more variable and include
down-dip compression
and extension,as well as many solutions
that are clearly neither (X's in Figure 2).
2. There is a correlationbetweenthe down-dipstresstype at intermediate
depthsand the grossnature of the distributionof seismicityas a functionof
depth:
STRESSES IN THE DESCENDING
LITHOSPHERE
117
Fig. 2. Global summary of
the distribution of down-dip
stresses in
inclined
zones. The
stress axis that
seismic
is approximately parallel to
the dip of the zone is represented by an untilled
(open) circle for the compressional or P axis and a
filled
(solid)
circle for the
5OOf
•
NORTH
CENTRAL
SOUTH
7001
-"•'"
tensional or T axis; an X
indicates
that
neither
the P
nor the T axis is approximately parallel to the dip.
The data plotted a•re tabulated in the second-to-last
column of Table 1. For each
700
F....
NCHILE
NE•'HEBRIDES
--
region the line represents
the seismic zone in a verti-
cal section aligned perpendicular to the strike of the
zone. The lines show approximately the dips and
3OO
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lengthsof the zoneand gaps
in the seismicactivity as a
functionof depth.
(b)
3.
Down-dip extensionis predominantin regionswhere either marked
gapsin seismicityare presentbetweendeep and intermediate-depth
eventsor where no deepearthquakesare present.
Down-dip compressionis predominant where the zone is continuous, but this correlation is tenuous.
'There is no obvious correlation between the bends in the vertical sections
shown in Figure 2 and the down-dip stresstype. Down-dip compressionis
dominantbelow 300 km regardlessof the configurationof the zone,whereasat
intermediatedepthsthe variations in down-dip stresstype are not related in any
obviousway to the structure of the zone.
Sinking o[ the Lithosphere
Sinkingat intermediatedepths. We claimedin ourpreviouspaperthat generalizations i and especially 2 listed above constitute evidence for the models
shownin Figure 3 in whichlithosphericplates,moredensethan the surrounding
mantle,sinkintothe asthenosphere
undertheir ownweigh[andaretherebyplaced
underextension.
Thusthe sections
shownin the middleand lowerpartsof Figure
2 correspond
to modelsa, b, or possiblyd in Figure3, whereasthosein the upper
par[ correspondto c (or possiblyb in the Kurile-Kermadecsections).The idea
that the suboceaniclithosphereis gravitationally unstable and can sink into
the asthenosphere
is physicallyplausibleand supportedby seismological
and
gravitydata (e.g.,Elsa,sser
[1967],Oliverand Isacks[1967,1968],McKenzie
ISACKS
118
a
b
AND
MOLNAR
c
d
LOW
INCREASING
STRENGTH
•) •o 0
HIGH
STRENGTH
Fig, 3. A model showingplausibledistributionsof stresseswithin slabs
wheregravitationalforcesact on excess
masswithin the slabs.A filledcircle
represents
down-dipextension,
an untilledcirclerepresents
down-dipcompression,and the size of the circle qualitatively indicatesthe relative
amount of seismicactivity. In (a) the slab sinks into the asthenosphere,
and the load of excessmassis mainly supportedby forcesapplied to the
slababovethe sinkingportion;in (b) the slabpenetrates
strongermaterial,
and part of the load is supportedfrom below,part from above; the stress
changes
fromextension
to compression
asa function
of depth.In' (c) the
entire load is supportedfrom below, and the slab is under compression
throughout.In (d) a piecehasbrokenoff. A gap in seismicityas a function of depthwouldbe expectedfor (b) and (d), whereasno deepearthquakesoccurbeneaSh(a). The horizontaldashedlinesin the figuresindicatepossiblephasechanges
in the uppermantlenear 350-400km and
650-700km [Anderson,1967]. (Figure after Isacksand Molnar, 1969.)
[1969b],Press[1969],Hath•rton[1969,1970],Jacoby[1970],and Griggs
[19711).
Mos• of •he seismiczonesshownin Figure 2 appear •o be in descending
pieces
of suboceanic
lithosphere.
Beneathseveralcontinental
regions
in theAlpide
bel•, suchas Burma,•he Hindu Kush,and Rumania,however,•he subcrustal
zonescanno•be •raced•o any nearbypieceof oceaniccrusLAlthough•hereis no
problemin havingoceaniclithosphere
sink in•o •he mantle,continental
lithosphereis probablybuoyan•andwould•end•o remainon•he surface.Geological
evidenceindicatesthat much of the Alpide bel• may be beneath or near the
suture where an ocean has closedand where collision of continental pla•es has
taken place [Holmes,1965]. Thus •he prevalenceof nearly verticalextensional
stresses
in the mantle beneaththeseregionssuggests
•ha• piecesof oceaniclitho-
sphere,remnantsof a consumed
suboceanic
pla•e,are nowpullingaway and
downwardfrom the continentalpiece•o whichi• is a•ached.
The imperfection
of the correlationbetweendown-dips•resstype at in•ermediatedepthsandthe distribution
of seismicity
asa functionof depth--notably,
the variability of s•resses
within•he Kurile arc and the southernNew Hebrides,
•he down-dipcompressions
in the Sou•hSandwichand Ryukyu arcs,and the
complexorientations
tha• are foundin severalotherregio.
ns--sugges•
tha• a•
intermediatedepthsstresses
other•han the simpletwo-dimensional
•ype implied
in Figure3 are presen•in certainregions.
This is no• surprising,
sinceeventsa•
intermediatedepthsare neares••he regionwherethe pla•,ebendsdownward
STRESSES IN THE DESCENDING
LITHOSPHERE
119
into the mantle; thus in certain regionsgeometricadjustmentsto this major
deformationmay locally controlthe orientationof stressinsidethe plate. Other
complicationsmay include effects of seamountsand other topographic features
on the forcesappliedto the descending
slab in the underthrustingzone; effectsof
fault zones and other mechanicalheterogeneitiesinherited from the formation
and deformation of the slab on the surface prior to its descent;variations in
the gravitational force due to a heterogeneous
distribution of excessmassin the
slab; and localizedeffectsof chemicalor phasetransitionsthat are not distributed
uniformly through the descendingslab. Someof these effectscould also expla.in
the nestsof intenseactivity or remarkablegapsin activity that characterizethe
distribution of earthquakesalongthe strike of many zones.
Down-dip compressionbetween300 and 700 km. The most consistentresult
of this investigation is the predominanceof down-dip compressioneverywhere
below about 300 km. One interpretation of this result is that the forces on the
lower portions of the slab that resist its motion become relatively important
below 300 km. The compressional
stressinsidethe slab would be the result of.the
downward-directed forces applied to the upper portions of the slab and the
upward-directedresistingforcesappliedto the lower parts of the slab. The cause
of this resistancemight be either an increasein strengthin the surroundingmantle
or a buoyant effect if the density of the slab were lower than that in the surrounding mantle.
The interval of depth in which down-dip compressionis prevalent is bounded
approximately by the depthsof 350-400 and 650-700 km at which major phase
changesare thought to occur in the normal mantle [Anderson,1967]. If the
shallower transition (near 350-400 km) occurs at shallower depths in the slab
than in the adjacentmantle [Griggs,1971], •he additionalload of densematerial
would further drive the lower portions of the slab downward and therefore
accentuatethe compressionalstressinside the slab below 300 km. This effect is
supportedby both the focal-mechanismdata and the estimatedparametersof the
phasechange[Anderson,1967; Akimoto and Fujisawa, 1968].
On the other hand, the parametersgivenby Andersonsuggestthat the deeper
phase change (near 650-700 km) would occur at greater depth in the slab than
in the surroundingmantle. This would provide an upward buoyant force that
resiststhe sinking of the lithosphere.The critical parametersin this caseare the
slope of the pressure-temperaturecurve at the phase boundary and a possible
increasein the iron-magnesiumratio with depth. Unfortunately, however,to our
knowledgethese parameters are not well determined.If the parameters of the
phase changeare suchthat buoyant resistancedoesnot occur,the prevalenceof
down-dip compressionprovidesstrongevidencefor an increasein strength in the
mantle. Alternatively, if the strength does not increase enough to support the
heavy slabs,then the focal-mechanismdat• supportthe idea that the buoyant
resistingforce is important.
In Figure 3 we suggesta zone of increasingstrength below depths of about
350 km to account for the compressionalstressesin sqismic zones that reach
depthsof only 500 km. This is reasonablein view of the indicationsof • minimum
of creep strength as a function of depth in the upper 300-400 km of the mantle
120
ISACKS AND MOLNAiR
[Gordon,1965; Anderson,1966; McConnell, 1968; Weertman,1970]. However,
the strength of the material through which the slab penetratesmust remain
significantlysmaller than the strengthof the slab if the plate is to maintain its
identity as suchto the depthsof the deepestearthquakes.'Strength'as usedhere
is creepstrengthasdefinedby Weertman[1970].
Gaps in the lithosphere. Figure 3 showstwo possibleexplanationsfor the
gaps in seismicactivity as a function of depth; in Figure 3b t.he lithosphere
is continuous,but the deviatoricstressgoesto zero in changingfrom compression
to extension.In Figure 3d the lithosphereis discontinuous,
and the gap in seismicity corresponds
to a gap in the lithosphere.In this model a piece of lithospherehas becomedetachedand is sinking independently.
There is evidencethat a[ least in someregionsa pieceof lithospheremay be
detached as shown in Figure 3d. The remarkable configurations of the zones
beneath Peru and western Brazil and the New Hebrides (Figures 10 and 14)
sugges[this. The deep earthquakesbeneath the North Island of New Zealand
[Adams, 1963] represen[a significantdiscrepancyin the otherwiseregular relationship between the lengLhof the seismiczone and the rate of convergencealong
the Tonga-I(ermadec-New Zealand island arc [Isacks et al., 1968; McKenzie,
1969b] and therefore may be located in a detachedpiece of lithosphere.The
solutionfor the Spanishdeep ear[hquake,in comparisonwith result•sfor nearly
every other region of deep-focusactivity in the world, is not explicable as
down-dip compressionin a lithosphericslab that is traceable t•o a presently
active island arc on the surface; this earthquakemay thus also be locatedin a
pieceof lithospherethat is detachedfrom a surfaceplate.
Observationsof the frequenciesand amplitudesof seismicshear waves from
deep earthquakesrecordeda[ suitable locationsmight indicate the presenceor
absenceof a gap in the lithosphere.In New Zealand [Mooney, 1970], the New
Hebrides (Barazangi, personalcommunica.tion,
1970), New Brit•ain,the Solomons,
and Chile [Molnar and Oliver, 1969; Sacks,1969], regionswherenotable gapsin
seismicity are present, high-frequencyshear waves pass through the gap. In the
caseof the Peru-westernBrazil region [Molnar and Oliver, 1969;Sacks,1969], the
results are as yet ambiguous.The clearly recordedhigh-frequencyphasesmean
either that the lithosphere is continuousthrough these gaps or that the zone of
large attentuation in the adacen[ normal mantle is confined primarily to the
region above the gaps,i.e., above about 300 km. Sincethe last alternative cannot
be excluded,the questionremains open.
Contortionsand Disruptionsof the.DescendingLithosphere
The two major world-encircling belts of island-arc or island-arc-like structures, the circum-Pacific and Alpide belts, are divided into more or less welldefiheddiscretesegments.The junctionsbetweenthesesegmentsare characterized
by abrup[ changesor offsetsin trend, marked changesin the dip of the seismic
zones,gaps in the seismicactivity, shallowingof the oceanictrench, and intersection of major transverse features. The curvature of the island arc on the surface,thoughgenerallyconvextowardthe oceanicside,variesconsiderablyamong
the segmentsof island arcs. In somecasesthe arc endsabruptly and the inclined
STRESSES IN THE DESCENDING
LITHOSPHERE
121
seismic zone has a well-defined lateral edge. These features imply that the
descendingedgesof lithosphericplates are contortedand segmented.Thus, it
would be surprisingif the distribution of stressesinside the descendingportions
couldbe completelyunderstoodin terms of the two-dimensionalmodelsof Figure
3. In particular, the solutionsexcludedfrom Figure 2 are for eventslocatednear
major junctions and contortededgesof seismiczones,and are discussedbelow.
Moreover, someof the solutionsshownin Figure 2 by an X (i.e., solutionsinterpretable as neither down-dipcompression
nor extension),as well as someof the
anomalousdown-diptypes, can be attributed to deformationsof the slabsthat
result from three-dimensionalgeometricaleffectsrather than the two-dimensional
effectsof Figure 3.
Bendingof lateral edgesof inclinedseismiczones. The distributionof hypocentersat the northern end of the Tonga island arc (Figure 11) and beneaththe
Banda Sea at the easternend of the Sundaisland arc (Figure 26) showthat the
lateral edgesof the seismiczonesin thesetwo regionsare bent upward. A simplified picture of such a deformation is shown in Figure 4. Limited evidencesuggeststhat bent edgesare also located at the northeasternend of the seismiczone
beneath Colombia, South America (near the Bucaramanga 'nest'), the northern
ends of the West Indies a.ndthe South Sandwicharcs, the southernend of the
New Hebrides arc, and the northern end of the Izu-Bonin arc.
Direct effectsof the bending,i.e., a compression
or an extensionperpendicular
to the axis of bending,are indicatedclearly by only two solutionsfor deepevents
beneaththe Banda .Seaand by solutionsfor intermediate-deptheventsin Colombia. In the latter regionthe interpretationis very uncertain.In contrast,the solutions for eventsnear the northern end of the Tonga arc appear to be merely
rotated versionsof the solutionsin the unbent part, whereinthe axis of rotation
corresponds
to the axis of bending.This relationshipimplies that the bending
Fig. 4. Diagram showing a bent edge of a
descendinglithospheric plate. The surficial
portion of the plate is to the left; on the
right side the plate bends down and descends into
the
mantle.
The
lower
corner
of the descendingpart of the slab curls upward, as shown in the lower right-hand side
of the model. Such a structure is postulated,
for example, for the eastern, northern, and
southern endsof the Sunda, Tonga, and New
liebrides arcs, respectively. (Figure after
Fitch and Molnar [1969].)
122
ISACKS AND MOLNAR
doesnot producethe stressreleasedby the earthquakesbut merely changesthe
orientationof the stressguide.
