84 -2 09

TECTONIC DEFORMATION OF THE ANDES

AND THE CONFIGURATION OF THE SUBDUCTED SLAB

IN CENTRAL PERU: RESULTS FROM A

MICRO-SEISMIC EXPERIMENT

G. Suarez, ,

5, J. Gagnepain2, A. Cisternas2, D. Hatzfeld ,

P. Molnar1, L. Ocola^ S.W. Roecker1 and J.P. Viode2,

6

1) Department of Earth and Planetary Sciences

Massachusetts Institute of Technology

Cambridge, MA 02139 USA

2) Institut de Physique du Globe

Universite de Paris VI

4 Place Jussieu

75006 Paris, France

3) Laboratoire de Geophysique Interne, IRIGM

Universite Scientifique et Medicale de Grenoble

Saint Martin d'Heres, France

4) Institute Geofisico del Peru

Lima, Peru

5) (present address) Lamont-Doherty Geological Observatory

Columbia University

Palisades, NY 10964 USA

6) Junior authors in alphabetical order

f.J.f.a.S.
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ABSTRACT

A microeartbquake study was conducted in the central Andes of Peru, east

of the city of Lima, to study the seismicity and style of tectonic deformation of

the Peruvian Andes. Although most of the stations forming our temporary

seismographic network were located on the high Andes, the vast majority of the

microearthquakes recorded occurred to the east: on the Huaytapallana fault in

the Eastern Cordillera or in the western margin of the sub-Andes. Thus, the subAndes appear to be the physiographic province subjected to the most intense

seismic deformation. Focal depths for the crustal events in this region are as

deep as 50 km. and the fault-plane solutions, as in the case of the teleseismic

earthquakes, show thrust faulting on steep planes oriented roughly north-south.

The Huaytapallana fault in the Cordillera Oriental also shows relatively high

seismicity along a northeast-southwest trend that agrees with the fault scarp

and the east-dipping nodal plane of two large earthquakes that occurred on this

fault on July 24 and October 1, 1969. Microearthquakes of intermediate depth

recorded during the experiment show a flat seismic zone about 25 km thick at a

depth of about 100 km. This agrees with the suggestion that beneath Peru the

slab first dips at an angle of 30° to a depth of 100 km and then flattens following

a quasi-horizontal trajectory (Grange et al 1983, Hasegawa and Sacks 1981).

Fault-plane solutions of intermediate-depth microearthquakes have horizontal T

axes oriented east-west.
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Introduction

The west coast of South America is the major active margin where an oceanic plate is being subducted beneath a continental plate. It represents a contemporary example of the tectonic regime that is often presumed to have

existed along the western margin of North America before the subduction of oceanic material ceased during the late Cenozoic (e.g. Atwater 1970, Hamilton

1969). Thus, understanding the style of deformation and evolution of the Andes,

an erogenic belt uncontaminated by a continental collision, may play a crucial

role in trying to decipher the complex tectonic history of previously active margins that are now dormant, or where an "Andean" margin has been followed by a

continental collision.

A useful tool to glean information about the style of deformation and tectonic regime of an active mountain belt is the study of the seismicity occurring

in it. The spatial distribution of seismic activity indicates where brittle deformation is taking place, while the style of faulting inferred from fault-plane solutions

allows inferences of the orientation of the stresses responsible for this

deformation. In the Andes, crustal seismic activity is low compared to that of

the subduction zone to the west, and the routine determination of hypocentral

locations published by the International Seismological Centre (ISC) and the

United States Geological Survey (USGS) are not accurate due both to the poor

geographical distribution of stations at teleseismic distances and to the sparse

coverage of the local stations; errors are specially large for focal depth determinations.

With the pupose of studying how the South American plate deforms due to

the subduction of the Nazca plate to the west, and to understand in more detail

the morpholgy of the subducted slab beneath central Peru we installed a temporary network of portable seismographs in the Altiplano and the Eastern
•*•

Cordillera of the central Andes, east of the city of Lima (Figure l).

