JOURNAL OF GEOPHYSICAL
RESEARCH,
VOL. 97, NO. B8, PAGES 11,889-11,903, JULY 30, 1992
The RegionalIntraplateStressField in South America
I•RCELO ASSUMPCAO
Departamento
de Geofistca,
Instituto
Astroni•mico
ß Geofisico,
Universidade
de SaoPaulo,Brazil
A compilationof lithosphericstressdirectionsfor continentalSouthAmerica and the inferredmajor patternsof the regionalintraplatestressfield are presented.Stressorientationsare basedprimarily on earthquake
focal mechanismsand Quaternaryfault slip inversionpublishedin the literature.Four new focal mechanisms
basedon short-periodP wave modeling,and selectedcentroid-moment-tensor
solutions(publishedby U.S.
GeologicalSurvey)consistentwith P wave first motionsat SouthAmericanand otherWorld-Wide Standard
Seismograph
Network stationsare alsoincludedin the database.The observedpatternsof intraplatestresses
may be usefulin constraining
numericalmodelsof the plate drivingforcesin the SouthAmericanplate.In
the Andeanplateau(altitudesgreaterthan 3000 m), N-S extensionalstressespredominate.E-W compressionalstressesare observedin the sub-Andeanand platform regionsextendingup to about 1000 km east of
the Andes.The maximumhorizontal
stress(SHmax)is uniformlyorientedin the E-W directionthroughout
western
SouthAmerica.Averages
of theSHmaxdirection
at gridpointsspaced
2.5ø weretakenasrepresenting the "regional"field. The E-W directionof this regionalfield is not affectedby the changeof strikeof the
Andeanchain nor by the contactof a flat subducted
slab underneath.
The regionalSHmaxdirectionis
oriented15ø more clockwisethan the directionof the Nazca plate convergence.
The differencebetweenthe
regionalSHmaxorientations
andthe absoluteplatemotionmaybe only about6ø. This may indicatethatcontact forceswith the Nazca plate may not be the only major contributorto the intraplatestresses
in western
SouthAmerica.The easternlimit of this Andeanstressprovinceseemsto coincidewith aseismicregionsin
the Upper Amazonbasinand in the Paramlbasin.In the centralAmazonianregion,seismicityand stressdata
suggesta seismicprovincewith N-S compressional
stresses.
The origin of thesestresses
is not yet clearly
understoodbut could possibly be related to lower crustal loading along the middle Amazon basin. In
northeastern
Brazil, seismicityis characterized
by uppercrustalstrike-slipearthquakes
borderingthe Potiguar
marginalbasin. A model is proposedfor this regionin which the stressfield is the resultof a superposition
of regionalE-W compressional
stresses
and local extensionalstresses
(orientedperpendicular
to the continental margin)possiblyrelatedto densitycontrastsand sedimentloadingin the continentalshelf.
INTRODUCTION
been used, coupledwith the gravitationaleffect of the Andean
The stateof stressin the lithospherecan be due to both local topography, to explain the N-S extensional normal stresses
forces (e.g., stress concentrationsdue to structure hetero- observedin the high Andes of Peru and Bolivia [Sebrier et al.,
geneities,crustalloadingand unloading,asthenosphericthermal 1985, 1988; Mercier et al., this issue].The Nazca plate "collianomalies)and regional,more uniform forcesdirectly related to sion" has seemed a natural explanation for any horizontal
plate motion (suchas ridge-push,negativebuoyancyof the sub- compressionin western South America even though other
ducted slab, and viscous shear forces in the asthenosphere- forces,suchas thosein the lithosphere-asthenospheric
boundary,
lithosphereboundary).The tectonicsettingof SouthAmerica is could also be consideredbecausethe absoluteplate motion is
unique:In no otherpart of the world doesa major oceanicplate approximatelyE-W. In fact, Zoback et al. [1989] have shown
subductbeneatha large continentalplate along a trenchalmost that the maximum horizontal stress (SH•,•,) in South America
6000 km long. Due to the large extent and relatively uniform tendsto be roughlyparallel to the absoluteplate motion.
The small difference between the convergenceand absolute
Andean tectonism(compared with other subductionzones) the
regional stress field in continental South America could be motiondirectionsin westernSouthAmerica (about 10ø, usually
expectedto be more uniform than in other smaller overriding less than the accuracyof any single stressdetermination)makes
plateswhere lithosphericheterogeneities,
such as in island arcs, it difficult to separatetheir contributionto the intraplatestress
may mask the effect of plate driving forces.For this reasonthe field. Also, the prominent Andean tectonic featuresrelated to
regionallithosphericstressesin the continentalpart of the over- the Nazca plate subductionseem to have overshadowedthe
boundaryforcesas potentialcontriburiding South American plate may be useful for inferring the lithosphere-asthenosphere
relative importanceof some of the driving forces that cause tors to plate stresses.The amountof stressdata compiledas
part of the World StressMap Projectmay begin to reveal the
plate motions.
In westernSouthAmerica, shallowearthquakesin the upper importanceof other sourcesof stressin SouthAmerica.
In central and eastern South America, few stress data are
plate usuallyhave thrustingmechanismswith P axesroughly in
the EoW direction,approximatelyparallel to the directionof the available.Due to the low midplateseismicityand low density
Nazca plate subduction.For this reason,since the pioneering of seismographicstations, focal mechanismscould be deterwork of Stauder [1973, 1975], it has been common to attribute mined for only a few events with magnitudesabout m•, 5
any compressional
stressesas being causedby the Nazca-South occurred in the last decades. The first focal mechanisms deterAmerica plate "collision" [Sudrez et al., 1983; Chinn and mined in midplate South America by Mendiguren and Richter
with a uniformcompressional
stressfield
lsacks, 1983; Assump½doand Sudrez, 1988]. A horizontal [1978] wereconsistent
compression
due to the convergence
of the Nazcaplate has also orientedroughlyNW-SE. However, the presentlyavailabledata,
althoughnot yet enoughfor a completedefinitionof the midCopyright1992 by the AmericanGeophysicalUnion,
Papernumber91JB01590.
0148-0227/92/91JB-01590 $05.00
plate stresspatterns,do show that the stressfield is not uniform,
suggestingthat local sourcesare important comparedwith
regionalforces.
11,889
11,890
A$$UlV!P•AO:
•ATB
S'rRBSSB•
IN SOUTHAMERICA
STRESS DATA BASE FOR SOUTH AMERICA
The intraplatestressdata for SouthAmerica (Figure 1) come
mainly from earthquakefocal mechanisms
(43%) andgeological
data (50%). Few breakout and in situ measurementsare available in this region. Figure 1 showsthe directionsof the maximum horizontalstress(SH•) throughoutSouthAmerica.This
single stressaxis is plotted to facilitate definitionof regional
stresspatterns;SHm•,,correspondsto the maximum principal
stressdirectionfor thrustand strike-sliptectonicregimesand to
the intermediateprincipal stressdirection for normal faulting
regimes(B axesof the focal mechanismsolutions).
EarthquakeFocal Mechanisms
The focal mechanismsolution of a single earthquakedoes
not give directly the orientationof the principalstressesacting
in thelithosphere,
butonlythedirections
of thatpartof the
1987, 1989; Sophia and Assumpcdo,1989; Assumpc&o
et al.,
1989, 1990] have providedmechanisms
of someeventsalong
the South Atlantic
coast.
All focal mechanisms have been examined and classified into
threequality categories(B, C, and D) accordingto the reliability of the estimateof the maximum horizontal stress,SHm•,,
[Zoback, this issue (a)]. In terms of the P and T directionsof
the earthquakefocal mechanism,the three classesroughly
correspond
to uncertaintiesof + 5ø(B), + 20ø(C) and + 40ø(D).
The highestquality A was only used for stresstensorinversion
usingseveralindependentfocal mechanismsin the samearea.
Figure 2 and Table 1 show four new focal mechanismsof
intraplate events (one near the Peru-Brazil border, one in
Argentina,one in Paraguay,and one near the Chile-Argentina
border)determinedby modelingthe relative amplitudesof the
short-perioddepth phases pP and sP, as describedby
Assumpcdoand Sudrez [1988]. All mechanismsare reverse
stress
released
by theearthquake
[McKenzie,
1969].However, faulting with roughly E-W oriented P axes. These four
experiencehasjshownthat the differencesbetweenthe orienta- mechanisms,
despitethe goodfit of the pP and sP amplitudes,
tionsof the P and T axesof the focal mechanismsolutionand havebeenclassifiedas qualityC becauseof the uncertainties
of
thePrincipal
d•eetions
of thestress
fieldarenotusually
more
the P axis direction.
than•about30ø [e.g., Raleigh et al., 1972]. Also, in areasunder
compression,
theaverage
ofP directions
ofvarious
independent
Harvard
earthquakes
tendto be closeto the S1 direction(maximumprincipal compression)of the regional stressfield [Zoback and
Centroid-Moment-
Tensor Solutions
In order to increase the data base for South America,
Zoback,1980;Zoback,thisissue(b)].
centroid-moment-tensor
solutions(CMT), determinedby HarFocal mechanismdata for South American intraplateearth- vard [Dziewonskiand Woodhouse,1983] and publishedin the
quakes come from three sources:(1) previouslypublished U.S. Geological Survey's EarthquakeData Report, were also
mechanisms
compiledfrom the literature,(2) four new solutions examined. The CMT solution is the result of simultaneous
presentedbelow, and (3) Harvardcentroid-moment-tensor
solu- inversionof long-periodwaveformdata to obtain the hypocentions checkedagainstP wave polarities.The compilationof tral parameters(epicenter,depth, and origin time) and the elepublisheddata did not includeeventsnear the Pacificcoast ments of the seismicmoment tensorof the best point source.
which, dependingon their focal depth, could be associated Body waves with periodsgreaterthan 45 s are generalyused.