Oblique-angledjunctions. Three notable oblique-anglejunctions, all concave toward the oceanicside, are located respectivelya• the junctions of the
Kurile, North I-Ionshu,and Izu-Bonin arcs and betweenthe island-arc-like structures of Peru and Chile in westernSouthAmerica.Only near the junctionbetween
the North I-Ionshuand Izu-Bonin segmentsdo we find goodevidencefor localized
effects on the focal-mechanismsolutions.The data suggesttha• the lithosphere
may be tearing beneath central I-Ionshuand descendingbeneaththe Sea of Japan
and southof I-Ionshuas separateslabs (Fig. 5b).
Wyss [1970] suggeststha• the low apparent stresscalculatedfor intermediate-depth events near the Peru-Chile corner may be due to a tear in the slab.
However, the solutionsfor intermediate-depthevents exhibit no obviouseffects
of the corner,bu• seemto be related to stressesinsideone of the segmentsrather
than to stressesattributable to the presumedtearing.
•C•'HINGE
FAULTING
Fig. 5. Contortion (a) or disruption(b) betweentwo segmentsof a slab with differentdips.
In (a) the portion between the two segmentsis bent and stretched.In (b) the two segments
tear apart along a vertical hinge fault. In this type of faulting the motion would be nearly
vertical and such that the more steeply dipping side moves down relative to the more gently
dippingside. An example
maybe locatedbeneathsouth-central
Honshu(Figures
21 and22).
STRESSES IN THE DESCENDING LITHOSPHERE
123
This may also be the casefor the junction betweenthe Kurile and North
Itonshusegments,
where•he solutionsseemmorerela•ed•o pa•ternson either side
than •o effectsof •he corneritself. However,•he interpretationfor •his regionis
more complexbecauseof •he variability of s•ressorientationswithin •he KUrile
zone.
Junctionsbetweenthe Tonga and Kermadec island arcs and the North Island
of New Zealand,and betweenthe Izu-Bonin and Marianas, are characterizedby
low seismicactivity; and few solutionsnear •hesejunctionswere obtained.The
distributionof hypocenterssuggests
•ha• the Tonga-Kermadeczone is disrupted
in•o two segmentscorresponding
to a Tonga slab and a Kermadec slab.
Convexcurvature. Frank [1968] pointsout that unlessa simplegeometrical
relationship holds between •he dip 'and the curvature of •he surface trace of the
seismiczone,surfacearea is no• conserved,
and •he descending
pla•e will be either
laterally compressed
or s•re•ched.$tauder [1968b] suggeststha• such an effe½•
could account for extension parallel to •he strike of •he seismic zone •ha• he
inferredfrom •he solutionof an intermediate-deptheven• in the Aleu•ans (Figure 19). AlthoughStauder'sargumen•is basedon a fia•-earthmodel (as depicted
in Figure 6), extensionis s•ill to be expectedthere becauseof •he s•eepdip of
the seismiczone [Davies and McKenzie, 1969;Murdock, 1969]. A similar effect
may accountfor solutionsof severaleventsin •he Sunda,Mindanao, and New
Hebrides
arcs.
Fig. 6. Illustration of how lateral extensioncan be producedin the descendingportion of a
plate near a corner (or bend) that is concavetoward the direction of dip of the slab. The
dotted line representsthe axis of the trench and the axis of bending.In 6A a horizontal,
undeformed
slabis shown.Bendingof the underthrust
edgeof the slabalongthe trenchaxis
produceslateral extension or separation near the corner; in 6B a separation is shown for
clarity. An example of possible lateral extension is shown in Figure 19 for the Aleutian arc
[Stauder, 1968b].
•
124
ISACKS AND MOLNA:R
On the other hand, if the dip of the seismiczoneis lessthan twice the radius
of curvature (in degrees)of the surfacetrace of the arc, then lateral compression
shouldresult. One solutionin agreementwith this is found at intermediatedepths
in the nearly straight Izu-Bonin arc.
By the samereasoning,lateral compression
shouldbe presentnear the concave cornersdiscussedin the last section.In those cases,however,the zoneson
either sidediffer appreciablyin dip sothat a contortionor disruptionof the types
depictedin Figure 5 may apply instead.
Bending as the source o• earthquake-producing stresses. The evidence
reviewed above, plus •he lack of effect on the mechanismsolutionsof the bends
in the seismiczonesdepictedin Figure 1, suggeststha• s•ressesdue •o bendingof
•he lithosphericplates do no• play an important role in the generationof earthquakesin the mantle. This conclusionis no• surprisingin view of the lack of any
appreciable seismic activity that can be associatedwi•h •he very pronounced
bending of the lithosphereas it turns downward beneath the island arc. The only
earthquakes•ha• appear •o be clearly associatedwith •his bend are shallowear,hquakeslocatedbeneathor slightly seawardof the axis of the •rench and characterized by horizontal extension perpendicular •o the trench. These are mos•
prominentin the Aleutian arc [Stauder, 1968b], althougha few examplesare
reportedfor o•her arcs [Molnar and Sykes, 1969; Katsumata a•d Sykes, 1969;
Fitch, 1970a; Isacks, 1970]. An even• a• 80 km beneaththe axis of the Kurile
trench may also be associatedwith compressionals•ress at the bottom of •he
bendinglithosphere,althoughin this case•he interpretation is uncertain (Figure
20). The par• of •he under•hrus•lithospherebetweenthe trench axis and •he
volcanoes,wherethe bendingis presumablymost severe,appearsto be relatively
aseismic.The relief of extensionalstressnear the upper par• of the ben• suboceaniclithosphereby earthquakesmus• thereforebe suppressed
whenthe upper
surfaceof the plate movesbeneaththe arc.
The implication of these observationsis that the generationof intermediatedepth and deep earthquakesis governednot only by the distribution of stress
within the lithospherebut also by unusualphysical propertiesor conditionswithin
the downgoingpart of the plate. This is further suggested
by the fact that earthquakesinsidethe lithosphereare apparently rare exceptin the downgoingpart.
Of three generaltypesof hypotheses
that have beenproposedto explainthe processof shearfracturingin a deepearthquake--(1) an embrittlementor weakening
[Raleigh and Paterson, 1965] or some other effect (e.g., Griggs and Handin
[1960] and Savage[196.9]) associated
with a fluid phase; (2) a high-temperature
instability in the creeprate (e.g.,Orowan [1965], Griggsand Baker [1969]);
and (3) a high-pressure,
low-temperaturemechanicalinstability [Byeflee and
Brace, 1969]--the first might be expectedto apply more to the sidesthan to the
centerof the descending
slab.The lack of eff•btsattributableto simplebending
of the slabssuggeststhat the earthquakesare locatednearer the centerthan the
sides, and further suggeststhat the earthquakesoccur in the coldest material.
Theseinferences
concurwith (3), althoughboththe theoreticaland the experimentalbasesfor that hypothesis
with respectto deepearthquakes
are uncertain.
STRESSES IN THE DESCENDING
LITHOSPHERE
125
Implications for Plate Tectonicsand Mantle Convection
In adding œur•hersuppor••o •he concep•oœsubcrustalearthquakezonesas
par•s oœcoheren•and s•rongpla•es of lithosphere,our resultscontribute•o •wo
major conditions•ha• a convection•heory shouldsafisœy.
The firs• is •ha• lithosphericmaterial can descend•o depthsoœ.a•leas• 650-700 km. Furthermore,•he
predominanceoœcompressive
s•ressesa• grea• depthsand •he correspondence
oœ
•he depth of •he sharp cutoff in world seismicityas a œuncfion
oœdepth wi•h a
major discontinuityor •ransi•ionregionin •he man•le [Engdahland Flinn, 1969]
sugges• bu• do no• prove •ha• •he lithospheric material does no• penetrate
deeper•han 700 km. The secondconditionis •ha•, because•he descendingslabs
would be impenetrable, the configurationoϥhe inclined seismic zones places
s•rong constraints on •he movemen• oœmaterial in •he as•henosphere;any flow
of material mus• •urn down near •he zonesand canno•pass•hrough•hem. By
•he same reasoning,•he contortionsoœ•he slabs contain inœorma•ionabou• •he
integrated history of •he relative motions between•he lithosphere,•he as•henosphere,and, possibly,•he mesosphere
in •he casesoœ•he deepes•zones.Although
•he contortionsare no• oœsufficien•magnitude•o obscure•he basic inclined-plane
asymme•ryoœmos• zones,•hey do a•es• •o relative mo•ionsbetween•he upper
and lowerpar•s oœslabs,includingbo•h •ranslafionsand ro•a•ions.
The forcesinvolvedin •he gravitational sinkinga• intermediatedepths,if
•ransmi•ed pas• •he bendingand under,brushing
par• oœ•he lithospherenear •he
•rench and island arc, couldbe an impor•an• contribution•o •he driving mechanism of sea-floorspreadingand continentaldriœ•.Moreover,•he increasedresistance •o •he downwardmo•ion oœ•he lithospherea• depthsgrea•er •han 300 km
migh• causealterationsin •he pa•ern oœmovementsoœsurficial pla•es oœlithosphere.Our results for •he Tonga region may have a direc• bearing on •hese
questions.The Tonga arc includes•he cleares• example oœa continuouszone
under compression
•hrough a range oœdepthsbetweenabou• 80 km and grea•er
•han 600 km; in •his case•he resisting•orcesapplied•o •he lowerpar• oœ•he slab
apparenfiy suppor• nearly •he entire load oœexcessmass within •he slab. Thus
(1) •he excessmassabove80 km is sutTicien•
•o drive •he slab downward; (2) the
slab is being pusheddownwardby forcesapplied mainly to the surfaceportions;
or (3) •he downwardmo•ionsare essentiallys•opping.The high level oœshallow
seismicity[Brine, 1968; Davies and Brune, 1971], •he lack oœsedimentsin •he
Tonga •rench,•he depthoœ•he •rench,and o•her geophysicaland geologicalobservations argue agains• bu• do no• disprove (3). The firs• is no• likely because•he
lithospherei•selœmay be nearly 80 km. Thus the results for •his region (and
probably for North I-Ionshualso) sugges••ha• •he gravitational sinking force
and resis[antea[ grea[ depth, allhoughprobably impor[an[ in [he dynamicsof
[he descendingpor[ionsof [he pla[es, eanno[be [he only importan[ forcesdriving
[he horizontal moJiohsof [he li[hosphere.
REGION-BY-REGION
PRESENTATION
OF
DATA
A de[ailed region-by-regionanalysis of •he rela[ionshipsbe[ween [he siress
axes inferred from the focal-mechanism
solu[ions and [he local s[rue[ures of [he
seismiczones is presen[edin •his see[ion.The eri[ieal information is •he dis[ri-
126
ISACKS AND MOLNAR
butionof hYpocenters
nearthe earthquakes
for whichœocal-mechanism
solutions
are available.Three basicsourcesof data usedœorall the regionsare Gutenberg
and Richter [1954],the locations
oœ•he Coastand Geode•icSurvey(ESSA)as
compiledby Barazangiand Dorman [1969], and Duda [1964]. In particular,
enlargedversionsof BarazangiandDorman'smapsof epicenters
within intervals
of 100-km depth were very helpful. These data are supplemented
by more
detailed s•udiesof particular regionsif available.
In eachregion•he solutionsare dividedinto groupsaccording•o their similarity and the locationsof •he earthquakesinvolved.If the structureof the
seismiczoneis reasonablywell determined,the orientationof •he stressaxescan
then be comparedwith that of a planethat bestrepresents
the localstructureof
the zone.This provides•he basictest of the hypothesis
that deepand intermediate-depthearthquakesindicatestresses
within thin lithosphericplatesas well as
of additionalhypotheses
discussed
in the previoussection.On the otherhand,if
•he structureof the seismiczoneis poorlyknown,the procedurecanbe reversed;
the stressaxesof •he solutionsdetermineplaneswhoseorientationscan then be
comparedwith the availableinformationon the distributionof hypocenters
and
wi•h other regional•rends.This comparison
yieldsadditionalweak testsfor the
hypothesisof stressedplates as well as informationon •he detailedregional
tectonics.
In addition, the featuresusedin •he constructionof Figure 2 are described
for each region; these include the maximum depth of hypocentersand the presenceand range of depthsof a gap in the distributionof hypocentersas a function
of depth. The determinationof gaps is subject to someuncertainties;it is not
possible
to discriminate
between• truly aseismic
gapand a gap•hat is actually
a minimum in a continuousbu• insufficientlysampleddistribution.
The presentations•ar•s with the New Guinea regionand proceedscounterclockwisearoundthe Pacific and •hencethroughIndonesiaand the HimalayanAlpide belt •o the MediterraneanSea.The numbersusedin Table I and the figures refer •o either the. earthquake or its mechanismsolution.
New Guinea-New Britain-SolomonIslands (Figures7 and 8)
Deep sea •renches,belts of volcanoes,and inclinedzonesof shallow- and
deep-focushypocentersform island-arc structuresparallel to •he New Britain
and SolomonIslands. These •wo structuresmeet and form a sharp cornereast
of the southeastern•ip of New Ireland. Like the New Hebrides arc farther east
and unlike all other circum-Pacificarcs, the New Britain and Solomonarcs are
convexto the south and face away from the Pacific basin. Apparently, lithospherebeneaththe Coral and SolomonSeasis being consumedin these arcs.
Southwestand west of New Britain subcrustalearthquakesoccur beneath New
Guinea,but •he distributionof hypocentersis not well defined.Severaladditional
zonesof shallow earthquakesare present in the region. The most striking is a'
well-definedlinear alignment of shallow epicenterstha• crossesthe Bismarck Sea
north of New Britain. Another follows the Bismarck archipelago,while a third
appears•o crossthe SolomonSea from the southeastern
end of New Guineato
the SolomonIslands.Thesezonessuggest•ha• in this region•he boundarybetween
STRESSES
IN
THE
DESCENDING
LITHOSPHERE
127
I
5-7
8-10
ß
x•
oo
o
ß
•00•
o
NEWBRiTAiN
NEW GUINEA
140 E
!
i
150
160
Fig. 7. The Solomonsand the New Britain island arcs and the region of New Guinea. The
following symbolsapply to Figures 7-29- The large circlesare equal-area projections of the
lower hemisphereof a focal sphere and are oriented accordingto the directions of the map
projection; in all casesthe top and right-hand side correspondto north and east, respectively.
