Geophys. J. Int. (2005) 163, 580–598
doi: 10.1111/j.1365-246X.2005.02759.x
Crustal deformation in the south-central Andes backarc terranes
as viewed from regional broad-band seismic waveform modelling
Patricia Alvarado,1,2 ∗ Susan Beck,1 George Zandt,1 Mario Araujo2,3 and Enrique Triep4
1 Geosciences
Department, University of Arizona, Tucson, AZ, USA
de Geofı́sica y Astronomı́a, Universidad Nacional de San Juan, Argentina
3 Instituto Nacional de Prevención Sı́smica, Argentina
4 Instituto Geofı́sico Sismológico Fernando Volponi, Universidad Nacional de San Juan, Argentina
2 Departamento
Accepted 2005 July 22. Received 2005 July 11; in original form 2004 December 10
GJI Seismology
SUMMARY
The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31◦ S. Earthquake locations define the subhorizontal subducted
oceanic Nazca plate at depths of 90–120 km. Another seismic region is located within the continental upper plate with events at depths <35 km. This seismicity is related to the Precordillera
and Sierras Pampeanas and is responsible for the large earthquakes that have caused major
human and economic losses in Argentina. South of 33◦ S, the intense shallow continental seismicity is more restricted to the main cordillera over a region where the subducted Nazca plate
starts to incline more steeply, and there is an active volcanic arc. We operated a portable broadband seismic network as part of the Chile–Argentina Geophysical Experiment (CHARGE)
from 2000 December to 2002 May. We have studied crustal earthquakes that occurred in the
back arc and under the main cordillera in the south-central Andes (29◦ S–36◦ S) recorded by
the CHARGE network. We obtained the focal mechanisms and source depths for 27 (3.5 <
M w < 5.3) crustal earthquakes using a moment tensor inversion method. Our results indicate
mainly reverse focal mechanism solutions in the region during the CHARGE recording period.
88 per cent of the earthquakes are located north of 33◦ S and at middle-to-lower crustal depths.
The region around San Juan, located in the western Sierras Pampeanas, over the flat-slab segment is dominated by reverse and thrust fault-plane solutions located at an average source depth
of 20 km. One moderate-sized earthquake (event 02-117) is very likely related to the northern
part of the Precordillera and the Sierras Pampeanas terrane boundary. Another event located
near Mendoza at a greater depth (∼26 km) (event 02-005) could also be associated with the
same ancient suture. We found strike-slip focal mechanisms in the eastern Sierras Pampeanas
and under the main cordillera with shallower focal depths of ∼5–7 km. Overall, the western
part of the entire region is more seismically active than the eastern part. We postulate that this
is related to the presence of different pre-Andean geological terranes. We also find evidence
for different average crustal models for those terranes. Better-fitting synthetic seismograms
result using a higher P-wave velocity, a smaller average S-wave velocity and a thicker crust
for seismic ray paths travelling through the crust of the western Sierras Pampeanas (Vp = 6.2–
6.4 km s−1 , Vp/Vs > 1.80, th = 45–55 km) than those of the eastern Sierras Pampeanas
(Vp = 6.0–6.2 km s−1 , Vp/Vs < 1.70, th = 27–35 km). In addition, we observed an apparent
distribution of reverse crustal earthquakes along the suture that connects those terranes. Finally, we estimated average P and T axes over the CHARGE period. The entire region showed
P- and T-axis orientations of 275◦ and 90◦ , plunging 6◦ and 84◦ , respectively.
Key words: Andes backarc, continental crust, earthquake-source mechanism, seismotectonics, subduction zone.
1 I N T RO D U C T I O N
∗ Corresponding author: Gould Simpson Building #77, 1040 E 4th St.,
Tucson AZ 85721 USA. E-mail: alvarado@geo.arizona.edu
580
Crustal seismicity in Argentina and Chile between 29◦ S and 36◦ S
is related to the Andean deformation (Fig. 1). The region is
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2005 RAS
Moment tensor inversion of Andean backarc crustal earthquakes
581
-1
Figure 1. PDE-NEIC seismicity during the CHARGE project seismic deployment. The CHARGE broad-band seismic instruments operated continuously
between 2000 December and 2002 May. Contours represent approximately the depth to the top of the subducted slab by Cahill & Isacks (1992). We studied
27 shallow earthquakes that occurred in the main cordillera of the Andes and backarc region with magnitudes M > 4.0 and depths <60 km (see the text and
Fig. 9).
characterized by the convergence between the oceanic Nazca plate
and the continental South America plate at a rate of 6.3 cm yr−1
and an azimuth of 79.5◦ (Kendrick et al. 2003). Major along-strike
changes in the Wadati-Benioff zone occur in this segment. The strike
of the Nazca plate changes in dip from subhorizontal to more inclined in the vicinity of 33◦ S (Barazangi & Isacks 1976; Cahill &
Isacks 1992) (Fig. 1). Approximately 490 earthquakes of magnitude greater than 4.0 and focal depth <50 km have occurred in
the last 20 yr in the main cordillera and backarc region between
29◦ and 36◦ S. In addition, large crustal earthquakes have caused
many deaths and great devastation near San Juan and Mendoza
(Argentina) in 1861, 1894, 1944, 1952 and 1977 (Castellanos 1944;
Groeber 1944; INPRES 1977; Triep 1979; Volponi 1979; Bastias et al. 1993). There is also a record of one shallow Ms = 6.9
earthquake that occurred in the main cordillera causing damage in
Chile in 1958 (Piderit 1961) (Fig. 2). Recently two crustal earthquakes with moment magnitudes of ∼6.4 have occurred in the main
Andean cordillera (PDE-NEIC location 34.93◦ S and 70.39◦ W, depth
16 km) and in Catamarca province (INPRES location 28.73◦ S and
66.20◦ W, depth 30 km) in 2004 August to September.
The backarc seismicity in the immediate eastern flank of the
Andes since 1963 has been studied using teleseismic data and shortperiod seismic networks (Stauder 1973; Barazangi & Isacks 1976;
INPRES 1977, 1985; Triep 1979, 1987; Chinn & Isacks 1983;
Triep & Cardinalli 1984; Giardini et al. 1985; Kadinsky-Cade 1985;
Langer & Bollinger 1988; Pujol et al. 1991; Smalley & Isacks 1990;
Regnier et al.1992, 1994; Assumpção & Araujo 1993; Smalley et al.
1993; Langer & Hartzell 1996). More recently, the Harvard catalogue provided more than 30 focal mechanism solutions for the
region since 1977 using the centroid moment tensor (CMT) method
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2005 RAS, GJI, 163, 580–598
(Dziewonski et al. 1981; Dziewonski & Woodhouse 1983). A compilation of these seismic data, including all available focal mechanism solutions, can be found in Alvarado et al. (2004). However,
continental seismicity that occurs in the easternmost part over the
flat-slab segment and south of 33◦ S of the Andean backarc is still
poorly known.
Small- to moderate-sized crustal earthquakes are more abundant
but difficult to study with global seismic data. Their study is important in understanding the seismotectonic processes and seismic
hazards of the region. Unfortunately, these events are often not well
recorded by global seismic networks. Regional broad-band portable
seismic networks can help to characterize this seismicity by providing focal mechanism determinations and stress orientation estimates, improving source-depth estimates, and permitting tests of
seismic velocity models for the region.
In this paper, we present a regional study of the moderate-sized
crustal seismicity in the Andean back arc between 29◦ S and 36◦ S
using broad-band seismic data collected during 18 months in the
Chile–Argentina Geophysical Experiment (CHARGE). Complete
three-component seismograms recorded at regional distances were
used to invert for the seismic moment tensor of 27 crustal earthquakes. We also determined their focal depths. In addition, we explored average crustal seismic velocity structures for the region.
These results are discussed in the context of previous geological
and geophysical studies.
2 R E G I O NA L T E C TO N I C S E T T I N G
Most of western South America between 29◦ S and 36◦ S is made up
of pre-Carboniferous accreted terranes (Ramos et al. 1986, 2000;
582
P. Alvarado et al.
Figure 2. Main geological provinces, terranes and historical earthquakes (represented by the year of occurrence and magnitude in parentheses) are taken from
all geological and seismic references cited in the text.
Dalla Salda et al. 1992; Astini et al. 1995). Geological studies have
documented how the major terrane sutures have experienced extensional and compressional deformation since the Jurassic (Ramos
1994; Rapela 2000; Ramos et al. 2002). In this seismic study we
also explore what control these recognized geological terranes might
have on present Andean deformation.