(rob 5 to 6) CMT inversionscan
directlywith slip betweenthe Nazea and SouthAmericanplates For moderatesize earthquakes
in the subduetionzone. Few studiesof intraplateearthquakes give resultswhich are quite different from the "correct"nodal
have beenpublishedfor SouthAmerica.Stauder [1973] deter- plane solutions.For example, Assumpcdoand Sudrez [1988]
mined focal mechanisms of 61 Andean events near the central
determineda reversefault focal mechanismfor a 23-kin-deep
and northernChile subduetionzone, but only one earthquake eventin the Amazonbasinusinga gooddistributionof P wave
was a shallowintraplateevent.In anotherstudyof the subdue- polarities together with P wave modeling and found 80ø
fion of the Nazea plate beneath Peru and Ecuador, Stauder difference in the P azimuth between their mechanism and the
[1975] determined40 focal mechanisms,10 of which were shallow intraplate.Sudrezet al. [1983] reexaminedthese10 events
and includedsevenother focal mechanismsin a studyof intraplate tectonicsin Peru, Ecuador,and southernColombia.Chinn
and lsacks [1983] determinedsourcedepthsand focal mechanisms of 28 shallow intraplate earthquakesfrom Ecuador to
Argentina;their data set includedthe same intraplateeventsstudied by Stauder [1973, 1975]. In westernSouthAmerica,additional intraplatefocal mechanismswere determinedfor shallow
events in Peru [Grange et al., 1984; Dorbath et al., 1986;
Carey-Gailhardisand Mercier, 1987; Doser, 1987; Deverchere
et al., 1989] and for the San Juan (Caucete,Argentina) 1977
earthquakes[Karlirish-Cate and Reilinger, 1985]. Focal
mechanisms
of 10 shallow(mostlystrike-slip)eventsin Colombia ;andVenezuelanear the Caribbeanplate [Pennington,1981;
Kafka and Weidner, 1981] were also includedbecauseof the
diffusenatureof the plate boundaryin that regioncharacterized
by a broad zone of internal deformation.East of the Andean
region, Mendiguren and Richter [1978], Assumpcdoet al.
[1985], and Assumpcdoand Sudrez [1988] determinedfocal
mechanismsof 11 midplate eventsin Brazil. Studiesof a daminducedactivity in southeastern
Brazil [Mendiguren,1980] and
someearthquakeswarmsin northeastern
Brazil [Ferreira et al.,
CMT solution (which was also a reverse fault mechanism).This
was probablydue to the instabilityof CMT inversionfor shallow dip-slip earthquakes[Sipkin, 1986; Anderson, 1988]. For
this reason, althoughCMT inversionsare valuable for global
statisticalstudiesand provide a good indicationof the general
style of deformationin an area, careful considerationof first
motion data is still necessaryfor detailed study of source
mechanicsandregionaltectonics[Anderson,1988].
Thirty-nine CMT solutionsfor intraplateevents,from January 1982 to June 1990, were checkedfor consistencywith P
wave polaritiesat World-Wide StandardSeismograph
Network
stationsand some high-gainBrazilian stations.From this comparison,two solutionswere found to be wrong (i.e., the CMT
seemedincompatiblewith the polarity data) and 14 CMT solutions were found to be uncertain(the quality and number of
polaritieswere not enoughto be used as a check of the CMT
solutions).The other 23 events were found to have CM'r solu-
tionsin generalagreementwith the P wave polaritiesand were
includedin the databasewith qualityC (seeFigure3 andTable
2). The two events with inconsistentP wave first motions are
shownin Figure 4. The event 870306C (mr, 5.5) was an aftershockof the mr, 6.5 Colombia earthquakeoccurredon the
sameday: the polarity data indicatethat the mechanismof the
ASSUMPCAO:
•ATE
i
I
STRESSES
IN SOUTHAMERICA
11,891
i
Fig. 6
Fig. 8
Fig. 7
F M .--o--,
G F .--o--BO
.•
IS--o--
-85
Fig. 1. Maximumhorizontal
stress(SHrmx)orientations
in SouthAmerica.I•rge symbols
corresponds
to dataquality'A'
and 'B'; intermediatesize to quality 'C'; smallsize to quality'D'. Only the boundaries
betweenthe majorgeotectonic
units
(Andeanchain, sub-Andeanforede• and platforms)are showntogetherwith the main, deep intracratonic
basins[Departamento Nacional de Producdo Mb•eral, 1978]. PB is Pamaiba basin. Saw-toothed lines mark the axis of the Peru-Chile
trenchand the easternAndeanfrontal fault zone [Pe•mb•gton,1981] which delimitsthe Andeanblock (AB) in Colombiaand
Venezuela.The stressdata are derivedfrom focal mechanisms
(FM), geologicalfaults(GF), b•.akouts(BO), and in siremeasurements(IS).
11,892
A$$UMIK::AO:
INTRAPLATB
STRI•$1• IN SOUTH
aftershock was not much different
from that of the main event
(dip-slip,shallow,reversefault, as shownin Figure3 andTable
2). The polarity data for the event 870913 indicatethat the NE
slrikingnodal plane (Figure 4b) is not correct.Two alternatives
are possible:a strike-slipsolutionwith vertical nodal planesor
a predominantlyreversefaulting with one nodal plane slriking
aboutN-S and steeplydipping to the east. An examinationof
the amplitudesof the depth phasespP and sP favors the dipslip alternativeas presentedin Figure 2d (the polarity misfits
correspondto less reliable data near one of the nodal planes).
Thus the two events with inconsistent CMT
solutions had shal-
low dip-slipmechanisms.
670809
GeologicalData
PERU- BRAZIL
c•
b
810509 ARGENTINA
SJ•
•
12s
In the Peruvianand Bolivian Andes, a large researchprogram to analyze Quaternaryand recent faults has been under
way for many years [Mercier, 1981; Sebrier et al., 1985, 1988;
½abrera et al., 1987; Bellier et al., 1989; Bonnot et al., 1988;
Mercier et al., this issue] with stress directions determined at
about40 sitescoveringmany different fault systems.Thus the
majority of the geologicaldata in Figure 1 are from Peru and
Bolivia. In the Puna plateau of northwesternArgentina (just
southof the sub-Andeanprovincein Figure 1), Allmendingeret
a/. [1989] have determinedstressdirectionsfrom Quaternary
fault slip data in five differentsites.Data setsfrom both PeruBolivia and NW Argentinashow that normal faultingpredominatesin the high Andes, whereasthrustfaults tend to occur in
the sub-Andean and Precordillera regions, a pattern also
observedin the earthquakedata.
Stress directions
have also been inferred
from
sets of fault
striae at some Pleistocenefaults in southeasternBrazil [Riccomini et al., 1989] and alignment of volcanic vents in southern
Chile [Katsui and Katz, 1967; Roa, 1976].
Breakout Measurements
The maximum horizontal stressdirections(SHm•) can be determined from tectonic elongationsobservedin deep boreholes
[e.g., Bell and Gough, 1979; Cox, 1983; Zoback eta/., 1985].
Theseelongationsare causedby shearfracturesof the borehole
wall ("breakouts")due to a redistributionof the stressesaround
the hole; thesestress-induced
breakoutsare observedon opposites sides of the borehole wall in the direction of the minimum
PARAGUAY
850412
•
..............
13s
horizontalstress[Zobacketa/., 1985]. Other boreholeelongationscausedby the drilling processwhich are not relatedto the
crustalstresses
can be distinguishedfrom the tectonicbreakouts
throughcarefulanalysisof the boreholelogs [Cox, 1983; Plumb
CHILEARG.
870913
and Hickman, 1985].
Breakoutshave been increasinglyusedto estimatethe crustal
stressdirections[e.g., Zoback and Zobaclg 1989; Zoback et al.,
1989]. Unfortunately,few breakoutmeasurements
are available
in South America.
Breakouts
from five oil wells in Colombia
[Castillo and Mojica, 1989] in the region of the CaribbeanSouth American plate boundary(Figure 1) are generallyconsistent with focal mechanism data. Cox [1983] published
breakout orientations
for two wells in the Bolivian
sub-Andes
and one in Guyana. In northeasternBrazil, two breakouts(C.
Lima, written communication,1990) ranked with qualifies C
Fig. 2. Additionalfocal mechanismsolutionsof intraplateearthquakes andD, complementthe focal mechanismdata.
(Table1) obtained
by modelingthe amplitudes
of theP, pP, andsP
phases.Upperand lower tracesare observedand syntheticseismograms, In Situ Stress Measurements
respectively.In the equal-areaprojection,lal'se and small symbols
denote more and less reliable polarities. Crosses and squaresare
compressional
anddilatadonal
P wavefirstmorions,respectively.
The
signal near the beginningof the time scale representsthe sourcetime
funcion.Numbersare eventdate (year,month,day).