A filled circle representsthe axis of tension, T; an untilled circle representsthe axis of compression,P; and an X representsthe null axis, B. A solid line in the projection is the trace
of the plane that best fits the orientation of the seismic zone; a dashed line is the trace of a
plane parallel to two of the stress axes (or the average positions of groups of axes) and
perpendicular to the third axis. If earthquakes occur in thin, slablike stress guides, this plane
should give the orientation of the slab. The numbers near the projections refer to the solutions plotted in the projection and listed in Table 1. On the map the contoursof hypocentral
depth (in kilometers) are shown by solid lines. The epicenters of the events are shown by
filled, upward-pointing triangles for hypocentral depths of 70-229 kin; untilled, upwardpointing triangles for depths of 300-499 kin; and filled, downward-pointing triangles for
depths of 500-700 kin. The landward side of coastlinesare shown by dots. In this figure only,
the dashedlines on the map representzonesof shallow earthquakes.
the large Australian and Pacific plates is complexand probably includesthree
o.r more additional small plates. An additional referenceused for this region is
Denham [1969].
Solomonarc. A zoneof intermediate-depth
earthquakes
with depthsmainly
lessthan 150 km parallelsthe southeastern
par• of the Solomons.
Deeper earthquakesappear•o be confinedto the westernmos•
parb (wesbof abou• 155øE,)and
definea very steeplydippingzone•ha5reachesdepthsof 500-600km (Figure8).
128
ISACKS AND MOLNAR
NW -.-*
v
o
oo
N E ---.•
¾
b
'
c
'
_
2OO
_
Cib
0
KM
400
-
4;;)
Fig. 8. Vertical sectionsthrough the New
Britain are (A) and the northwestern end
of the Solomons are (B). Sections in-
600
I
I
I
clude the most accuratelylocated events
based on data reported in the Earthquake Data Reports for 1961-1965.Events
within
100 km
of the sections A and B
are plotted.
!
145
E
i
1•5
The deepestshocksare displacednortheastof the nearly vertical distributionof
hypocentersaboveabout 400 km. Denham'ssectionsand Figure 8 suggestthat a
gap or a minimumin seismicitymay be presentbetweendepthsof about200 and
350 km. Becauseearthquakesare few in number at all depthsbelow 150 km,
however,discriminationbetweenthe two possibilitiesis quite uncertain.
Earthquake I is locatedin the easternpart of the arc (Figure 7) wherethe
inclinedzoneappearsto dip steeplynorth.The T axisof I is nearlyverticaland
thus couldindicateextensionparallel to the dip of a nearly vertical zone.The B
axis is approximatelyparallel to the trend of the arc. Althoughthe depthgivenin
the PreliminaryDeterminationof Epicentersof the Coastand GeodeticSurvey
(ESSA) is 95 km, this depth is revised to 52 km in the Coast and GeodeticSurvey's Seismological
Bulletin. The shallowerdepth is in better accordwith the
characterof the seismograms
for this event.Moreover,the solutionfor I is consistent with solutionsfor othershallowearthquakesin the region(Molnar, unpublished data, 1969). Thus, the alternate and preferableinterpretationis that 1
indicatesunderthrustingof the Coral Sea beneaththe Solomons;i.e., slippage
between two lithospheric plates.
Solutions2-4 for earthquakesin the westernmostpart of the zone exhibit a
marked variation with depth. The solutions3 and 4 for the events below 300-km
depth have nearly vertical P axes,whereasthe solution for the event at 118 km
has a nearly vertical T axis. The zone above400 km, includingearthquakes2
and 3, is nearly vertical and strikes approximatelynorthwest;thus solutions2
and 3 indicate,respectively,
compression
and extension
parallelto the dip of the
zoneat depthsof 118 and 392 km, respectively.
A plane parallel to the T and P axesand perpendicularto the B axis of 4
dips steeplybut has a morenortherly strike than that of the zoneat intermediate
depths.The distributionoœhypocenters
near4, althoughnot well defined,suggests
STRESSES IN THE
DESCENDING
LITHOSPHERE
129
such• changein s•rike. The da• m•y •hus be ½onsis•en•
wi•h •n inœerence
of
down-dip compression
near •he bottom oœ•he zone.
New Britain arc. Figure 8 and the sectionsgiven by Denham show an
inclinedseismiczone •h•t dips north-northwesterlybeneath•he New Britain
islands.Thesed• plus recentlo½•fionsnot plo•ed in •he figureshowthat this
zonere•½hesdepthsgre•er •h•n 500km, •l•houghhypocen•ers
deeper•h•n •bou•
200 km occurvery inœrequen•ly.
Toward the wes• •he line oœ•½tive volcanoes
•nd •he bel• oœshallow •nd intermediate-depthe•rthqu•kes bend •round •o •
more westerlytrend •nd in•erse½••he northern½o•stoœNew Guinea. In •his
regionthe zone appearsto steepenin dip.
Earthquakes 5-7 are located beneath eastern New Britain, where the
seismiczonedipsnorth-northeas•at about45ø. The T axesof 5 and 6 are parallel
to the dip of the zone,and the B axesare parallel to the strike oœthe zone.These
resultsthus indicate down-dip extensionalstress.
The T axis of 7, however,is nearly vertical, whereasone of the nodal planes
is most nearly parallel to the seismic zone. This relationship is mos• simply
accountedfor as lithospherebeneath •he SolomonSea being thrust under New
Britain. The. depth oœthis earthquake (103 km) appearsto be reliable. The solution for 7 may thus represen• a downward continuation of the thrust œaulting
•ypical of shallow events in the region (Molnar, unpublisheddata, 1969).
The solutionsfor 8-10 for events located in the western par• oœthe arc are
similar to one another. A plane through the B and T axes strikes approximately
parallel to the local alignmentoœearthquakesand volcanoesand dipsvery steeply
north. This plane is also a reasonablerepresentationof the distributionof hypocentersshownby Denham. These solutions,like those for 5 and 6, are thus consistentwith down-dipextensionalstressa• intermediatedepths.
New Guinea. The seismicity of New Guinea is very complex. A zone oœ
subcrustal earthquakes with depths less than 200 km trends west-northwest
beneaththe northern and westernparts of New Guinea and appearsto dip sou•h
or southwest.This zone also appearsto be nearly continuouswith the steeply
dipping zone of the New Britain arc. Near the intersectionoœ•hese•wo zones,
where the dip apparently reverses,the pattern is complicatedby a •hird, very
poorly definedzonethat •rendseast-southeast.
If •he solutionfor 11 is indicative
oœdown-dipextension,then this zonewould have a westwarddip. However, the
solutionsfor three, closelygrouped,intermediate-depthshocks(12-14) are qui•e
differen• from one another, althoughone of these (12) is similar •o 11. The data
plotted in the appendixshowthat thesedifferencesare real. The variability in
orientationsexhibitedby 11-14 and the unresolvedcomplexitiesin the configuration of the seismiczonemake œur•herinterpretation pointless.
New Hebrides (Figures9 and 10)
Near the Santa Cruz Islandsthe seismicbel• of •he Solomonsbendsabruptly
and continuesas the linear south-southeasterlytrending seismiczone of the New
Hebrides island are. The inclined seismic zone is very active a• intermediate
depths.
Figure10shows
tha•it hasa slablike
structure
witha dipoœ
a'bout
65-70ø.
The zoneis remarkably linear in strike •nd is parallel to •he line of active volca-
130
ISACKS AND MOLNAR
I
I
i
_ '"'•....Lx._
15
,
-
'
/" •
NEW600-650
KM
•
•
i
\_%• HEBRIDES
Fig. 9. The New Hebrides
island arc. In this figure and
in others, except where
noted, the heavy dashedline
on the map represents the
-
axis of the oceanic trench
associated with the arc. The
FIJI
•
light dashed line encloses
the epicentersof deep earthquakes with depths between
20
about
/
J
\ ,,
X
4- F,
'•-•
600 and 650 km.
\ -•
[ '-o./_' ,
I- '----:{
_x''-----t-....
/
\,'
25
,
I
160 E
165
WSW
170
Trench
o ob e'
1'75
•Volconic
line
(N20øW,
thru
155øS,
168
OE)I
I
i
I
ENE
I
I
o• o•O
o
;
o• ¸
I00 '
KM
2O0 '
15
-
:50O
20
4OO -
CALED*ONIA
i""' "" 175_
f
INEW
l•o5E
ITO
500-
60(
o
-
o
o
o-
o Ooø 8 o o
,,
->_00
I .... 0I,
-I00
I
+100 KM
I,
200
•)0
I
400
I
500
I
600
,
Fig. 10. Verticalsectionthroughthe New Hebridesarc,includingthe most
accuratelylocated eventsthat occurredbetween1963 and 1967,selected
from the samesourcesusedin Figure 8 plus •ykes [1964]. The sectionincludesevents within the rectangulararea shownon the map inset.
STRESSES IN THE DESCENDING
LITHOSPHERE
131
noeson the surface.Near its southernend the zoneappearsto bend slightly east
and then terminatesabruptly; many featuresof the seismicityand bathyme•ry
[Sykeset al., 1969] suggest
thai the are is •ermina•edby a transformfaull Irending east-northeast.
The evidencefor a gapin activity betweendepthsof about300 to 500km is
good.At greaterdepths•he distributionof hypocen•ers
is qui•eremarkable(Figure 10). Figure 10 showsthai hypocenterswith depthspredominanfiybetween
about600 and 650 km occurin a nearly horizontalplanar zonetha• in map view
(Figure 9) is elliptical and has i•s longestdimension•rending west-northwest.
Only one deep earthquake (32) is known to have occurredsouth of •his zone.
Intermediate-depthearthquakes.Solutions15-23 for earthquakesin •he
northern half of the linear belt of intermediate-depthearthquakesall have very
steeplydippingT axes•hat are morenearly parallel •o the dip of •he zone•han
are the polesof •he nodalplanes.Thesesolutions
are thereforein agreement
wi•h
an inferenceof extensionalstressparallel to •he dip of •he s•eeplydippingslab.
Although four of these solutionshave B and P axes approximatelyparallel,
respectively,
to •he s•rikeandnormaldirectionsof the inclinedzone,the B andP
axesof 17 and three othersare disfin½fiynonparallel•o thosedirections.
The solutions24-29 for earthquakesin •he southernpar• of •he zone differ
appreciablyfrom thosein •he northern part. Solution24 is nearly identical •o 25
and 26, even though earthquake24 is locatedclosest•o earthquakesof the northern group and separated by more •han 300 km from earthquakes 25 and 26.
Solutions24-26 are similar •o 15-25 in •hat •he T and B axes are parallel •o •he
inclined zone,but differ in that •he T and B axesare distinctly not parallel •o the
dip and strike directions.
Earthquakes 28 and 29 are locatednearestthe southernend of •he intermediate-depth zone,wherethe arc curveseastward.Althoughthe B and P axesinterchangepositionsfrom one solutionto •he o•her, both indicate extensionals•ress
parallel •o •he s•rike of the zone,and may •hus indicale the type of deformation
shownin Figure 6.
Solution 27 is different from •he o•hers in •ha• •he T axis is perpendicular
rather than parallel to the inclined zone. The solution indicatesthat the zone is
under compression
parallel •o i•s s•rike. The depth of •he shock,249 km, is about
100 km greater than the depthsof the o•hersin the intermediate-depthgroups
except17. Bo•h 27 and 17 are near the lower par• of •he intermediate-depthzone,
i.e., near •he gap in seismicactivity.
Although •he solutionsfor 24-29 differ remarkably, •he s•ressaxes remain
closely grouped and oriented parallel or perpendicula.rto •he over-all planar
geometryof the inclined zone. This result offers strong support•o the idea that
the earthquakesresull from stressesin a thin plate. The causesof •he spatial
variations of the stressorientations,however, are not known.
Deep earthquakes. Solutions 30-31 for •wo earthquakes located in the
nearly horizontal zone of deep 'earthquakesalso exhibit a remarkable variation
in orientation. Although all of •he s•ress axes interchange positions from one
solution•o •he other, the axesremain approximatelyparallel or perpendicularto
the planar geometryof the zone.A plane throughthe more nearly horizontal axes
132
ISACKS
AND MOLNAR
(the dashedline in the projectionin Figure 9) actually dips abou• 10ø wes• •nd
is consis•en•wi•h the distribution oœhypocen•ersshown in Figure 10. These
resultssuppor•the concep•oœa s•ressedplate and sugges•a nearly horizontal,
possiblyisolated, piece oœlithosphere.The causesoœ•he spatial variations in
stress are no• known.
The solution œorthe singledeep even• (32) in the southernpar• of •he region
is shownin Figure 9 togetherwi•h •he solutionœor33, the westernmos•deepeven•
of •he Tonga zone.Earthquake 33 appearsto be locatedin a bel• oœdeepear•h-
quakes(34-38) •ha• trendswestward•rom •he northernend oœ•he Tonga deep
zone,and its solutionis similar •o those (34-38) in •ha• zone (seeFigure 11). The
orientationoœsolution32 wi•h respec•to the southernend oœthe •NewHebrides
arc is similar to tha• oœ33 (and 34-38) wi•h respec•to •he northern end oœthe
Tonga arc. These relationshipsmay thus indicate a bend in •he lower southern
edge oœa •New Hebrides deep zone similar to •ha• a• •he northern end oœ•he
Tonga zone.
In summary, the da•a œorthe northern par• oœ•he •New Hebrides arc clearly
indicate down-dip extensiona• intermediate depths. In the southernend oœthe
zone and a• grea• depthsbelow a marked gap in seismicactivity, the orientation
oœs•ress,althoughhighly variable, is still parallel or perpendicularto •he planar
geometryoœthe zone,aswouldbe expectedœors•resses
in a •hin plate.
Tonga-Kermadec-New Zealand Region
The nearly linear island-•rc system •ha• ex•ends•rom Samoa •hrough •he
North Island oœNew Zealand can be divided in•o •hree distinc• segmentson the
basisof seismicityand o•her •ec•onicœea•ures:
•he Tonga arc, the Kermadec arc,
•nd •he •Nor•hIsland oœNew Zealand [Sykes, 1966; Hamilton and Gale, 1968].
Tonga a'rc (Figure 11). Figure 11 summarizesthe results of Isaclcs et al.
[1969] plus eigh• new solutionsno• reported in •ha• paper. The con•oursin the
figure summarize•he loc•tions oœSykes [1966] and Sykeset al. [1969] and show
tha• the zone can be representedby an inclined plane tha• has contor4ionsalong
its lower and lateral edges.Betweendepthsof 450 and 650 km the northernpart
of the zone bends sharply toward the northwest.At depths lessthan 450 km the
northwesterlybending is probably present but considerablyattenuated. The distribution of seismicity is continuousat depths lessthan about 680 kin. Partly as
a resul• of the contortionsalong the lower edge of the zone, hypocentraldepths
decreasesouthward to less than 600 km and then abruptly increaseto greater
•han 600 km near 26øS.