Fig. 2 shows the morphological and structural provinces and terrane boundaries of our study area. These provinces have developed
since ∼25 Ma in response to the subduction conditions that produced a progressive eastward migration of the orogenic front and
the volcanic arc (Ramos 1988; Kay et al. 1991). The Principal
Cordillera, which forms the main part of the Andes, is composed of
Mesozoic marine deposits and the basement is involved in their
deformation. The northern and southern parts are characterized by
thick-skinned tectonics, while the central segment, with peaks with
elevations higher than 6700 m, is characterized by thin-skinned
tectonics (Ramos et al. 1996). The Frontal Cordillera includes
Palaeozoic–Triassic magmatic rocks that behaved as a rigid block
during the Andean deformation (Ramos et al. 1996). The Precordillera forms the foothills of the Andes between 29◦ S and 33◦ S.
It is a fold and thrust belt developed on a Palaeozoic carbonate platform (Baldis et al. 1982). East of the Andes, the Sierras Pampeanas
province is composed of a series of crystalline basement-cored
Precambrian–Early Palaeozoic rocks uplifted and tilted by compression during the formation of the Andes and separated by broad
and relatively undeformed basins (Jordan et al. 1983; Jordan 1995).
These uplifts are considered a modern analogue to the Laramide
uplifts of the western USA (Jordan & Allmendinger 1986). The
western Sierras Pampeanas are composed principally of abundant
crystalline calcites, amphiboles, basic and ultrabasic rocks, and
scarce granitic bodies (Caminos 1979). In contrast, the eastern
Sierras Pampeanas exhibit schists and gneisses of a mainly sedimentary origin, granitic rocks, some of which reach batholitic dimensions, and abundant migmatites and granulites (Kraemer et al.
1995; Varela et al. 2000). Proterozoic–Early Palaeozoic sutures separate the Rı́o de la Plata, Pampia and Famatinia terranes in the eastern
Sierras Pampeanas (Aceñaloza & Toselli 1976). The western Sierras
Pampeanas contain rocks as old as 1.1 to 1.0 Ga (McDonough et al.
1993) that correlate in age with the Grenville orogeny. These rocks
are part of the Cuyania terrane, which also includes the Precordillera
(Dalla Salda et al. 1992; Ramos et al. 2000). The boundary between
the Cuyania and Famitinia terranes is interpreted as a major Early
Palaeozoic suture (Dalla Salda et al. 1992; Astini et al. 1995; Ramos
et al. 2002). However, the origin and evolution of these terranes are
still debated (Aceñaloza et al. 2002).
South of 33◦ S, a very active volcanic arc (Fig. 1) coincides
with a decreasing age of the subducted plate and hence a higher
geothermal gradient (Yañez & Cembrano 2004). Crustal seismicity
is very active in the foreland over the flat-slab segment (Smalley
& Isacks 1990; Regnier et al. 1992; Smalley et al. 1993; Gutscher
et al. 2000). The Precordillera and Sierras Pampeanas have produced large crustal earthquakes (Costa et al. 2001; INPRES 2005)
with the most destructive having magnitude ∼7.0 and occurring
near San Juan in 1944 (Castellanos 1944; Groeber 1944) and 1977
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2005 RAS, GJI, 163, 580–598
Moment tensor inversion of Andean backarc crustal earthquakes
(INPRES 1977; Volponi 1979; Kadinsky-Cade 1985; Costa et al.
2000) (Fig. 2). In contrast, south of 33◦ S very shallow (depth
<20 km) earthquakes are restricted to the high cordillera
(Barrientos et al. 2004) (Figs 1 and 2).
3 D AT A A N D R E G I O N A L WAV E F O R M
MODELLING
The CHARGE network consisted of 22 seismic stations with 10
STS2 and 10 Guralp-esp sensors from IRIS-PASSCAL (USA), and
two Guralp-40T sensors from the Instituto Nacional de Prevención
Sı́smica (INPRES) and the Universidad Nacional de San Juan
(Argentina), respectively. They were deployed in two transects from
the Chilean coast to central Argentina at about 30◦ S and 36◦ S with
several stations in between (Fig. 1). Stations JUAN, USPA, HUER
and LLAN were installed coincident with INPRES sites. The rest of
the instruments were deployed inside a temporary vault located on
hard-rock sites. The instruments operated continuously recording at
40 samples per second for 1.5 yr (2000 December–2002 May). In
that period, we recorded 75 shallow earthquakes having body wave
magnitudes M ≥ 4.0 including four events of 5.0 < M < 6.0, with
locations in the backarc and main Andean cordillera region (Fig. 1).
We performed a linear least-squares seismic moment tensor inversion (SMTI) for 27 crustal earthquakes that were well recorded
using at least four CHARGE stations. We used available global
data from station PEL (Geoscope) in Chile to study three events
located near Mendoza. We also determined the best focal depth. We
will refer to the events by the year and Julian day of occurrence.
The SMTI technique (Randall et al. 1995) consists of modelling
the complete three-component seismic-displacement records at regional distances to obtain the seismic moment tensor for a point
source. We considered epicentral distances up to 727 km.
Accurate seismogram modelling depends upon reliable seismic
locations and a seismic velocity structure. We used the epicentre information determined by the Argentine seismic network (INPRES)
operating in the region. Data from more than 23 seismic stations
that use short-period, broad-band and acelerometer sensors were
used to constrain the hypocentres. The determination encompasses
more than 10 phases with more than two S-wave arrival times. The
earthquake locations were calculated using HYPO71 (Lee & Valdes
1985) and a simplified crust-upper mantle model. Horizontal epicentral errors are smaller than 10 km. Vertical hypocentre errors
are in general larger, but we determined the focal depth using the
complete broad-band regional waveforms in the SMTI.
In order to assess the effect of epicentral mislocations on our
focal mechanisms, we performed several tests using the epicentre
determinations. Before the inversion, the north-south and east-west
seismic components at each CHARGE seismic station were rotated into the orthogonal radial and tangential components using the
INPRES epicentre information. For each event, we produced plots
of the horizontal particle motion at each station for a ∼1.5-s time
window around the first P-wave arrival. For a correct epicentre location, there should be no energy on the tangential component, and
horizontal particle motion should be maximum in the radial component. We found that horizontal hypocentral determination errors
<6 km produced more than 90 per cent of the seismic energy concentrated in the radial direction with respect to the tangential direction.
As another way to test the impact of epicentral mislocations, we
determined the focal mechanisms with and without data from the
closer CHARGE stations. Data from closer stations are more impacted by epicentral mislocation because the mislocation is a larger
percentage of the total path length. Hence it introduces larger errors
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2005 RAS, GJI, 163, 580–598
583
Table 1. Crustal velocity structures used in the SMTI.
Thickness
(km)
Vp
(km s−1 )
10
35
Half-space
5.88
6.20
8.15
45
Half-space
6.20
8.15
Vp/Vs
Density
(gr cm−3 )
1.70
1.70
1.80
2.70
2.85
3.30
1.80
1.80
2.85
3.30
Model 1
Model 2
in the fitting of the data and consequently in the seismic moment
tensor results. Finally, we compared synthetic and observed data
aligned at the first P-wave arrival for each station. This reduces the
dependence on location uncertainties, origin time and the chosen
seismic velocity structure. Thus, from 75 earthquakes of magnitudes
M ≥ 4.0 and depths <50 km that occurred during the CHARGE
operation period (Fig. 1) we chose the 27 events with the best
locations.
In determining the seismic moment tensor, we used two seismic
velocity structures for the region, summarized in Table 1. These
structures are based on receiver function analysis (Shearer 2002;
Gilbert et al. 2005) and Pn studies (Fromm et al. 2004) that also
used the CHARGE data and forward modelling in this study. Model 1
represents the eastern Sierras Pampeanas, and Model 2 characterizes the western Sierras Pampeanas, Precordillera and Cordillera
terranes. The main difference in the models is the Vp/Vs ratio in
the crust (especially Vs). Therefore, for cases involving ray paths in
both terranes we combined both structures to obtain a better fit to
the data.
We used a bandpass filter between 15 and 50 s for larger earthquakes and 15–30 s for smaller events to compare synthetic and observed data. As a result, we obtained the seismic moment tensor and
corresponding best double-couple fault-plane solution for a point
source that had the smallest amplitude misfit. We also estimated
the compensated linear vector dipole (CLVD) component for each
moment tensor. This parameter measures how different the source
is from a pure double-couple solution. Thus CLVD = 0 per cent
means that the moment tensor is 100 per cent a pure double-couple,
producing a dislocation which in theory has a geometry identical
to the faulting. A discussion about how non-double-couple components affect the solution using the same SMTI technique can be
found in Mancilla et al. (2002). The amplitude-misfit normalized
error is the sum of the square difference between synthetic and observed waveforms, divided by the sum of the squares of all observed
seismogram amplitudes considered in the inversion. The seismic
displacements are not linearly dependent on the focal depth. Therefore, we performed a grid search for the source depth, producing
synthetic seismograms at a series of fixed hypocentral depths.
