Journal of Seismology 5: 519–539, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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Source mechanism of earthquakes in Peru

H. Tavera & E. BufornDpto. de Geofı́sica, Universidad Complutense, Madrid, Spain

Received 29 December 1999; accepted in revised form 18 December 2000

Key words: earthquakes in Peru, reverse faulting, source rupture process, subduction

Abstract

The source mechanism of 19 earthquakes that occurred in Peru (1990–1996) is studied using broad band data.Focal mechanisms are obtained using polarities of P wave and body wave form inversion. Shallow earthquakesshow complex source time functions, intermediate and deep depth shocks have simpler ones. Stress distributionshave been obtained from focal mechanisms estimated in this study and previous studies. Shallow earthquakes showreverse faulting with an ENE-WSW to E-W oriented pressure axes. Intermediate depth shocks indicate horizontalextension on E-W direction, normal to the Peru-Chilean trench. Earthquakes with foci at very deep depth showhorizontal extension in the E-W direction in Peru-Brazil and N-S in Peru-Bolivia borders. This difference in stressorientation may indicate a different origin for deep activity at each region.

Introduction

Peru is located in a region with a high level of seis-micity associated with the subduction of the Nazcaoceanic plate beneath the western continental borderof the South American plate. As a consequence ofthis process, where the Nazca plate moves with anapproximate convergence velocity of 10 cm/yr in dir-ection N◦80 (Minster and Jordan, 1978), this regionhas a high level of deformation. The Peruvian Andesrepresent the most significant expression of this de-formation. Two characteristics of the subduction inthis area are alternative regions of normal subduction(dipping 30◦) and flat subduction (nearly horizontal)at a depth of about 100 km and the absence of earth-quakes in the depth range from 300 to 500 km. Thezone of normal subduction is located in southern Peruand is associated with active volcanism. The flat sub-duction is located in northern-central Peru where thereis an absence of volcanism.

The subduction process in Peru has been the sub-ject of many studies. Some are based only on seis-micity data (Barazangui and Isacks, 1976, 1979;Hasegawa and Sacks, 1981; Jordan et al., 1983; Bevisand Isacks, 1984; Grange, 1984; Suarez et al., 1990;Cahill and Isacks, 1992; Lindo et al., 1992; Norabuena

et al., 1994), while others also consider the focalmechanisms of earthquakes (Chandra, 1970; Isacksand Molnar, 1971; Abe, 1972; Fukao, 1972; Stauder,1975; Petersen, 1976; Dewey and Spence, 1979; Pen-ington, 1981; Chinn and Isacks, 1983; Suarez et al.,1983; Doser, 1987; Beck and Ruff, 1989; Hartzelland Langer, 1993; Ihmle et al., 1996; Tavera, 1998;Spence et al.;1999). Only three of these studies (Ihmléet al., 1996; Tavera, 1998; Spence et al., 1999) usebroad band data which can give more details about thesource rupture process.

The Peru-Chile trench, located offshore near west-ern Peru, borders the Nazca and South Americanplates. In land the main geological feature in Peru isthe Andean Cordillera, which extends from north tosouth formed in five morphological units (Audebaudet al., 1973; Mégard, 1978; Dalmayrac et al., 1980;Suarez et al., 1983) (Figure 1): 1. The coastal zoneextends from north to south in a narrow belt no widerthan 40 km. It is limited to the west by the coastlineand to the east by the Cordillera batholith. 2. TheCordillera Occidental (W. COR) is formed mainly byvolcanic and plutonic rocks of Mesozoic and Cenozoicage, extending from north to south parallel to the coastline. 3. The Cordillera Oriental (E. COR) is a unitof lower altitude than the Cordillera Occidental and

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Figure 1. Morphological units of Peru (Audebaud et al., 1973; Daylmarac et al., 1987). W. COR = Cordillera Occidental; E. COR. = CordilleraOriental, ALT = Altiplano. Topography is show as white (h<1500 m), grey (1500<h<4000 m) and black (h>4000 m). The arrow shows plateconvergence direction (Minster and Jordan, 1978). Numbers and symbols correspond to earthquakes studied in this paper.

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corresponds mainly to an wide anticline comprissingPrecambrian deposits north of 12◦S and Paleozoic de-posits to the south. There is a change in strike of theCordillera from N-S to E-W south of this unit. 4. TheAltiplano (ALT) consists of a sequence of Mesozoicintermountainous plateaux, bounded by the CordilleraOriental to the east and by the Cordillera Occidentalto the west, up to the Bolivian border. At approx-imately 9◦S, the central high plateau disappears andthe Cordillera Occidental and Oriental are adjacent.5. The Sub-Andean region represents the eastern act-ive boundary of the Andean Cordillera and is locatedbetween the Andean Cordillera and the Amazonianplane.