The main consistencyamong•he solutionsof bo•h deep and intermediatedepth shocksis •he parallelism between•he P axes and •he local dip of •he
seismiczone. This is shown clearly by solutions40-48 and 51-55 for the ear•hquakes located away from •he northern and southernlateral boundariesof •he
zone.
The solutionsfor earthquakes
near •he lateral boundariesare morecomplex.
The orientationsof solutions33-39 can be explained,however,by a simple
geometriceffect.The s•ruc•ureof •he zonenear earthquakes33-39, •houghcompleg in detail, can be representedapproximatelyas a bend of •he lower northern
STRESSES IN THE DESCENDING
i
i
i
I
LITHOSPHERE
i
I
133
i
i
15
i
5(
5
%
i
I
FIJI
!
I
,-•,
...
..:
,1:3:3
"'•
...'
I
•,•
I
,
I
i
I
TONGA
40
Fig. 11.
Tongaisland
are. 2o
I TRENCH
•
I
I
i
.Jr_
•
•49
•49
?
I
180
I
'
175 W
cornerof the inclinedzone (as in Figure 4). The plane shownby the dashed
line in the equal-area,
projectionof 33-39 (Figure11) is approximately
parallel
to the groupings
of P andB axesand perpendicular
to the groupingof T axes.
This planeis a reasonable
representation
of the part of the inclinedzonebent
aroundto the west.Thus, the main differences
betweenthe solutionsfor 33-39
and thosefor 40-48 in the unbentpart can be accountedfor by a rotation about
the axis of bendingand not by stressescausedby the bending.
Solution37, in the denseclusterof hypoeentersthat includes36 and 38, is
differentin respectto an interchangeof the B and T axes.This relative instability
of the orientation of the B and T axes is also characteristic of the shocks in the
main part of the are. However,39, locatednearestthe bendand deeperthan the
others,differsfrom the solutionsfor nearbyeventsin that the B and P axeshave
interchanged.
This onesolutionmay be relatedto the deformationnear the end.
The solution for 33 is similar to others in the group 34-38, although the
location of 33 is more than 400 km to the west.of 34-38. Several deep earth-
quakesreliably locatedbetween33 and 34-39 indicate that the zone is continuousto 33; this is shownby the 600-km contourin Figure 11. If this bend
were straightenedout, the deep zone would extend considerablynorth of the
edgeof the zoneas definedby the distributionof hypocentersabove450 km. It
is interestingthat if the whole inclined zone were unbent and moved to the
surface,the part of the zone corresponding
to the 600-km contourwould then
extend considerablynorth of the presentlyactive transform fault located along
the 15øSlatitude line. That part corresponding
to the zone at depths lessthan
134
ISACKS AND MOLNAR
450 km would,however,nearly coincidewith the transformfault. Theserelationships,as well as •he discrepancy
between•he locationof the presentlyae•ive
transform fault and the expectedlocationfor this feature farther north along
the borderof the Fiji plateau,can be explainedby a changein •he directionof
relative motionbetweenthe Pacific and Australianplatesfrom a west-northwesterly directionperpendicularto the Tonga trench •o a westerly direction
parallel[o •he seismiczonealong15øS.The slipvectorsinferredfrommechanism
solutions
of shallowearthquakes
[Isackset al., 1969]do not clearlydiscriminate
betweenthesetwo directions.The appropriatelengt.
h alongthe northernedge
of the Tongaslabis about750km, which,if dividedby LePichon's[1968] rate
for the northernend of the are, yieldsa time of approximately
8 m.y. ago for
the changein direction.Insteadof a changein direction,C. Chasesuggested
to
the authorsthat the hinge-transform
fault systemthat terminatesthe northern
endoftheTonga
lares'migrated
southward
andthereby
shortened
thelateral
dimensionof the deseelnding
slab.
Earthquake50 is locatedat in[e}mediatedepth near the northernedgeof
the zone,wherethe westwardcurvatureis presentbut not pronounced.
Although
the stressaxesof this solutiondo not appearto be closelyparallelor perpendicular to the seismiczone,their orientationswith respectto the northernend of
the Tongaare are similarto thoseof 28 (Figure9) with respectto the southern
end of the New Hebrides
are.
Solution49 is anomalous
in that the T axis,ratherthan the P axis,appears
to be parallelto the seismiczone(but not to its dip). This solutionmay indicate
a localizedreorientationof stressnear the loweredgeof the zone.
Solutions56-58 for intermediate-depthevents located nearestthe junction
betweenthe Kermadee and Tonga ares resembleothersin the region and indicate
no profound effect of the junction, although the northerly deflectionof the P
axesof 56 and 58 from the dip of the zone may indicate a minor effect.
In summary,the contortionsof the seismiczonedo not seemto have large
effectson the focal-mechanismsolutions;instead, the stressaxes tend to follow
the local structureas if embeddedin the zoneand suggesta compressional
stress
transmittedthroughnearly all parts of the slab.This is contraryto what would be
expectedif the mechanismswere related primarily to a localizedstressnear a
contortion
of the slab.
Kermadec
arc (Figure12). The inclinedzonein thisregiondipsmoresteeply
than that of the Tonga are and doesnot appearto extendto depthsgreaterthan
500-550 km [Sykes, 1966]. A plane striking N 20øE and dipping 60ø west is a
goodrepresentationof this zone.
Solutions60-61 for earthquakesat depths near 230 km both have T axes
parallel to the dip of the seismiczone,whereas59, for an earthquakelocated
north and 120 km deeperthan 60-61, showsdown-dip compression.
Although
the data are limited,theseresultssuggesta changefrom down-dipextensionto
down-dip compressionsomewherebetween230 and 350 km.
An unusualfeatureis that the other stressaxes,althoughcloselygrouped,
are not parallel to the strike and normal directionsof the seismiczone.
North Island,New Zealand(Figure 12). Hamiltonand Gale [1968,1969]
STRESSES
IN THE DESCENDINGLITHOSPHERE
ix
135
,rl! /
;5s/
.
I
// TRENCH
Fig. 12. Kermadec island arc and New
Zealand.
NEW
I
i?o E
/
i
180
showedthat the seismicactivity at intermediatedepthsbeneaththe North
Islandof New Zealandhasa slablikestructure.
Belowdepthsof about350km
onlythreeearthquakes
are known[Adams,1963].Thesethree,near a depth
of 600kin, are locatedalmostverticallybeneath
the southern
part of the intermediate-depthzone.
Near 62 the zonecan be approximately
represented
by a planestriking
aboutN 45øEanddipping
northwesL
Boththe T andtheP axesof 62 appear
to parallelthe zone;the T axisis approximately
parallelto the dip, andthe P
axis is parallel to the strike.
Adams [1963] reported a solution different from 62 for an event located
in the southernpart of the intermediate-depth
zone. Adams found that first
motionsrecorded
at)New Zealandstationsfromthe threedeepshocksare generallythe opposite
in polarityof thosefromthe intermediate-depth
shocks.
This
resultmay indicatea variationin mechanisms
with depthsimilarto that in the
Kermadec
region.The predominantly
dilatationalreadings
reportedby Adams
for the deepshocks
are consistent
with a P axisorientedmorenearlyvertical
thanhorizontal,
i.e.,a P axisparallelto a steeplydippingdeepzone.
South America
The disSribuQonof earthquakesand o•her island-arc-like tectonic features
indicates•hat •he coas•region of western South America north of the Chile rise
is a majorzoneof convergence
between
an oceanic
anda continental
lithospheric
pla•e.Mechanisms
of shallowearthquakes[Isacks,1970] indicate•hat between
Ecuador and the Chile rise the oceanicNazca plate underthruststhe oontinen•
of South America in an easS-northeasterly
direction. North of Ecuador an
136
ISACKS AND MOLNAR
island-arc-like structure continuesthrough Colombia, but the plate tectonics
of this region are complexand have yet to be fully worked out [Molnar and
Sykes, 1969]. In the following,South America is consideredin two parts, one
north and one southof Ecuador.Southof the Chile rise the seismicitydecreases
markedly and provides little evidence of the nature of the connectionbetween
the South Sandwich arc and the main seismically active structuresof Chile.
Region of the Peru-Chile trench (Figures 13 and 1•).
Additional sources
of informationon the distributionof earthquakesin this regionare Ocala [1966]
and Santo [1969a]. A• intermediatedepthsthe seismiczoneis slablikeand dips
gently beneath the continent. The sectionin Figure 13 showsthis for the inclined
zone beneathPeru; the slablike zone of intermediate-depthhypocen•ers,separated from a zone of shallow,probably crustal earthquakesthat lies above i•,
has a dip of only 10ø to 15ø and reachesdepthsof about200 km. Santo [1969a]
showsthat. the zone beneath northern Chile dips more steeply a• abou• 25-30 ø
and reaches depths of abou• 300 to 350 km. Between latitudes of abou• 16øS
and 25øS the zone of earthquakes with depths between abou• 70-150 km is
extremelyactive; •his is illustrated by the large number of earthquakesin this
region for which focal-mechanism solutions were obtained. Near the border
betweenChile and Peru the seismiczone, topographicfeatures,and the line of
active volcanoes all appear to curve around toward the northwest. Just north
of this bend, near 16øS in southernPeru, the seismicactivity at intermediate
depths decreasesmarkedly and the zone of historically active volcanoesterminates.
Beneath depths of about 200 km in Peru and abou• 300 km in Chile the
seismicactivity decreases
remarkably.Santo'splo• of •he frequencyof earth-
Fig.
13. The Peru-Chile
trench and the adjacent region of western South America. Coastline shown extends
from Ecuador
to central
Chile. The northern deep
zone (600-km contour) is
2O
beneath westernBrazil, and
the southerndeep zone (500ß
o
õ
ß
!
o
ern Argentina.
3O
CHILE
TRENCH
, :•90
80 W
70
km contour) is beneath west-
,
60
STRESSES IN THE DESCENDING LITHOSPHERE
_AXIS
At
OF
PERU
-200
I
0(B)
I
i
+200
I
I
øOoo%
Fig. 14. Vertical
through
Peru
events within the area shown
in the inset. The events are
+400
I
o
0
7600
i
i
C
i
i
I
I
øø
o
oo ooo•O
oQ••
•
O0
section
including
137
TRENCH
0
o(P ø
o
200
KM !
those with magnitudes
greater than about 5 reported
in the Preliminary Determination o• Epicenters [19611967].
I
80 W
75
I
70
quakesas a functionof depth showsa pronouncedgap betweenabout 350 and
500km. This gapis especially
prominent
because
the seismic-energy
releasea't
great depths in this region is so high; Santo estimatesthat during the past 50
years it is larger in South America than that at great depths in any other
single region. Santo tabulates only six small earthquakeslocated by the Coast
and GeodeticSurvey (ESSA) with depthsbetween300 and 500 km. These locations, however, are determined by few and poorly distributed data and could
probably be moved either above or below the gap in seismicity.These six are
all in the Chile-Argentina region; none have been reported for the gap in the
P'eru-western
Brazil
zone.
At depths of 500-650 km, beneath the gap in seismicactivity, the hypocenters are confined to •two' remarkably linear belts, one beneath western
Argentina and one benBathwestern Brazil. These belts are parallel and trend
slightly west of nor•l•, but the one beneath western Brazil is offset about 500
km west of that beneath westernArgentina. Between these zonestwo large earthquakes occurredin 1958 and, 1963 (no. 91) at nearly the same location beneath
the Peru-Bolivia border.These eventsare closestto the deep •arthquakes beneath
western Brazil and may thus be on a Southeastwardextensionof that zone.
Recent studies of focal-mechanismsolutions reported by Stauder [1969]
and Ritsema [1970] appear to 'be in goodagreementwith our analysis, although
Ritsema placesa different interpretation on the results.
The zone of intense intermediate-depth activity beneath northern Chile
and southern'Peru, including earthquakes70-84, is representedon the equal-area
plot in Figure 13 by two planes. One has the N 7øE strike of the linear coast
of Chile, and the secondhas the northwest strike of southeastPeru;-both have
a dip of 28ø. The orientation of the zone in the bend between Chile and. Peru
probably varies between these two extremes.Although the stressaxes scatter
somewhat,partially as a result of the inclusionof solutionsof varying quality
(seeappendix),the T and P axesof 70-84 showa distinct grouping.The plunges
'of the T axes vary between 7ø and 54ø but have an average of 31ø, which is
138
ISACKS AND MOLNAR
very closeto Santo'sestimateof 28ø for the dip of the zone.The trends of the
T and B axes, although somewhatscattered,do not vary systematicallywith
respec••o •he locationof the event and •hus do not follow the major bendof the
coastline and other tectonic features in the region of the Peru-Chile border.
Instead, the axes trend approximatelyparallel or perpendicularto a direction
between the strikes of the coastlines of southern Peru and northern
Chile. The
east-northeasterlytrend of the T axesis approximatelyparallel to the direction
of underthrustinginferred from the solutionsof shallow earthquakes [Isacks,
1970] and approximatelyperpendicularto the alignment of active volcanoesin
northern Chile. These results are in good agreement with down-dip extension
in the zone but do not reveal any marked effect of the bend in the coastline
between Chile and Peru.
Although less closelygroupedthan 70-84, the T axes of 64-69 also tend to
parallel the dip of the intermediate-depthzonebeneathPeru. From the epicentral
locationsalone it is not clear whether earthquakes63 and 64 (beneathEcuador)
are at the northern end of the Peruvian zone, at the southernend of the north-
eastward-trending
zonei• Colombia,
or in a bendbetween
twozones.
The simiIarity between 64 and those for other events in Peru suggeststhat this earthquake may be in the northernpart of a coherentslab beneathPeru. The solution
for 63 is quite different in orientation.This event could be related to deformation
near a junction between the inclined zonesof Peru and Colombia.
The solutionfor 67 is also different from othersbeneathPeru, althoughthe
earthquake is located in the central part of Peru. Nevertheless,the B and T
axes,althoughnot parallel to the dip and strike directions,remain approximately
parallel •o the inclined seismiczone.
Included in this analysis are mechanismsof several earthquakes (68, 69,
72, 79, 82, 84) whosedepthsvary ñ20 km around the 70-km borderline between
intermediate and shallow depths.In some casesthe depths are controlled •airly
well by pP readings,but in other casesthey are uncertain.In all cases,however,
someevidenceo• • depth greater than normal was •ound. The additional basis
for inclusion in the analysis is the similarity between the solutions •or these
earthquakesand those for the well-located events at greater depths.Becauseof
the gentle dip of the seismiczones,these solutionshave a distinctly different
orientation •rom that characteristico• •he underthrus• •ype o• shallow earthquakesin the region.