Fig. 3 shows the results for event 02-117 (MD = 5.3) that occurred northeast of San Juan on 2002 April 27. We modelled this
event at ten CHARGE stations for all three components of displacement using different trial depths. The stations used relative to the
epicentre show good azimuthal coverage (Fig. 3c). The results of
the focal depth versus normalized amplitude misfit, CLVD component and focal mechanism are shown in Fig. 3(a). The amplitudemisfit error curve shows a single minimum around the best depth
(21 km). This is also coincident with a minimum for the CLVD
(0 per cent). The focal mechanism solution remains robust for the
depth range investigated with the inversions. Fig. 3(b) shows the
fit of the data and synthetic for the best depth using a bandpass
584
P. Alvarado et al.
Figure 3. Results of the regional seismic moment tensor inversion (SMTI) for event 02-117. (a) Amplitude-misfit error between synthetic and observed seismic
displacements versus source depth (curve with the focal mechanisms). The minimum occurs at a depth of 21 km. We ran an inversion every 1 km testing also
the CLVD percentage in the solution (dashed line curve). The minimum in the CLVD component coincides with the minimum in the amplitude-misfit error
curve. Note how stable the focal mechanism solution is for the depth range explored. (b) Matches between synthetic and observed waveforms for the best depth
(21 km). Epicentral distance and azimuth of the stations used are indicated below the station name. Waveforms were bandpass filtered between 15 and 50 s. (c)
Location map of broad-band stations used in the SMTI showing the ray paths relative to the epicentre. For comparison, we present the Harvard-CMT solution.
filter between 15 and 50 s. The SMTI solution is M 0 = 5.81 ×
1016 N m; M w = 5.1; depth = 21 km; fault plane 1: strike = 34◦ ,
dip = 40◦ , rake = 98◦ ; fault plane 2: strike = 204◦ , dip = 50◦ , rake =
83◦ . We used the Aki & Richards (1980) convention for each faultplane solution, which gives the azimuth of the fault, the dip of the
fault considering the block located at the right when looking at the
azimuthal direction, and the rake, which is the angle of displacement measured in the fault plane from the azimuthal direction. The
Harvard-CMT solution for this event (M 0 = 8.02 × 1016 N m;
M w = 5.2; depth = 49.3 km; fault plane 1: strike = 341◦ , dip =
41◦ , rake = 31◦ ; fault plane 2: strike = 227◦ , dip = 71◦ , rake =
127◦ ) has a similar magnitude with a strike-slip component for the
focal mechanism and a deeper source for the centroid located southwest of the INPRES location. Even though our focal mechanism is
different from the CMT solution, the P-axis orientation does not
significantly change within 8◦ (Table 2). Modelling higher frequencies (as using the regional SMTI technique) than those considered
using the CMT inversion method should give us a better estimate
of the depth of the event. There are only two CMT solutions in the
region during the CHARGE network operation period that we can
compare with (events 02-117 and 01-285). Overall, both solutions
have small differences (their focal mechanisms are mainly reverse
or mainly strike-slip solutions), although the CMT depths tend to
be larger (Table 2).
Fig. 4 shows another example for event 02-107 (MD = 4.4) located
north of event 02-117. We modelled this earthquake using the closer
stations and data filtered with a bandpass between 15 and 30 s.
We present these results using Model 1 and Model 2 separately.
The focal mechanism solutions do not change significantly around
the best depth. However, a better fit is obtained using the seismic
velocity structure described in Model 2 (Fig. 4a). This reflects the
fact that most of the seismic ray paths travelled in the western Sierras
Pampeanas for the stations considered in the SMTI (Fig. 4b). We
show in Fig. 4(c) the synthetic and observed data match for the best
depth (25 km) that produced the solution M 0 = 2.01 × 1015 N m;
M w = 4.2; depth = 25 km; fault plane 1: strike = 31◦ , dip = 31◦ ,
rake = 124◦ ; fault plane 2: strike = 173◦ , dip = 65◦ , rake = 72◦ .
We also present the SMTI results for event 02-005 (MD = 4.8)
with an epicentral location near the Barrancas anticline, the structure on which the 1985 (M w = 5.9) earthquake is thought to have
occurred as discussed in Section 5.3 (Figs 2 and 5). The best results
show the minimum in the amplitude-misfit error curve as a function
of the source depth occurs for a minimum in the CLVD curve at
26 km depth (Fig. 5a). 10 CHARGE stations were used in this inversion (Fig. 5b). Station AREN recorded the largest amplitudes
of the three-component seismic displacements used. In order to test
how much this station dominates the SMTI we ran the inversion taking out data from AREN. We found similar results (M 0 = 1.18 ×
1016 N m; M w = 4.7; depth = 26 km; fault plane 1: strike = 61◦ ,
dip = 22◦ , rake = 175◦ ; fault plane 2: strike = 156◦ , dip = 88◦ ,
rake = 68◦ ). Fig. 5(c) shows the synthetic-observed data comparison for the best depth (26 km).
4 S E N S I T I V I T Y T O C RU S T A L M O D E L S
The seismic modelling for the source depends to second order
on the seismic velocity structure used to calculate the Green’s
functions (Sileny 2004). In order to test the sensitivity of the solution
to the chosen crustal model we selected two earthquakes (events
01-138b and 02-117) with epicentral locations in the eastern and
western Sierras Pampeanas, respectively. For each seismic event,
we performed a grid search for the crustal thickness, P-wave crust
velocity, and crustal Vp/Vs ratio using the SMTI method described
previously.
First, we constructed the Green’s functions for event 01-138b with
magnitude M w = 4.3 located near Córdoba (Table 2 and Fig. 6c)
for a set of simple average models consisting of a crustal layer
over a half-space. The models had a crustal thickness varying from
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2005 RAS, GJI, 163, 580–598
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2005 RAS, GJI, 163, 580–598
04:21:31
22:37:24.3
01:17:50.4
02:27:35
11:36:26.8
11:14:18
01/12/15
01/10/12
01-285
14:25:03
01-349
01/09/17
01-260
17:16:26
01/12/14
01/07/08
01-189
00:37:35.2
01-348
01/06/28
01-179
22:57:11.06
01/12/05
01/06/20
01-171
13:10:13.3
01-339
01/05/18
01-138b
04:38:23.3
01/11/29
01/05/18
01-138a
15:00:26.10
01-333
01/05/10
01-130
05:40:06.10
01/11/24
01/05/07
01-127
11:06:19.99
01-328
01/03/15
01-074
01:11:50.4
13:15:9.1
01/02/09
01-040
20:18:40.2
01/11/12
01/02/06
01-037
00:49:27.15
01-316
00/12/25
00-360
06:13:12.60
04:21:37.9
00/12/21
00-356
GMT
Origin
Time
hr:mm:ss
(∗ )
Date
yr/
month/
day
Date
yr-Julian
Day
−69.815
−33.286
−66.857
−67.73
−31.596
−29.337
−68.999
−33.194
−68.02
−67.873
−31.247
−31.787
−69.012
−33.211
−64.435
−64.93
−31.0
−32.72
−67.729
−31.756
−66.963
−67.956
−32.072
−32.113
−69.080
−33.021
−67.214
−69.56
−34.82
−33.346
−67.87
−31.383
−67.534
−69.495
−33.459
−33.131
−68.015
−31.533
−69.04
−68.935
−29.474
−33.16
Longitude
degree
Latitude
degree
51
5
80
20
10
15
(fixed)
15
10
20
16
30
65.9
40
23
11
10
33
19
33
22.3
54
Location
depth
km
20
(20–30)
16
(10–20)
6
10
(20–32)
15
13
17
(13–17)
5
15
(18–24)
(13–15)
21
22
(21–24)
7
20
14
3
22
20
(20–30)
15–20
14
Modelled
depth
km
151
356
191
353
334
236
34
190
34
193
181
15
276
115
45
218
344
251
133
22
23
204
15
185
149
19
301
36
222
314
55
215
98
218
16
238
60
231
20
198
14
204
Strike
36
57
42
49
83
46
48
44
51
41
19
71
53
39
39
51
83
71
53
64
40
50
49
41
57
46
73
75
78
81
48
43
27
76
46
52
48
42
64
26
41
49
Dip
69
104
103
78
47
169
106
72
103
74
77
94
79
104
95
86
19
173
33
138
89
91
96
83
56
130
−15
198
171
12
104
75
147
66
58
119
96
83
91
88
82
97
Slip
290
110
145
308
326
135
87
259
262
100
294
80
116
311
14
101
114
113
96
91
76
Az
4
19
3
3
27
3
2
23
6
4
5
6
9
6
7
26
5
2
25
3
11
Pl
P-axis
Focal mechanism solution
162
292
23
210
99
31
178
168
3
328
118
343
209
103
143
292
359
13
207
202
305
Az
84
71
85
67
53
80
15
2
62
84
85
47
18
83
79
63
78
77
37
81
74
Pl
T-axis
966
10 800
512
1060
269
1510
105 000
56 300
950
334
209
569
2990
574
229
1590
20 400
1230
405
743
1060
Seismic
moment
1012 N m
4.0
3.2
3.2
5.6
3.4
3.0
4.4
3.2
3.3
3.3
Ms
4.1
5.3
4.2
4.3
3.8
4.2
5.1
3.9
4.2
4.0
4.0
4.5
3.4
4.1
4.2
4.8
4.2
4.1
4.0
4.5
MD
4.0
4.4
3.8
4.0
3.6
5.3
(stat.)