Several systems of faults are located within thesefive units. Some of them correspond to large thrustfaults (Moyabamba system, Satipo, Huaytapalla, Ay-acucho, Salvación-Mazuko) and others to normalfaults (Quiches, Cordillera Blanca and Tambomachaysystem) with strike parallel to the Andean Cordillera(Goller, 1949; Audebaud et al., 1973; Sébrier et al.,1982, 1985; Dorbath et al., 1986; Doser, 1987; Sébrieret al., 1988; Cabrera, 1988; Deverchère et al., 1989;Dorbath et al., 1990, 1991; Cabrera et Sébrier, 1998).These units are a consequence of an active subduc-tion process that produces the present high level ofseismicity.

In this paper we present focal mechanisms of 19selected earthquakes which occurred in Peru duringthe period 1990–1996 (mb≥5.8) in order to obtainthe stress distribution in this area. These shocks aretypical of seismicity in Peru, with focus at shallow(h ≤ 60 km), intermediate (60<h≤350 km) and at verydeep depth (h>350 km) (Figure 1 and Table 1). Focalmechanisms have been determined from the inversionof body wave forms using broad band data.

Seismicity

The first reported earthquakes in Peru are from theXVI and XVII centuries. Silgado (1968, 1978, 1985)and Dorbath et al. (1990) give exhaustive informa-tion about historical seismicity. Historical earthquakeswith maximum intensity larger than VIII MM aremostly located along the coast, but epicenter loca-tions may be due to concentration of the populationin this part of the country. Many of these large earth-quakes produced tsunamis, which argues for offshoreepicenters (Montessus de Ballore, 1911; Dorbath etal., 1990).

In Figure 2 the distribution of epicenters for theperiod 1964–1996 (mb≥5) is shown (Engdahl etal., 1998). Earthquakes with focus at shallow depth(h ≤ 60 km) are mainly located offshore between thecoast and the trench (Figure 2a). Main characteristicof the shallow seismicity is the frequent occurrenceof shocks with large magnitudes (M>7). Inside thecontinent a significant shallow seismic activity areais located in northern and central Peru, between theCordillera Occidental and the Subandean zone andparallel to the Cordillera. Earthquakes in this areaare of lower magnitude, in general less than 6.5.An important concentration of shocks is found in theMoyabamba region (6◦S, 77◦N) in northern Peru.

Earthquakes with focus at intermediate depth(60<h≤350 km) are mainly located in three areas(Figure 2b). The first is offshore and parallel to thecoast, between latitudes 9◦S and 18◦S. The secondarea is located within the continent, between the Cor-dillera Occidental and the Subandean zone. The thirdregion, located in the southern part of the Altiplano, isthe most active region with a continuous seismic activ-ity parallel to the Peru-Chile trench. The maximumdepth of earthquakes is 100–150 km in the two firstregions and 300 km in the third.

Seismic activity at very deep depth (h>350 km) islocated in two areas (Figure 2b). The first, with thehigher activity, is located on the border of Peru-Brazil,with epicenters showing an N-S linear distribution.The second is located on Peru-Bolivia border, with alower level of activity.

Three transverse cross sections, each with a totalwidth of 300 km, are shown in Figure 3. The azimuthsof these cross sections are approximately perpendicu-lar to the trench, with similar orientation for AA’ andBB’ sections and rotated 20◦ to north for CC’ section(Figure 2b). On section AA’ in northern Peru (Fig-ure 3a), depth of earthquakes increase from west toeast with a maximum depth of 150 km. An apparentgap exists in the interior of the continent, between200 and 450 km of distance from the trench. From500 km from the trench, the number of focus increase,but maximum depth remains practically constant at150 km. On section BB’, in central Peru (Figure 3b),distribution of hypocenters follows the pattern de-scribed previously, but the seismic gap is not observed.These two distributions of earthquakes show the limitof the subducted plate, which descends at an angle of30◦ to an approximate depth of 150 km, where it flat-tens and is subducted horizontally. Further east, nearthe Peru-Brazil border, a nest of deep depth earth-

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Table 1. Hypocentral data of the nineteen earthquakes studied in this paper