Solutions 85-90 for the deep earthquakes located beneath western Brazil
are nearly identical to one another.The P axes are all nearly vertical, whereas
the B and P axes are approximatelyparallel and perpendicularto the linear
trend of the deepseismiczone.A plane •hroughthe B and P axesis morenearly
parallel•o the N 10øW•rend of the belt o• deepearthquakesthan •o the northwest trend of the Peru trench and coastline.This relationshipsuggeststhat the
plane throughthe B and P axesrepresentsthe local structureof •he deepzone
and is thus consistentwith an inferenceof down-dipcompression
within a nearly
vertical zone.The locationof the deepeventsnearly vertically beneaththe lower
par• of the intermediate-depth
zonefurther supportsthis interpretation.
•
Solutions92-97 •or deep earthquakesbeneathwesternArgentina h•ve B •nd
STRESSES IN THE DESCENDING LITHOSPHERE
139
T axesthat are alsomorenearly parallel to the belt of deephypocenters
than
to the Chile trenchand coastline,but the differencein this caseis small. A plane
throughthe B and P axesdips about 70ø east and is approximatelyparallel to
a plane joining the deep and intermediate-depthearthquakesthrough the gap
in seismicity.These resultsare in agreementwith an inferenceof down-dip
compression
within a steeplydippingdeepslab.
Solution 91 for the large multiple event beneath the Peru-Bolivia border
[Chandra, 1970b] appearsto indicatedown-dipcompression
in a steeplydipping
zone. The B and T axes,however,have significantly different trends than those
in the deep zonesbeneath either western Brazil or western Argentina. The simplest interpretationis that the seismiczone near 91 strikes northwest,and the
T rather than the B axis is parallel to the zone. A straightforward connection
between91 and the zone beneathwesternBrazil would require this strike. The
parallelism of the T axis and the inferred strike of the zone may be related to a
stretchingor someother contortionin this region.
In summary, the most outstandingresult for the Peru-Chile region is the
changefrom down-dip extensionat intermediate depthsto down-dip compressionat great depths.
Colombia: the Bucaramanga nest (Figure 15). The island-arc-like features
of the Peru-Chile region bend around in Ecuador and trend east-northeast in
westernColombia.Earthquakes98-100 are locatedin the zoneof intenseactivity
at depthsnear 150 km beneathBucaramanga.This zonemay be at the northern
end of a continuous inclined zone that trends parallel to the island-arc-like
featuresof Ecuadorand Colombia.Molnar and Sykes [1969], for example,show
15
Fig. 15. Region of northwestern
South
America
showing the zone of intermediate-depth earthquakes
in
Ecuador
and
Colombia.
Earthquakes 98-100 are located in the Bucaramanga
nest. The lines of alternating
long and short dashes represent major
mountain
ranges and structural trends.
140
ISACKS AND MOLNAR
a zonedippingeastwardbeneaththe coas•of Colombianear 5øS. Althoughlarge
numbersof earthquakesare very concentratedspatially near earthquakes98-100
[Trygsvasona•d Lawson, 1970], shallowerbut subcrustalearthquakesare reliably located northwestto north-northeast of the Bucaramangaregion, and deeper
earthquakes (depths near 200 km) are located slightly southeastof the region
[Sykesand Ewiag, 19•5]. A vertical sectionalignednorthwest-southeast
through
the Bucaramanga 'nest' [Santo, 1969a] also shows a zone dipping southeast.
These locationssugges•tha• near Bucaramanga the zone may bend around to
a more easterly strike than in Ecuador and southern Colombia.
Solutions98 and 100 are nearly the same; the T axes could be parallel to
a zone dipping southeast.The third solution (99), however, is not in agreemen•
with tha• interpretation. Alternatively, a plane perpendicularto the closegroup-
ing of northward-plungingaxes (representedby the dashedline in Figure 15)
is parallel to all the other axes.The orientation of •his plane is consistentwith
the other evidence cited above for a zone near Bucaramanga tha• dips more
•oward the sou•h than toward the eas•. If this is the case,then the data indicate
compressional
s•ressorientedmore nearly parallel to the strike •han to the dip
of the zone. Moreover, the approximateinterchangeof •he positionsof •he B
and T axes of 99 wi•h respec•to 98 and 100 suggeststhai; compressional
s•ress
is dominant. One interpretation of •hese relationshipsis tha• the lateral compressionresults from a bending of the northern end of the slab.
Atlantic
Island Arcs
South Sandwicharc (Figure I6).
The seismicityof •his region is poorly
known, partly becauseof •he lack of nearby seismographs•ations. As in other
arcs, most of the shallow earthquakes appear to occur beneath the landward
wall of the trench; and in general,the intermediate-deptheventsare locatedwest
of the main zone of shallow activity. Earthquakes appear to be restricted to
I
55S
I
.I.
I
EORGIA
:,
":)
Fig.
16.
South
San
/
wich island arc.
60-
",....,•x •1o5
I
I
50
I
W
!
20
STRESSES IN THE DESCENDING
LITHOSPHERE
141
depthslessthan about 200 km. At the northernpart of the are, wherethe features bend toward the wes[, an intenseconcentrationof hypoeentersis located
at a depth of about 100 km (near earthquakes101-103). A vertical section
through this part of the are, oriented about N 60øE, shows an inclined zone
dipping very steeply southwest[Engdahl et al., 1969, and E. R. Engdahl, personal communication,1970]. The data are no[ adequate to define the dip of
the zone farther south along the are.
Solution 104 indicatesdown-dip extension.Solutions101-103 for the earth-
quakes located doses[ to the westward bend also indicate extensionalstress
parallel to the zone, but the axes are not parallel to the dip. This could be an
effectof the westwardbend,i.e., an effectof a contortionat the northernend of
the zone.
In contrast, solution 105 for an earthquake located in the middle of the
are exhibits simple down-dip compressionand is nearly the opposite of 104.
Solution 105, plus two others (148-149) for earthquakesin the Ryukyu are, are
the only easesin which down-dip compressionwith the B axis parallel to the
strike of the zone occurs in a seismic zone that does not reach depths greater
than 300 km.
West Indian arc (Figure 17). The distributionof hypoeenters[Sykesand
Ewing, 1965] and the focal mechanismsof shallow earthquakes[Molnar and
Sykes, 1969] demonstratean eastward rigid-body motion of the Caribbean with
respectto North America, SouthAmerica, and the westernAtlantic. In the Lesser
Antilles
the Atlantic
sea floor underthrusts
the Caribbean
toward
the west.
Near the Puerto Rico trench, the Atlantic sea floor is also underthrustingthe
Caribbean, but the motion is nearly parallel to the strike of the trench. The
seismicity of the Lesser Antilles portion of the are clearly shows a westwarddippingzoneof aet.ivity,but at the northernend of the are the activity changes
orientation abruptly and trends east-west, parallel to the Puerto Rico trench.
This east-wes[belt has a more complexstructure.No earthquakesin either portion of the are have been locateddeeperthan 250 km.
An intensesourceof intermediate-depthearthquakesis located beneath the
easterntip of Hispaniola.Althoughthe distributionof hypoeenters
is not simple,
Sykesand Ewing'ssectionsuggests
a nearly vertical distributionof hypoeenters
nearthat source.Molnar and Sykesreportsolutionsfor threeintermediate-depth
I
I
I
I
.i'.'•_
•=½
HISPANIOLA
......................
Fig. 17. West Indies island are.
I
\
"-t
ß '"•'..lk•"-'•,
70 W
'• •'
•0
142
ISACKS AND MOLNAR
earthquakesin this region,but only one (106) is well determined.A plane
throughthe B and T axesoœ106hasa strikethat is Slightlynorth o• the westerly
trend o• the Puerto Rico trench but is approximatelyparallel to the belt o•
seismicactivity alongthe northerncoastof Hispaniola.The vertical dip of this
planeis consistent
with the distributionof hypocenters.
A nearly vertical slab
underextension
approximately
parallel•o its dip may thus be locatedbeneath
the easternend of Hispaniola.It is interestingthat both the locationsand the
solutionsof 106 and 101-103 (Figure 16) have approximatelysimilar relationshipsto the West Indian and the SouthSandwicharcs,respectively.
Molnar and Sykes'two other solutions,thoughnot well determined,are
different from one another and from that of 106. This variability plus the com-
plexity in the distributionof hypocenters
in the regionmay alsoindicatea contortion or disruptionof the slab.
Sykesand Ewingreportedeventswith reliablydetermineddepthsbetween
70 and 120 km located at the southern end of the Antilles arc near Trinidad.
Molnar and Sykesfoundevidence
that someof the eventsin this regionmay be
relatedto hingefaulting,wherebythe portionof the westernAtlantic-South
Americanplate that underthrusts
and descends
beneaththe Antillesarc tears
awayfromtheportionremaining
onthe surface.It is not clear,however,
whether
the intermediate-depth
eventsare involvedin this type of faulting.The hinge
faultingat the northernend of the Tonga arc occursat shallowdepths[Isacks
et aI., 1969].
Middle America Arc (Figure 18)
Molnar and Sykes[1969] showthat the oceaniclithosphere
is underthrusting Mexicoand CentralAmericain a northeastdirection.Betweenabout86øW
and 96øW the structure of the arc is particularly simple.The surface features
are nearlystraight,andthe slablikezoneof subcrustal
earthquakes
is relatively
planarand free of major contortions.
This regionprovidesa particularlygood
,
I
I
I
Fig. 18. Middle America
trench and adjacent region
of Central
America.
STRESSES IN THE DESCENDING LITHOSPHERE
143
tessof the idea that, as a result of gravitational body forceson excessmassin
the sinkinglithosphere,extensionalstresspredominates
in the slabstha5 are
relativelyuncontorted
and that do not reachgreat depths(Figure3a).
The plane that fits the closegroupingsof T and B axes of 107-110 strikes
N 40øW and dips about 60øNE. The 20-25 ø difference between the trends of
the B axesand the strike of the trench and seismiczoneappearsto be significant.
The T axis of 111 has a plungeof about 40ø. The differencebeSweenthe plunges
of 111 and 107-110 is determinedby numerousreadingsin the central parts of
the focal spheresand is probably real. Earthquake 111, at 85-kin depth, is
shallowerthan the other four. Theseresultsare consistentwi•h down-dipextension in a zone tha5 increasesin dip from about 40ø to 60ø a5 depths greater
•han 85 km. Althoughsucha changecannot be resolvedby the data of Molnar
and Sykes,their resultsdo not excludeit. The data are thus in agreementwith
the model shownin Figure 3a, where the slab is sinking bus has not descended
beneaShthe asthenosphere.
Cascades
In the northwesternUnited Statesan island-arc-likestructuretrendsparallel
to the coastsof Oregonand Washingtonand is definedapproximatelyby the
volcanoesof the Cascaderange.Subcrustalearthquakesmay occurin this region
[Tobin an.dSykes, 1968], but the data are inadequateto define an inclined
seismiczone. Several studiesof the motionsof the lithosphericplates in this
region indicate that liShosphere
has been consumedin the pass and that the
downgoingslab will dip east [Morgan, 1968; McKenzie and Morgan, 1969;
Silver, 1969]. Althoughthe depthof earthquake112 (59 km) [Algerm.issen
a•d
Harding, 1965] is shallowerthan other eventsin this study, it is significantly
deeperthan othersin the westernUnited States.•/Ioreover,its solutionhas an
eastward-dippingT axis and thereforecould indicate extensionalstressparallel
to an eastward-dippingzone.
Aleutian Arc (Figure 19)
The underthrustingof the Pacific plate northwestwardbeneaththe Aleutians
andAlaskais described
by McKenzieandParker [1967] andStauder[1968a,b].
Focal depths are generally lessthan 200 kin. A5 the western end of the zone of
active volcanismand subcrustalseismicity(near 178øE) the dip of the zone,
althoughnot too well determined,appearsto be abou560ø [Davies and McKenzie, 1969; Murdock, 1969]. A plane throughthe P and T axes of 113 is
approximatelyparallel to the seismiczone,and the solutionindicatescompressionparallel to the dip or extensionparallel to the strike of 5he zone.Sincethis
earthquakeis locatedwherethe curvatureof the arc increases,
Stauder [1968b]
suggested
that the extensional
stressparallel to the strike resultsfrom the type
of deformation
of the descending
slabshownin Figure6. Solutionsof this type
are found in other regionswhere convexcurvatureis pronounced.
Solution204 is similar to those for shallow events in the region and is
interpretedby Stauder [1968b] as underthrustingon a nearly horizontal fault
plane. A15hough
the depShof 60 km for 204 is no5 qui•e intermediate,its loca-
ISACKS
144
AND
MOLNAR
60
Fig. 19. Aleutian islandarc.
55
N
5O
I
1'75'E
I
180
I
1'75W
1'70
160
tion is well determined;and the eventis probablylocatedin the part of the
seismiczonethat has a dip considerably
greaterthan that requiredfor the
underthrusting
interpretation.
The solutionis thereforemoreconsistent
with an
inferenceof down-dipextension
insidea northward-dipping
slab.The dip of the
zoneinferredfromthe plungeof the T axisis about40ø. Down-dipextension
is
alsoa reasonable
interpretationof the solutionof the January1, 1963,earthquake [Stauder,1968b]locatedeastof 204. The intermediatedepthof 75 km
for this eventis determined
by numerous
pP readingsand is preferableto the
depthof 51 km listedby the InternationalSeismological
Summary.Thus,downdip extension
may be prevalentin the part of the descending
lithosphere
eastof
earthquake113 that has a more nearly linear strike.
Northwest Pacific' Kamchatka-Japan-Marianas
A nearly continuous
zoneof deepand intermediate-depth
earthquakescan
be tracedfrom KamchatkathroughJapanand thenceto the Marianas.This vast
descending
edgeof the northwestPacific lithospherecan be divided into four
segments,
each characterized
by distincttrendsof the oceanictrench,of the
line of activevolcanoes,
and of the inclinedzonesof earthquakes.
The KurileKamchatka,North Ito•shu, and Izu-Boninsegments,
trendingnortheastward,
north-northeastward,
and north-northwestward,
respectively,form two obliqueangledcornersnear Itokkaido and eastcentralHonshu.The fourth segment,
the
Marianasarc, continues
the trend of the Izu-Boninarc but is offsetto the east
and is separatedfrom the Izu-Bonin segmentby a gap in seismicityand by a
shoalingof the oceanictrench. The dips of the inclined seismiczonesdiffer
notablyamongthe four segments.