4.1
5.1
4.0
3.6
3.5
3.8
4.3
3.8
3.5
4.1
4.8
4.0
3.7
3.9
4.0
Mw
0.389
0.241
0.569
0.221
0.516
0.280
0.237
0.264
0.438
0.469
0.445
0.184
0.394
0.410
0.303
0.239
0.154
0.530
0.200
0.331
rms
23
27
14
15
14
25
10
30
14
14
13
22
16
20
13
25
15
18
20
11
17
#comp
2
2
1
1–2
2
2
1–2
2
2
2
2
1
2
2
2
2
2
2
2
2
Crustal
Model
Table 2. Parameters of intraplate earthquakes of the Andean backarc between 29◦ –36◦ S and 64◦ –71◦ W. Origin time, epicentral location (error ≤6 km), and duration magnitude MD are from the local INPRES
catalogue. Magnitude Ms is from the NEIC catalogue. Modelled depth (error ≤2.5 km), focal mechanism, seismic moment, moment magnitude M w (with errors given by the seismogram fitting shown in column
rms), and the last three columns are from this study.
Moment tensor inversion of Andean backarc crustal earthquakes
585
36
(stat.)
2
30
5.2
0.316
5.1
5.3
291
17
178
50
80 200
4.8
58 100
83
72
5
299
2
18
0.171
4.2
4.4
3.9
2010
65
51
18
277
1
15
0.300
4.0
1070
75
176
4
280
287
8
85
82
966
2.7
4.0
2
21
0.207
4.0
3.9
2
10
0.190
4.0
3.9
3.2
1210
11
145
30
242
30
0.254
4.7
4.8
4.1
11 800
43
45
39
4.5
4.3
3.5
5860
88
44
2
257
88
92
175
68
−15
210
95
86
69
109
124
72
98
83
31
127
47
43
22
88
61
77
37
53
43
50
31
65
40
50
41
71
(∗ ) Harvard-CMT solutions for the centroid.
49.3
−67.78
−31.72
(∗ )
23:53:58.8
21
16
−67.6
−30.998
23:53:53
02/04/27
02-117
12.3
−67.827
−30.314
02/04/17
02-107
02:11:09.2
18
(16–21)
25
13.3
−64.090
−30.908
02/03/15
02-074
21:35:47
15
10
−67.965
−31.65
02/03/10
02-069
11:13:51.5
(5–9)
23.6
−66.175
−30.36
04:23:14.8
02/01/19
02-019
27
−68.61
−33.313
02/01/05
02-005
12:02:57.3
26
166
349
61
156
279
17
21
194
356
204
31
173
34
204
341
227
15.4
−66.859
−32.124
01/12/18
01-352
09:16:27.3
17
Pl
Az
Pl
Az
T-axis
P-axis
Slip
Dip
Strike
266
1–2
0.310
29
#comp
rms
Mw
MD
Ms
Seismic
moment
1012 N m
Focal mechanism solution
Modelled
depth
km
Location
depth
km
Longitude
degree
Latitude
degree
GMT
Origin
Time
hr:mm:ss
Date
yr/
month/
day
Date
yr-Julian
Day
Table 2. (Continued.)
2
P. Alvarado et al.
Crustal
Model
586
Figure 4. Results of the SMTI for event 02-107. (a) Curves with focal mechanisms showing the amplitude-misfit errors between synthetic and observed
seismograms versus hypocentral depth for two velocity structure models
(Table 1). The minimum is observed for a CLVD = 7 per cent (dashed line)
at a source depth of 25 km using Model 2 (western Sierras Pampeanas). (b)
Location map of the stations used in the SMTI showing ray paths from the
epicentre to the CHARGE stations. (c) Synthetic and observed waveforms
for the best fit (25 km) using a bandpass filter of 15-30 s. Epicentral distances
and azimuth of the stations used are shown below the station name.
25 to 65 km in steps of 5 km, a P-wave crustal velocity varying from
5.8 to 6.6 km s−1 in steps of 0.2 km s−1 , and a Vp/Vs ratio fixed
at 1.70. We maintained the P-wave velocity at 8.15 km s−1 for the
half-space with a Vp/Vs ratio of 1.80. We ran an inversion matching
synthetic and observed seismograms using a series of source depths
for each crustal model. We used a bandpass filter between 15 and
30 s. Fig. 6(a) shows the results for the minimum normalized misfit
errors obtained for each model. Accepting misfit errors with rms
<0.25 constrains the crustal thickness to be <45 km and P-wave
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2005 RAS, GJI, 163, 580–598
Moment tensor inversion of Andean backarc crustal earthquakes
Figure 5. Results of the SMTI for event 02-005. (a) Amplitude-misfit errors between synthetic and observed seismograms versus hypocentral depth
(curve with focal mechanisms). The minimum with a CLVD = 0 per cent
(dashed line curve) occurs at a depth of 26 km. (b) Map of the stations used
in the SMTI showing the ray paths from the epicentre. (c) Synthetic and
observed seismic waveforms for the best depth (26 km) using a bandpass
filter of 15–40 s. Epicentral distances and azimuth of the stations used are
shown below the station name.
crustal velocities to be ∼6.0–6.2 km s−1 . The misfit increases rapidly
(>75 per cent) using a high crustal Vp and/or a thickened crust
(Figs 6a, d and e).
The second step consisted of fixing Vp at 6.0 km s−1 and initiating
a grid search around crustal thickness and Vp/Vs ratio using the
SMTI for the same earthquake. This means that only the S-wave
velocity and crustal thickness varied. We ran an inversion for each
model of crustal thickness varying the thickness from 25 to 65 km
in steps of 5 km and varying Vp/Vs ratio varying from 1.60 to
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2005 RAS, GJI, 163, 580–598
587
1.85 in steps of 0.05. Again, changes in Vp/Vs ratio correspond to
S-wave crustal velocity variations only. No modifications were made
in the half-space described above. Fig. 6(b) displays the results of the
inversions obtained using each seismic model. A crustal thickness
of <44 km and Vp/Vs values of 1.65–1.70 produce misfit errors
with rms <0.25.
Therefore, models using a low crustal Vp of 6.0–6.2 km s−1 , a
low Vp/Vs ratio of 1.65–1.70, and a crustal thickness corresponding to a normal crust (30 km), or at least no thicker than 45 km,
predict the best results. Nevertheless, the seismic ray paths for the
earthquake considered in this grid search (event 01-138b) recorded
at the CHARGE seismic stations used in the inversion test travelled mostly through the eastern Sierras Pampeanas (Figs 2 and 6c).
The focal mechanism solutions did not change significantly around
the best depth (Fig. 6e) and remained stable for the entire range of
parameters tested.
The same grid-search process was carried out for event 02-117
with magnitude M w = 5.1 and epicentre location in the western
Sierras Pampeanas (Table 2 and Fig. 3). For this test, we only considered stations located in the western part of the CHARGE network
region (compare Fig. 7c to Fig. 3c).
We started testing crustal models with P-wave crustal velocity
varying from 5.80 to 6.60 km s−1 in steps of 0.2 km s−1 , Vp/Vs =
1.80, and thickness varying from 25 to 65 km in steps of 5 km. We
maintained the mantle parameters Vp = 8.15 km s−1 and Vp/Vs =
1.80. We ran an inversion for each model and a set of fixed source
depths using a bandpass filter of 15–50 s. Fig. 7(a) shows a map of
the SMTI minimum amplitude-misfit error results for each crustal
model. The best fit (>75 per cent) occurs around a crustal Vp =
6.40 km s−1 and a thickness of 45–52 km.
Fig. 7(b) shows SMTI results for a grid search around Vp/Vs
and crustal thickness for the same event. We tested models fixing
the crustal P-wave velocity at 6.20 km s−1 and varying the crustal
S-wave velocity to obtain Vp/Vs ratios in the range 1.60–1.85 in
steps of 0.05. The crustal thickness varied from 25 to 65 km in
steps of 5 km. The mantle parameters were the same as before.