Methods of Analysis

From May 13 to June 12, 1980. we operated a network of up to sixteen portable seismographs complementing the less dense permanent seismic network

operated by the Institute Geofisico del Peru (Figure 1 and Table 1). Our goal was

to monitor the seismic activity in the High Plateaus and the Eastern Cordillera

in the vicinity of the city of Huancayo (Figure 1). The geographic coordinates

and elevation above sea level of the portable stations were determined using

topographical maps published by the Institute Geografico Militar of Peru at a

scale of 1:100.000 (Table 1). We estimate the uncertainty of the positions of the

stations to be in the order of 250 meters.

We used Sprengnether MEQ-800 seismographs with Mark Products L4-C vertical seismometers. The amplifiers were generally set to 84 db, corresponding to

a magnification of about 5x 105 at 10 Hz. Earthquakes were recorded by a fine

stylus on smoked paper at a speed of 60 mm/min. Records were changed every

48 hours and the drift of the internal clock of each instrument was checked at

this same time interval, by directly recording time pulses transmitted by the

WWV station in Colorado, USA. Clock drifts during these 48 hour periods were

always less than 0.1 sec.

The arrival times of the phases of interest were digitized on a table-top digitizer. The accuracy of the readings depended on the level of background noise

and the sharpness of the onsets. On the average we estimate an accuracy of

about 0.1 seconds for P arrival times. S waves were also picked when they could

be confidently identified, and we estimate the error in S wave arrival time to be

in the order of 1 second.
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The earthquakes were located using the computer program HYPOINVERSE

developed at the USGS by Klein (1978). In the first iteration. P arrival times were

assigned a weight of 1 and S arrival times a weight of 0.5. The smaller weights of

the S waves compensate the larger uncertainty in arrival-time readings of S

waves. The algorithm assigns weights to the observed arrival time as a function

of the residual from the previous iteration. This is useful for damping arrival

times that are grossly in error, as in the case of misidentified phases. Arrival

times showing large residuals were reread.

A time correction (station delay) compensating for differences in elevation

was applied to all arrival times at a given station. The station corrections were

applied assuming a reference elevation of 3500 meters (the average elevation at

the center of the array) and assuming a mean P-wave velocity of 5.0 km/sec for

the top crustal layer.

Accuracy of the Locations

Effects of the Velocity Structure Utilized

The velocity structure in central Peru is poorly known. Although seismic

refraction studies have been undertaken in southern Peru and northern Bolivia

(e.g. Ocola et al 1971, Ocola and Meyer 1973. Tatel and Tuve 1958) little work has

been done in central Peru to constrain the velocity structure there. To examine

the uncertainties in our locations due to ignorance of the velocity structure, we

calculated locations for selected events using three alternative velocity models

(Table 2). The velocity structure inferred by Ocola and Meyer (1973) from a

refraction survey across the Andes in southern Peru, that obtained by James

(1971) from the dispersion of Rayleigh and Love waves along various paths in the

Central Andes, and the velocity structure based on results of a deep crustal

refraction experiment carried out in Colombia by Meissner et al (1977).
- B -

A random selection of 29 earthquakes occurring at various depths were

located using these three seismic velocity structures. The difference in epicentral coordinates and focal depths of these 29 earthquakes located using the

three alternative velocity models are shown in histogram form (Figure 2). In

general, epicentral locations do not appear to depend strongly on the choice of

the velocity structure; changes in epicentral coordinates are usually less than

four to six kilometers. Focal depths, however, are more sensitive to changes in

the velocity structure and may-change by as much as B to 14 km depending

upon the location of the event relative to the network. In both cases, however,

those earthquakes that exhibited changes in epicentral coordinates larger than

about 10 km are events that are poorly constrained because they lie outside or

on the periphery of the network.

The velocity model obtained by Ocola and Meyer (1973) generally produced

the smallest root-mean-square (rms) values of travel time residuals and was

adopted to locate all the events recorded. A ratio of Vp/Vs of 1.73, corresponding to a Poisson's ratio of 0.25 was used in all cases. Other studies have shown

that varying the ratio of Vp/Vs by about 0.3 produces changes in hypocentral

locations that are usually not larger than 2 km (e.g. Chatelain et al 1980, Prevot

et al 1980. Trehu 1982).