No Date Origin time Latitude Longitude Depth (km) Mb

1 Jan 07, 1990 0906:43.4 –15.94 –74.24 48 5.9

2 May 30, 1990 0234:05.8 –6.01 –77.23 24 6.1

3 Oct 17, 1990 1430:13.1 –10.97 –70.77 598 6.7

4 Apr 04, 1991 1523:20.7 –6.04 –77.13 20 6.0

5 Apr 05, 1991 0419:49.5 –5.98 –77.09 19 6.5

6 May 24, 1991 2050:55.8 –16.50 –70.70 127 6.3

7 Jul 06, 1991 1219:49.5 –13.11 –72.18 104 6.2

8 Jul 13, 1992 1811:33.7 –3.92 –76.60 96 6.1

9 Jan 10, 1994 1553:50.1 –13.34 –69.44 596 6.4

10 Jan 20, 1994 0906:52.7 –6.00 –77.05 123 5.8

11 Dec 12, 1994 0741:55.4 –17.47 –69.59 148 5.9

12 May 02, 1995 0606:05.6 –3.79 –76.92 97 6.5

13 Jun 12, 1995 0335:48.8 –8.30 –75.91 34 5.8

14 Sep 23, 1995 2231:56.3 –10.58 –78.58 60 6.0

15 Oct 03, 1995a. 0151:23.9 –2.77 –77.88 24 6.5

16 Oct 03, 1995b. 1244:58.0 –2.81 –77.89 17 6.0

17 Feb 21, 1996 1251:04.3 –9.62 –79.56 33 6.0

18 Mar 10, 1996 0856:22.3 –12.97 –69.42 33 5.8

19 Nov 12, 1996 1659:43.0 –14.99 –75.67 33 6.5

quakes exists at depths between 500 and 600 km. Insouthern Peru, section CC’ (Figure 3c), hypocentersare distributed along a zone of constant dip of 30◦with some isolated events at 600 km of depth, at thePeru-Bolivia border. Changes in the distribution of hy-pocenters in the three cross-sections show the changein the subduction process that has been interpreted bysome authors as a rupture of the oceanic plate, witha vertical displacement at northern and central Peru(Barazangi and Isacks, 1979; Snoke et al., 1989).Other authors propose an abrupt change in curvatureof the subducted slab, from convex to concave up-ward (Hasegawa and Sacks, 1981; Bevis and Isacks,1984; Boyd et al., 1984; Grange, 1984; Rodriguez andTavera, 1991; Cahill and Isacks, 1992). Consideringthe very deep activity (h∼600km), there is a lowerlevel of seismic activity at the Peru-Bolivia border inprofile CC’ than at the Peru-Brazil border in profileBB’.

Source mechanism

Nineteen earthquakes (mb≥5.8) for the period 1990–1996 have been selected for a study of their focalmechanism (Figure 1 and Table 1). Ten of them cor-respond to shallow depths (h ≤ 60 km, events 1, 2,

4, 13, 14, 15, 16, 17, 18 and 19), seven have foci atintermediate depth (60<h≤350 km, events 6, 7, 8, 10,11, 12 and 14) and two at very deep depth (h∼350km,events 3 and 9).

First motions of P waves have been used to es-timate a preliminary fault-plane orientation (Annex 1)using the algorithm of Brillinger, Udías and Bolt. Thealgorithm uses a maximum likelihood function to es-timate the orientation of the principal stress axes (Tand P), nodal planes and their standard errors from po-larity data (Brillinger et al., 1980; Udías and Buforn,1988). The score is defined as the proportion of correctpolarities to the total number of observations. Dataused mainly correspond to digital broad band seismo-grams recorded at teleseismic distances, but in orderto obtain a better control of the solutions, polaritiesof non-broad band stations at regional distances havealso been used.

Wave forms of body waves (P and SH) from broadband stations at teleseismic distances (30◦ to 90◦) havebeen used in order to obtain fault-plane orientation,depth, scalar seismic moment and source time functionusing the Nabelek inversion method (Nabelek, 1984).Records have been amplitude-equalized to a commondistance of 40◦ and selected using a criterion of sig-nal/noise level greater than 100. Seismograms were

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Figure 2. Seismicity map of Peru and surrounding areas for the period 1964–1996 (Engdahl et al., 1998) and shallow focus (h≤60 km,Figure 2a), intermediate and deep depth (60<h≤350km and h>350 km, Figure 2b). Lines AA’, BB’ and CC’ correspond to vertical crosssections shown in Figure 3.

weighted according to azimuthal distribution of sta-tions, such that stations clustered together were givensmaller weights than those of isolated stations (Mc-Caffrey et al., 1991). Green’s functions are computedfor a homogenous half-space at the source region withvelocity of P =6.0 km/s and density of 2.8 g/cm, forevents at shallow depth, and P velocity of 6.8 km/sand density of 3.0 g/cm3 for intermediate and deepdepth events. Poisson’s ratio of 0.25 and an averageattenuation factor t∗ = 1 for P and t∗ = 4 for SH areused.

In general, solutions obtained from polarities arewell constrained due to the large number of observa-tions. For this reason, fault plane solutions obtainedfrom first motions have been used as preliminary ori-entations in the wave form inversion process. The finalsolution corresponds to minimum values of RMS ofthe residuals of observed and synthetic wave forms.Results of wave form inversion show good agree-ment between synthetic and observed seismograms for

earthquakes with mb≤6.0, but for larger magnitudes(mb≥>6.0), it is sometimes impossible to model allphases. This may be due to Nabelek’s method itselfwhich considers a point source. For earthquakes withmb>6.0 this assumption is no longer valid and it isnecessary to take into account the directivity effectsdue to the source dimensions.