In contrast to the test results for the event in the eastern Sierras
Pampeanas (Fig. 6b), the best results for event 02-117 occur when
considering a crustal structure with Vp/Vs ratio greater than 1.80
and thickness between 40 and 57 km. Fig. 7(d) shows examples of
the amplitude-misfit error and CLVD versus source depth for a set
of 45-km-thick crust models. We conclude that the best results are
observed using a higher Vp/Vs ratio (1.80–1.85) if the ray paths
travel in the western Sierras Pampeanas geological terranes.
Fig. 8 shows seismic ray paths for variations in S-wave velocity
used in the SMTI of the 27 earthquakes here studied. The P-wave
velocity that we used in all inversions was practically the same for
both models (Table 1). However, based on the SMTI results (Figs 6a,
d, e and 7a), it seems that Vp is smaller in the eastern than in the
western part of the region. Fig. 8 shows that most of the ray paths are
sampling the western geological terranes. However, even with less
sampling in the eastern Sierras Pampeanas the results indicate that
Model 1 is more appropriate for this region. The last two columns
in Table 2 show the total number of seismic components and the
crustal model used in the SMTI of each event, respectively.
We did not modify the structure for the upper mantle in this
grid-search analysis. Previous studies in the Altiplano using the
same technique and band-period ranges of 15–50 s and 15–100 s
demonstrated that the SMTI technique is less sensitive to changes
in the upper mantle seismic velocities (Swenson et al. 2000).
Our results are in agreement with the Vp/Vs estimates based on
receiver function analysis (Gilbert et al. 2005) and crustal thickness
588
P. Alvarado et al.
−1
−1
0.25
−1
0.85
−1
0.25
0.70
−1
−1
−1
−1
−1
Figure 6. Grid search results obtained for event 01-138b using the SMTI technique. (a) Synthetic and observed data misfit errors for different crustal Vp and
crustal thickness combinations. We maintained crustal Vp/Vs = 1.70, mantle Vp = 8.15 km s−1 and Vp/Vs = 1.80. Dots represent different crustal model
parameters used in (e). (b) Synthetic and observed data misfit errors for different crustal Vp/Vs ratio and crustal thickness pairs maintaining crustal Vp =
6.0 km s−1 . The mantle parameters were the same as in (a). (c) CHARGE stations used in the inversions with respect to the event location. (d) Synthetic
and observed seismogram fit for the best depth using the model with crustal Vp = 6.00 km s−1 and thickness = 40 km. (e) Amplitude-misfit errors versus
hypocentral depth for different crustal Vp model parameters showed as dots in (a) for a 40-km-thick crust. Dashed lines are the CLVD component in the solution.
Note how the focal mechanism solution does not significantly change around the best depth (∼5 km).
from Pn studies (Fromm et al. 2004) that also used CHARGE data.
Previous studies using a dense local network (1987–1988 PANDA
experiment) in the region around Sierra Pie de Palo (Fig. 2) covering an area of roughly 150 km north–south by 100 km east–west
showed the westward increase of the Moho depth between the Sierra
Pie de Palo (48–55 km) and the Precordillera (55–60 km) from
P-and S-wave converted phases (Regnier et al. 1994). Interpretations of JHD PANDA station corrections also predict higher crustal
velocities (>6.4 km s−1 ) near Sierra Pie de Palo than to the west of
the Eastern Precordillera (Pujol et al. 1991). These studies found differences in the crustal structure between the Cuyania and Chilenia
terranes that we cannot resolve because we have very few pure
Chilenia paths (Ramos 1994). Our estimates in the western terranes
are in close agreement with those results, although our surfacewave-based average crustal models do not discriminate between the
properties of the Pampeanas and Eastern Precordillera terranes.
5 R E S U LT S A N D D I S C U S S I O N
Our results for the events in this study are listed in Table 2 and shown
in Fig. 9. The table includes the SMTI results for both fault-plane solutions, the orientation (azimuth and plunge) of the P- and T-axis, the
seismic moment, the moment magnitude and the amplitude-misfit
error. Values for Ms and MD are from the global NEIC and local
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2005 RAS, GJI, 163, 580–598
Moment tensor inversion of Andean backarc crustal earthquakes
589
−1
0.55
−1
0.30
−1
0.55
0.30
Figure 7. Grid search results obtained for event 02-117 using the SMTI technique. (a) Maps of the synthetic and observed data misfit errors for each crustal Vp
and crustal thickness pairs explored. Mantle parameters are as in Fig. 6. (b) Same as (a) but now varying Vp/Vs ratio and maintaining crustal Vp = 6.20 km s−1 .
Dots show the crust models used in the inversions shown in (d). (c) Ray paths, seismic station and epicentre location used in all of the inversions shown in (a)
and (b). Note that the ray paths are travelling in the western geological terranes for this grid searching. The same event, analysed with more station-components,
is shown in Fig. 3. (d) Amplitude-misfit error and CLVD component (dashed line) versus source depth for different crustal Vp/Vs models (dots in diagram b).
INPRES catalogues, respectively. Even though in theory single
three-component station waveforms are sufficient to estimate the
moment tensor solution and focal depth, we limit the minimum number of stations to four (Fig. 8). The more stable solutions, however,
are related to higher numbers of station/components used (see the
number of used components (column #comp) in Table 2). We also
observed that the CLVD component depends on azimuthal coverage.
Hence we present solutions with a minimum misfit error around the
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2005 RAS, GJI, 163, 580–598
best depth coincident with a minimum or an acceptable (<20 per
cent) CLVD component.
In some cases, we observed a broad minimum instead of a more
local minimum in the amplitude-misfit error curve around the best
depth. However, the focal mechanism solutions did not change for
the depth-range explored. In general, the depth curves were broader
for events located outside of the CHARGE network with magnitudes
<3.9. For example, event 00-360 in Table 2 shows a minimum at
590
P. Alvarado et al.
Figure 8. Ray paths showing the different seismic velocity models (Table 1) used during the inversion of the 27 crustal earthquakes in this study. Crustal
S-wave velocity variations are responsible for Vp/Vs changes. Higher Vp/Vs ratios produced better fits for ray paths mainly travelling in the western geological
terranes (see Fig. 2) while lower Vp/Vs ratios seem to better characterize the Eastern Sierras Pampeanas.
20 km depth, but errors are still acceptable for a depth-range of 20–
30 km. Higher frequencies are required to constrain the depth better.
However, at higher frequencies this is complicated because the solution becomes more sensitive to the complex shallow structure of
the crust. Nevertheless, our determinations are still very useful to
discriminate between crustal and deeper or intraslab seismic events,
and to characterize their focal mechanism solution. We obtained
good depth constraints for 80 per cent of the crustal earthquakes
studied with an estimated error ≤2.5 km.
Overall, we find the backarc has mainly reverse or thrust focal
mechanism solutions with average P-axis and T-axis oriented at azimuths of 275◦ and 90◦ , and plunges of 6◦ and 84◦ , respectively.
However, the earthquakes could be representing different styles
of deformation as reflected by crustal seismicity variations as we
discuss below.
A larger number of events modelled in this study have locations in
the western terranes Cuyania and Famatina (Figs 1, 2, 9, 10 and 11).
The Pampia terrane does not show seismic activity for earthquakes
of magnitudes larger than 4.0 during the CHARGE time period.
However, palaeoseismic studies along Quaternary faults point out
significant earthquakes that have occurred in the southern part of this
terrane (Costa & Vita-Finzi 1996; Costa et al. 2000). Interestingly,
we also observed crustal earthquakes further east at about 800 km
from the trench near Córdoba in central Argentina related to the Rı́o
de la Plata terrane (events 02-074, 01-138b and 01-339). Another
distinction already mentioned was observed in the seismic velocity
structure for the region. We could fit the observed data better when
using an S-wave seismic velocity that was 6 per cent lower in the
crust (40–57 km thickness) in the western Sierras Pampeanas than
in the 25- to 45-km-thick crust of the eastern Sierras Pampeanas.
This result could be related to a different composition or a more
fractured crust in the west or a combination. Regional surface-wave
tomographic studies also show lower S-wave velocities beneath the
thicker Andean crust than in the eastern shields for this region (Feng
et al. 2004).
Earthquakes with epicentre locations in the western Sierras
Pampeanas show focal mechanisms with pure thrust or reverse faultplane solutions. These earthquakes are at depths of between 15 and
25 km, which represent middle-to-lower crustal levels. The only exception is event 01-348, which we will discuss later. In the eastern
Sierras Pampeanas, we find strike-slip fault-plane solutions (events
02-019 and 01-138b) at shallower crustal depths. We were still able
to model event 02-074 (MD = 4.0) located ∼50 km north of Córdoba
using CHARGE data at epicentral distances up to 727 km with poor
azimuthal coverage. Its solution shows a reverse focal mechanism
and a source depth of ∼18 km. This example shows that it is possible
to estimate the moment tensor for M ∼ 4.0 events with locations
outside the network using the SMTI technique. Event 01-285 located in the main cordillera had the largest magnitude (Ms = 5.6) of
the studied crustal earthquakes. Its focal mechanism solution indicates strike-slip motion at a very shallow depth (5 km). The region
around Mendoza showed deeper crustal earthquakes in the range
of 10–26 km and reverse focal mechanisms with some strike-slip
components. The southernmost located earthquake we modelled is
event 01-074, which occurred on 2001 March 15 in the eastern flank
of the Andean cordillera with a moderate magnitude (MD = 4.9).