The Quality of Hypocentral Locations

As a result of the four weeks of recording a total of 344 earthquakes were

located (Figure 3). The events located span an area of about 400 by 400 km. Most

of these earthquakes occurred within the crust in the sub-Andes, east of the

Andean Cordillera. Seismically. the high Andes appear to be relatively inactive

with the exception of a linear northwest-southeast belt of earthquakes near the

Huaytapallana fault in the Eastern Cordillera (near 12 S, 75 W). Intermediatedepth events were also located beneath the high Andes and the sub-Andes. Very
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few events, however, were recorded along the main thrust contact between the

Nazca and the South American plate possibly because this area is far from our

network.

Many of the events lie outside the network and the maximum azimuthal gap

in recording stations is sometimes quite large. A rule of thumb for judging the

accuracy of hypocentral depth determination is that the distance from the

event to the closest station should be less than or roughly equal to the focal

depth. For most of the crustal earthquakes in the sub -Andes this is clearly not

the case since many of them lie outside of our network. Focal depths of most of

these crustal events appear to be less than 40 km and the distance to the

closest station is often as large as 100 to 150 km. Consequently. HYPOINVERSE

assigned large errors to the epicentral coordinates (ERH) and focal depths (ERZ)

of many events located in the sub-Andes. Therefore, it is necessary to establish

criteria for separating well located earthquakes from poor and unreliable locations. We consider as reliable locations those events for which a minimum of five

arrival times, including at least one S phase, were used, those for which the calculated errors in the horizontal (ERH) and (ERZ) directions are less than 10 km.

and earthquakes with rms travel-time residuals of less than 0.5 sec. The errors

in the hypocentral parameters of earthquakes meeting these criteria are probably not larger than about 5 to 10 km. Among the 344 events that were located,

only 114 events met these criteria (Figure 4).

Seismicity in the High Andes

Earthquakes in the Altiplano

It is clear that crustal seismicity in the Altiplano is relatively low (Figures 3,

4, and 5). Although the majority of our portable stations were deployed in the

western part of the Altiplano, only about fifteen crustal earthquakes were
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located beneath the network; the overwhelming majority of the crustal earthquakes occurred in the eastern margin of the Cordillera and in the sub-Andes

(Figures 3 and 4). This pattern of seismicity is consistent with studies of teleseismic events showing that most large, shallow events occur in the western part

of the sub-Andes (e.g. Stauder 1975. Suarez et al 1983). Thus, insofar as both the

microearthquakes reported here and the historical seismicity reflect the degree

of active tectonics, the Altiplano appears to be more stable and at present is not

subjected to erogenic deformation as intense as that affecting parts of the

Eastern Cordillera and the whole of the western sub-Andes. Otherwise, the deformation must either be episodic with large periods of quiescence or be absorbed

by creep or viscoelastic behaviour.

Those few events that occurred in the Altiplano were all very small and their

fault-plane solution could not be obtained. Thus, we could not address one of

our original objectives: whether of not normal faulting occurs in the Altiplano of

central Peru, as it does in the Cordillera Blanca of the high Andes in northern

Peru (Dalmayrac 1974. Yonekura et al 1979) and in the Bolivian Altiplano

(Lavenu 1978, Lavenu and Ballivian 1979, Mercier 1981).

The Huaytapallana Fault

The Huaytapallana fault is located in the Eastern Cordillera of central Peru,

a few kilometers east of the city of Huancayo (Figure l). Fault-plane solutions of

two large earthquakes that occurred on this fault on July 24 and October 1, 1969

show reverse faulting (Stauder 1975). The earthquake on October 1 produced a

steep fault scarp dipping east, with a vertical displacement reaching 1.6 m (Deza

1971. Philip and Megard 1977). Philip and Megard interpret the fault as one of a

group of reactivated parallel faults trending NNW-SSE (Paredes 1972) (Figure 6).

The existence of a large number of small microearthquakes shows that the

Huaytapallana fault (or system of faults) is still active (Figures 4 and 6). Station
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HYT. installed at the southeastern end of the mapped surface rupture,

registered an average of about 30 events per day with S-P time differences of

less than 4 seconds. The epicenters of earthquakes located in the vicinity of the

Huaytapallana area follow a trend in a NNW-SSE direction, parallel to the

mapped surface faults and to the strike of the east-dipping nodal plane of the

fault-plane solutions of the two larges Huaytapallana earthquakes (Stauder 1975.