The structureof the zonebetweenthe segments,whether continuous(and bent) or disrupted,cannot be clearly determined by the available data on locationsalone. Additional sourcesused in
determiningthe contoursshownin Figures20, 21, and 22 includeFedotov [1965,
1968] and Sykes [1966] for the Kurile-Kamchatkaregion; Katsumata [1966,
1967a,b] and Ichikawa [1961] for North Itonshuand the Sea of Japan; and
Katsumataand Sykes [1969] for the Izu-Bonin and Marianas regions.
STRESSES IN THE
DESCENDING
LITHOSPHERE
145
As a resultof the pioneeringresearchof It. Honda and his colleagues,
reliable
solutions of many earthquakes that occurred during the last 40 years are
available for the Japaneseregion. Although only solutionsbased on the most
numerousand consistentdata are listed in Table 1, other data, summarizedby
Balalcina [1962] for the Kurile-Kamchatka region and by Honda et al. [1956,
1967], Ichilcawa [1966], and Alci [1966] for the Japaneseregion, are in substantial agreement with the results selectedfor Table 1.
The Kurile-Kamchatka arc (Figure 20). Between Kamchatka and the
Hokkaido-Sakhalin region the inclined seismic zone appears to be relatively
simplein structureand can be representedby a plane with a dip of 45ø and a
strike varying from about N 35øE near Kamchatka to N 60øE near Hokkaido.
Nearly all the large,well-locateddeepeventsnortheastof the Itokkaido-Sakhalin
regionhave depthsbetween400 and 500 km. Examination of reported locations
suggeststhat the maximum depth of this zone is probably lessthan 600 km and
may be near 500 km. A minimum in activity as a function of depth occurs
betweenabout 200 and 400 km, but apparently reliable locationsof eventswith
depthsin this range (e.g., Savarenslcyand Golubeva [1969]) indicate that this
minimum is not an aseismicgap in activity.
In nearly all the solutionsfor this region either the P or the T axes tend to
x
.o
128-154
!
!
5O
N
,/
!
118
'4-
I
I
I
--]--
I
I
x
i
125-127
/
140 E
o
o
150
16 0
Fig. 20. Kurile island arc and the region near Hokkaido, North Honshu, and the Sea of
Japan. The discontinuity in the contours of hypocentral depths representsa possible disruption between the Kurile
and North
Honshu
seismic zones.
146
ISACKS AND MOLNAR
beparallelto thedipof thezone,whiletheB axesareapproximately
parallelto
i•s strike.However,
the pa•ternof down-dip
s•ress
in the regionis complex
and
not jus• a œunction
oœdepth.Solutions123-127œoreventsnear the junction
between
•heKurileandNorthHonshuzones
all indicatedown-dip
extension,
as
do 119-121for threeintermediate-depth
eventslocatedsouthoœKamchatka.In
contrast,solutions117-118œorintermediate-depth
eventslocatednorth and
southoœ119-121and•wosolutions
(114-115)œor
deepeventsall showdown-dip
compression.
Solution116 for a deepearthquakehas • differen•orientationfrom the
othersin the regionanddoesnot appear•o be closelyparallelto the over-all
planargeometry
oœthe zone.However,the parameteroœthe solutionthat is
mostnearlyparallelto •he dip oœthe zoneis s•ill •he P axis.
The peculiar location oœ122 a• 80 km beneaththe axis o• the Kurile trench
doesnotappearto bean errorin •helocation.
Shallow
eventscharacterized
by
solutions
oœ
thenormalœaulfing
typearefoundnearthe axisoœtrenches
in other
arcs,notablythe Aleutians[Startder,
1968b],and canbe interpreted
as extensionneartheupperpar• oœthe oceanic
plateasit bendsbeneath
the islandare.
By thesamereasoning,
compression
wouldbe expected
nearthe lowerpart oœ
theplate.Although
the•rendoœthe P axisoœ204is notperpendicular
to the
axisof thetrench,theP axisis morenearlyhorizontal
thanthe T axisandcould
indicate
compression
oœ
the lowerpar• oœ
theplateasit bendsdownward
near
the trench.
The Hotet•aidocor•er (Figure20). Solutions123-129for the eventslocated
in •heregion
between
thegentlydipping
NorthHonshu
zoneandthemoresteeply
dippingKurile zoneare oœtwo types.Solutions123-127for eventslocatedon
theKurilesideoœ
thecornerall indicate
down-dip
extension
andarethussimilar
to 119-121 œorevents in the northern Kuriles. Solutions 128 and 129 •or events
located on the North I•Ionshuside oœthe corner are similar to 130-134 and are
thusapparently
relatedto the down-dipcompression
in the NorthHonshuzone.
The solutions,
thereœore,
appearto berelatedmoreto patternson eithersideof
•he cornerthan to any specialeffectof the corneritself.
Hondaet al. [1956]andlchit•awa[]966] repor•o•hersolutions
(no•included
in Table1) •orearthquakes
near•hecorner,
someoœ
whichcanbeinterpreted
to
suppor•the lateral s•retchingor tearingoœ• slab as shownin Figure5. For
example,•hree oœtheir solutionsœorearthquakes
near 123 have T axesthat
plungebetween
the dip direction
andthes•rikedirection
oœ
the zone;thesesolutionscouldbe taken to indicatethe obliques•retchingoœa slab as shownin
Figure5a. Also,oneof •he solutions
hasa verticalnodalplanestrikingnorthnorthwes•that, iœtaken as the œaultplane,indicatestha• the Kurile slabmoves
downwithrespe½•
to theNorthHonshu
slab.Thisis in agreement
with•hehinge
faultingshownin Figure5b, and,iœcorrect,i• couldindicatea disruption
oœ
the zone.However, examinationoœthe solutionsoœHonda et al. and Ichikawa
in these casessuggeststo us tha• the differencesbetweentheir solutionsand
thoseœor123-127may no• be real.In fact, it seemspossible
to fi• mos•oœthe
datagivenby Hond•e• al. andIchikawa,withminormodifications,
to thesolutions œor123-127.
STRESSES IN THE DESCENDING LITHOSPHERE
147
The distribution of earthquakesplus the remarkable difference between
solutions127 and 128 suggestthat the main junction betweenthe North Itonshu
and the Kurile zonesis located beneath western Itokkaido (between 127 and 128)
and beneaththe northern Sea.of Japan east of 129. Whether the zone is continuousor disruptedin this regionis not dear from the distributionof hypoeenters. The reorientationof stressbetween127 and 128, however,may be easierto
understandif the eventsresult from stresses
in separateslabs.
Although 125 and 126 are near the zone of shallow underthrusting,their
depthsnear 80 km are reliably determined,and their solutionsare quite similar
to those for the deeper events. Solutions 125 and 126 thus appear to indicate
stresswithin the descending
plate rather than slip betweenplates.Moreover,the
directionsof slip inferred from the solutions,were the earthquakesshallower,
indicatemotionof the Pacificplate relative to the Asianplate that is significantly
more northerly than the northwesterly direction indicated by McKenzie and
Parker [1967]; Isacks et al. [1968]; Le Pichon [1968]; and Stauder [1968b].
Inclusion of such events probably accountsfor the diserepan[ directions near
Hokkaido shownin the worldwideplot of slip directions(Figure 3 of Isacks et al.
[1968]).
North Honshu (Figures 20 and 21). The deep •nd in•ermediMe-depth
earthquakes•ha[ occur beneath [he Sea of Japan are par[ of an inclined zone
rela[ed to •he ac[ive volcanoesand offshoreIrerich thai parallel the nor•heas[
coast;of Honshu.Ichikawa [1961] and Katsumata [1967a] show•hat; •he zone
has a dip of abou[ 30ø. Allhough seismicac[ivi[y betweendep[h• of 200 and 500
I
128-134
c)
o
/
i
40
N
i
l
i
x
Fig. 21. Izu-Bonin island
are and the region of Honshu
and the Sea of Japan. The
area near earthquakes 135-
o
o
137 is shown in more detail
in Figure 22.
50
ß
•0
144-
130E
140
150
148
ISACKS AND MOLNAR
kmissparse,
thezoneappears
to becontinuous.
It does
notappear
to reachdepths
much greater than about 600 km.
Nearly all of the P axesof 128-134are approximately
parallelto the dip
of theNorthItonshuzone.Threesolutions
showan interchange
of B andT axes
with respectto the others.The consistency
amongthe solutions
is fi•rtherevidencefor the coherency
of a singleinclinedzonedippingbeneaththe Seaof
Japan.Othersolutions
reportedby Hondaet al. and Ichikawaagreewith the
resultsshown
in Figures20-21.In addition,
theirresultssuggest
that down-dip
compression
may alsobe presentat depthsof 100-200km beneaththe northern
coastof Itonshu,butthe dataare limited.Thusthe NorthItonshuslabmay be
undercompression
at all depthsbelow100km.
Thejunction
between
theIzu-Bonin
andtheNorthHonshu
segments
(Figures21 and22). Both the distributionof hypocenters
andthe focal-mechanism
solutionssuggestthat the junctionbetweenthe Izu-Bonin and the North Itonshu
zones
is.located
near135-137(Figures
21-22).Thedistribution
of earthquakes
shallower
than200km,theoceanic
trench,andthevolcanic
front[Sugimura,
1965]forma well-defined
cornerin the regionof Tokyo and centralItonshu.
Subcrustal
activity is especiallyintenseat depthsof 50-100km near 135 and
at depths
of200-300
kmnear136.Earthquake
137,located
at a depthof360km,
appears
to beat thenorthern
endoftheIzu-Bonin
zone;tothenorththeactivity
decreases
andappearsto be part of the gentlydippingNorth ttonshuzone.As
shown
in Figure21 anddescribed
in thenextsection,
solutions
for deepearthquakes
in theIzu-Bonin
zonelocated
south
of 136and137arecharacterized
by
down-dipcompression
andaresimilarin thisrespect
to thosein the Northttonshu
arc.Solutions
135-137
for theevents
located
between
thetwozones,
however,
havea distinctlydifferentorientation
and therefore
probablyreflectlocalized
effectsnear the junction.
The solutions
for 135-137are in eachcaserepresentative
of solutions
for
other
shocks
inthevicinity
ofeach
hypocenter.
Honda.
etal.andIchikawa
report
twoothersolutions
thatagree
with137andfivesolutions
thatagree(orforwhich
40
Fig. 22. The corner between the
Izu-Bonin and the North tIonshu
zones. The dotted lines in[ the
55
N
projectionof solution136represent
the nodal planes of the solution.
U (up) and D (down) show the
sense of
motion
on the
more
nearly vertical nodal plane taken
as the fault plane. The solution is
30
consistent
with hingefaulting.
STRESSES IN THE DESCENDING
LITHOSPHERE
149
the data are not inconsistent)with 136. Solution 135 is an average of a group of
solutions reported by Ichikawa [1966]. The individual solutions combined in
135 are quite similar to one another and agree with independentresults of Aki
[1966] for small earthquakeslocatedin the region.
Solution 136 is consistentwith the hinge faulting depictedin Figure 5b; in
this casethe steeply dipping Izu-Bonin slab tears away and moves downward
with respectto the moregently dippingNorth Honshuslab.This type of deformation, however,doesnot accountfor the orientation of 135 or 137. If the tearing
and hinge faulting are located in the zone of intensesubcrustalseismicactivity
near earthquake136, then at greater depthsthe two slabs will be separated.
Hence, earthquake 137 may be in the northern edge of a separate Izu-Bonin
slab. The differencebetween 137 and 138-146 may be attributable to a rotation
of the axes due to a bend at the edge similar to that at the northern end of the
Tonga arc (Figures 4 and 11). Further study of the detailed distribution of
hypocentersin the region beneath central Honshu is required to test this interpretation.
The earthquakescomprising135 may result from stressesin the portion
of the slababovethe tearingand disruptionnear 136.The orientationof 135,
however,is most simply interpretedas a faulting parallel to the inclined seismic
zone, i.e., thrust faulting between the convergingPacific and Asian plates. if
this is correct,then the earthquakescomprising135 would be an anomalously
deep extensionof the shallowthrust zone characteristicof the inner margins of
the trenches.Most of the earthquakescomprising135 have depthsgreaterthan 60
km, and many have depthsbetween80 and 100 km. In most casesthese depths
are accurately determined by numerous arrival times from stations within a
degreeof the epicenter.
Izu-Bonin arc (Figure 2I). Katsumata and Sykes [1969] show that the
inclinedseismiczone south of Honshu,though slablike in structure,varies as a
function of depth and location along the strike. The zone appears to be continuousto.depthsof about 550 km. In the separateequal-areaplots of Figure 21,
the data are shown relative to the local orientation
of the zone. The P axes for
the deep earthquakes138-147 all appear to parallel the local dip of the seismic
zones;i.e., the P axestend to follow the local variations in dip. The T axestend
to be perpendicularto the zone except in two casesin which the B and the T
axes interchange.
The data are sparsefor intermediatedepths.Ichikawa reportstwo solutions
for earthquakesat depthsof 150 and 130 km locatedsouthwestof Tokyo, just
southof the junction describedin the precedingsection;both indicate down-dip
compression.
Solution148 for an even[ at 80-km depth, however,indicatescompressionparallelto the strike of the zone.Mechanismsof this type are rare; the
only other clear exampleis solution27 for an intermediate-deptheven[ located
at the southernend of the New Hebrides arc (Figure 9).
Marianas arc (Figure 23). Katsumata and Sykes [1969] show that the
inclined zone beneath the northern par[ of the Marianas arc has a remarkable
structure.From shallowdepthsnear the trench the slablike zone curvesdown-
wardandbelow250km hasa nearlyverticaldip.The zoneis continuous
to a
ISACKS
150
AND
MOLNAR
50
2C
Fig. 23. Marianas island arc.
N
,5•x
•/'
, IR.....
I0
140E
150
depth of 680 kin. Most of the hypocentersbelow500 km are confinedto.s narrow
range alongstrike betweenabout 19ø and 20øN; few 'deepeventsoccurbetween
this zone and the southern end of the Izu-Bonin
zone near 27øN. There is evi-
dence [Gutenbergand Richter, 1954], however,for the location of earthquakes
at depthsof 500-600 km near about 24øN; a short solid segmentof the 500-kin
contour in Figure 23 indicates the location of these events. A continuous and
nearly straight 500-km contour that trends north-northwest and connectsthe
Izu-Bonin and Marianas deep zonespassesthrough the deep hypocentersnear
24øN. The distributionof the deepesteventsdoesnot thereforeappearto reflect
the offsetthat is prominentin the surfacefeaturesof the Marianas arc.