Our results indicate a pure-thrust fault-plane focal mechanism and
a very shallow depth of 3 km for this event. Notably, the events
located south of 33◦ S in the high cordillera showed very shallow
depths (<5 km) during the CHARGE period.
5.1 Western Sierras Pampeanas–Sierra Pie de Palo
and Valle Fértil Megashear
The region around the Sierra Pie de Palo (Fig. 2) is of particular
interest because of the last large damaging (Ms = 7.4) earthquake in
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2005 RAS, GJI, 163, 580–598
Moment tensor inversion of Andean backarc crustal earthquakes
591
Figure 9. Results of the SMTI showing lower hemisphere projection focal mechanisms and source depths in parentheses (Table 2). Dark quadrants are
compressional motions. Observe the distribution of the crustal seismicity with respect to the presence of geological terranes and their boundaries. Location of
the seismic data showed in the cross-section in Fig. 11 is indicated by the rectangle. Brackets at 31◦ S indicate the location of the geological profile shown in
Fig. 11.
1977 linked to different interpretations of its causal fault (Fig. 10)
(Kadinsky-Cade et al. 1985; Langer & Bollinger 1988; Langer &
Hartzell 1996). This Precambrian block is one of the westernmost
ranges of the Sierras Pampeanas and the most seismically active
in this geological province (Figs 1 and 2). Previous seismic studies
show along-strike variation of structures in this area (Langer &
Bollinger 1988; Regnier et al. 1992) making it difficult to associate
microseismicity with single planar faults (Smalley & Isacks 1990).
The 3-D structural geometry of this range is very complex, and
its relationship with the adjacent areas is interpreted as different
blocks bounded by nearly north-trending thrust faults with important
E–W transverse megafractures (Bastias & Weidmann 1983; Perez
& Martinez 1990; Regnier et al. 1992; Zapata & Allmendinger
1996; Zapata 1998) (Fig. 10). Ramos et al. (2002) suggested an
alternative interpretation for the southernmost block of the Sierra
Pie de Palo, a passive roof duplex related to a basement mid-crustal
wedge. We found earthquake sources deeper than 15 km for seven
events with epicentral locations to the east of the major axis of the
main anticlinorium of the Sierra Pie de Palo. Almost all of the faultplane solutions have trends oriented parallel to this axis and indicate
pure thrust or reverse faulting (Fig. 10).
Given uncertainties in the earthquake locations, it is very speculative to relate any one event to one structure. However, because of
the increasing interest in relating regional seismicity with structural
system or crustal volumes associated with structures in active zones,
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2005 RAS, GJI, 163, 580–598
we think that it is important to briefly put our results in the context
of known structures in and around the Sierra Pie de Palo (Figs 10
and 11).
Event 02-117 (Figs 3, 9 and 10) is very intriguing because its
epicentre is in the vicinity of the Valle Fértil major reverse fault that
borders the western side of this north-northwest trending range and
is responsible of its uplift (Jordan & Allmendinger 1986; Snyder
et al. 1990) (Figs 2 and 10). This 600-km-long fault zone has rocks
characteristic of an ophiolitic belt (Ramos et al. 2000). It seems to
have generated several earthquakes recorded by the global seismological network (Stauder 1973; Chinn & Isacks 1983; Dziewonski
et al. 1988), which also detected the moderate-size (M w = 5.1) event
02-117. However, the SMTI fault-plane trends of our solution and
the depth suggest it may be associated with the main north-northeast
oriented axis of the Sierra Pie de Palo and other non-exposed structures located to the north of this range (Zapata & Allmendinger
1996; Zapata 1998). In fact, seismic reflection interpretations at the
intersection of the Sierra Pie de Palo and the next range to the east,
Sierra Valle Fértil, predict a buried anticline as the northern termination of the Sierra Pie de Palo named the North Pie de Palo
anticline (Fig. 10). We speculate that event 02-117 may be related
to the fault that bounds the North Pie de Palo anticline to the west.
This evidence has two implications: the Sierra Pie de Palo western
fault branches from the Sierra de Valle Fértil fault (Fig. 10, inset) as
predicted by Zapata (1998). Second, this structure correlated with
592
P. Alvarado et al.
Figure 10. Major thrust faults around the Sierra Pie de Palo according to the geological references cited in the text and earthquakes in this study with maximum
epicentre-location errors of ±6 km (Table 2). Approximate locations of the historical large crustal earthquake in 1944, and the foreshock (F) and mainshock
(M) of the Ms = 7.4 1977 earthquake are also shown. Fractures and faults divide the Sierra Pie de Palo in several blocks. Event 02-117 (M w = 5.1) at 21 km
depth (Fig. 3) is likely related with the northern continuation of this oldest Pampean range in the buried North Pie de Palo anticline (Zapata 1998) (see the
vertical projection of this event in the inset). Projections and labels for the seismic events are the same as in Fig. 9.
the northernmost part of a suture between the Precordillera and the
western Sierras Pampeanas (Fig. 10) of Grenville age (Zapata 1998)
is seismically active.
Another moderate-sized earthquake (event 01-348; MD = 5.3)
occurred in the southeast of the Sierra Pie de Palo (Fig. 10) on
2001 December 14. Its focal mechanism solution indicates a very
shallow west-dipping plane or a vertical east-dipping plane and
6 km focal depth. Previous large events in this area like the second
shock of the 1977 (Ms = 7.3) earthquake (Kadinsky-Cade 1985)
and the 1941 (M = 6.7) earthquake (INPRES 2005) have large
location uncertainties and do not show a clear correlation with exposed faults (INPRES 1977; Volponi 1979; Langer & Bollinger
1988; Langer & Hartzell 1996). Although a smaller earthquake,
event 01-348, is very shallow so we might expect it to correlate with
an exposed fault. This earthquake could be related to some continuation to the southwest of active faults seen on the east side of the
southern block of the Sierra Pie de Palo known as the Nikizanga fault
system (Ramos & Vujovich 1995; Ramos et al. 2002) (Fig. 10). The
east-dipping Nikizanga fault showed a 10 km surface rupture during
the large deep-crustal earthquake of 1977, and for this reason it is
interpreted as a secondary fault (Costa et al. 2000). Local seismicity
studies by Regnier et al. (1992) also define a cluster of seismicity
around 5 km depth for this area. However, a potential occurrence of
event 01-348 along the west-dipping fault-plane solution cannot be
ruled out, suggesting its relationship with a shallower décollement
under Sierra Pie de Palo like that modelled by Ramos et al.
(2002).
There are other events (00-356, 01-107, 01-333, 01-352 and 01328) located near the terrane boundaries that separate Cuyania from
the Famatina and Pampia terranes. They also separate the region
in terms of seismicity. Very few crustal earthquakes of magnitudes
>4.0 had locations in the Pampia terrane during the CHARGE period (Figs 1, 2, 9, 10 and 11).
Deformation rates based on moment tensor summation (Kostrov
1974) using the Harvard-CMT solutions for events (M w > 5.0) that
occurred in the last 27 yr around the Pie de Palo range (Fig. 2) predict
31.8 mm yr−1 rate of crustal shortening (Siame et al. 2002, 2005).
This estimate results from multiplying the horizontal component
of the maximum shortening strain rate (oriented in the direction
N93◦ E) times the width of 93 km for the seismically deformed
volume of 123 × 93 × 30 km3 , which covers the Sierra Pie de Palo.
This calculation is dominated by the 1977 earthquake sequence.
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Moment tensor inversion of Andean backarc crustal earthquakes
593
Figure 11. SMTI results in a vertical projection along a cross-section around 31◦ S (see Fig. 9 for location) including: (a) GPS velocities (yellow circles) around
this latitude from Brooks et al. (2003), and (b) Geological interpretation modified from Regnier et al. (1992), Ramos et al. (2002) and Siame et al. (2002). The
western Sierras Pampeanas are more seismically active than the eastern basement blocks. Focal depths with errors of ±2.5 km are in good agreement with the
proposed décollements under Sierra de Pie de Palo based on geological studies. Note the different horizontal and vertical scales used in the geological cross
section but not for the fault-plane focal-mechanism projections.