Suarez et al 1983) (Figure 6).

Both the fault-plane solution of one of the largest microearthquakes and a

composite fault-plane solution for four other events show reverse faulting with

some left lateral strike-slip motion occurring on a fault plane dipping east with a

strike of about 30° west of north (Figure 7). This agrees with the sense of motion

observed at the fault (Philip and Megard 1977) and with the fault-plane solution

of the two large 1969 events.

In the basins just west of the Huaytapallana fault. Dollfus and Megard (1968)

reported folding of Quaternary glacial morraines. The strike of these folds

N30°W and the orientation of the P axes of the fault-plane solutions of earthquakes in the Huaytapallana fault (Figure 7) suggest this region is being

deformed by a regional horizontal stress oriented roughly east-west, in the

direction of plate motion.

Based on a study of the Huaytapallana fault scarp. Philip and Megard (1977)

suggest that the overall length of the fault is approximately 3.5 km long. The

length of the fault of the October 1, 1969 earthquake can be estimated by considering the average displacement u on the fault to be 1.6 m (Philip and Megard

1977). the rigidity /z equal to 3.3x 1011 , a fault width of between 10 to 15 km, and

a seismic moment, M0, of l.Ox 1086 dyne-cm (Suarez et al 1983). From the formula M0=fj.uS (Aki 1966), where S is the area of the fault, the October 1 earthquake requires a fault about 12 to 20 km long. Thus, it is likely that the 1969
- 10-

event ruptured a fault that extends north of the surface break, to where the

microearthquake activity occurs today. The epicenters of most of the earthquakes recorded in this area lie about 10 km east of the projected surface location of the fault (Figure 6). .

A cross section across the fault shows that the depths of the earthquakes

tend to increase towards the east (Figure B), consistent with an east-dipping

fault plane. The locations are not accurate enough to determine, however,

whether the earthquakes take place on a single fault plane or on a series of

imbricate faults.

Seismicity of the sub-Andes

Description of the Seismicity

The sub-Andes of central Peru are seismically more active than the high

Andes and appear to be the physiographic province in the Peruvian Andes

currently undergoing the most intense seismic deformation. The vast majority of

earthquakes in the overriding continental plate are concentrated in the subAndean region over an area roughly 100 km wide. These events lie east of the

main Andean Cordillera and occur beneath areas of low topographic elevation

(Figures 3 and 4). Most of these events occurred at large distances from the

eastern margin of the network and. therefore, the focal depths of these events

are very poorly resolved. Station POC operated by the Institute Geofisico del

Peru is the only station located in the sub-Andes, but due to its low gain it could

not be used to locate most of the microearthquakes that occurred in the area.

In the western part of the sub-Andes there are enough well located events

to show that here the seismicity affects most of the crust, at least to depths of

about 40 to 50 km, in agreement with the depths of two nearby teleseismic

events (Chinn 1982. Suarez et al 1983). Although most of the microearthquakes
-11-

seem to occur at mid-crustal depths of between 15 and 25 km, one event clearly

took place at a depth of 50 km. This earthquake occurred beneath the network

and was located by 15 stations; the horizontal distance to the nearest station

was about 30 km. Even though the travel-time residuals are relatively large

(about 0.4 s). the estimated focal depth changes by less than 5 km for the

different velocity models used, and the calculated errors in the vertical direction (ERZ) are always less than four kilometers. Below a depth of about 50 km

there seem to be no earthquakes within the South American plate. An aseismic

zone extends to a depth of about 90 km where intermediate-depth earthquakes

associated with the subducted slab occur (Figure 5).

Fault-Plane Solutions and Tectonic Interpretation

A composite fault-plane solution from six events located just east of the network at depths of between 15 and 26 km shows thrust faulting with a component

of strike-slip motion (Figure 9a). This solution is very similar to that of the

larger May 15, 1976 event that occurred about 25 km east of this group of events

at a depth of 20 km (Chinn 1982. Suarez et al 1983) (Figure 10). A fault-plane

solution obtained for an event in the sub-Andes suggest nearly pure reverse

faulting (Figure 9b). Both of these solutions have the P axes oriented northeastsouthwest, in a direction perpendicular to the mountain range (Figure 10).