The P axis of 150 plungesvertically and is thus parallel to the local dip
of the Marianas deepzone.The B and T axesare horizontaland trend parallel
and perpendicularto the north-northwesterlytrend inferred above for the
regionaldistributionof deephypocenters.
This relationshipis evidencethat the
deep seismic zone is continuousbetween the Izu-Bonin and the Marianas arcs.
Solution149 for an event at intermediatedepth indicatesextensionalstress
approximatelyparallel to the strike of the zone.Note that althoughthe P and
the B axesare not parallelto the zone,neitherare the nodalplanes.The solution
is similar to others for earthquakesin regionswhere the surface features are
curved,and can be explainedby the lateral stretchingeffect shownin Figure 6.
Ryukyu Arc (Figure 24)
Katsumata and Sykes [1969] showthat an inclinedzone dips beneaththe
Ryukyu Islandsat an angleof about45ø and reachesdepthslessthan 300 kin.
Their solutionsfor shallowearthquakes
indicateunderthrusting
oœthe Philippine
Seaplate beneaththe Ryukyus.In Taiwanthe seismicity
changes
trendabruptly
and trendssouthalongthe easterncoast.Subcrustalearthquakesare also found
beneathTaiwan andmay be part of an inclinedzonethat dipssteeplyeastw•ard.
Earthquakes151-152are locatedaway from eitherendof the arc. The solutions are similar to one anotherand indicate down-dipcompression.
In our
previouspaper [Isacksand Molnar, 1969] we suggested
that thesesolutions
are similar to 113 in the Aleutiansin respec••o indicatingextensional
stress
STRESSES IN THE DESCENDING LITHOSPHERE
151
Fig. 24. The Ryukyus island arc and the region near
Taiwan.
parallelto •he s•rike.However,asshownin Figure23, •he T axesfor the Ryukyu
solutionsare morenearly perpendicularto.the zoneand the B axesmore nearly
parallel to i•s strike. Thus, we canno5in•erpre55hesesolutionsby 5he lateral
sSretchingeffec• shown in Figure 6.
Earthquakes153-155 are locatednear the intersectionof zonesin Taiwan.
Two of thesehave depthsof 67 and 70 km and so are near the borderlinebetween
shallowand in•ermediaSedepths;the third (153) has a depth of 119 kin. The T
axesof all •hree plungesteeplynorthward,and the B and P axestend to be nearly
horizontal.The B and P axesof 154 and 155 approximatelyinterchangepositions.
The daSafor the three solutionscan thus be 5aken •o sugges5exSensionparallel to
•he dip of a nearly vertical and possiblycontortedslab near the Taiwan comer.
Earthquake 156 is locatedat a depth of 72 km in •he easSward-dipping
zone
sou•h of the Taiwan intersection. The s[rucSureof the zone is no5 well enough
known to interpre• this solution.
Philippines-CeIebes-Halmahera
(Figure 25)
The •ectonicsof •his region are especiallycomplexand are no5 well unders•ood.Fitch [1970a] summarizesevidencefor a subdivisionof 5he region into
three disfinc5island-arc-like s•ruc•ures: •he Luzon arc, •he Mindanao arc, and
•he Talaud-ttalmahera arc. Although all three have zones of subcrustal eaFchquakes, only •he Mindanao arc has a, fairly well-defined inclined zone tha5
reaches grea5 depths. The solutions for subcrustal earthquakes are also moss
simply groupedaccordingto 5hesedivisions.
Luzon arc. A northward-strikingStench[Hayes and Ludwig, 1967; Ludwig
et al., 1967] and the presenceof volcanoesand shallow and in•ermedia.•e-depSh
earthquakesindicaSe5ha5 an island-arc structure exists on •he wesSernside of
152
ISACK$ AND MOLNAR
,x/5B-161
x
Fig. 25. Region of deep and intermediate-depth
earthquakes
near the
Philippines,I-Ialmahera,and Celebesislands.Talaud Island is locatedbetween the dashedrectanglesthat encloseearthquakes158-161and 163-166.
Luzon.An inclinedseismiczoneappearsto dip east,but the data are inadequate
to determine
its dip.The onlysolution(157) for thiszonecanbe interpreted
as
extension
parallelto a steeplydippingzoneor thrust faultingalonga zone
dippingabout45ø.Because
of thedepthof theevent(166km), theformerinterpretationis moreconsistent
with the data for otherregions.
Mindanao
arc.
The inclined seismic zone associated with the Mindanao
trenchdipswestwardand reachesdepthsgreaterthan 650 km (M. Katsumata,
personal
communication).
A gapin activityappearsto occurbetweendepthsof
about 300 and 500 km. Shallow earthquakemechanismsin the region indicate
a westwardunderthrusting
of the PhilippineSeaplate beneathMindanao [Fitch,
1970a].In the southern
part of the structurethe zonesof earthquakes
andactive
volcanoes
appearto bendwestwardandparallelthe east-west-trending
peninsula
of northernCelebes.Fitch a.n.d
Molnar [1970] showthat the zonemay dip northward near Celebes.
Earthquakes158-161are locatedin the westward-dipping
zone,and 167-168
are located where the arc bendsand has a more northerly dip. Earthquake 163,
thoughlocatedcloseto 160 and 161,appearsto be part of the eastward-dipping
zone beneathTalaud and Halmahera; i• is considered
in the next section.The
STRESSES IN THE DESCENDING LITHOSPHERE
153
T axes of 158-161 all dip westwardand are consistentwith down-dip extensionalstress.Note that 161is interpretedby Fitch as indicativeof motionbetween
plates,i.e., underthrusting
of the PhilippineSeaplate beneaththe CelebesSea
plate.The depthreportedin thePrelimiaaryDeterminatioa
of Epicenters
of the
Coastand GeodeticSurvey[1963] is 64 kin, but it is revisedto 83 km in the
Seismological
Bulletinof the Coastand GeodeticSurvey[1963] and 122km in
the InternatioaalSeismological
Summary[1963]. The data controllingdepthfor
this eventare poor,but examination
of the seismograms
suggests
that the depth
is greaterthan normalbut probablynot as greatasthat listedby the ISS. If the
solutionsfor thrust faults in the shallowportionof the seismiczonehave the
same orientationas onesindicatingextensionalstressparallel to the zone at
intermediate
depths,the seismiczonemustdecrease
in dip near the thrustzone.
Althoughsucha changein dip is difficultto resolvein the distributionof hypocenters,Mitronovaset al. [1969] demonstrateit for the Tonga arc.
The solutionof 162 indicatescompression
parallel to the dip and extension
parallelto the strike of the zone.This resultmay thus indicatean effectof the
convexcurvatureof the arc near Celebes (Figure 6). The solutionsfor 167 and
168 for events located where the arc bends westward are consistent with exten-
sionalstressparallel to a zonedippingsteeplynorthwestward.
In summary,solutions158-161 and 167-168 are consistentwith an inference
of down-dipextensionat intermediatedepths,whereas162 can be interpretedas
an effect of the bend near Celebes.The results for great depths are uncertain;
Ritsema's solutions [1964] for two deep events in the Mindanao zone are not
clearly interpretablein terms of stressparallel to the deepzone.
Talaud-Halmahera arc. The distributions of volcanoes and earthquakes
with depthsbetween70 and 200 km suggestan island-arc-likestructurestriking
south-southeastbetweenthe islands of Talaud and Italmahera [Hatherton and
Dickinson,1969;Fitch and Molnar, 1970]. The apparenteastwarddip of the zone
is oppositeto that of the adjacentMindanao zone.
The T axesof 163-166 tend to be more nearly vertical, whereasthe P and B
axes tend to be more nearly horizontal. A nearly vertical plane approximately
parallel to the island-arc-likesurfacefeaturescan be fitted throughthe stress
axesof three of the solutions.This result suggeststhe presenceof down-dipextensionin a steeplydippingslab.
Celebes. Earthquake169,locatedbeneathsouthernCelebes,is isolatedfrom
the seismicallyactive island-arcstructuresor from other zonesof shallowearth-
quakesin the region.The planethroughthe B and T axesdipsnearlyvertically
and strikesslightly west of north. This would be consistentwit.h extensionin a
vertically sinking slab. Alternatively,the solutioncould indicate horizontal
compression
within a thick lithospheric
plate on the surface.Examinationof the
seismograms
and the locationdata for this eventindicatesthat the depthof 81
km is reliable.
SundaArc (Figure 26)
Sumatra-FloresSea. The Sunda arc is a nearly continuousstructureextend-
ing fromBurmato the BandaSea.The portionshownin Figure26 includes
the
154
ISACKS AND MOLNAR
Fig. 26. Sunda island arc.
main belt of deepand in[ermediate-depthearthquakes.The distributionof earthquakeswith depth appearsto vary abrup[ly alongthe are; near the SundaStrait
depthsincreasefrom lessthan 200 km beneathSumatra to grea[er than 600 km
beneaththe Java Sea [Fitch and Molnar, 1970]. Be[weenabou[ 105øEand 126øE
[he zone reachesdepths greater [han 650 km and possiblygreater than 700 km
[Gutenbergand Richter, 1954]. Benea[hthe Flores and Java Seasa pronounced
minimumin activity is presentbe[weendepthsof abou[ 300 to 500 km.
The T axes of 170-171 plunge beneath Sumatra and can be in[erpreted as
down-dipex[ension.Moreover,Fitch [1970a] reportsa solution(his number1)
for an event at the northern end of Suma[ra which, thoughnot well determined,
is alsoin agreemen[with extensionparallel •o [he seismiczone;[he solutionis no•
in agreemen[with under[hrust faulting. The in[ermedia[e depth of [hat even[
(70 km) appearsto be reliable.
The T axis of 173 plungesgently north, whereas[he dip of the zone near
173 appears to be steeper. The structure of [he seismiczone, however, is not
suftieientlywell resolved[o excludethe possibilitythat the zonedips more gently
near 173 and steepensin dip at greaterdepth [Fitch and Molnar, 1970;Engdahl
et al., 1960; E. R. Engdahl, personaleommuniea[ion,1970].
The solution of 172 indicates extensionparallel to [he strike of [he zone.
This earthquake is near the changeof s[rike of the are, where the distribution
of hypoeentral depths abruptly changes.Filch and Molnar suggestedthat the
convexcurvature of the are in this regionmay be responsiblefor the anomalous
orientation of 169 and can be explainedby the model of Figure 6.
The solutions174-176 for three deep shocksbeneath[he Java Sea are in good
agreementwith an inferenceof simpledown-dip compression.
The ver[ieal orien-
STRESSES IN THE DESCENDING
LITHOSPHERE
:155
tation of the seismiczoneshownin Figure26 is basedon sections[Engdah•et al.,
1969;F•tch a•d Mo•nar, 1970] that showa well-definednarrow zone•ha• dips
nearly vertically betweendepthsof abou•500 and 650 km.
The three solutions177-179 for deep eventsbeneaththe Flores Sea also
indicatedown-dipcompression
parallel to a nearly vertica! zone,but the trends
of the B and T axessuggestthat •he zonemay have a more nearly east-northeasterlystrike rather than the easterlystrike of •he surfacefeaturesand of the
alignment of deep hypocentersfarther west. This relationshipplus reliable locations of deep events northeast of 177-179 suggestthat the deep zone bends
around in a fashion similar to the shallower zone beneath the Banda Sea (see
next section).Solutionsfor three deep earthquakesreportedby Ritsema, [1964]
are in approximate agreementwith the solutionsof 174-179.
Banda Sea. Fitch and Molnar [1970] argue that both the spatial distribution and the focal mechanismsof earthquakeslocated beneath the Banda Sea
can be explained by the upward bending of the western edge of a lithospheric
slab dipping beneaththe Sundaare (Figure 4). The earthquakesare not as deep
beneath the Banda Sea as those farther west; furthermore, as a resul• of the
bending, the activity at intermediate depths and at depths of 300 to 500 km is
especiallyintense.This explanationimplies that the gap in activity west of the
bend correspondsto a minimum in stresswithin a continuousslab rather than an
actual gap in the slab, i.e., implies model b rather than model d in Figure 3.
Solutions 180-187 indicate two predominant stressorientations in the zone'
180-185 indicate down-dip extensionand an interchangeof the positionsof the
B and P axes, whereas 186-187 have compressionalaxes oriented between•he
strike and dip of the zone and an interchangeof the T and B axes.Solutions
186-187,for the two eventslocatedat depthsgreaterthan 300 km, couldindicate
a ½ompressive
stressin the upperpart of a bent slab; this might also explainthe
parallelismof the P axesand the strike directionsin the solutionsof someof the
eventsa• intermediatedepths.The predominanceof down-dipextension,however,
suggests
tha• a• intermediatedepthsthe stressresultsmainly from a stretching
rather than a bending of the slab.
In summary, the down-dip extensionat intermediate depths and down-dip
compressionat great depthspredominatein the Sunda are. The solutionsfor the
two deepeventsin the bent edgeof the zone beneaththe Banda Sea are among
the few caseswe have found in which the effectsof bendingmay be important.
Alpide Belt: Asia
The mos• active zones of subcrustal seismic activity in the Alpide bel•
betweenSumatra and the Mediterranean region are located beneath Burma and
the Hindu Kush Mountains. Although subcrustal hypocentershave also been
reportedfor the regionof North Sumatraand the AndamanIslands,the Himalayan Front betweenBurma and the Hindu Kush, the Zagrosbel• of Iran, and
the regionof the Caspian Sea, focal-mechanismdata for the regionsare lacking.
Burma (Figure œ7). The zoneof intermediate-depth
earthquakeslocated
near the arcuaterange of mountainsbetweenAssamand Burma appearsto dip
eastward and s•rikes parallel to the north-northeasterly•rend of •he mountain
156
ISACKS AND MOLNAR
Fig. 27. Zone of intermediate-depth earthquakes in Burma. The lines of long and
short dashes represent major mountain
ranges.
oo
belt. As Santo [1969b] notes,the level of shallowactivity is low relative to that
at intermediatedepths.Focal depthsprobablydo not exceed200 km.