We calculated the earthquake moment tensor sum with the same
technique using the eight closest crustal events to the Sierra Pie de
Palo in the 1.5 yr CHARGE period. These events are located in
an area of approximately 90 × 38 km2 with a seismogenic depth
range of 19 km (Figs 9 and 10). In this particular volume, the main
contraction axis is oriented along an azimuth 297◦ and has a plunge
of 1◦ , whereas the minimum contraction axis has an azimuth of
46◦ and a plunge of 86◦ . The rate of crustal deformation along the
maximum shortening strain orientation over the width of 38 km and
the 1.5 yr period is about 0.45 mm yr−1 .
As expected, when comparing Siame et al.’s (2005) estimation
with our estimate of earthquake crustal shortening, we find that their
rate of deformation is nearly two orders of magnitude greater. This
is not surprising given the Ms = 7.4 1977 earthquake (Fig. 2). We
believe that our estimate is less likely related to the 1977 earthquake
sequence since event 02-117 on 2002 April 27 (Table 2 and Figs 3, 9
and 10) in our calculation is the largest magnitude >5.0 earthquake
that occurred in this area over a period of 14 yr. However, neither
estimate is likely to represent the long-term rates.
Interestingly, the GPS velocity vectors observed in the vicinity of
the Sierra Pie de Palo and the Sierra Valle Fértil over a time span of
∼3–5 yr show a decay towards the east along an azimuth of ∼82◦
(Brooks et al. 2003) (Fig. 11), which agrees with the plate convergence direction (N79.5◦ E-Kendrick et al. 2003). Our estimate for
the maximum shortening axis orientation and that determined by
Siame et al. (2005), both based on the moment-tensor sum of earth
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2005 RAS, GJI, 163, 580–598
quakes in the area during different periods of time (1.5 and 27 yr),
are ∼35◦ and ∼11◦ clockwise rotated with respect to GPS orientation observations. This might indicate that other tectonic processes
are occurring besides crustal earthquakes that affect geodetic measurements. However, it could also indicate that even though the main
shortening orientation is that observed by GPS data, the pre-existing
fractures and faults associated to the Sierra Pie de Palo (Fig. 10) are
controlling the moderate-to-large earthquake deformation.
5.2 Eastern Sierras Pampeanas
Focal mechanisms for this region are either strike-slip or reverse
fault-plane solutions (Figs 9 and 11). The strike-slip events have
very shallow hypocentres. Since our determinations provide the first
seismic source characterizations in this region, we consider it important to discuss their tectonic implications in order to exploit their
seismic information.
Our results for the largest size (MD = 4.8) earthquake in this region, event 01-138b (Table 2) that occurred close to Soto, Córdoba,
indicate a shallow depth (7 km) and a strike-slip solution (Figs 6
and 9). No movement along strike-slip Quaternary faults has been
recognized in the region (Costa et al. 2001). However, interpretations of the orogenic evolution mainly during the Pampean cycle of
the Sierras de Córdoba describe eastward dipping, NW–SE trending, sinistral wrench faults for the studied region (Baldo et al. 1996).
This observation is coincident with the movement along the N16◦ W
594
P. Alvarado et al.
nodal plane of the SMTI solution for event 01-138b. Tectonic studies
by Kraemer et al. (1995) and Martino et al. (1995) also find evidence
of north-south trending shear zones related to the suture between Rı́o
de la Plata and Pampia terranes (Fig. 2). Their studies also suggest
the reactivation of old faults since the Pliocene. Thus it is likely that
the historic 1908 (M = 6.5) earthquake that occurred very close
to the event 01-138b epicentre and affected the same population
centres (INPRES 2005) was generated by similar structures in the
region. These findings are important in helping to identify structures that generate seismicity at very shallow depths that might affect cities like Córdoba, which has one of the larger populations in
Argentina.
We also constrained two reverse fault-plane focal mechanisms
for events 01-339 and 02-074 with magnitudes of ∼4.0 and depths
of 16–18 km, respectively (Fig. 9). They represent the easternmost
Andean compression acting over a region where the flat slab finally
resumes its inclination to the east (Cahill & Isacks 1992) (Fig. 1).
Magnetotelluric studies revealed a strong contrast in resistivity for
crustal structures that could be responsible for the uplift of the
Sierras de Córdoba along east-dipping faults (Booker et al. 2004).
The same studies also predict their extension into deeper levels
(40 km).
Event 01-349 (MD = 4.1) in the northern region is close to the
Sierra Velasco (Figs 2 and 9) and has a dip-slip source mechanism.
Both fault-plane trends are parallel to the main NE-trend of the eastern front of this range. This deformation contributes to the uplift of
the basement of the eastern Sierras Pampeanas, sometimes generating larger reverse focal mechanisms such as the recent Catamarca
earthquake (2004 September 7, Harvard-CMT solution: depth =
20 km; M w = 6.1; fault plane 1: strike = 216◦ , dip = 37◦ , rake =
80◦ ; fault plane 2: strike = 48◦ , dip = 54◦ , rake = 97◦ ) that occurred
∼90 km northeast of event 01-349.
Our results in the eastern Sierras Pampeanas are consistent with
GPS observations. According to Brooks et al. (2003) the crustal seismicity deformation observed far away from the trench contributes to
recover the elastic loading component, which is the entire GPS velocity field observed in the eastern Sierras Pampeanas (green curve
in Fig. 11a). Their study assumes that the locking of the converging Nazca and South America plates as far as 600 km to the west
generates this elastic component.
5.3 Mendoza
Mendoza has been the site of several destructive earthquakes
(Fig. 2). The 1985 (M w = 5.9) earthquake took place ∼15 km south
of the city of Mendoza in the Barrancas anticline (Fig. 2) above blind
thrust faults (INPRES 1985; Triep 1987; Chiaramonte et al. 2000).
The region is characterized by oil fields hosted by the Triassic Cuyo
basin hydrocarbon reserves (Moratello 1993; López-Gamundi &
Astini 2004). Tectonic analysis based on seismic imaging suggests
that the faults involve the basement and the productive traps could be
the result of the reactivation of older structures (Dellapé & Hegedus
1993; Ramos et al. 1996).
At this latitude, the Precordillera loses its surface expression,
and the Cuyo rift basin as well as a series of low-elevationed anticlines continue to the south (Fig. 2) with less east–west contraction
(Ramos et al. 1996; Brooks et al. 2000). An estimate of a shallow décollement at about 15 km has been identified to the west of
the Cuyo basin (Fig. 2) using balanced cross-sections and seismic
reflection data (Kozlowski et al. 1993; Ramos et al. 1996). The westdipping thrust fault-plane solutions for events 01-127 and 01-189
(MD = 4.2) at similar depths might be related to this décollement
system (Table 2 and Fig. 9).
One intriguing (MD = 4.8) earthquake that we modelled in this
region is event 02-005. The focal mechanism has a very low angle southeast-dipping fault plane or a high-angle west-dipping fault
plane. Also, it has the deepest depth estimate (26 km) in our study
(Table 2 and Fig. 5). Its epicentral location lies below the gentle
domes of the Cuyo basin (Figs 2 and 9). Based on seismic reflection data, a tectonic suture in the Cuyania terrane between the
Precordillera and the western Sierras Pampeanas basements has
been proposed further north (∼100 km) at similar middle-to-lower
crustal depths (Cominguez & Ramos 1991). Perhaps the very low
angle fault-plane solution of event 02-005 is related to the same
proposed suture providing evidence of its present activity. This coincides with a major change in the geometry of the subducted Nazca
plate near this latitude where the plate changes from a near horizontal
dip to a steeper dip of 30◦ (Fig. 1).
The seismic moment tensor solutions for this region give average
P- and T-axis orientations of azimuth 82◦ and 156◦ , and plunge of
11◦ and 55◦ , respectively.
5.4 High Cordillera
Shallow seismicity that occurs in the high Cordillera is absent north
of 33◦ S for magnitude 4.0 and larger events (Fig. 1). An interesting
concentration of crustal seismicity began in 2001 October around
Tupungatito volcano (Fig. 2), generating 80 aftershocks in approximately one month (Barrientos et al. 2004). This location marks
the beginning of the crustal seismicity to the south. Coincidentally,
the downgoing plate changes to a steeper subduction angle, an active volcanic arc starts to the south, and the Precordillera and the
basement-cored faulting of the Sierras Pampeanas are absent in
the foreland (Figs 1 and 2) (Ramos et al. 2002). We modelled the
main shock (Ms = 5.6) of this sequence (event 01-285) that occurred on 2001 October 14 southwest of Cordón del Plata in the
Frontal Cordillera (Fig. 2). This was the largest continental crust
event recorded during the CHARGE experiment. Its focal mechanism corresponds to a near-vertical strike-slip fault-plane solution
with a very shallow focal depth of 5 km (Fig. 9). The CMT focal
mechanism solution, constrained for a fixed depth of 15 km, is very
similar (Table 2).