The existence of earthquakes in the sub-Andes at middle and lower crustal

depths of between 20 and 50 km is unusual. In general, brittle deformation in

the form of earthquakes is confined to the top 15 km of the crust (Chen and Molnar 1983, Meissner and Strelhau 1982). Below this depth, the absence of earthquakes is inferred to be due to the sharp decrease in strength of the typical crustal minerals at depth under normal geothermal gradients (e.g. Brace and

Kohlstedt 1980, Caristan 1980). To explain both the concentration of intense

deformation along the western margin of the sub-Andes and the unusually deep
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seismic deformation of up to 40 or 50 km. Suarez et al (1983) suggested that the

seismicity results from the underthrusting of the cold Brazilian shield beneath

the eastern margin of the Cordillera Oriental.

It appears that the stresses applied to the western coast of the South American plate by the subduction of the Nazca plate are not elevating the Andean terranes that already have a high elevation. Instead, they seem to break and shorten the crust to the east where it is probably thinner. This crustal shortening

would thicken the crust and uplift the topography of the eastern Cordillera causing the Andean mountain range to grow eastward. In this scenario, the relatively

deep microearthquakes and the large teleseismic events in the western subAndes would reflect the tectonic deformation of the underthrust Brazilian shield

(Suarez et al. 1983). The age and intensity of the sedimentary rocks in the sub

Andes is generally shown to decrease progressively to the east (Dalmayrac et al

1980, Megard 1978) and may be interpreted as the result of the a detached

wedge of sedimentary rocks lying east of the zone of intense deformation in the

western sub Andes. The large number of shallow microearthquakes occurring in

the sub Andes proper would then reflect the thin-skinned deformation of this

accretionary prism.

Intermediate Depth Microearthquakes

Shape of the Subduction Zone beneath the Central Andes

Based on the study of a carefully selected catalog of teleseismically located

events. Barazangi and Isacks (1976, 1979) inferred that the seismic zone dips at

about 10° beneath Peru. As James (1978) pointed out. most of the events used

by Barazangi and Isacks to infer this dip are shallow events along the coast or

from a cluster of intermediate-depth earthquakes in northern Peru. Using

earthquakes recorded by networks in southern Peru. Grange et al (1983) and
- 13-

Hasegawa and Sacks (1981) inferred that the seismic zone beneath the northern

part of their network dips at about 30° down to a depth of 100 km and then

flattens. Because of the locations of their stations, neither study could

confidently locate events more than 150 km from the coast to determine how far

the seismic zone remains horizontal.

During our field investigation we located twenty intermediate-depth earthquakes ranging in depth from 85 to 110 km with uncertainties of less than about

10 km. These earthquakes seem to define a flat seismic zone about 25 km thick

beneath the Andes and suggests that the seismic zone remains flat at a depth of

about 100 km for at least a distance of 450 km from the trench (Figure 5). The

slab does not seem to dip a constant slope of 15" as inferred by Barazangi and

Isacks {1976, 1979). Very few events, however, were located on the main thrust

contact between the Nazca and the South American plates and we were unable

to constrain the dip of the initial portion of the subduction zone.

One of us (L. Ocola) prefers to interpret the seismicity as two slabs dipping

at 30°; an active subduction zone along the western coast of South America and

a now dormant and deeper one beneath the sub-Andes connected by a low dip

"detachement" zone (Ocola. 1983).

Fault-Plane Solutions and the Continuity of the Slab

The fault-plane solutions determined for four of these intermediate -depth

microearthquakes show normal faulting with T axes lying almost horizontally

and parallel to the direction of relative plate motion (Figures 11 and 12).

The orientation of the T axes in the direction of the dip of the slab at intermediate depths in other subduction zones of the world has usually been interpreted as a result of the gravitational body force acting on the downgoing slab

(Isacks and Molnar. 1969). If the intermediate part of the slab lies flat, however.
- 14 -

Bind it is decoupled from its deepest segment as suggested by" the absence of

earthquakes at depths of between 200 and 550 km in Peru, it is difficult to

ascribe the horizontal T axes to gravitational body forces acting on the slab

where the earthquakes occur.