Only two direct measurements
of stresshave beencompiled,
one hydraulicfracturingin centralBrazil and an overcoringin
northeastern
Brazil (Figure 1). In centralBrazil, measurements
A$$Ubfi:'CAO:
INTRAPLA•S'TRBSSr•
IN SOUTHAlVfiLRICA
11,893
TABLE 1. FocalMechanismParameters
of the Earthquakes
in Figure2
Date
Aug. 9, 1967
May 9, 1981
April 12, 1985
Sept 13, 1987
Time
UT
0714:08
0950:40
1434:57
2008:52
Latitude Longitude Depth ms
deg
deg
km
-8.45
-26.57
-23.94
-34.33
-73.83
-64.90
-60.55
-69.97
42
15
21
13
5.1
5.0
5.3
5.8
Azimuth/Plunse,
del[
P Axis
T Axis
262/00
108/07
259/06
51/10
169184
227/76
4/68
303160
Peru-Brazil
Argentina
Paraguay
Chile-Argentina
,
821119
8110603
8110605
850322
850126
8501t 10
850726
8508
8601t05
860619
860811
Fig. 3. Harvardcentroid-moment-tensor
solutions(bestdoublecouple)consistent
with polaritydataas recordeat
by WWSSN
and SouthAmericanstations.Crossesand squaresare compressional
and dilatatioualfirst motions;smallersymbolsdenote
lessreliablepolaritydata.Identificationnumbersare the earthquakedatesas shownin Table 2.
with hydraulic fracturing were made in two nearby shallow information
wasavailableon localtopography
at thesite.
wells (100 and 150 m deep) drilled in granitefor engineering is greaterthanthe lithostatic
pressure,
implyinga compressional
studiesat a dam site [Captoni and Armelin, 1990]. The average state of suess in that area. In northeastern Brazil, several
NW-SE direction of the two orientations (with 25ø difference) overcoringmeasurements
were madein an underground
mine at
was given a C qualitybecauseof the shallowdepthsampled;no depthsrangingfrom 280 to 490 m [Cipriani, 1990]. The result-
11,894
Asstnvl•no:
Isrmm•nTE
STR•S• IS SottinAlvinatari
8'70322
8'70922
8801i ! 2
8903 ! 6
8901i ! 5
ß
69050q
p
P
90050
Fig. 3 (cont.).
ing magnitudes
and orientations
had large uncertainties
and an averageE-W to SE-NW orientedSHm• seemsto be present
werevery scattered;
the five bestresults(with magnitude
errors in the Andean block which would be consistent with the ESE
lessthan40%) gavean averageazimuthfor SHm•,,of N64øE motionof the Caribbeanplate relativeto the SouthAmerican
plate[DeMetset al., 1990]asshownin Figure6.
(+27ø).This was given a D quality.
STATE OF STRESSIN THE ANDEAN REGION
Northwestern
South America
Stress Patterns in Western South America
Throughout
the Andeanregion,from about3øN to 40øS,
SH• is orientedpredominantly
E-W. The histograms
of Figure
5a comparethe SHm• orientations
with the directionof the
SouthAmericaabsoluteplate motion,whichis essentially
E-W
[Minsterand Jordan, 1978]. The geologicaldatagenerallycluscatedin Figure 1. Deformationis accommodated
by mainly ter aboutthe local absoluteplate motionazimuth;however,the
strike-slipmotionon many differentfaults.Despitethe few orientationsshow a secondarypeak roughly oriented N-S
to upperPliocene-lower
Pleistocene
faultsfrom
availablestressdatafor a detaileddescriptionof the stressfield, corresponding
The southernlimit of the Caribbeanplate in the region of
Panama,Colombia,and Venezuelais characterizedby a broad
zone of internal deformation[Pennington,1981; Karlcaand
Weidner,1981]. This zoneis called "Andeanblock"andis indi-
ASSUMIn7AO:
INTRA•ATE STRF3SF_.S
IN SOUTHAMERICA
11,895
TABLE 2. FocalMechanismsDeterminedby Harvard(Centroid-Moment-Tensor
Solutionas
Published
in the Earthquake
DataReportsof the U.S. GeologicalSurvey)Compatible
with P Wave First Motion at South American and WWSSN
Date
Time
UT
Latitude Longitude Depth mb
deg
deg
km
Stations
Azimuth/Plunge,
deg
P Axis
Nov.11,1982
0427:14 -10.61
-74.69
14'
6.3
June 3, 1984
June5, 1984
0410:27
0415:24
-76.78
-76.71
34
25
5.3
5.8
92/12
91/26
224/72
251/62
Jan.26, 1985
0306:58
-33.07
-68.58
15'
5.8
86/09
325/72
Argentina
March 19, 1985
March 22, 1985
April 10, 1985
1028:36
1402:41
2015:37
-18.60
-18.63
+1.56
-63.58
-63.56
-77.02
19
9
10
5.4
5.4
5.3
89/07
271/06
113/79
307/81
21/73
342/07
Bolivia
Bolivia
July 26, 1985
Aug. 13,1985
April 5, 1985
1756:58
0529:19
2014:29
-5.38
-15.04
-13.41
-78.65
-75.47
-71.78
18
25
51
5.3
5.4
5.3
77/14
262/11
152/70
274/75
124/76
14/15
June 19, 1986
2157:24
-16.94
-65.43
20
5.4
226/01
320/79
Aug.11,1986
Feb.3, 1987
2206:43 -30.92
1642:41 -37.75
-67.68
-72.89
25*
32*
5.4
5.8
50/20
263/29
155135 Argentina
80/61 Chile
March 6, 1987A
March 6, 1987B
0154:50
0410:42
-77.65
-77.82
14
10
6.1
6.5
86/27
100/19
244/61
268/71
March22, 1987 0323:58
-7.80
-7.82
+0.05
+0.15
-24.06
252/14
Region
T Axis
24/70
-70.14
40*
5.8
271/23
Sept.22, 1987
Nov. 15, 1987
April 12, 1988
1621:35
2200:50
1526:20
-1.08
-9.43
-2.79
-78.13
-75.66
-77.65
10
32
27
5.9
5.4
5.5
80/09
95134
281/12
190/63
262/55
162/66
38/55
March 16, 1989
1712:22
-16.93
-65.00
44
5.1
48138
217/52
April 15, 1989
May 4, 1989
May 30, 1990
1426:41
1030:07
0234:06
+8.45
-6.61
-6.02
-61.04
-75.76
-77.23
23
36
24
5.8
5.5
6.1
20/51
57/12
73/24
149/28
287/72
223/63
P•ru
Peru
Peru
Colombia
Peru
Peru
Peru
Bolivia
Colombia
Colombia
Chile
Ecuador
Peru
Peru
Bolivia
Venezuela
Peru
Peru
See Figure3.
*Depth
calculated
withpPphases
bytheauthor
orbyU.S.G.S.
southernPeru (about 14øSin Figure 1). This may be due to a
changeof SHmaxdirectionduringthe Quaternary[Bonnotet al.,
1988; Mercier et al., this issue].Figure5b showsSHm• orien-
convergencerate of the Nazca plate). From late Quaternaryto
presentan intermediatesituationresultsin N-S extensionin the
High Andes and E-W compressionin the sub-Andes.
tations in western South America from both focal mechanisms
It is interestingto note in Figure5a that the recentgeological
andrecentgeologicaldata (i.e., excludingthe upperPlioceneto data have an averageSHm• orientationabout 8ø more clocklower Pleistocene
data) separatedinto two classes:reverseand wise than data from focal mechanisms.
This could be explained
normalfaultingregimes.In the high Andes(elevationsgreater by the different distributionof the two data sets:the geological
than 3000 m), normal faulting tendsto prevail with extension data cover mainly the High Andes of Peru and northernBolivia
usuallyin the N-S direction (E-W SHm•) which has been inter- where the Altiplano trends roughly in the NW-SE direction,
pretedas due to the topographical
load of the high Andesover- whereas the focal mechanism data are not restricted to that
comingthe regionalE-W compression
making S1 vertical and region (Figure 5b). As mentionedabove, the plateau-induced
S2 E-W [Sebrier et al., 1985, 1988; Froidevaux and Ricard, extensionalstresseswould be perpendicularto the NW-SE axis
1987;Mercier et al., this issue].Reversefaultingtendsto occur (north of 18øS) somewhatoblique to the E-W compression
at the lower altitudesin the sub-Andes.In Figure 5b the few
focal mechanisms
availablein the high Andesreflectthe lower
seismicityof the Altiplano-Punaplateau[Sudrezet al., 1983].
In a high plateau such as in the Andes, extensionalstresses
may developperpendicular
to the axisof the plateauas a result
of the combinedeffectof thehightopography
and'thebuoyance
of the compensatingcrustal root [Fleitout and Froidevaux,
1982; Froidevaux and Ricard, 1987]. These stressestend to
causenormalfaultingwith SHm• parallelto the axisof the plateau. The resulting tectonicregime dependson the balance
betweentheseplateau-induced
stressesand the compressional
stressesdue to the plate convergence.Bonnot et al. [1988] and
Mercier et al. [this issue] have shown that this balance has
Fig. 4. Centroid-moment-tensor
solutions(best doublecouple)inconsistentwith polarity data as recordedby WWSSN and other South
the Pliocene,E-W to NE-SW extensionoccurredin the High American stations.Crossesand squaresare compressionaland dilataAndeswith the apparentdominanceof the topographical
effect tional first motions;smallersymbolsdenoteless reliablepolarity data.
over horizontalcompression
beinginterpretedas associated
with (a) Event of March 6, 1987, 0814 UT, occurredin Colombia with rnb
a low convergence
rate of the Nazcaplate.In early Quaternary, 5.5; this was an aftershockof the main event with m• 6.5 at 0410 UT
on the sameday (event "870306B" in Figure 3). (b) Event of Sepmajor E-W compressionaldeformation(and also minor N-S
tember 13, 1987, 2008 UT, occurrednear the Chile-Argentinaborder
trendingcompression)was observedin the whole Andes and with m• 5.8; the preferredfocal mechanisra
solutionis shownin Figure
varied since the Pliocene in the Andes of Peru and Bolivia.