Focal mechanism of shallow earthquakes

Fault-plane solutions obtained from first motion ofP wave are shown in Table 2 and Annex 1. Mostsolutions have a well-constrained predominantly ver-tical plane (in general with errors less than 15◦). Thesecond plane is nearly horizontal, in general with lar-ger errors. Observations are well distributed over thefocal sphere, with a minimum of 24 (event 1) and amaximum of 67 (event 15). Scores are greater than0.90 with the exception of event 12 (score of 0.84).

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Figure 3. Vertical cross sections for the period 1964–1996 (Engdahl et al., 1998). T = trench, C = coast.

Results of wave form inversion are shown in Fig-ure 4, Annex 2 and Table 2. For events 1, 2, 4, 5 and13, data of P and SH waves were only available on aquarter of the focal sphere. For this reason, solutionsobtained from wave form inversion are not well con-strained. Due to its low magnitude (5.8), for event 18S waves were very noisy and it has not been possibleto use them.

Fault-plane orientations obtained from polaritiesand wave form inversion show similar results. Differ-

ences are generally less than 15◦ in the orientation ofthe planes, with the exception of events 2 and 15. Thesolutions obtained correspond to thrusting faults withplanes oriented in a N-S direction. Offshore earth-quakes (Figure 4), between the coast and the trench,have a nearly vertical plane trending N-S and a secondplane nearly horizontal and dipping to the east (events17 and 19). This second plane may be associated withthe subduction process at shallow depths (Beck andRuff, 1989; Langer and Spence, 1995). Earthquake 1

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Figure 4. Focal mechanisms of shallow (Figure 4a) and intermediate and deep depth (Figure 4b) earthquakes studied in this paper. Numberscorrespond to Table 1.

has similar plane orientations, but the plane dipping tothe east is more vertical. Within the continent, nearthe Ecuador-Peru border, earthquakes show thrust-ing mechanisms with planes trending to NNE-SSW(events 15 and 16) parallel to the Cordillera. In north-ern Peru, in the Moyabamba region (6◦S, 77◦W), event5 shows a complex faulting mechanism formed by twosubevents with different fault-plane orientation. How-ever, the focal mechanisms for Moyobamba events (2,4 and 5), show a common plane, nearly horizontal andstriking N-S.

In central Peru, earthquake 13 has a thrustingmechanism with planes dipping about 45◦ and strik-ing N-S, parallel to the Cordillera. Event 18, locatedin southern Peru near the Peru-Bolivia border, has athrusting mechanism with planes striking WNW-ESEin agreement with the change of strike of the Cor-dillera in this region. The NNW-SSE plane present inalmost all solutions inside the continent agrees withthe azimuth of main geological faults in the Subandeanzone.

Event 5, with a complex solution, may be ex-plained in terms of two subevents, both with thrustingmechanisms and depths of 16 and 23 km respectively.The second subevent has larger scalar seismic momentand longer duration of the source time function (STF).For events 15 and 19 the STF (Figure 5) starts witha low release of energy and after 8 s and 40 s, re-spectively, the largest amount of seismic moment isreleased. Earthquake 19 has been studied by Ihmléet al., (1996) and Spence et al., (1999). These au-thors obtained similar focal mechanism to that of ourstudy with some differences. The depth that we ob-tain (18 km) lies between those obtained by Ihmlé etal. (9 km) and Spence et al. (21–24 km). The solu-tion obtained by Spence at al. (1999) corresponds toa complex event formed by three subevents, whereasthe solution obtained in our study and in that by Ihmléet al. formed by only one event. The scalar seismicmoment obtained in the present study (4.4 × 1020 Nm)is larger than that obtained by Ihmlé (1.9 × 10 20 Nm)but lower than Spence’s value (8.6 × 10 20 Nm). In the

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Figure 5. Map showing the distribution of source time function (STF) of earthquakes studied in this paper (Table 1). Black STF correspondto shallow shocks, grey to intermediate depth and white to very deep depth. Numbers correspond to mb magnitude. The amplitude scale arenormalized to the maximum scalar seismic moment for each earthquake (as in Annex 1).

three studies the maximum energy is released between25 and 45 s after the initiation of the rupture. We havestudied the three larger aftershocks of this earthquake,trough due to their low magnitude (mb≥ 5.5–5.4) onlyP wave data were available for inversion. Fault-planesolutions estimates correspond to thrust faulting sim-

ilar to that of main shocks and depths of 25, 11 and21 km. The scalar seismic moments are lower thanthat estimated for the main shock (1.6, 2.8 and 0.3 ×1018 Nm) and shorter duration of STF (4, 9 and 3srespectively).

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Table 2. Focal mechanisms of the earthquakes studied. Atleft solution obtained from polarities. At right solution frombody-wave modelling.