Hasegawa and Sacks (1981) suggest that the horizontal T axes could be

explained by a continous slab descending to a depth of 600 km with an aseismic

portion between 200 and 550 km. Our results suggest that the seismic zone

remains horizontal to a distance of about 650 km from the trench, where it

appears to bend again and continue to greater depth with a relatively steep

angle (Figure 13). This interpretation is supported by the presence of a cluster

of intermediate-depth events occurring at depths of about 150 km beneath

eastern Peru (see cross section B in Figure 3 of Barazangi and Isacks 1978). One

could speculate that a large number of earthquakes occur here because of the

stresses produced by the forces bending the slab, in a situation analogous to

that of a plate bent at a trench.

It is unclear what could cause these contortions of the downgomg slab: the

unbending of the plate to underplate the Andes horizontally over a distance of

about 300 km. and the second change in dip thereafter (Figure 13). A possible

culprit for this behaviour may be the buoyant nature of the Nazca ridge that

resists subduction due to its lighter composition. Based on reconstructions of

past plate motions. Pilger and Handshumacher (1981) suggest the Nazca and

Tuamotu Ridges represent mirror images of hot-spot traces formed at the

Pacific-Farallon plate boundary. The former resting on the Nazca plate and the

latter on the Pacific plate. In order to be a mirror image of the Tuamotu Island

Chain, the subducted eastern continuation of the Nazca Ridge must bend to the

northeast and lie now beneath Peru (Pilger 1981). The buoyancy added to the

slab by the ridge might induce the very shallow dip of the subduction zone in
-15-

this area. South of the Nazca Ridge, in southern Peru and northern Chile, the

subducted slab does not have this positive buoyancy and the subduction process

reverts to a steeper and more normal dip of about 30°. The more steeply dipping slab lying to the east of the flat seismic zone (Figure 13) may be interpreted as a remnant of a normal episode of plate subduction prior to the arrival

of the easternmost extension of the Nazca Ridge to the trench.
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Summarj

A microseismic study of the Peruvian Andes east of the city of Lima was

conducted during the summer of 1930 deploying a portable seismic network for

three weeks. The seismicity reveals that the most intense seismic deformation

of this portion of the Peruvian Andes occurs in the sub Andes. The majority of

the microearthquakes located occur to the east of the high Andes beneath areas

of low topographic relief. In the western margin of the sub Andes the seismicity

reaches a depth of up to 50 km. The fault-plane solutions obtained for events in

the western sub-Andes show high-angle reverse thrusting on planes oriented

roughly north-south, in agreement with the orientation of the stresses expected

from the direction of relative plate motion. The high Andes appear to be relatively aseismic suggesting that if they are being currently deformed by the

stress that produces the high rate of seismicity in the sub Andes, the deformation is plastic and not brittle in nature. In the high Andes, only the Huaytapallana fault in the Eastern Cordillera shows a high rate of seismic activity. Here,

the fault-plane solutions obtained from the microearthquakes are similar to

those of the two large earthquakes that ruptured the fault in 1969. The microseismlc activity in the vicinity of the Huaytapallana fault follows a trend parallel to

mapped surface faults and the earthquakes become deeper from west to east

suggesting an east-dipping fault plane. The intermediate depth events located

range in depth from about 85 to 100 km. They define a flat seismic zone that

underplates the Peruvian Andes horizontally up to a distance of 650 km from the

trench. This is compatible with results obtained in southern Peru which suggest

that beneath Peru the slab dips initially at an angle of about 30° down to a depth

of 100 km and then flattens following a horizontal trajectory (Grange et al 1933,

Hasegawa and Sacks 1981). The fault-plane solutions of the largest of these

intermediate depth events show quasi-horizontal T axes oriented in the direction
- 17-

of relative plate motion.

Acknowledgements

We thank R. Benites, A. Flores. and F. Pardo for ably assisting us in the field

and D. Huaco. F. Megard. and M. Sebrier for their advice and help in logistics. We

gratefully acknowledge the support provided by Ing. A. Giesecke, F. del Castillo,

and J. Lanar at the Observatorio de Huancayo, Peru. This work was supported by

NASA contract NAG5-300 and a grant from the Institut National d'Astronomie et

de Geophysique.
18

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Geophys. Res., 76, 3246-3271.