In
interpreted
as due to predominance
of platecontactforces(high
2d.
11,896
ASSUM•AO: IlgrRAPLATESTRESSES
IN Solrill AMERICA
related to the Nazca plate convergence.Superpositionof these
two sourcesof stresscould result in an SHm• orientationin the
High Andes with a slight tendencyto be alignedalongthe axis
of the Altiplano. Alternately,the 8ø differencebetweenthe geological and focal mechanismdata may be a result of local
effects in the areaswith high concentrationof geologicaldata
suchas in the Cuzco area of Peru (Figure 1).
In addition to the changes of stress orientationsduring
Quaternarytimes, structuralheterogeneities
may be expectedto
causesome additionalscatterin SH•,• orientation.For example,
beneaththe high peaksof the easternflank of CordilleraBlanca,
in central Peru, the stress tensor determined with microearth-
quakesindicatesa normal faulting regime with $W-NE extension [Deverchereeta/., 1989], whereasthe major fault planeof
Figure 6 showsthat the orientationof the "regional"SHm•
(i.e., the grid point average)is remarkablyconstantin western
SouthAmerica. The changeof strike of the Andeanchain and
the Peru-Chiletrench(from roughlyN-S southof 18øSto NWSE north of 18øS) apparentlyhas no significanteffect on the
directionof the regionalfield. A comparisonof the orientations
of this regional field with the directionsof relative motion
betweenthe Nazca and SouthAmericanplates (calculatedwith
the NUVEL1 pole of DeMets et al. [1990]) showsthat the
observeddirectionsare about15ø more clockwisethanthe plate
convergencedirection(Figure 6b, histogram"N"). Taking into
accountthe 95% confidencelimit of the NUVEL1 pole that
differencecould vary between 12ø and 18ø. If the relative
the Cordillera Blanca shows N-S extension, consistent with the
regionalstressfield, as indicatedby the most recentfault striae
[Sebrier et al., 1988; Bonnot et al., 1988]. Mercier et al. [this
issue] suggestedthat the presence of a granitic batholith
changesthe stressfield locally so that the focal mechanismsof
the microearthquakesin or beneath the batholith are not
representativeof the regional field.
RegionalStressField in WesternSouthAmerica
Scatter in SHm• orientation due to local heterogeneities
needsto be averagedout to emphasizethe directionof a more
uniform, regional component.This was done in Figure 6a by
taking the averageorientationaroundgrid pointsspacedevery
2.5ø. For each grid point all data within a searchradiusof 2.5ø
was used. Besidesweighing the data accordingto the quality
criteria (full weight for quality A, 0.7 for B, 0.35 for C, and
0.18 for D), linearly decreasingweight was assignedfor distancesbetweenhalf and one searchradius.The averageorientation was calculated
with
-10
the sum of the vectors with double
angle [Davies, 1986]. The standarddeviationof the orientations
was then used to assign a quality to the grid point average.
Most of the standard deviations (25 out of 31) were less than
25ø (largebarsin Figure 6a).
-18-
3Or I•IUPPER
PLIOCENE-F•
io
L ! .....
-90
-60
-30
0
30
60
..,,.,,,
90
ao [•1 D
,o
- 90
FM
-60
- 30
0
30
60
,•GF
90
•
Fig. 5b. SHm• orientations
for westemSouthAmerica.The contour
line is the 3000 m elevation.Solid and open symbolsdenotecompressional stresses(S1 horizontal) and normal stresses(S1 vertical); strikeFig. 5a. Distribution
of theSHmaxorientations
in westemSouthAmer- slip earthquakesare indicatedby solid circles as well. Circles and
ica with respect to the absolute plate motion [Minster and Jordan, squaresrepresentdata from earthquakefocal mechanisms(FM) and
1978]. A positiveanglemeansthat the observedSHmaxdirectionis recent geological fault slip (GF), respectively. Only recent (Midorientedmore clockwisewith respectto the directionof absolutemotion. Pleistoceneto PresenO geological data are shown. Large symbols
Top histogramshowsgeologicalfault-slipdata (GF); bottomhistogram representdata with qualitiesA and B; intermediatesize qualityC; small
showsfocal mechanismdata (FM), with qualityD datadiscriminated.
sizequalityD.
DEVIATION
ANGLE
{o)
ASSUMI•AO:INTRAI•ATESTRESSES
IN SoLrrHAMERICA
motion betweenNazca and South America is taken only from
the slip vectorsof shallowthrusteventsin the subductionzone
("best fitting" pole of DeMets et al. [1990]), then the regional
SHm• orientationsare 11ø more clockwise instead of 15ø. So
the consistent,clockwise difference between average SHm•
directions and the Nazca-South America plate convergenceis
small but probablysignificant.If the observedregionalfield is
comparedwith the absoluteplate motion (using the AM1-2
poles of Minster and Jordan [1978] based on the hot spot
frame), an averagedifferenceof only 6ø + 11ø is found (Figure
6b, histogram"A"). This may be an indication that asthenospheric forces have an important contributionto the state of
su'essof the overlying plate. However, becauseof the large
standarderrorsin the pole for absolutemotion, the closercorrelation of the regional SHm• with the absolutemotion may not
be significant.In fact, usingthe pole NUVEL1-HS2 for absolute
motion,more recentlydeterminedby Gripp and Gordon [1990],
a difference
of 13 ø + 11 o is found.
Contourlines of the Wadati-Benioffzone (i.e., averagehypocenlxaldepths)in the subductingslab were compiledbasedprimarily on the Ixendsurfacesof Bevisand Isacks [1984] comple-
0 -
-5-
-10-
-15-
-20-
11,897
mented with more detailed data for Colombia [Pennington,
1981], southern Peru [Grange et al., 1984; Schneider and
Sacks,1987], centralChile, and northwestemArgentina[Sinailey and lsacks, 1987; Pardo and Fuenzalida, 1988]. The subductingplate is subhorizontalin northernand centralPeru (from
about 3øS to 14øS) and near the San Juan region of
northwestemArgentina (from about 28øS to 33øS). If we consider that the hypocenterstend to occur, on average,about 1020 km beneath the top of the subductingslab ISmalley and
Isacks, 1987] and that the thicknessof the overlying lithosphere
shouldbe about 100 km or more, we could expect that west of
the 140 km depthcontour,the SouthAmericanplate is in contact with the underlyingplate (or there is very little asthenospheric material in between). However, the orientation of the
regional SHn• in areas of presumedplate contact does not
appearto be significantlydifferent from areasto the east where
the South American plate directly overlies the asthenosphere
(Figure 6b].
The E-W direction of the regional SHm• field in western
South America is remarkablyconstant.It does not seem to be
significantlyaffectedby the changeof slxike of the Andes nor
by the presence of a fiat slab underneath. The difference
between the observedorientationsand the Nazca plate convergence direction is small but probably significant.This means
that the Nazca plate "collision"does not seem to be the only
direct conlxibutorto the lithosphericstressesof the overlying
South American plate. Besides plate boundary forces, lithospheric density heterogeneitiescan give rise to large tectonic
slxesses.Fleitout and Froidevaux [1982] showed that a heavy,
cold lithosphericroot can induceregionalcompressireslxesses
in the uppercrust.Also, the downpullof the dipping,denseslab
could producecompressionalslxessesin the upper lithosphere
associatedwith downflexingof the upper plate [Bott et al.,
1989]. However, it is not clear how these other sourcesof slxess
would produce a regional field which is independentof the
strike of the Andean plateau and the subductedslab. The state
of slxessin the lithospheredoes not dependonly on the forces
acting at its nearestplate boundarybut also on the balanceof
the forces actingon all its borders.Therefore,only numerical
experiments[e.g., Richardsonet al., 1979; Stefanickand Jurdy,
this issue; Meijer and Wortel, this issue] can show the combined effect of other forces such as ridge-pushand asthenosphericshearslxesses
and help understandthe origin of this uniform su'essprovincein westernSouthAmerica.
N
A
15-
-25-
IO-$0-
I
-35
-g0
-,s
-,0
-ds
-g0
Fig. 6a. RegionalSHIn,,,orientations
calculated
astheaveragedirection
15
,
,
25
35
-15
-5
5
15
25
over B.Z. < 140 Km deep
aroundgrid pointsspaced2.5ø apart.Large and intermediatetracesindicate standarddeviationsless than 25ø and 40ø, respectively.Small symbols correspondto standarddeviationsin the range 40o-50ø (note that
the value for a random distributionis 52ø). The Benioff zone is indicatedby the 80- to 140-km isodepths.The "N" and "C" arrowsindicate
the directions of convergenceof the Nazca and Caribbean plates
regionalSHmax (Figure 6a) and the directionsof the Nazca-South
America convergence(histogram"N") and the absoluteplate motion
[DeMets et al., 1990]; the "A" arrows indicate the South America absolute motion [Minster and Jordan, 1978].