Another larger (M w = 6.5) earthquake (event 04-241 in Fig. 9)
with an epicentre located 190 km southwest of event 01-285 occurred on 2004 August 28, generating 85 aftershocks in 2 days. The
Harvard-CMT focal mechanism solution also indicates a strike-slip
solution (M 0 = 7.18 × 1018 N m; M w = 6.5; depth = 16 km;
fault plane 1: strike = 21◦ , dip = 61◦ , rake = −178◦ ; fault plane 2:
strike = 290◦ , dip = 88◦ , rake = −29◦ ). If this earthquake and event
01-285 are being generated by the same major strike-slip Andean
structure, we infer that the most likely ruptured plane is very approximate to the NE-striking nodal plane of both solutions. A detailed
study including the aftershocks detected for this earthquake following a methodology similar to that used by Fromm et al. (2005) may
lead to a determination of the activated fault plane.
Although not in an organized pattern, recent GPS data show a
slight clockwise rotation of the backarc vectors with respect to the
forearc vectors relative to the stable cratonic South America (Brooks
et al. 2003). Studies in other segments (18◦ S–28◦ S and 39◦ S–43◦ S)
of the Andean cordillera have documented strike-slip faults more
than 1000 km long (Armijo & Thiele 1990; Hervé 1994). The strikeslip faults in the High Cordillera are also related to the location of
an active volcanic arc and a more inclined position of the subducted
C
2005 RAS, GJI, 163, 580–598
Moment tensor inversion of Andean backarc crustal earthquakes
Nazca plate segments. Interestingly, those faults do not show high
seismic activity (Siame et al. 1997) as it is observed in the 33◦ S–
36◦ S Andean segment.
Reverse and thrust faults are the main mechanisms that account for shortening in the Principal and Frontal Cordillera over
the steep subduction zone (Kozlowski et al. 1993; Ramos et al.
1996; Cristallini & Ramos 2000). However, the slightly oblique
convergence direction between the Nazca and South America plates
could also provide a potential source for right-lateral slip parallel to
the Andes (Siame et al. 1997). Previous workers have recognized
Palaeozoic strike-slip movements along a main fault that limits the
Principal and Frontal Cordilleras (Fuentes et al. 1993). The same
structure is believed to have been reactivated as a thrust fault that
uplifted the Frontal Cordillera as a block (Kozlowski et al. 1993).
The possible occurrence of event 01-285 along this major structure
would be an indication of a modern reactivation of this structure
as a right-lateral strike-slip fault. In addition, other strike-slip deformation in the High Cordillera has been recognized constraining
focal mechanisms in the Chilean side (Alvarado 1998). The 1958
(Ms = 6.9) Las Melosas, Chile, earthquake is another example of a
strike-slip seismic source in the Cordillera (Piderit 1961; Barrientos
et al. 2004) (Fig. 2).
We also modelled another moderate-sized earthquake, event
01-074 (M w = 4.8), that occurred at the orogenic front of the Principal Cordillera (Figs 2 and 9). Its location lies 25 km east of the
Sosneado volcano in the Malargüe fold and thrust belt (Fig. 2). This
thick-skinned deformation was developed by tectonic inversion of
a pre-Andean rift system during late Cenozoic times (Ramos et al.
1996). Our focal mechanism solution indicates a shallower westdipping or a more inclined east-dipping fault plane and a very shallow focal depth of 3 km. The main dip-slip movement found is
consistent with the active structures recognized in the area.
Previous seismic studies cited above and our analysis show that
moderate earthquakes occur at very shallow depths in this region.
Therefore, seismic studies can have important implications in the
evaluation of seismic hazards in Mendoza, as other authors have
already indicated (Bastias et al. 1993; Zamarbide & Castano 1993;
Moreiras 2004).
6 C O N C LU S I O N S
Seismic moment tensor inversions using regional broad-band
CHARGE waveforms were determined for 27 moderate-sized
(3.5 < M w < 5.3) earthquakes located in the Andean backarc crust
over the flat-slab segment (30◦ S–33◦ S) and in the high Cordillera
south of 33◦ S. The SMTI solutions show mainly reverse or thrust
events with average P- and T-axis orientations of azimuth 275◦ and
90◦ , and plunge 6◦ and 84◦ , respectively. This is also in agreement
with previous results around San Juan from the PANDA experiment
(Regnier et al. 1992).
The basement of the Cuyania terrane had the highest seismic
activity during the 2000–2002 CHARGE deployment. The Valle
Fértil fault, which is also a terrane boundary, separates the region
in terms of level of seismic activity. Very limited crustal seismicity
of M > 4.0 occurs in the Pampia terrane over the flat subduction
region. However, this terrane transfers deformation into the adjacent
cratonic areas. We observed reverse-fault crustal earthquakes in the
Rı́o de la Plata terrane as far as 800 km east of the oceanic trench.
This is also observed in the historical seismicity of the region.
Most of the reverse or thrust earthquakes occurred at mid-crustal
focal depths (>21 km) in the western Sierras Pampeanas and near
C
2005 RAS, GJI, 163, 580–598
595
Mendoza. They are probably associated with brittle behaviour on
deeper décollements that are related to reactivation of old faults
and terrane sutures. Although strike-slip focal mechanisms are not
dominant features in the region, we found two earthquakes with very
shallow strike-slip solutions, one in the eastern Sierras Pampeanas
and the other in the high Cordillera. The event in the Cordillera
as well as another crustal earthquake that occurred in 2004 in its
vicinity could be accommodating strike-slip deformation produced
by the slightly oblique plate convergence.
Our tests of different average seismic velocity models for the crust
indicate that a higher P-wave seismic velocity (6.2–6.4 km s−1 ), a
lower S-wave seismic velocity (Vp/Vs = 1.80–1.85) and a thicker
crust (45–52 km) represent the western Sierras Pampeanas, while a
Vp of 6.0–6.2 km s−1 , a Vp/Vs ratio of 1.65–1.70, and a thickness
of 27–37 km characterize the eastern Sierras Pampeanas. Exposed
basement rocks in the eastern and western regions also exhibit different compositions. The western Sierras Pampeanas have a more
mafic composition with very little granitic rocks compared to the
eastern Sierras Pampeanas. The high seismic activity and different
seismic properties in the western Sierras Pampeanas terrane may be
due to its different basement composition, to a more fractured crust
in the western part, or to a combination of these factors over the flat
subduction region.
AC K N OW L E D G M E N T S
We are especially grateful to Robert Fromm, who passed away in
July 2004, for his generosity and helpful assistance with scripts
and constructive suggestions. We thank the GSAT group at the
University of Arizona for fruitful discussions and Rick Bennett
for his help in interpreting GPS data. Mauro Saez helped us
with the preparation of the figures of this paper. National Science
Foundation grant EAR-9811878 supported the project CHARGE.
The instruments used in the field program were provided by the
PASSCAL facility of the Incorporated Research Institutions for
Seismology (IRIS) through the PASSCAL Instrument Center at
the New Mexico Institute of Mining and Technology. Processing of the seismic data was performed using Seismic Analysis
Code SAC2000 software. Maps were generated using Graphic
Mapping Tools software. We also used seismological data retrieved from the International Seismological Centre, Harvard-CMT,
and INPRES on-line bulletins. We acknowledge our gratitude
to the Universidad Nacional de San Juan and the Instituto Nacional de Prevención Sı́smica of Argentina, the Departamento de
Geofı́sica at the Universidad de Chile (Chile), and all personnel
for their support to the CHARGE project (www.geo.arizona.edu/
CHARGE).
We are very grateful for the valuable help during the field work
in Argentina from Ministerio de Obras y Servicios Públicos, San
Juan Government; Escuadrón 25 ‘Jáchal’ (San Juan) and Escuadrón
29 ‘Malargüe’ (Mendoza) from Gendarmerı́a Nacional; Embalse
Las Pichanas and familia Farroni, DIPAS-Repartición Cruz del
Eje (Córdoba); Parque Nacional Ischigualasto-Valle de la Luna
(Ischigualasto, San Juan); Dique Nivelador Pachimoco (San Juan);
Municipalidad and Secretarı́a de Turismo de Malargüe (Mendoza);
Puesto ‘Los Rincones’ (familia Ontiveros, Ischigualasto, San Juan),
‘Puesto Las Peñas’ (familia Pérez, Malargüe, Mendoza), ‘Puesto
Vega-Miranda’ (familia Moyano, Malargüe, Mendoza), ‘Puesto La
Niebla’ (familia Zambrano, Malargüe, Mendoza).
We thank the editor John Beavan, and two anonymous reviewers
for their comments to improve this paper.
596
P. Alvarado et al.
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