No First motion P and SH-waveform

�◦ �◦ �◦ �◦ Depth Mo

(km) (Nm)

1 P: 49 76 P: 49 76 45 4.2 × 1017

T: 204 15 T: 204 15

2 P: 45 70 P: 62 69 23 5.3 × 1018

T: 245 21 T: 218 22

3 P: 111 8 P: 80 7 612 4.1 × 1019

T: 242 85 T: 260 83

4 P: 102 69 P: 102 69 21 2.4 × 1018

T: 275 21 T: 275 21

5 P: 62 79 P1: 53 76 16 1.9 × 1018

T: 194 16 T1: 178 23

P2: 106 73 23 4.0 × 1018

T2: 339 26

6 P: 51 20 P: 237 20 121 2.5 × 1019

T: 72 70 T: 72 70

7 P: 81 86 P1: 86 81 98 0.2 × 1019

T: 177 83 T1: 176 89

P2: 283 32 98 1.1 × 1019

T2: 181 82

8 P: 73 3 P: 13 1 107 6.0 × 1018

T: 287 88 T: 279 90

9 P: 8 28 P: 18 29 607 1.5 × 1019

T: 167 63 T: 167 64

10 P: 44 7 P: 14 9 107 3.9 × 1017

T: 243 83 T: 246 83

11 P: 232 46 P: 232 45 152 1.7 × 1018

T: 88 50 T: 88 50

12 P: 28 2 P: 12 15 110 1.3 × 1019

T: 246 89 T: 252 83

13 P: 265 76 P: 258 88 31 1.2 × 1017

T: 115 16 T: 157 92

14 P: 273 23 P: 282 25 71 3.2 × 1018

T: 82 67 T: 79 66

15 P: 292 89 P: 123 83 21 3.3 × 1019

T: 36 3 T: 228 26

16 P: 100 74 P: 99 70 17 4.4 × 1018

T: 281 16 T: 254 22

17 P: 260 52 P: 253 54 9 8.7 × 1019

T: 78 37 T: 78 35

18 P: 214 74 P: 204 81 32 3.5 × 1017

T: 66 19 T: 44 9

19 P: 242 59 P: 245 60 18 4.4 × 1020

T: 81 33 T: 85 32

� = 90-plunge; � = trend; Mo = Scalar Seismic Moment, T =tension, P = presion.

Focal mechanism of intermediate and deep depthearthquakes

Fault plane solutions have been obtained from polar-ities of P-waves for seven earthquakes at intermediatedepth (60<h≤350 km) and are shown in Table 2 andannex 1. Number of data are between 22 (event 7)to 50 (event 14) with scores higher than 0.95. Sixsolutions correspond to normal faulting, with a nearlyvertical plane striking to NNW-SSE (events 6, 11 and14) on planes dipping about 45◦ and trending to NNW-SSE (events 10 and 12), or with strike in NNE-SSWdirection (event 8). Event 7 shows a solution of strikeslip motion with nearly vertical planes and striking toNW-SE and NE-SW, respectively. A strike-slip solu-tion is unusual for intermediate depth focus, however,in this case the fault-plane solution is well constrained,with data covering the total focal sphere.

Results of the inversion of body wave forms areshown in Table 2 and Annex 2. Fault plane orientationsagree with the results obtained from polarities withdifferences less than 15◦ in the orientation of planes.Intermediate depth earthquakes show a predominantfocal mechanism of normal faulting with planes ori-ented in approximately N-S direction. Event 7, locatedin continental southern Peru, shows a complex solu-tion formed by two subevents with foci at the samedepth (h = 98 km) but different fault-plane orienta-tion. The first one corresponds to a strike slip solutionwith vertical planes oriented in NW-SE and NE-SWdirection, which agrees with solution obtained frompolarities. The second subevent has a normal faultsolution, with planes dipping about 45◦ and trendingE-W, and larger energy release and longer STF thanthe first (Figure 5).

Depths estimated in the inversion process arebetween 71km and 152 km. In general, source timefunctions correspond to triangles with short duration(less than 5s, Figure 5). Only for events 6 and 15 hasa longer STF been obtained. Earthquake 6 has beenstudied by Bos et al. (1998), who obtained a depth of126 km, very close to our solution (121 km). Both,STF and scalar seismic moment (8 s and 2.12 × 1019

Nm) are very similar to our solution (9s and 2.4 × 1019

Nm).The two very deep depth earthquakes present dif-

ferences on focal mechanisms (Table 2, Figure 4 andAnnex 1, 2). The fault plane solution of event 3,near the Peru-Brazil border, obtained from polarities,shows normal faulting with planes trending N-S indic-ating a nearly vertical plane and a second horizontal

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plane. Solution obtained from waves form inversiongives planes dipping at about 45◦. Earthquake 9, onthe Peru-Bolivia border, shows a focal mechanismwith planes oriented in an E-W direction, one nearlyvertical and the second nearly horizontal using bothpolarities and wave form inversion. Depths obtainedin the inversion process for earthquakes 3 and 9 are612 and 607 km, respectively. Source time functionsare simple with a total duration of 6s and 7s and scalarseismic moment values of 4.1 × 1019 and 1.5 × 1019

Nm, respectively (Figure 5).