(histogram"A"). Hachured data correspondto grid points above a
Benioff zone deeperthan 140 km.
over B.Z. > 140 Km deep
Fig. 6b. Distributionof the differencesbetweenthe orientationsof the
11,898
ASSUMIK2AO:
I.NTRAPLATE
STRESSES
IN SOUTHAMERICA
STATE OF STRESS AND SEISMICrrY
axis of the Amazonian basin as due to loading effects of dense
materialintrudedin the lower crustas evidencedby a chain of
gravity highs (+40 to +90 mGal) along the axis of maximum
IN MIDPLATE SOUTH AMERICA
Northern
Amazonian
Craton
thicknessof the basin [Nunn and Aires, 1988]. The two reverse
With the presentlyavailabledata it is not possibleto deter- faultingeventsin the centralpart of the Amazonbasinoccurred
mine the exact eastern limit of the E-W orientedSHm• proat 23 and 45 km depth [Assmnpc•oand Sudrez,1988]. Numerivince in westernSouth America. At about 20øS this stresspro- cal modelingby Richardsonand Zoback [1990] indicatedmaxvince may reach abouthalf the continent(Figures1 and 6a). At
imum compressionaldeviatoric stresses,of the order of 100
lower latitudes,two reversefault earthquakes
with N-S oriented MPa, at midcrustaldepths,consistentwith the observeddepths
P axes in the middle of the Amazonian basin and one break-out
of the two earthquakes.
measurement
in Guyana(Figures1 and 7) suggestthe existence
of a different stressprovince in the north central part of the SeismicityAroundthe Parand Basin
Amazonian craton. All epicentersfor events with mr, greater
Figure 8 showsall epicenterswith magnitudesabovemb 3.0
than 3.5 are shown east of the dashedline in Figure 7. These
in and around the intracratonic Paleozoic Paran• basin. The
seismicitydata were taken from the catalogof Berrocal et al.
data from the
[1984] coveting up to 1982, complementedwith the Brazilian seismicitydata includehistoricaland •nstrumental
SeismicBulletins (1984-1990) up to 1989. The completeness same catalog used for Figure 7. Completenessthresholdsare
thresholdof this data set is about mr, 4.5 since 1963 (from the
worldwide network) and about mr, 3.5 since 1980 with the
regional seismic stationsin the Amazonian region. The stress
data showingN-S SH• orientationare locatedwithin a cluster
of epicenterswhich are separatefrom the sub-Andeanseismicity
and which may characterizea different seismicity/stress
province in the Middle Amazonianbasin and the centralpart of the
Guyana shield. Figure 7 showsthat betweenthe sub-AndeanEW compressive stresses and the central Amazonian N-S
compressire stress province there is a region of very low
seismicity. This may indicate that the transition from the
western South America stressprovince to the central Amazonian stressprovince occursby a decreaseof the stressmagnitudesandnot just a changeof orientation.
The origin of theseN-S stressesin the Amazoniancraton is
not known yet. Althoughthe relative motionbetweenthe Caribbean and South American plates has a small N-S component
[DeMets et al., 1990] the Caribbean border seems to be too far
(Figure 1) to be the main sourceof stressin the centralAmazonian region. Zoback [this issue(a)] and Richardsonand Zoback
[1990] interpretedthe compressirestressesperpendicularto the
about mb 4.5 since 1963 (with the worldwide network) and
aboutm• 3.0 since 1978 with the regional seismicstationsin
southeastern
Brazil. The largesteventin Figure 8 was mr, 6.3 in
1955 (at 12.4øS,57.3øW). The Paran• basin is characterizedby
very low seismicity.West of the basinthe focalmechanisms
are
consistentwith the E-W compresslyestressprovinceof western
South America.
North
and east of the basin the available
are rather scatteredto define any pattern in the stress field
except to say that horizontal compressionpredominates.The
focal mechanisms"f" and "g" refer to shallow (1-2 km) daminduced activity. Quaternaryfault slip data near the Atlantic
coasthave shown both extensionaland compressional
faulting;
the extensionalfaulting being interpreted as relaxation of
compressionalcycles [Riccominieta/., 1989]. If the scatterof
the stresses
east of the Paran•ibasinis not just a resultof poor
quality data, then local sourcesmay predominanceover plate
wide forces.
It is not known why the imerior of the Paran•ibasin is much
lessseismicthan its borders.Gravity and isostaticstudiesin the
northern part of the basin have indicated that lower crust
densification(by upper mantle intrusions),rather than crustal
-2
-6
-IO= 76
-72
-6B
-64
data
-60
-56
-52
-48
Fig. 7. Seismicityand stressdata in the Amazonianregion. Epicenterseast of the dashedline cover the period 1950-1989.
Symbolsizesdenotemagnitudesrangingfrom mr, 3.5 to 5.5. Epicentersare completefor magnitudes
aboveaboutmr, 4.5
since1963 and abovemr, 3.5 since1980 (seetext). West of the dashedline only stressdirectionsare shown(samedatapoints
asin Figure1). Convergent
anddivergent
arrowsaretheP andT axes,respectively.
Forreverse(or normal)faultingearthquakes,
onlytheP (or T) axesare shown;for strike-slip
events,bothP andT areshown.The hachured
areain theMiddle
Amazon basin representspositiveBougueranomalieswhich roughly correspondto the locationof lower crest densematerial
[Nunn and Aires, 1988]. SA is the sub-Andesof Peru; CA and EA are the centraland eastemCordillerasof Colombia.
ASSUMI•AO:INTRAPLATE
STRESSES
IN SOLrFH
AMERICA
i
I
11,899
I
I
I
-2
/
/
//
/
ß
/
/
-4
/
/
.
PA RNAI'BA
BASIN
-6
d"26
.ß•
•
-3oe>•> •,
-62
-58
-5q
-50
lines indicate
SHmax
30 - 40 mb
ß
41 - 52m b
I
-IO
-44
-q6
-q2
Fig. 8. Seismicityin and around the intracratonicPaleozoicParaml
basin. The dot-dashed
BO
ß
the Asunci6n
arch and the West
Pampeanarch which separatethe Paramlbasin from the Chacosbasin.
Solid dots are historicaland instrumentalepicentersfrom 1861 to 1989
from the Brazilian catalog(Berrocal et al. [1984] and Brazilian Seismic
Bulletins, 1984-1989). Symbol sizes denote magnitudesranging from
m/, 3.0 to 6.3. Completenessthresholdsare about m/, 4.5 since 1963
and m/, 3.0 since 1978. Focal mechanisms(lower hemisphere)have
dark compressional
quadrantsfor solutionswith qualityC and hachured
for qualityD. Divergentand convergentarrowseastof the Paramlbasin
denoteextensionaland compressional
stresses
from Quatemaryfault slip
-42
-40
-38
-36
-34
-32
Fig. 9. Seismicityand focal mechanismsin northeastemBrazil. Historical and instrumentalearthquakedata (from 1808 to 1989) taken from
the Brazilian catalog(as in Figures7 and 8) are completeabovern• 4.5
since 1963 and above rn• 3.0 since about 1978. Maximum and
minimum horizontal stressesas deducedfrom the earthquakefocal
mechanisms
are indicatedby convergentand divergentarrows:for solu-
tions"a" and "b" they are the P andT axes;for solutions
"c" and"d"
(with identifiedfault plane, "F") they are plottedat 30ø and 60ø to the
fault plane,insteadof 45ø. The continentalshelfis shownby the 200-m
and 2000-m bathimetry.
was found in the Jo• Cftrnaraarea nor at any other epicentral
area of northeasternBrazil. However, evidenceof strongHolocene
deformationhas been inferred from tilted coastal peat
[Riccomini et al., 1989]. The stressdirection at about 14øS was obtained
depositsabout 50 km northeastof the Jo•o C•_rnaraepicentral
with hydraulicfracturing[Caproniand Armelin, 1990].
area [Gusso and Bagnoli, 1989]. It should be noted that the
seismicity observed in Figure 9 occurs mainly in areas of
thinning,is a preferredmodel for the basin formation [Molina
exposedPrecambrianbasementor at the edgesof the Potiguar
et al., 1988]. However, free-air anomaliesin the northernpart
basin. No event greater than rna 4.0 has been observedsouthof
of the basinare usuallynegative(average-20 mGal), indicating
6øS, althoughno obviousdifferenceis found in the Precambrian
that the lower crustal densificafiondoesnot compensatefor the
geologicalfeaturesnorth and southof this line. However, there
thicknessof the low-densitysediments.Thus, the resultingcruseemsto be a changeof strike of structuraltrendswithin the
stal mass deficit in the basin, as comparedwith neighboring
Precambrianbasement:north of 6øS the predominanttrend is
areas, may cause extensional stressesin the upper crust
SW-NE, whereasto the south, E-W directionsseem to prevail
[Fleitout and Froidevaux, 1982] which might balance the
[Assump½cio
et al., 1985, Figure 5].
regionalcompressional
stresses.The southernpart of the Paran•i
Four
focal
mechanismsavailable for this area consistently
basin,however,doesnot shownegativefree-air anomalies.
show
strike-slip
motions with a small componentof normal
Whateverthe causeof the aseismicityof the Parangbasin,it
faulting
(Figure
9).
For two of these mechanisms,the fault
is interestingto note that the E-W compressional
stressprovince
of westernSouthAmerica seemsto be limited here also by a plane could be clearly identified by the trend of hypocenters
determinedin microearthquake
surveys(nodalplane "F" in Figregionof low seismicitysimilarlyto the Amazonianregion.
ure 9), and the SHm• estimate was taken at 30 ø to the fault
Northeastern
Brazil
Earthquakesin northeasternBrazil tend to occur aroundthe
Potiguar Mesozoic sedimentary basin (Figure 9) with focal
depthsusually less than 10 km [Assumpcaoet at., 1989]. The
maximumobservedmagnitudein this region was rnb 5.2 (event
b in Figure 9). Earthquakesin this region tend to occur in
swarmslastingfor many monthsor even years.The best documentedseriesis the Jo•o Camaraearthquakesequence("event"
d in Figure 9) which startedin August 1986, had two main
events(November30, 1986, and March 10, 1989, both with rnb
5.0) and is still active with magnitudesup to rn, 3.0 in 1991).