Fitch [1970b] reports solutionsfor two intermediate-depthevents in this
region.Althoughonly one (188) is fairly well determined,the data for the other
are in agreementwith it. The T axisplungeseastwardand is thus probablyparallel to the dip of the inclined seismiczone. Although limited, the data are thus
consistentwith down-dip extensionalstresswithin the eastward-dippingseismic
zone.Note that the P axis, not the B axis, is more nearly parallel to the strike
of the zone. This orientation, plus the depths of the events, arguesagainst an
interpretation of the data as thrust faulting along the inclined seismiczone.
Hindu Kush (Figure 28). The remarkable concentrationof intermediatedepthhypocentersbeneaththe Hindu Kush is locatednear a complexjunctionof
tectonic belts, and no simple island-arc-like structuresare apparent. Near. the
Hindu Kush the seismicityis greatestat intermediatedepths,whereasthe most
active nearby zone of shallow earthquakes is associatedwith the east-northeast-trending block-faulted structures of the Tien Shah located farther north.
Beneaththe Hindu Kush the largestearthquakesappearto be remarkably
concentrated
in space.Gutenbergand Richter'smostaccuratelylocatedevents,
includingmostof the largestshocksthat occurredbetween1907 and 1950,all
have depthsbetween210 and 240 km and occur within a degreeor less of
36•øN and 70%øE. This spatial concentrationof the largest shockshas con-
tinuedduringthe periodfrom 1954to the present.Locationsof earthquakes
of
smaller magnitude show that the zone has a discernibleextent and structure and
is not a pointlikesourceof earthquakes.
The AtlasZemletryasenii
SSSR[1962]
andNowroozi[1971]showthat earthquake
hypocenters
are distributed
throughout the rangeof depthslessthan 300 km. Nowroozi,in particular,clearlydelineatestwo thin, slablikezones;oneis locatedbetweendepthsof about70 and
175 km and trendsnortheast,and the secondis lo.catedat depthsbetweenabout
STRESSESIN THE DESCENDING LITHOSPHERE
157
4O
55
N
Fig. 28. Zone of intermediate-depth earthquakes beneath the Hindu Kush. The names
refer to the major mountain belts in the
region.
50-
65
70E
75
175 and 250 km and trends east-west.In Figure 28 thesetwo zonesare indicated
by the 100-kmand200-kmcontours
oœhypocentraldepths,respectively.
Focal-mechanism
solutions
for the regionare summarized
by Ritsema[1966],
Stevens[1966],Shirokova[1967],andSoboleva[1967,1968].Stevensshowsthat
solution189 is representative
oœmost oœthe data, if not all oœthe solutions,
reportedby other workers.Ritsema derivesa commonsolution (192) for earthquakesthat occurredduringthe period1949-1965.Both are plotted in Figure 28
togetherwith well-determinedsolutionsfor two other earthquakes(190 and 191).
The resultsoœSobolevaand Shirokovaare in generalagreementwith the pattern
exhibited by thesedata.
Earthquakes 189-191, as well as most oœthe events comprising192, are
probably located in the lower east-west-strikingzone. The T axes dip steeply
north, but the polesoœone set oœnodal planes dip as steeply south.From this relationshipalonewe couldequallywell inœereither (a) shearingor slip parallel to the
zone or (b) stressinside a thin slab. However, the interchangeoœthe B and P
axes oœ190 relative to the other solutions,although difficult to explain in terms
oœvertical shearingor slip, is consistentwith extensionalstressinside a slab that
dips steeply north. Nowroozi'sdata do not excludea northward dip oœthe zone
as small as, say, 75ø. Thus, althoughfurther investigationis required,down-dip
extensionin this zoneis the preœerred
interpretation oœthe data.
AlpideBelt: MediterraneanRegion(Figure29)
Known subcrustal earthquakes are confined to •our separate areas in the
regionoœthe MediterraneanSea: the VrancearegionoœRumania,the southern
AegeanSea, the Tyrrhenian Sea northwestoœCalabria and Sicily, and southern
158
ISACKS AND MOLNAR
193-197
x
201-202
/
40
N
'xx
\
[
'
-I-- ;
I
+
X
!
50
o
,
I
0
'
03
I
0
199-2(
I
IOE
I
20
30
Fig. 29. The Alpide belt in and near the MediterraneanSea, includingthe Vrancearegion
of Rumania (193-197),the Hellenic islandarc (198-200),the Calabrianislandare (201-202),
and the Spanishdeep earthquake (203).
Spain.Solutionsbasedon WWSSN data are availableonly for the Aegeanarea.
Becauseof the numerousstationsthat have long beenin operationin Europe,
however,reliablesolutionsare alsoavailablefor previousearthquakes.
¾rancearegionof the easternCarpathians. A concentratedsourceof earthquakeswith depthsbetweenabout 100 and 150 km is locatedjust east of the
sharpbendof the CarpathianMountains.Althoughthe detailedstructureof this
zoneis not well determined,Karnik [1969], in a summaryof Rumanianwork,
suggests
that it reachesdepthsof about 200 km and dips steeply.Radt• [1965]
showsan inclinedzonestrikingnortheastanddippingnorthwest.
The five solutions193-197 plotted in Figure 29 were selectedfrom the literature as the most reliable that we were able to find. The solutions 193-196 are
nearlyidentical;197 differsin that the P andB axesapproximately
interchange
positions
with respectto others.The T axesof all five dip nearlyvertically,and
the nearly horizontalB and P axesare either parallel or perpendicular
to the
apparentstrikeof the subcrustal
zone.The resultsthussuggest
down-dipextensional stress within a nearly vertical slab.. This inference is similar to that
obtainedfor the concentratedsourceregionbeneaththe Hindu Kush.
Hellenicarc. The distributions
of shallowand intermediate-depth
earthquakesand volcanoes,
focal mechanisms
of shallowearthquakes[McKenzie,
1970],andthe data from marinegeology[Ryan eta!., 1970]indicatean island-
STRESSES IN THE DESCENDING LITHOSPHERE
159
arc structureboundingthe southernand southwestern
AegeanSea.Solutionsfor
shallowearthquakesalongthe arc indicatenortheasterlyunderthrustingof the
Mediterraneansea floor beneaththe Aegean[McKenzie,1970]. Becausethe
seismicity
is ratherlow for an islandarc,the distributionof hypocenters
is poorly
known;the availabledata sugges•
a zonedippingnorth bo northeastbeneath
the Aegean.Karnik [ 1969] findsbhabhypocenbral
depthsdo not exceed200 km.
Solutions199 and 200 are consistent
wi•h down-dipextensionwibhina zone
dippingabout60 degrees
northeast.The T axisof 198 dipsgentlynorthandthus
may indicateextension
within a gentlydippingzone,although•he P andB axes
arenot parallelor perpendicular
5othe eas•-west
strikeof the westernendof the
arc.
SouthernItaly: Calabrian arc. Gutenberga•d Richter [1954] and Peter-
schmitt[1956] showthat intermediate-and deep-focus
earthquakes
beneaththe
Tyrrhenian Sea are associatedwith an island-arc-likestructurenear Calabria
and Sicily (seealsoRyan et al., 1970). The inclinedseismiczonedipsnorthwest
and reachesdepthsof 450 km. Peterschmittfinds evidencetha• seismic-wave
velocitiesnear this zoneare anomalously
high,a resultnowbhoughtto be charac5erisSicof •he island arcs of the Pacific [Oliver and Isacks, 1967; Utsu, 1967;
Davies and McKenzie, 1969; Mitronovas, 1969].
Solution202 is an averagesolutionfor subcrusbal
earthquakesderivedby
Ritsema [1969]. Bo•h solutions201 and 202 have P axes•hat plungenortheast
and B axesthat are nearly horizontaland parallel•o the s•rikeof the Calabrian
arc. The data are bhusin good agreementwith down-dip compressionalstress
within •he northwests-dipping
zone.
Southern Spain. A large deep-focus earthquake occurred beneath the
Sierra Nevada of southernSpain in 1954 at a depth of 650 km. No other deep
seismicactivity is known •o be located in this region, and the event doesnot
appear
t• berelated
•oanobvious
island
arcorisland-arc-like
sbructure.
Solution 203 is well determined by numerous firs5 motions reported by
nearby Europeanstabions.In Figure 28 two planesare drawn throughthe stress
axesof this solution.Extrapolationof resultsfrom o•herregionssuggests
that •he
plane •hroughthe P and B axes,a plane bha• s•rikesnorth and dips 45ø eas•,
would be •he mos• likely orientation of a seismiczone near 203. In this casean
east-dipping
slabof lithosphere
is undercompression
parallelto its dip. The surface brace of •he inferred slab would be located near or wes• of Portugal and
Gibraltar and would strike north-south. Tectonic trends of southern Spain are
sometimesshownto be curvedaroundthrough Gibraltar •o join the riff s•ructures
of northern Africa; near Gibraltar such a belt would have a north-southstrike
and could thus be related to •he surface trace of bhe postulabedseismiczone.
Alternatively, a vertical plane throughthe P and T axesis parallel to the pre-
dominantlyeas•-wes•alignmen•of the belbof shallowearthquakesin •he western
Mediterraneanand alongthe Azores-Gibraltarridge.McKenzie [1970] claims
•ha• •his bel• marks •he boundarybetween•he convergingAfrican and Eurasian
p!a•es.Thus 203 may be :rela•ed•o lithospheric
consumption
alongthat zone,
although•he isolationof the event as well as the orientationof stressare
anomalous.Until the complex•ech)nichistory of the wesSernMediterraneanis
160
ISACKS AND MOLNAR
better known,the relationshipof the Spanishdeepearthquaketo the near-surface
structure remains a puzzle.
APPENDIX
Table
1
Table I lists the locations and focal-mechanismsolutions for the earthquakesplotted in
Figures 1, 2, and 7 through 29. The location listed in each case is the most reliable one
reported in the literature. If a relocation is not available from a special investigatio.n of
seismicity,or from the International SeismologicalSummary and the Bulletins o• the International SeismologicalCenter, the location is taken from the SeismologicalBulletin of the
Coast and Geodetic Survey (ESSA); if none of these is available (mainly for the more recent
earthquakes), the location is taken from the Preliminary Determination o• Epicenters of the
Coast and Geodetic Survey.
The trends and plunges listed for the focal-mechanism solutions are measured in degrees
clockwisefrom north and downward from the horzontal, respectively. The second to last
column lists the data plotted in Figure 2. 'P' and 'T' indicate down-dip compressionand
tension, respectively; 'X' indicates that neither the P nor the T axis is approximately
parallel to the dip of the zone; and 'n.p.' indicates that the solution is not plo.tted in Figure 3.
The data not plotted are for earthquakes in regions where the structure of the zone is
poorly known or very complex and no clear determination of down-dip stress type can be
made.
In the last column o.n the right, the reference from which the solution is taken is
tabulated
as follows:
BJ
Berckhemerand Jacob[1968]
M
D.P. McKenzie, personalcommunication
CE
Ch
CRE
Constantinescuand Enescu [1962]
Chandra [1970b]
Constantinescu
et al. [1966]
Md
MS
Mendiguren[1969]
Molnar and Sykes[1969]
Fa
Fdch [1970a]
N
New solutionobtainedby us for this paper
Fb
FM
YI
Fitch [1970b]
Fitch and Molnar [1970]
Honda and Masatsuka [1962]
and Honda et al. [1956]
Hodgsonand Cock[1956]
Hirasawa [1966]
Hedayati and Hirasawa [1966]
Ichikawa [1966]
Isacks et al. [1969]
Katsumata and Sykes[1969]
Ra
Rb
Rc
S
St
SB
TB
VP•
Ritsema [1965]
Ritsema [1966]
Ritsema [1969]
Stevens[1966]
Stauder[1958b]
Stauder and Bollinger [1965]
Teng and Ben-Mer•ahem[1965]
Vvedenskayaand R•prechtova[1961]
HC
I-Ii
I-It{
I
ISO
KS
The numbers following these abbreviations in the last column are those emplo,yedby the
authors in each case to identify the particular earthquake.
New Focal-Mechanism
Solutions
The new solutions ('N' in Table 1) are illustrated in appendix plates 1-4 on equal-area
projectionsof the lower hemisphereof the focal sphere.In all casesthe upper and right-hand
side of the projections correspo.ndto north and east, respectively. Filled or solid circles and
triangles indicate compressionalfirst motions, and-• untilled or open circles and triangles
indicate diiatational first motions. Triangles are for data on the upper hemisphere (pP or
P at small distances) projected thro.ugh the center onto the lo,wer hemisphere; circles are
for data on the lower hemisphere. X indicates compressional-wavedata judged to be near
a nodal plane. The direction of first motion of S is shown by an arrow. A small circle,
triangle, or x and a dashedarrow indicate uncertain determinations.
Solutionsfor some of the events listed in Table I as new have also been reported by
other investigators[e.g., $tauder and Bollinger, 1964, 1965; Chandra, 1970a; and Mikumo,
STRESSES IN THE DESCENDING LITHOSPHERE
161
162
ISACKS AND MOLNAR
o
o
o
o
oo
o
•
STRESSES IN THE DESCENDING LITHOSPHERE
163
164
ISACKS AND MOLNAR
STRESSESIN THE DESCENDING LITHOSPHERE
165
166
x <]
ISACKS AND MOLNAR
•
STRESSESIN THE DESCENDING LITHOSPHERE
167
168
ISACKS AND MOLNAR
STRESSES IN THE DESCENDING LITHOSPHERE
169
1969]. In most casesthe agreement of their solutions and ours is excellent and attests to
the reliability of solutionsbased on WWSSN data. The largest discrepanciesare associated
with certain of Stauder and Bollinger's solutions for earthquakesin the Peru-Chile region.
We attribute these discrepanciesto their use of first-motion data for P waves that a•e reported
in bulletins (and are often inconsistent) and their primary, (nearly exclusive in some cases)
reliance upon the polarization of shear waves.
Acknowledgments.Discussionswith E. R. Engdahl, T. Fitch, D. T. Griggs, T. Hanks,
V. Karnik, D. P. McKenzie, J. Oliver, A. R. Ritsema, C. Scho.lz,and L. R. Sykes were
especiallyhelpful. E. R. Engdahl, D. P. McKenzie, and A. Nowroozi kindly supplied data
prior to publication of their work. We thank J. Oliver and L. R. Sykes for critically reading
the manuscript.
This researchwas supportedby NOAA, Department of Commerce, Grants E22-53-69G
and E22-73-70G, and by National ScienceFoundation Grants NSF GA-11152 and NSF
GA-1537. Contribution 1615 from Lamont-Doherty Geological Observatory.
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