James, D.E., 1978. Subduction of the Nazca plate beneath Central

Peru, Geology, _7, 174-178.

Klein, F.W., 1978. Hypocenter location program Hypoinverse, Open-file

report 78-694, United States Geological Survey, Menlo Park,

113PP.

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nord de 1'Altiplano bolivien (Region de la Paz-Ayo-Ayo-Umala),

Cah. O.R.S.T.O.M., ser. Geol., X, No. 1, 115-126.

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107-129.

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de la Recherche Scientifique et Tecnique Outre-Mer, Mem. 86, 310

pp.

Meissner, R. & Strehlau, J., 1982. Limits of stresses in continental

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shallow earthquakes, Tectonics, J_, 73-89.

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with the Aegean Arc: comparison with the Andean Cordillera of

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337-355.
21

Ocola, L.C., Meyer, R.P. & Aldrich, L.T., 1971. Gross crustal

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Tectonophys., 38, 259-278.

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92, 448-456.

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22

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Geography, Tokyo, 88, 1-19.
TABLE 1

STATION COORDINATES

23

NAME

AGO

SAC

MAR

UNC

COS

CUL

LAI

PAC

HYT

PAM

PAG

ATO

MOL

PAR

COC

COM

YAU

PA1

PR1

VIS

CHA

GUA

CAL

QUI

SJU

NNA

LM2

HUA

VES

ANC

AYA

POC

LAT

-11.981

-11.778

-11.604

-11.235

-12.140

-12.203

-12.308

-11.778

-11.960

-12.435

-12.003

-12.341

-11.726

-11.666

-11.890

-11.699

-11.714

-11.782

-11.672

-12.592

-12.011

-13.993

-12.627

-12.943

-15.356

-11.998

-12.068

-12.038

-12.213

-11.775

-13.080

-11.250

LONG

-75.095

-75.191 '•

-75.653

-75.374

-75.562

-75.211

-75.359

-75.727

-75.039

-74.870

-74.917

-75.087

-75.409

-75.085

-75.305

-75.082

-75.469

-75.721

-75.083

-74.961

-75.374

-75.789

-75.978

-76.437

-75.189

-76.843

-77.033

-75.323

-76.937

-77.150

-74.250

-74.600

DELAY

(sec)

0.08

0.16

0.11

-0.36

0.23

0.06

0.07

0.03

0.23

0.04

0.01

0.08

0.02

0.12

-0.01

-0.12

-0.02

-0.03

-0.12

0.12

0.00

-0.64

-0.42

-0.68

-0.77

-0.67

-0.67

-0.05

-0.77

-0.78

-0.17

-0.63

LOCATION ELEV (m)

ACOPALCA

SACSACANCHA

MARCAJASHA

UNCUSH

COSMOS

CULHUAS

LAIVE

PACHACAYO II

HUAYTAPALLANA

PAMPAS

PAGUA

ATOMPAMPA

MOLINOS

PARCO II

CONCEPCION

COMAS

YAULI

PACHACAYO I

PARCO I

VISCAPATA

CHAMBARA

GUADALUPE

CALACOCHA

QUILMANA

SAN JUAN

NANA

LIMA2

HUANCAYO

V.EL SALVADOR

ANCON

AYACUCHO

PTO OCOPA

3900

4250

4050

1925

4600

3800

3850

3700

4600

3750

3600

3900

3650

3000

3500

3000

3450

3700

3000

4100

3350

678

1655

510

75

555

127

3313

110

56

2800

750
TABLE 2

VELOCITY MODELS

Ocola and Meyer (1975)

Depth to

Vp (km/sec) layer (km)

5.0 0.0

5.6 10.0

6.0 22.0

7.9 65.0

8.0 100.0

Meissner et al (1977)

Depth to

Vp (km/sec) layer (km)

5.9 0.0

6.2 10.0

6.7 30.0

8.1 50.0

James (1970)

Depth to

Vp (km/sec) layer (km)

5.0 0.0

6.0 8.0

6.6 25.0

7.9 60.0

8.0 100.0
25

Figure Captions

Figure 1 . Main physiographic units of the central Andes of Peru

(after Megard 1978). Symbols show the location of the seismographic

stations; filled symbols are the stations of the permanent Peruvian

Network.