The eventsof the Jo• C•nara sequencedefine a 30-km-long
rupturestrikingSW-NE with maximumdepthof about8-10 km
[Ferreira et al., 1987; Takeyaet at., 1989]. No surfacerupture
plane. Only two breakoutmeasurements
(qualitiesC and D) are
availablein this region from boreholesin the Potiguarbasin.
The
stress orientation
inferred
from
breakouts
in the offshore
well was not well constrained(only 38 m total length).The five
reliable estimatesof SHm• in the Potiguarbasin region (four
focal mechanismsand the onshorebreakout)have an average
orientationof 102ø (with standarddeviation of 26ø). The direction of the absoluteplate motionin this part of the plate is 83ø
[Minster and Jordan, 1978] and the Mid-Atlantic ridge-push
direction,usingthe NUVEL-1 rotationpole betweenAfrica and
SouthAmerica[DeMetset al., 1990], is 89ø. Althoughonly five
measurements
are not enoughto definethe regionalstressfield,
it seemsunlikely that the strike-slipmotionsobservedin Figure
9 could be causedsolely by either of those two plate-wide
sourcesof stress(asthenospheric
shearstressesand ridge-push)
11,900
Ass•^o:
I•m•a,u^• STreSses• SOUTHA•rauc^
especiallytaking into accountthat most focal mechanisms
have
a smallcomponentof normalmotion.
The T axes (roughlyequivalentto the least principalstress
direction,S3) of the four earthquakes,and the Sh•andirection
from breakoutsin the onshorewell, are all orthogonalto the
northern coastlineand continentalmargin, suggestingthat the
horizontalextensionalcomponentcouldbe due to stresses
generated by the continent-oceanstructuraltransition.Bott and
Dean [ 1972] have shownthat the lateral variationin the density
structureacross continentalmargins can produce extensional
stresses(30-40 MPa) perpendicularto the coasfiineon the continentalside (and smallercompressional
stresses
on the oceanic
side). The continentalshelf in northeasternBrazil is very narrow (Figure 1), which may be an indication of a sharp
continent-oceantransition.In addition to lateral density variations, sedimentloading at the continentalshelf and continental
rise coupledwith subsidenceof the coolingoceaniclithosphere
also causesextensionalstressesin the upper part of the continentalcrust.Cloetinghet al. [1984] modeledthe flexureof a
100 Ma oceaniclithospheredue to a load of sedimentswith 8
Someproblemsstill need to be clarified for a better understandingof the seismicityin the Potiguarbasinregion.If this
model of superpositionof local and regional stressesis
appropriate,seismicityshouldoccur along the whole northern
coast. The intracratonicPaleozoic Parnaiba basin (Figure 9),
however, seems to be aseismic like the Paleozoic Paran•i basin
[Ass,um•½&o,
1989].
Highheat
flowvalues,
greater
than
100
mW/m, are commonlyobservedin northeastern
Brazil, eastof
the Parnaibabasin [Carneiro et al., 1989]. This high heat flow
may indicatea reducedthicknessof the brittle u•
crustwith
a resultingconcentrationof stresses.The extensionalstresses
induced by lateral crustal density contrast [Bott and Dean,
1972] are an orderof magnitudesmallerthanthosepredictedby
sedimentloading of an elastic lithosphere[Cloetingh et al.,
1984]. However, slow sedimentation rams on a viscoelastic
lithospherecan reducethe flexuralstresses
by an orderof magnitude[Steinet al., 1989]. Flexureof the lithospheredue to sediment loadingat the continentalshelf and rise would alsocause
extensional stresses in the oceanic upper crust, but no
significant seismicity has yet been detected offshore in
km maximum thickness and obtained extensional stresses in the
northeastern
Brazil (Figure 9). Probably,variationsof the theouppercontinentalcrustof about270 MPa a few hundredkilom- logicalpropertiesof crustalmaterials[Fleitout and Froidevaux,
1982; Stein et al., 1989] and the presenceof weak zonesin the
eters from the edge of the continentalshelf. If allowanceis
made for relaxation of stresseswith faulting at the ocean- uppercrust[Zoback,this issue(b)] are alsoimportant.
Zoback [this issue (b)] showed that most intraplate earthcontinentboundaryduring the evolutionof the marginalbasin,
the extensionalstressesin the upper continentalcrustmay be quakesseemto occur in zonesof weakness(preexistingfaults)
in responseto the ambientcrustalstresses.
This would give supsmaller, about 100 MPa [Turcotte et al., 1977], but still
et al. [1985] that in the
significant.Perhapsa superposition
of theselocal extensional port to the observationof AssureperVo
stresseswith a regionalE-W compression(either due to ridge- more seismic area (north of 6øS) the structuraltrend is SW-NE,
more favorableto rupturing,whereasin the aseismicarea to the
push or to asthenospheric
drag) could produce the type of
stresses
which inducestrike-slipmotions(i.e., S1 roughlyparal- south the structural trend is oriented E-W, parallel to the
lel to the northerncoast and S3 orthogonalto it). This simple presumedregional stress,inhibiting slipping.However, both in
model would also explain why the seismicityin northeastern Jo•o Cfunara(epicenter"d" in Figure 9 [Takeyaet al., 1989])
and in Palhano(epicenter"c" [AssureperVo
et al., 1989]) the rupBrazil is muchreducedalongthe N-S trendingcoast(Figures9
tures
occurred
in
planes
not
related
to
any geological fault
and 10): in this regionthe presumedE-W regionalcompression
mapped at the surface,which indicatesthat more detailed stuwould be balancedby the local extension(also roughlyE-W)
due to lateral densitycontrastsand sedimentloadingat the con- dies are necessaryto understandthe relation between seismic
tinental shelf.
"zonesof weakness"and surfacegeologicalfeatures.
Fig. 10. Model of superposition
of local and regionalstresses
in the uppercrust.Openarrowsdenotelocalextensional
stresses
causedby lateral densitycontrastsand sedimentloadingat the continental
margin.Solid arrowsdenoteregional
compressional
stresses
(possiblyrelatedto platedrivingforces).Area "A", higherseismicactivitywith stresses
inducing
strike-slip
faulting;area"B",lowerseismic
activitywithlocalandregionalstresses
balancing
eachother.
ASSUMP•AO:
INTR•A•
STRESSES
IN SOUTHAMERICA
11,901
CONCLUSION
Acknowledgments.
I speciallythank Mary Lou Zoback for continuous
encouragement
during the World StressMap Projectand many helpful
discussions
during the preparationof th;s paper. Data providedby
The compilation of stress data in South America, based INPRES, Argentina,and many otherobservatories
is greatlyappreciated.
mainly on focal mechanisms
of intraplateearthquakes
and geo- This work was supportedby Brazilian grantsCNPq 30.0227/79 and
logicalfault slip data, is beginningto revealthe main character- FAPESP 88/4163-8andby an ICL travelgrant.
icticsof the regionalstressfield. Althoughlarge areasin South
Americaremaindevoidof data, the presentdata baseshouldbe
helpful in constraining
theoreticalmodelsof the plate driving
forces.
In westernSouth America, predominantlyN-S extensional
stresses
in the High Andes(SHm• beingthe intermediateprincipal stress)and E-W compressionalstressesin the sub-Andes
(SHm• being the major principal stress)are now well established(seeMercier et al. [thisissue]for a detaileddescription
of Quaternarystressesin the Peruvianand Bolivian Andes). The
orientation
of theregionallithospheric
SHm• fieldis remarkably
constantand does not seem to be affectedby the changeof
strike of the Andean chain nor by the presenceof a fiat slab
underneath.
The averagedifferencebetweenthe regionalSHm•
orientationand the directionof the Nazca plate convergence
is
only about 10ø-15ø but seems to be significant.The closer
alignment
with the absolute
platemotiondirection,althoughstatisticallynot significant,may indicatea substantialcontribution
of shear stressesfrom asthenosphere-lithosphere
interactions
eastof the Andes.Alternatively,the coincidence
of the regional
SHm• directionwith the absoluteplate motionmay be fortuitous with the regionalfield being the result of variousplate
boundary forces (and also gravitationalbody forces in a
laterallyvaryinglithosphere)
which can only be modeledby
REFERENCES
Allmendinger, R.W., M. Strecker, J.E. Eremchuk, and P. Francis,
Neotectonicdeformationof the southernPuna Plateau,NW Argentina, J. S. Am. Earth Sci., 2, 111-130, 1989.
Anderson,H., Comparisonof centroid-momenttensor and first motion
solutionsfor westernMediterraneanearthquakes,Phys. Earth Planet.
Inter., 52, 1-7, 1988.
Assump•o, M., Patternsof focal mechanismand seismicprovincesin
Brazil, Proc. I Congr. Braz. Geophys.Soc., 1,467-472, 1989.
Assump•o, M., and G. SuSxez,Source mechanismsof moderate-size
earthquakesand stressorientationin mid-plate South America, Geophys.J., 92, 253-267, 1988.
Assump•lo, M., G. Smirez,and J.A.V. Veloso, Fault plane solutionsof
intraplate earthquakesin Brazil: Some constraintson the regional
stressfield, Tectonophysics,
113, 283-293, 1985.