Interpretation and discussion

From results of focal mechanisms obtained in thisstudy we present the horizontal projection of T andP axes (black arrows) for Peru in Figure 6. Results ob-tained by previous studies (listed in Table 3) are shownas white arrows. The deduction of regional stress fromfault plane solutions of earthquakes is not exempt fromambiguity, in fact the maximum compressive stressmay have an orientation anywhere within the dilata-tional quadrant, and not necessarily at 45 degreesof the fault plane. However, the stress axes derivedfrom fault plane solutions of large earthquakes mayserve as an indication of their general trend for agiven region, in this case Peru. For shallow earth-quakes located offshore, between the trench and thecoastline, horizontal pressure axes (Figure 6a) strik-ing ENE-WSW are obtained. These stresses may beassociated with the convergence motion between theoceanic and continental plates. This regime extendsto 60 km depth. At the Ecuador-Peru border, pressureaxes trend NW-SE, normal to the Cordillera. In north-ern Peru, pressure axes are rotated to ENE-WSW toEW. In the Moyabamba region there is a concentra-tion of horizontal pressure axes with azimuths between53◦ and 106◦ (the average is 81◦). This variation inthe trend of P axes, may suggests the response to E-W compression by local heterogeneous conditions. Incentral Peru pressure axes are rotated in an E-W direc-tion, which may be associated with local deformationprocesses. Finally, in the Peru-Bolivia border pressureaxes are rotated NNE-SSW (event 18), perpendicu-lar to main faults of the Cordillera. Summarising, achange on stress orientation from north to south canbe observed. In the Cordillera, north of 5◦S, pres-sure axes trend in a NNW-SSE direction; between 5◦Sand 13◦S the direction is NE-SW and finally, south of13◦S it is NNE-SWS orientated. This change in stress

orientation may be related with different amounts ofdeformation, being greater in the north than to thesouth

Intermediate depth earthquakes show horizontaltension axes in an E-W direction, parallel to the plateconvergence direction (Figure 6b). Events located off-shore show horizontal tension axes striking SW-NE,normal to the coastline, which could be associated,with internal deformation of the oceanic plate. Withinthe continent, tension axes have similar orientationwith a more E-W direction in northern Peru in agree-ment with results obtained by Astiz et al., (1988).Only for event 7, tension axis presents a different, N-S, orientation. This orientation may be interpreted interms of stresses that produce a plate contortion as itchanges from flat to normal subduction. Deep eventsshow horizontal tension axes in a N-S direction nearthe Peru-Bolivia border, and in an E-W direction atthe Peru-Brazil border, that agree with results obtainedby Stauder and Bollinger (1966), Isacks and Molnar(1971), Fukao (1972), Kikuchi and Kanamori (1994),Kirby et al. (1995) and Wu et al. (1995). This differ-ence in tension axes orientation suggests a differentorigin for deep events in the Peru-Brazil and Peru-Bolivia regions (Kirby et al., 1995; Creager et al.,1995; Wallace, 1995).

Source time function (STF) shows different timecharacteristics of energy release (Figure 5). Earth-quakes 1, 3, 4, 5, 6,12, 16, 18 have larger momentrelease in the first part of the process, while for shocks15, 2, 17, 7, 19 this occurs in the last part. Shallowearthquakes (Figure 5, shown in black) with mb<6and epicenter inside the continent show simple STF,with a total duration less that 3s (events 13 and 18).These shocks can be associated with a single rup-ture. Earthquakes with larger magnitude (mb≥6) showlonger duration of STF (between 6s and 12s) and com-plex STF (events 2, 4, 5, 15, 16). Events 2 and 15initiate with small release followed, after 2s, by alarger one. Event 5 shows a complex rupture processformed by two subevents, with a shorter STF for thefirst one. Events with epicenters offshore, betweenthe trench and the coastline, show very complex STF(events 17, 19 and 1), with larger duration (25 and 50sfor events 17 and 19). Event 17 shows four pulses, thelargest being the third. Event 19 shows a small energyrelease at the beginning. All these STF may representcomplex rupture process.