-Figure 2. a) Histograms showing changes in calculated epicenters and

focal depths for 29 events using the velocity structures proposed by

Ocola and Meyer (1973) and James (1971) for the Central Andes.

b) Changes in calculated epicenters and focal depths for

the same events using the velocity structures proposed by Ocola and

Meyer (1973) for southern Peru and by Meissner et. al. (1977) for

southern Colombia.

Figure 3. Epicenters of all 344 earthquakes recorded during the field

experiment. Crosses indicate calculated depths shallower than 50 km

and the triangles events deeper than 50 km. Closed symbols show locations of stations used to locate the earthquakes. Dashed lines show

the 1000 and 3000 meter topographic contours of the central Andes.

Figure 4. Epicenters of screened catalog of well located microearthquakes. Symbols as in Figure 5.

Figure 5. Hypocenters of screened microearthquakes shown in Figure 6

projected on the cross section A-B. Vertical lines show the projected

positions of the stations.
26

Figure 6. Epicenters of shallow events in the vicinity of the

Huaytapallana fault plotted on a geological sketch-map of the Eastern

Cordillera (after Philip and Megard 1977). Closed circles show

epicenters and filled diamonds the seismic stations. Stars are the

epicentral locations of the July 24 and October 1, 1969 earthquakes

given by the ISC.

Figure 7. a) Composite fault-plane solution of four earthquakes b)

Solution for single event located near the Huaytapallana fault, and c)

Fault-plane solution for the July 24, 1969 earthquakes (Suarez et.

al. 1983). These are lower-hemisphere equal-area projection. Open

symbols indicate dilatation and closed symbols compression.

Figure 8. Hypocenters of the events shown in Figure 6 projected onto

cross section A-B (Figure 6). Arrows indicate the positions of the

stations and the dashed line is the assumed dip of the Huaytapallana

fault from the fault-plane solution of the 1969 events (Stauder 1975).

Figure 9. a) Comparison of the composite fault-plane solution for six

events in the sub-Andes and the fault-plane solution of the May 15,

1976 earthquake (Suarez et. al. 1983) that was located about 20 km

east of the events used in the composite solution.

b) Fault-plane solution of an earthquake located in the western

sub-Andes. Symbols as in Figure 9.
27

Figure 10. Map of the central Andes summarizing lower hemispheric

projections of fault-plane solutions of shallow crustal events obtained in this study. Dark areas represent quadrants of compressional

first motion. Numbers near fault-plane sulutions correspond to event

numbers in Figure 7 and 9. Also shown are the fault-plane solutions

of two large events (stars) studied by Suarez et. al. (1983) in this

area.

Figure 11 . Fault-plane solutions for four intermediate-depth events

recorded in this study. Note T axes are nearly horizontal and

oriented roughly east-west in all cases.

Figure 12. Map of central Peru summarizing lower hemisphere projections of the fault plane-solutions of intermediate-depth earthquakes

obtained in this study. Numbers near fault-plane solutions correspond

to event numbers shown in Figure 11. Shown as stars are the locations

and faultplane solutions of the intermediate-depth events studied by

Stauder (1975). Dark areas indicate quadrants of compressional first

motions.

Figure 13. Cross section showing the inferred geometry of the subducted slab beneath Peru. To the earthquakes located in this study

(shown as circles) we have added the best located intermediate-depth

events selected by Barazangi and Isacks (1976) in Peru shown as

squares). T axes of intermediate-depth events in this area are shown

as arrows.
28

12'

13 ••

Pacific

Ocean

77 °W 76° 75

Figure 1
29

Vp/Vs-1.75

6 8 10 12 14 16 18 20

Epicentral Location Change (km)

22

i

24

I

12

i

8 10 12 14 16 18

Focal Depth Change (km)

20

I

22 24

Figure 2a
30

2 4 6 8 10 12 14 16

Epicentral Location Change (km)

Vp/Vs-1.75

8 10 12 14 16 18

Focal Depth Change (km)

32 34

Figure 2b
31

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36

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