Assump•o, M., J.M. Ferreira, J.M. Carvalho, M.L. Blum, E.A.
Menezes, D. Fontenele, and A. Aires, Seismic activity in Palhano,
CE, October 1988, preliminaryresults,Rev. Braz. Geofis.,7, 11-17,
1989.
Assump•[o, M., J.A. Veloso, J.R. Barbosa,M. Blum, E. Neves, J. Carvalho, and A. Bassini, The earthquakesof Manga, MG, of March,
1990 (in Portuguese),Proc. Braz. Geol. Congr. 36th, 5, 2154-2159,
1990.
Bell, J.S., and D.I. Gough, Northeast-southwest
compressivestressin
Alberta: Evidence from oil wells, Earth Planet. Sci. Lett., 45, 475482, 1979.
Bellier, O., J. Machar•, and M. Se•brier,Extensi6n actual del nor Pen1:
estudioda la falla activa de Chaquilbamba(norte del Departamento
numericalcalculation[e.g., Richardsoneta/., 1979; Fleitout
de la Libertad,Peru), Bol. Soc. Geol. Perd, 80, 1-12, 1989.
and Froidevaux,1982, 1983; Bott et al., 1989; Stefanickand
Berrocal,J., M. Assumpg•o,R. Antezana,C.M. Dias Nero, R. Ortega,
Jurdy,thisissue].At any rate, it seemsunlikelythat the Nazca
and H. Franga,Sismicidadedo Brazil, 320 pp., InstitutoAstron6mico
plate"collision"shouldbe the only majorsourceof the regional
e Geofisico, Universidade de S[o Paulo, Brazil, 1984.
stress field in western South America.
The easternlimit of the AndeanE-W stressprovinceseems
to coincidewith regionsof very low seismicity,in the Amazoniancratonandin the intracratonic
Paran•ibasin.This may indicate that the AndeanE-W stressfield decreases
in magnitude
eastwardand is replacedby stresses
of differentorigin at the
middleof the continent.More stressdata in mid-plateSouth
America,however,arenecessary
to substantiate
thishypothesis.
In the centralAmazonianregion, the distributionof epicentersand the stressdata suggesta differentseismicand stress
provincecharacterized
by roughlyN-S compression.
It is not
clear yet whetherthis compressional
field could result from the
balanceof the variousplate boundaryforcesor couldbe due to
local sourcessuch as lower crustalloadingas suggested
by
Richardsonand Zoback [1990].
Bevis,M., and B.L. Isacks,Hypocentraltrend surfaceanalysis:Probing
the geometryof Benioff zones,J. Geophys.Res., 89, 6153-6170,
1984.
Bonnot,D., M. S6brier, and J. Mercier, EvolutiongeodynamiquePlioQuatemairedu bassinintra-cordilleraindu Callejon de Huaylaset de
la r•gion de ia Cordill,re Blanche,P6rou, Geodynamique,3, 57-83,
1988.
Bou, M.H.P., and D.S. Dean, Stresssystemsat youngcontinentalmargins,Nature Phys.Sci., 235, 23-25, 1972.
Bott, M.H.P, G.D. Waghorn,and A. Whittaker,Plateboundaryforcesat
subductionzones and trench-arccompression,Tectonophysics,
170,
1-15, 1989.
Cabrera,J., M. S•brier, and J.L. Mercier, Active normal faulting in the
high plateausof centralAndes:The Cuzco region (Peru), Ann. Tectonicae, 1, 116-138, 1987.
Caproni, N., Jr., and J.L. Armelin, Instrumenta•o das escavac,
sSes
subterffmeas da UHE Serra da Mesa, in Simpdsio sobre
Instrumentacdo
Geot•cnicade Campo - SINGEO'90, vol. 1, pp.249257, Associaq•oBrasileirade Geologia de Engenharia,S•o Paulo,
In northeastern
Brazil, strike-slipfocal mechanisms
and the
1990.
distributionof seismicitysuggesta combinationof plate-wide Carey-Gailhardis,E., and J.L. Mercier, A numericalmethodfor deter"regional"forces(suchas asthenospheric
drag or ridge-push) mining the state of stressusing focal mechanismsof earthquake
populations:
Applicationto Tibetanteleseismsand microseismicity
of
with localsourcesof stress(due to densitycontrastsacrossconSouthem Peru, Earth Planet. Sci. Lett., 82, 165-179, 1987.
tinentalmarginsand sedimentloadingat the marginalbasins).
Camelto, C.D.R., V.M. Hamza, and F.F.M. Almeida, AtivaOo
A similarcombinationof regionaland local stresseswas also
observed in the Atlantic
continental shelf of North America
[Dart and Zoback, 1987; Zobackand Zoback, 1989].
Certainly,more stressdata are necessary,especiallyeast of
the Andes,for a betterdefinitionof the stressprovincesin midplate South America. Nevertheless,the data available so far
shouldbe a usefulconstraintto the variouspossiblemodelsof
the forcesthat drive the plates.
tectfnica, fiuxo geotdrmico e sismicidade no nordeste oriental
brasileiro, Rev. Bras. Geoc., 19, 310-322, 1989.
Castillo,J.E., and J. Mojica, Orientacionesde esfuerzosactualesa partir
de elongaciones
tectonicas("breakouts")en algunospozos petroleros
de los LLanosOrientalesy del Valle Medio del Magdalena,Colombia, Proc. I Congr.Braz. Geophys.Soc.,1, 457-460, 1989.
Chinn,D.S., andB.L. Isacks,Accuratesourcedepthsandfocal mechanisms of shallow earthquakesin western South America and in the
New Hebrides Island Arc, Tectonics,2,529-563,
1983.
11,902
ASSUMI•AO:INTRAPLATE
STRESSES
IN SOUTHAMERICA
Cipflani, J.C., In-situ stressesin the Caraibaunderground
mine (in Portuguese),Rep. IPT-DIGEM 28170, 168 pp., Inst. of Technol.Res.,
Sgo Paulo, Brazil, 1990.
Mercier, J.L., Extensional-compressional
tectonicsassociatedwith the
AegeanArc: Comparisonwith the AndeanCordilleraof southPerunorth Bolivia. Philos. Trans. R. Soc. London, Ser. A, 300, 337-355,
1981.
Cloetingh,S.A.P.L., M.J.R. Wonel, andN.J. Vlaar, Passivemarginevolution, initiation of subductionand the Wilson cycle, Tectonophysics, Mercier, J.L., M. Sebfler, A. Lavenu, J. Cabrera, O. Bellier, J.-F.
109, 147-163, 1984.
Dumont, and J. Machar6, Changesin the tectonicregime above a
subductionzone of Andeantype: The Andesof Peru and Bolivia durCox, J.W., Long axis orientationin elongatedboreholesand its correlaing the Pliocene-Pleistocene,
J. Geophys.Res.,this issue.
tion with rock stressdata, Trans. SPWLAAnn. LoggingSyrnp.,24th,
Minster, J.B., and T.H. Jordan, Present-day plate motions,
1-14, 1983.
J.Geophys.Res.,
83, 5331-5354, 1978.
Dart, R.L., and M.L. Zoback, Principal stressdirectionson the Atlantic
continentalsheff inferred from orientationsof boreholeelongations, Molina, E.C., N. Ussanti, N.C. S•, D. Blitzkow, and O.F. Miranda
Filho, Deep crustal structureunder the Parana basin (Brazil) from
U.S. Geol. Surv. OpenFile Rep. 87-283, 1987.
gravity studies,in The Mesozoic Flood Volcanism of the Parand
Davies, J.C., Statisticsand Data Analysisin Geology,2nd ed., 646 pp.,
Basin: Petrogeneticand GeophysicalAspects,edited by E.M. PicJohnWiley, New York, 1986.
ciflllo, and A.J. Melfi, pp. 271-283, Instituto Astron6mico e
DeMets, C., R.G. Gordon, D.F. Argus, and S. Stein, Current plate
Geofisico,Universidadede Sgo Paulo, Sgo Paulo, 1988.
motions,Geophys.J. lnt., 101,425-478, 1990.
DepartamentoNacional de ProducgoMineral, Tectonicmap of South Nunn, J.A., and J.R. Aires, Gravity anomaliesand flexure of the lithosphereat t,heMiddle Amazon Basin, Brazil, J. Geophys.Res., 93,
America, scale 1:5,000,000, Brasilia, Brazil, 1978.
415-428, 1988.
Deverchere,J., C. Dorbath,and L. Dorbath, Extensionrelatedto a high
Pardo,M., and A. Fuenzalida,Estmcturacorticaly subducci6nen Chile
topography:Resultsfrom a microearthquake
surveyin the Andesof
Central,in Proceedingsof V CongresoGeoldgicoChileno,vol. lI, lap.
Peru andtectonicimplications,Geophys.J. Int., 98, 281-292, 1989.
F247-F265, SociedadGeo16gicade Chile, Santiago,1988.
Dotbath, C., L. Dorbath, A. Cistemas,J. Deverchere,M. Diament, L.
Ocola, and M. Morales, On crustal seismicityof the Amazonian Pennington,W.D., Subduction of the Eastern Panama Basin and
seismotectonics
of northwesternSouth America,J. Geophys.Res., 86,
foothill of the centralPeruvianAndes, Geophys.Res.Lett., 13, 1023-
10,753-10,770, 1981.
Plumb, R.A., and S.H. Hickman, Stress-induced
boreholeelongation:A
comparisonbetweenthe four arm dipmeterand the boreholein the
Auburngeothermalwell, J. Geophys.Rec.,90, 5513-5521, 1985.