Intermediate depth earthquakes mainly showsimple STF (Figure 5 shown in grey), in general cor-responding to a simple impulse of triangular form with

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Table 3. Hypocentral data and references of earthquakes with focal mechanism for this area

Date Latitude Longitude Depth (km) mb References

May 24, 1940 –11.6 –77.5 20 8.0 Beck and Ruff (1989)

Nov 10, 1946 –8.4 –77.8 15 6.5 Doser (1987)

Apr 18, 1962 –10.0 –79.0 39 6.7 Stauder and Bollinger (1966)

Apr 13, 1963 –6.2 76.5 125 7.0 ”

Aug 15, 1963 –13.8 –69.3 543 8.0 ”

Aug 29, 1963 –7.1 –81.6 23 6.5 Stauder (1975)

Sep 17, 1963 –10.6 –78.2 60 6.7 Wagner (1972)

Sep 24, 1963 –10.6 –78.0 80 7.0 Stauder (1975)

Nov 03, 1963 –3.5 –77.8 18 6.7 Suarez et al. (1983)

Nov 09, 1963 –9.0 –71.5 600 7.0 Wagner (1972)

Nov 10, 1963 –9.2 –71.5 600 6.7 ”

Jan 26, 1964 –16.3 –71.7 119 6.1 Isacks and Molnar (1971)

Nov 02, 1964 –4.1 –76.9 91 6.0 ”

Nov 28, 1964 –7.9 –71.3 650 6.0 ”

Jul 30, 1965 –18.1 –70.8 72 6.1 Stauder (1973)

Aug 03, 1965 –7.7 –81.3 49 5.8 Stauder (1975)

Nov 03, 1965 –9.1 –71.3 593 6.2 Wagner (1972)

Dec 30, 1965 –16.6 –71.6 112 5.8 Isacks and Molnar (1971)

May 01, 1966 –8.5 –74.3 165 5.8 Wagner (1972)

Jun 07, 1966 –14.9 –75.8 48 5.7 Stauder (1975)

Oct 17, 1966 –10.7 –78.7 38 6.3 Wagner (1972)

Feb 09, 1967 –2.9 –74.9 58 6.3 ”

Feb 15, 1967 –9.0 –71.3 597 6.2 ”

Sep 03, 1967 –10.6 –79.8 38 6.7 ”

Oct 11, 1967 –10.3 –71.1 590 5.8 Petersen (1976)

Jun 19, 1968 –5.5 –77.2 20 6.1 Suarez et al. (1983)

Jun 20, 1968 –5.5 –77.3 33 5.8 ”

Jul 30, 1968 –6.9 –80.4 37 5.8 Wagner (1972)

Sep 28, 1968 –13.2 –76.3 70 6.4 ”

Oct 31, 1968 –16.4 –73.4 47 5.7 Stauder (1975)

Feb 04, 1969 –8.2 –80.2 16 5.9 Wagner (1972)

Jul 19, 1969 –17.3 –72.5 54 5.9 Stauder (1975)

Jul 24, 1969 –11.9 –75.1 1 5.9 ”

Oct 01, 1969 –11.9 –75.1 4 5.9 ”

Feb 14, 1970 –9.8 –75.5 28 5.8 Suarez et al. (1983)

May 31, 1970 –9.3 –78.8 43 6.6 Stauder (1975)

Jun 02, 1970 –9.3 –79.8 49 5.7 ”

Jun 02, 1970 –10.1 –78.6 62 5.8 ”

Jun 04, 1970 –9.8 –78.6 57 5.8 ”

Jun 17, 1970 –15.8 –71.8 91 5.9 ”

Dec 10, 1970 –3.9 –80.6 32 6.3 Suarez et al. (1983)

Oct 15, 1971 –14.2 –73.4 8 5.7 ”

Jan 12, 1972 –6.9 –71.8 580 6.0 Stauder (1975)

Mar 20, 1972 –6.7 –76.7 38 6.1 ”

Jan 05, 1974 –12.4 –76.3 93 6.3 Dewey and Spence (1979)

Oct 03, 1974 –12.3 –77.6 10 6.6 Beck and Ruff (1989)

Nov 09, 1974 –12.6 –77.5 20 6.0 ”

May 15, 1976 –11.6 –74.4 18 5.9 Suarez et al. (1983)

Apr 18, 1993 –11.6 –76.5 106 6.0 Tavera (1995)

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Figure 6. Distribution of P and T axes for shallow (circles) and intermediate (squares) and deep (triangles) depth earthquakes. Black arrowscorrespond to results obtained in this study and white arrows by other authors (Table 3).

short time duration, less than 5s for events 8, 10, 11and 14. Events 6, 7 and 12 (mb>6) show severalimpulses that can be interpreted as a more complexrupture process. In events 12 and 6 large moment re-lease takes place in the first part of the process andfor event 7 in the second. Deep depth events (3 and 9)present simple STF (Figure 7 shown in white) witha total duration of about 8s. These values of STFduration agree with results obtained by other authors(Chung and Kanamori, 1980; Kanamori and Given,1981; Fukao and Kikuchi, 1987; Ekström and Eng-dahl, 1989, Bos et al., 1998; Campus and Das, 2000).In general, we can conclude that with the same mag-nitude, shallow earthquakes have longer duration andmore complex STF than shocks at intermediate andvery deep depth.