Raleigh, C.B., J.H. Healy, and J.D. Bredehoeft,Faulting and crustal
stressat Rangely, Colorado,in Flow and Fracture of Rocks,Geophys.Monogr. Ser., vol.16, editedby H.C. Heard et al., pp. 275-284,
AGU, Washington,D.C., 1972.
3271, 1983.
Ferreira,J.M., M. Takeya, J.M. Costa, J.A. Moreira, M. Assump•go, Riccomini,C., A.U.G. Peloggia,J.C.L. Saloni,M.W. Kohnke,and R.M.
Figueira, Neotectonicactivity in the Serra do Mar rift system (SE
J.A. Veloso, and R.G. Pearce, A continuingintraplateearthquake
Brazil), J. S. Am. Earth Sci., 2, 191-197, 1989.
sequence
near Jo•o C•mara, NE Brazil, preliminaryresults,Geophys.
Richardson,R.M., and M.L. Zoback, Amazonasrift: Modeling stress
Res. Lett., 14, 1042-1045, 1987.
arounda paleozoicrift in South America (abstract),Eos Trans. AGU,
Ferreira,J.M., M. Takeya, E. Menezes,A. Ayres, and M. Assump•go,
71, 1606, 1990.
The Groairas,CE, earthquakes
of March 31, 1988 (4.1 and 3.9 mb):
Another example of strike-slipfaulting in northeasternBrazil, paper Richardson,R.M., S.C. Solomon,and N.H. Sleep, Tectonicstressin the
plates,Rev. Geophys.,17, 981-1019, 1979.
presentedat I Congress,Brazilian Geophys.Soc., Rio de Janeiro,
Nov. 20-24, 1989.
Roa, H.M., The Upper Cenozoicvolcanismin the Andes of Southern
Chile (from 40.0 to 40.5 S.L.), in Andeanand AntarcticVolcanologiFleitout, L., and C. Froidevaux,Tectonicsand topographyfor a lithocal Problems, edited by O. Gonzales-Ferran,pp. 143-171, Internaspherecontainingdensityheterogeneifies,
Tectonics,1, 21-56, 1982.
tional Associationof Volcanologyand Chemistryof the Earth'sInteFleitout,L., and C. Froidevaux,Tectonicstressesin the lithosphere,Tecflor, Naples,Italy, 1976.
tonics,2, 315-324, 1983.
Schneider,J.F., and I.S. Sacks, Stress in the contortedNazca plate
Froidevaux,C., and Y. Ricard, Tectonicevolutionof high plateaus,TecbeneathsouthernPeru from local earthquakes,J. Geophys.Res., 92,
tonophysics,
134, 227-238, 1987.
13,887-13,902, 1987.
Grange,F., D. Hamfield,P. Cunningham,P. Molnar, S.W. Roecker,G.
Se'bfler,M., J.L. Mercier, F. Megard, G. Laubacher,and E. CareySuarez, A. Rodflgues,and L. Ocola, Tectonic implicationsof the
Gailhardis,Quatemarynormal and reversefaulting and the state of
microearthquake
seismicityand fault plane solutionsin southernPeru,
stressin the central Andes of Peru, Tectonics,4, 739-780, 1985.
J. Geophys.Res.,89, 6139-6152, 1984.
Se'brier,M., J.L. Mercier, J. Machar6, D. Bonnot, J. Cabrera, and J.L.
Gdpp, A.E., and R.G. Gordon, Current plate velocitiesrelative to the
Blanc, The stateof stressin an overridingplate situatedabovea flat
hot spots incorporatingthe NUVEL-1 global plate motion model,
slab:The Andes of central Peru, Tectonics,7, 895-928, 1988.
Geophys.Res.Lett., 17, 1109-1112, 1990.
Gusso, G.L.N., and E. Bagnoli, Evid•ncias de intensa deformaOo Sipkin, S.A., Estimationof earthquakesourceparametersby the invertectbnica em sedimentos costeiros holoc•nicos
do Rio Grande do
sion of waveformdata: Global seismicity,1981-1983,Bull. Seismol.
Soc.Am., 76, 1515-1541, 1986.
Norte,paperpresented
at I Congress,
BrazilianGeophys.Soc.,Rio de
Janeiro, Nov. 20-24, 1989.
Smalley, R.F., Jr., and B.L. Isacks, A high-resolutionlocal network
studyof the Nazca plate Wadati-Benioffzone under westernArgenKadinsky-Cade,K., and R. Reilinger, Surface deformationassociated
tina,J. Geophys.Res., 92, 13,903-13,912,1987.
with the November 23, 1977, Caucete, Argentina, earthquake
Sophia, C.M., and M. Assmix;go, Padr•o de mptura da falha de
sequence,
J. Geophys.Res.,90, 12,691-12,799,1985.
Samambaia,RN, na reativa•o de fevereirode 1987, Proc. I Congr.
Kafka, A.L., and D.J. Weidner, Earthquakefocal mechanismsand tecSoc.Bras. Geofis.,1, 350-356, 1989.
tonicprocesses
alongthe southernboundaryof the Caribbeanplate,J.
Stauder,W., Mechanismand spatialdistributionof Chileanearthquakes
Geophys.Res., 86, 2877-2888, 1981.
with relationto subductionof the oceanicplate,J. Geophys.Res., 78,
Katsui,Y., and R. Katz, Lateralfissureeruptionsin the SouthernAndes
5033-5061, 1973.
of Chile, J. Fac. Sci. Hokkaido Univ., Ser. IV, 13(4), 433-448, 1967.
McKenzie, D.P., The relation between fault plane solutionsfor earth- Stauder,W., Subductionof Nazca plate under Peru as evidencedby
focal mechanismsand by seismicity,J. Geophys.Res., 80, 1053quakes and the directionsof the principal stresses.Bull. Seismol.
1064, 1975.
Soc. Am, 59, 591-601, 1969.
and driving forces
Meijer, P.T., and M.J.R. Wortel, The dynamicsof motion of the South Stefanick,M., and D.M. Jurdy, Stressobservations
modelsfor the SouthAmericanplate,J. Geophys.Res.,this issue.
Americanplate,J. Geophys.Res.,this issue.
Mendiguren, J.A., A procedureto resolve areas of different source Stein,S., S. Cloetingh,
N.H. Sleep,andR. Wortel,Passive
marginearthmechanismswhen using the methodof compositenodalplane soluquakes,stresses
and theology,in Earthquakesat North-AtlanticPastion, Bull. Seismol.Soc. Am., 70, 985-998, 1980.
siveMargins:Neotectonics
and Postglacial
Rebound,editedby S.
Mendiguren,J.A., and F.M. Richter, On the origin of compressional
Gregersen
and P.W. Basham,pp. 231-259, Kluwer Academic,Bosintraplatestresses
in South America,Phys. Earth Planet.Inter., 16,
ton, Mass., 1989.
1026, 1986.
Doser, D.I., The Ancash,Peru, earthquakeof November 10: Evidence
for low angle normal faulting in the high Andes of northernPeru,
Geophys.J. R. Astron.Soc.,91, 57-71, 1987.
Dziewonski, A.M., and J.H. Woodhouse,An experimentin systematic
studyof global seismicity:Centroid-moment
tensorsolutionsfor 201
moderateto large earthquakesof 1981, J. Geophys.Res., 88, 3247-
318-326, 1978.
Smlrez,G., P. Molnar,and B. Burchfiel,Seismicity,
fault planesolu-
ASSUMI•AO:
INTRAPLATE
STRESSES
IN SOUTH
AMERICA
11,903
tions,depthof faulting,andactivetectonics
of the Andesof Peru, Zoback,M.L., and M.D. Zoback,Stateof stressin the conterminous
Ecuador,and southernColombia,J. Geophys.Res., 88, 10,40310,428, 1983.
Takeya,
M., J.M.Fen'eLm,
R.G.Pearce,
M. Assuml•ffo,
J.M.Costa,
and
C.M. Sophia,The 1986-1989intraplateearthquake
sequence
near
Jo8oC•xnara,northeast
Brazil -- Evolutionof activity,Tectonophysics,167, 117-131, 1989.
UnitedStates,J. Geophys.
Res.,85, 6113-6156,1980.
Zoback,M.L., andM.D. Zoback,Tectonicstressfield of the continental
UnitedStates,in Geophysical
Frameworkof the Continental
United
States,editedby L. PakiserandW. Mooney,Mere.Geol.Soc.Am.,
172, 523-539, 1989.
Zoback,M.L., et al., Global patternsof tectonicstress,Nature, 341,
Turcotte,D.L., J.L. Ahren,and J.M. Bird, The stateof stressat con-
291-298, 1989.
tinental
margins,
Tectonophysics,
42, 1-28,1977.
Zoback,M.D., D. Moos, L Mastin,and R.N. Anderson,Well bore
breakouts
andin situstress,
J. Geophys.
Res.,90, 5523-5530,1985.
Zoback,M.L., First-and second-order
patternsof stressin the litho-
M. Assumpq•o,
Depto.de Geofisica,
IAG-USP,S8oPaulo,P.O. Box
sphere:
TheWorldStress
Mapproject,
J. Geophys.
Res.,thisissue
(a).
Zoback,M.L., Stressfield constraints
on intraplateseismicity
in eastern
NorthAmerica,J. Geophys.
Res.,thisissue(b).
9638, Brazil 01065.
(ReceivedMay 17, 1990;
revised June 7, 1991;
acceptedJune16, 1991.)