A 3D scheme for the subduction process obtainedfrom the results of this study is presented in Figure 7.Maximum depth of subducted plate has been estim-ated from earthquake depths along 25 vertical crosssections normal to the coastline. Two types of sub-

duction are present in our scheme: flat subduction innorthern and central Peru and normal subduction insouthern Peru. The flat subduction in northern andcentral Peru has a maximum depth of 100–150 km.Normal subduction has a constant dip angle of 30◦ till350 km depth. These differences may be due to thewidth and depth of the Cordillera: 250km and 75 kmin southern Peru and 50 km in north and central re-gions (James, 1971; Jordan et al., 1983). T axes withsimilar dip as the subducted plate have been plotted(Figure 7) showing a tensional process along the sub-ducted plate in northern and southern regions, wherethe lithosphere material is stretched and it is pushedto deeper zones. About latitudes 14◦S to 15◦S, T axeschange in trend to a N-S orientation, and this may ex-plain the contortion of the plate and the change fromflat to normal subduction.

Summarising, the horizontal compression ob-served at shallow depth (h<60 km) is due to theconvergence of oceanic and continental plates. A sec-ondary effect of this is observed at the eastern part

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Figure 7. 3D scheme for the subduction processes in Peru. Arrows show tensional axes dipping parallel to the subducted plate.

of the Cordillera with a continental collision due tothe underthrusting of the Brazilian shield under theCordillera Oriental. The extension observed at in-termediate and deep depth is a consequence of thestretching of the subducted Nazca plate at intermediatedepths.

Conclusions

The study of the seismicity and focal mechanismsof 19 earthquakes in Peru reveals a different beha-viour for shallow, intermediate and very deep depthshocks. Spatial and depth distribution of the earth-quakes has permitted a better constraint for the geo-metry of the subduction process in Peru. The northernand central regions present flat subduction with max-imum depths of 100–150 km. The southern regionshows normal subduction dipping 30◦ with maximumdepths of 300 km. The deep earthquakes are distrib-uted between 500 and 700 km depth and are locatednear the Peru-Brazil and Peru-Bolivia borders.

Solutions obtained for shallow earthquakes showreverse faulting with planes striking NW-SE to N-S.For intermediate depth earthquakes solutions corres-pond to normal faulting. Deep earthquakes presentfocal mechanisms of normal faults with planes ori-

ented in NW-SE to N-S direction for earthquakeslocated in the Peru-Brazil border, and E-W near thePeru-Bolivia border.

Source time functions show different characterist-ics depending on the magnitude and depth of the earth-quakes. Shallow earthquakes located offshore withlarge magnitude show complex STF and long timeduration. Within the continent shallow earthquakesshow less complexity and STF have shorter duration.Intermediate and deep earthquakes show simple SFT,with time duration proportional to their magnitude.

The stress regime inferred from our study, may beexplained in the following way. At shallow depths ahorizontal compressional regime is present perpendic-ular to the Peruvian-Chilean trench, that may be dueto convergence of the Nazca and Sudamerican plates.Inside the continent compressional stresses are per-pendicular to the direction of the Andean Cordillera.This process of compression may be produced bythe underthrusting of the Brazilian Shield under theCordillera Oriental, greater in the northern and cent-ral regions of Peru. A second stress regime may beinferred from the intermediate and deep depth seismi-city. At intermediate depth the stress pattern regime isof extensional type, parallel to the direction of plateconvergence. The anomalous N-S extension, present

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in the southern region of Peru, may be explained byplate contortion. The absence of deep earthquakesbetween 200 and 500 km depth in the central re-gion, and between 300 and 500 km in the southernregion and the different orientation of the tension axesobtained for the very deep shocks in both regions, sug-gests a different origin. A possible explanation may bein terms of a detached portion of the subducted slab,still cold and rigid that generates the very deep activity.

Acknowledgements

The authors wish to thank to Prof A. Udías and DrA. Negredo, Universidad Complutense de Madrid,Dr G. McInctosh, University of Manchester and DrM. Bezzeghoud, Universidade de Evora for helpfulcomments on the manuscript. This work has beensupported in part by the CICYT, Project AM97-0975-C02. Publication 388; Department of Geophysics,Universidad Complutense de Madrid.

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Annex 1

Fault-plane solutions obtained from first motion of P wave. Lower hemisphere of focal sphere has been represented.On top of each solution reference number and date are shown. Black circles correspond to compressions and whitecircles to dilatations.

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Annex 2

Results of body waves modelling. On top data of event, fault-plane orientation (strike, dip, slip), scalar seismicmoment and depth. Stations are shown as a letter over focal sphere. Observed seismograms on top and syntheticin bottom. Vertical scale is given in micron, horizontal scale in seconds. Black circle shows tension axes, in whitepressure axes.

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